Spontaneous emission of polaritons from a Bose-Einstein condensate

小学上册英语第1单元期末试卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.We have a lot of ________ (作业) to finish.2.What is the capital city of Canada?A. TorontoB. OttawaC. VancouverD. MontrealB3. A goldfish is a popular _______.4.The book is on the ___. (table)5.We can hear the ___ of the birds. (song)6.What is the name of the famous scientist who developed the theory of evolution?A. Albert EinsteinB. Charles DarwinC. Isaac NewtonD. Nikola Tesla7.My sister loves to ________.8. A ___ (小狐狸) is clever and sneaky.9.The tiger is a powerful __________ (掠食者).anic chemistry studies compounds that contain _____.11.The first modern Olympics were held in ________ (1896).12. A __________ is a small furry animal that often lives in burrows.13.My mom, ______ (我妈妈), always helps me with my homework.14.What is the capital of Denmark?A. CopenhagenB. OsloC. StockholmD. Reykjavik15.We have a ______ (快乐的) gathering for Thanksgiving.16.What is the fastest land animal?A. LionB. CheetahC. HorseD. LeopardB17.The first man to walk on the moon was ______ (尼尔·阿姆斯特朗).18.The _______ (The Boston Massacre) intensified anti-British sentiments in the colonies.19. A ______ is a chart that shows relationships among elements.20.What is the term for a baby chicken?A. CalfB. DucklingC. ChickD. LambC21.What is the name of the famous clock tower in London?A. Big BenB. The ShardC. The GherkinD. The Tower of LondonA22.In chemistry, a solution is a _______ mixture.23.What do we call a scientist who studies the environment?A. EcologistB. BiologistC. ChemistD. GeologistA24.What do we call a person who studies rocks?A. GeologistB. BiologistC. ChemistD. Astronomer25. A plant’s _____ (生命周期) includes germination, growth, and reproduction.26.The _____ (rainbow/sun) is colorful.27.My favorite drink is ______. (juice)28.I planted a __________ (花坛) in my backyard.29.The chemical symbol for thulium is _____.30. A ______ is a large, flat area of land at a high elevation.31.What is the name of the famous clock tower in London?A. Elizabeth TowerB. Big BenC. Tower BridgeD. London BridgeB32.I also want to take my pet on ______. We could explore new places and meet other animals. It would be an adventure for both of us! I hope to find parks where pets are allowed to run and play freely.33.The process of evaporation occurs when a liquid turns into a ______.34.The cat is ______ on the window. (sitting)35.What is the smallest continent?A. AsiaB. AfricaC. AustraliaD. EuropeC Australia36.I have a collection of ________ from different countries.37.We have a family tradition of making ________ (饼干) during the holidays. It’s a sweet ________ (传统).38.What do you call a young male sheep?A. RamB. LambC. KidD. PupB39.Mountains usually have steep ______.40.The bat flies at ____.41.The book is _____ (interesting/boring).42.I love to ride my ______ (bike) in the park.43.What do we call a young goat?A. CalfB. KidC. LambD. CubB44.What do we call the outer layer of the Earth?A. CoreB. MantleC. CrustD. AtmosphereC45.Minerals are naturally occurring ______ with a definite chemical composition.46.The vulture often eats _________ (尸体).47.I like to _______ (观察) the stars at night.48.My ________ (玩具名称) is inspired by my favorite character.49.She enjoys ________ (种花).50.What is the name of the famous physicist who discovered the law of gravity?A. Albert EinsteinB. Isaac NewtonC. Galileo GalileiD. Nikola TeslaB51.What do we call the main character in a story?A. AntagonistB. ProtagonistC. HeroD. VillainB52.He is very _____ (细心) about his work.53.Batteries convert chemical energy into ______ energy.54. A ______ is a nocturnal animal.55.The chemical formula for silver bromide is _____.56.invasive species) disrupt local ecosystems. The ____57.I love to play ______ (户外运动) with my friends. It keeps us active and healthy.58.The __________ (历史的推动) inspires change.59. A __________ is an area of land that is very mountainous.60.The first female aviator to fly solo across the Atlantic was _______ Earhart.61.Indicators are substances that change color in response to _____ (pH changes).62.My mom makes the best ______ (soup).63. (33) Mountains separate Europe and Asia. The ____64.The Sun is classified as a yellow ______ dwarf.65.I like to watch the ________ (日出) in the morning.I like to watch the ________ bloom.66.My teacher is __________, and she/ he helps us __________.67.What do we call the process of turning milk into cheese?A. FermentationB. CoagulationC. PasteurizationD. Homogenization68.What do you call a group of singers?A. ChoirB. BandC. OrchestraD. EnsembleA69.The __________ was a significant event in American history for women's rights. (妇女选举权运动)70.小狼) runs fast in the wild. The ___71.What do we call a small insect that can fly?A. ButterflyB. BeeC. AntD. Ladybug72.The process of mixing a solute into a solvent is called ______.73.I want to ___ (visit/see) the zoo.74.My sister enjoys singing in the ____ (choir).75.What do you call a place where you can borrow books?A. BookstoreB. LibraryC. ClassroomD. OfficeB76.The chemical symbol for sodium is _____ (Na).77.What is the chemical symbol for gold?A. AuB. AgC. FeD. PbA78. A snake can be very ______.79.My uncle is a skilled ____ (potter).80.ssance began in ________. The Rena81. A horse makes a ______ sound.82.The _______ of a circuit can be affected by resistance.83. A _____ (植物园) showcases various species.84.The __________ (附近) has many interesting places to explore.85.Which gas do humans breathe in?A. Carbon DioxideB. OxygenC. NitrogenD. HeliumB86.Which fruit is yellow and curved?A. AppleB. BananaC. OrangeD. GrapeB87.What do you call a young giraffe?A. CalfB. FoalC. PupD. Kit88. A _______ is a measure of how much mass is present in a certain volume.89.My friend has a ________ that likes to dig.90.What is the capital of Argentina?A. Buenos AiresB. SantiagoC. LimaD. CaracasA91.Which animal is known for building dams?A. BeaverB. OtterC. SquirrelD. RabbitA92.The _____ of an element is determined by the number of protons in its nucleus.93.__________ are used in baking for their leavening properties.94.The chemical symbol for copper is _______.95.We have ________ (lunch) at noon.96.The chemical symbol for scandium is ______.97.The Earth's surface is covered by various types of ______ ecosystems.98. A _____ (植物联盟) can promote conservation efforts.99.The bird is ___ in the sky. (soaring)100.What do you call a person who plays golf?A. GolferB. PlayerC. AthleteD. SportspersonA。

ReviewSurfaced-enhanced cellular fluorescence imagingQi Hao a ,Teng Qiu a ,b ,⇑,Paul K.Chu b ,⇑aDepartment of Physics,Southeast University,Nanjing 211189,China b Department of Physics and Materials Science,City University of Hong Kong,Tat Chee Avenue,Kowloon,Hong Kong,China a r t i c l e i n f o Keywords:Surface-enhanced spectroscopy Surface plasmons Cellular fluorescence imaging Fluorescence microscopy Plasmonic nanostructuresa b s t r a c tThe novel and burgeoning technique of surfaced-enhanced cellularfluorescence imaging has tremendous potential in the monitoringand investigation of intracellular processes at the single-molecularlevel,for instance,high-resolution cellular imaging,long-termin vivo observation of cell trafficking,tumor targeting,and diagnos-tics.The success hinges on the development and fabrication ofplasmonic nanostructured surfaces with size and shape compatiblewith cell interactions because they are crucial to enhanced cellularimaging.In this review,the mechanism of surface-enhanced cellu-lar fluorescence imaging is discussed in view of metal-enhancedfluorescence.The design of nanostructured surfaces with evenlydistributed plasmonic fields suitable for enhanced cellular fluores-cence imaging such as nanoparticle superlattice coatings,litho-graphically-based substrates,and alumina-templated surface aredescribed.Ó2012Elsevier Ltd.All rights reserved.Contents1.Introduction .........................................................................241.1.Conventional cellular fluorescence imaging methods and limitations......................241.2.Surfaced-enhanced cellular fluorescence imaging:a novel and burgeoning techniquefor cellular imaging.................................................................252.Mechanism of surface-enhanced cellular fluorescence imaging ................................262.1.Surface plasmons (SPs)...........................................................262.2.Surface-enhanced cellular fluorescence imaging.......................................280079-6816/$-see front matter Ó2012Elsevier Ltd.All rights reserved./10.1016/j.progsurf.2012.03.001⇑Corresponding authors.Address:Department of Physics,Southeast University,Nanjing 211189,China and Department of Physics and Materials Science,City University of Hong Kong,Tat Chee Avenue,Kowloon,Hong Kong,China (T.Qiu),Department of Physics and Materials Science,City University of Hong Kong,Tat Chee Avenue,Kowloon,Hong Kong,China (P.K.Chu).E-mail addresses:tqiu@ (T.Qiu),paul.chu@.hk (P.K.Chu).24Q.Hao et al./Progress in Surface Science87(2012)23–453.Design of surfaces suitable for enhanced cellular fluorescence imaging (29)3.1.Distance dependence (29)3.2.Surface patterns (31)4.Nanostructured surfaces with evenly distributed plasmonic fields compatible with enhanced cellularfluorescence imaging (33)4.1.Nanoparticle superlattice coatings (33)4.2.Lithographically-based substrates (34)4.3.Alumina-templated surface (35)5.Major applications of surfaced-enhanced cellular fluorescence imaging (36)5.1.Single molecule detection(SMD) (36)5.2.Long-term imaging with high resolution (38)6.Conclusion and future challenges (39)Acknowledgements (40)References (41)1.Introduction1.1.Conventional cellularfluorescence imaging methods and limitationsThe history and development offluorescence and associated techniques is replete with indepen-dent inventions spanning almost two centuries.In1845,thefirst observation offluorescence from a quinine solution was reported[1].To describe the phenomenon and explain the mechanism,George Gabriel Stokes initially used the term‘‘dispersive reflection’’in his article‘‘On the Change of Refran-gibility of Light’’in1852[2].The20th century witnessed ground breaking discoveries such as excita-tion spectrum of a dye(1905),fluorescence quenching(1919),fluorescence polarization in a dye solution(1923),determination offluorescence yields(1924),direct measurement of nanosecond life-time(1926),Jablonski diagram(1935),and quantum mechanical theory of dipole–dipole interactions (1948).These and other studies established the theoretical basis,and being highly-sensitive,specific and non-invasive,fluorescence techniques have been quickly adopted by scientists and engineers in many areas of molecular biology,chemistry,geology,gemology and so on.Consequently,related applications pertaining to analysis,sensing,lighting,equipment,dyes,and biological detection have been spawned[3–6].These newfluorescence applications strongly promoted development of basic and applied life sci-ences including genomics,proteomics,bioengineering,medical diagnostics,and industrial microbiol-ogy[7–12].At present,some relatively simple and convenient methods such asfluorescence resonance energy transfer[13],fluorescence lifetime imaging[14,15],fluorescence polarization-re-solved imaging[15],and total internal reflectionfluorescence[16,17]are commercially viable.These techniques not only render the detection and monitoring of protein activity in cells possible,but also provide sensitivity sufficiently high for medical diagnostics and genomics[18–21].In addition,the development offluorescent probes has spurred the studies of protein localization and functions in liv-ing cells.Thesefluorescent probes provide convenient markers in the in vivo study of gene expression and protein targeting in single cells and whole organisms[22].These and other discoveries have rev-olutionized cell biology by allowing scientists to monitor molecular activities inside living cells in real time,that is,cellular imaging.Cellular imaging is especially useful to non-invasive monitoring of diseases,evaluation of drug ef-fects,assessment of the pharmacokinetic behavior of drugs,and identification of molecular biomark-ers for diseases[23].In order to accurately study the progress,a multi-color biological labeling method is needed for membrane protein imaging.Cellularfluorescence imaging meets the requirement due to the large range offluorophores with distinctive spectral characteristics suitable for clinical practice. Moreover,it offers the specific,targeted imaging contrast needed for the study of specific cellular processes.Hence,cellularfluorescence imaging,particularlyfluorescence microscopy,is now an irreplaceable tool in biomedical science.Fluorescence microscopy,which allows the detection ofQ.Hao et al./Progress in Surface Science87(2012)23–4525 single molecules,has good compatibility with living cells and is used in dynamic and minimally inva-sive imaging experiments.Many differentfluorescent dyes can be used to stain various structures or chemical compounds[24].One particularly powerful method is to couple afluorophore such asfluo-rescein or rhodamine to antibodies in immunostaining.Fluorescent dyes can be chemically bound to antibodies and coupled to a specific protein in cells thereby providing microscopic contrast with high specificity[25].Cellularfluorescence imaging methods can be used to quantify cellular phenotypic changes and investigate the role of a particular target in in vitro disease studies.Moreover,the advent offluores-cence microscopy and other sophisticated techniques has enabled routine studies of dynamic pro-cesses in living cells.Thesefluorescence techniques can deliver the necessary resolution to image certain cellular organelles and track proteins in for example,the nucleus,endoplasmic reticulum, and Golgi apparatus,and other biomolecules in living cells[26],so that valuable information about the dynamics of intracellular networks,signal transduction,and intercellular interactions can be ob-tained.Versatilefluorescence-based techniques,particularly confocal microscopy and wide-field microscopy,excel in cell imaging due to theflexible resolution and different magnification options [27].Specially,methods such as magnetic resonance imaging and optical coherence tomography can provide real-time cellular imaging but can hardly achieve a resolution of less than$10l m.By contrast,electron microscopy can provide almost molecular-level spatial resolution.However,dy-namic imaging is not available by electron microscopy because this method is invasive.Between these two resolution extremes,fluorescence microscopy is better suited to cell imaging.In spite of recent advances,the spatial and temporal resolution of cellularfluorescence microscopy is limited by traditional organicfluorescent dyes.When usingfluorophores as dyes,the signal cannot be easily discerned[28,29]from the background of autofluorescence,which is natural emission from biological entities[30].Autofluorescence can be problematic influorescence microscopy due to sev-eral reasons.First of all,the unwanted signals may interfere with specificfluorescent signals especially when the latter are very dim.Secondly,the emission lifetime on the order of2–4ns is very close to that of the cell autofluorescence background,and thirdly,organicfluorophores are known to emit sig-nals with poor stability and strong blinking[31,32].In many instances,the sensitivity is limited by autofluorescence from the sample rather than lack of signal.Hence,one of the main challenges is to improve the sensitivity and photostability.1.2.Surfaced-enhanced cellularfluorescence imaging:a novel and burgeoning technique for cellular imagingIn order to further improve the sensitivity,there are continuous attempts to lower the detection limits.Detection of afluorophore is usually limited by the quantum yield,autofluorescence from the sample,and photostability of thefluorophore.In addition,the complex milieu encountered inside living cells requires substantial adaptation of current in vitro techniques.Hence,in order to enhance the performance of existing imaging techniques,there is increasing use of metallic nanostructures to modify the spectral properties offluorophores and to overcome some of these photophysical constraints.As a spontaneous emission process,fluorescence involves the interaction between the emitter and its environment and is thus subjected to external factors[33].It creates the possibility of tailoring the fluorescence process to increase the emission intensity[34].In early studies onfluorophore-metal interactions in the1970s[35,36],afluorophore was used to interact with a smooth silver or goldfilm typically about40nm thick and thefluorescence intensity and lifetime were not altered dramatically. However,in contrast to a smooth metallic surface,rough surfaces seem to interact more strongly with light[37].The generally accepted view is that intensity amplification arises mainly from the electric field enhancement that occurs in the vicinity of small(in relation to the light wavelength),interacting metal particles illuminated with light in resonance or near resonance with the localized surface-plas-mon(SP)frequency of the metal structure.Silver or gold colloids are typically sprayed onto a substrate to obtain a suitable surface with the appropriate electromagnetic(EM)resonance orfilms with silver islands are produced by chemical means.Depending on the geometry and the distance between the metal andfluorophore,these surfaces can result influorescence enhancement by factors of up to26Q.Hao et al./Progress in Surface Science87(2012)23–451000[38,39].Even though spraying silver or gold colloids onto a substrate produces highfluorescence signals from some local‘‘hot spots’’,it is not easy to obtain reliable,stable,and uniform signals span-ning a wide dynamic range from the metal surface due to particle aggregation.Hence,methods that can produce as evenly distributed nanoscale surface are necessary in order to make sure that different parts of the sample can have the same degree of amplification and enable the use of surfaced-en-hanced cellularfluorescence imaging in the study of intracellular processes at the single-molecule le-vel including high-resolution cellular imaging,long-term in vivo observation of cell trafficking,tumor targeting,and diagnostics.Nevertheless,although this technique boasts the aforementioned advanta-ges,many issues are still not well understood and require more research,for instance,reproducible control of the nanoparticle-surface properties.Furthermore,the variable response of different cell types,nonspecific serum-protein adsorption on the nanoparticle surface,and different experimental conditions render systematic studies of surfaced-enhanced cellularfluorescence imaging challenging. Fortunately,recent advances in nanobiotechnology have made it an achievable and worthwhile goal. In this review,we discuss the mechanism and applications of surfaced-enhanced cellularfluorescence imaging and describe the different methods to produce plasmonic nanostructures to overcome the aforementioned problems.The advantages and limitations of existing techniques are summarized and ideas about further improvement are presented.2.Mechanism of surface-enhanced cellularfluorescence imaging2.1.Surface plasmons(SPs)Metal-enhancedfluorescence is the major mechanism in surface-enhanced cellularfluorescence imaging.Studies about the effects of metallic particles and surfaces onfluorescence enhancement da-ted back to the reports of Drexhage in1970[35].Afluorophore in the excited state has the properties of an oscillating dipole(Fig.1)[40]and the excitedfluorophore can induce electron oscillations in the metal.The electricfield created by the metal can interact with the excitedfluorophore and alter its emission.This interaction is bidirectional,so that light-induced oscillations in the metal can affect thefluorophore by offering decay channels.A reduced lifetime is found when the reflectedfield is in phase with the oscillating dipole of thefluorophore and conversely,an increase is observed when the reflectedfield is out of phase with thefluorophore dipole.This discovery has spurred many subsequent theoretical and experimental studies on the interac-tions between the oscillating dipole and metallic surfaces/particles[41–45]and readers are referred to several excellent review articles for these important developments during this period[46–48].It wasplasmon polaritons in the following discussion.SPs at the interface between a metal and dielectric materials have combined EM wave and surface charge characteristics.The EM wave has a transverse magnetic characteristic and the generation of surface charge requires an electricfield normal to the surface.These two effects enhance thefield component perpendicular to the surface near the surface and it decays exponentially with distance away from it[46].The perpendicularfield is evanescent or near-field in nature being a consequence of the bound,non-radiative nature of SPs because the wave vector of a polariton is normally larger than that of a free photon at the same frequency and prevents power from propagating away from the surface.Another important effect arising from the interaction between the surface charge density and EMfield is the momentum of the SP mode,⁄k SP.Solving Max-well’s equations using the appropriate boundary conditions yields the SP dispersion relationship[51], that is,frequency-dependent SP wave-vector,k SP,as:k SP¼k0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiede medþe m r:The frequency-dependent permittivity of the metal,e m,and dielectric material,e d,must have oppo-site signs in order for SPs to be possible at the interface.This condition is satisfied on metals because e m is both negative and complex(the latter corresponding to absorption on the metal).The increase in momentum is associated with binding of the SPs to the surface,and the resulting momentum mis-match between the light and SPs with the same frequency must be bridged if light is to be used to generate SPs.To provide the missing momentum,we may make use of scattering from subwavelength protrusions or holes on the surface to generate SPs locally[52,53].A periodic corrugation in the metal surface can also provide the momentum[54]and some other techniques are also available to solve this problem[55,56].We now discuss how SPs propagate on the metal surface.After light has been converted into an SP mode on aflat metal surface,it will propagate but be gradually attenuated due to losses from absorp-tion by the metal.The degree of attenuation depends on the dielectric function of the metal at the oscillation frequency of the SPs.A classical theoretical approach to calculate the relaxation processes induced by the modified environment presents accurate results on the process in the case of a planar interface[46,48].However,if theflat metal surface is changed to a rough or corrugated one,the results can be difference.Calculation[57]demonstrates that absorption by the metal can be overcome when the surface is periodically textured on the scale of the light wavelength.This is important tofluores-cence enhancement because it provides an effective way to maximize radiative relaxation as all relax-ation channels compete with each other.Besides,silver exhibits the smallest loss in the visible spectrum and can lead tofluorescence enhancement.When the periodic length of the nanostructure is smaller than half of the effective wavelength,reduced absorption by the metal is observed together with strong localized SP resonance[58].Simultaneously,the electric and magneticfields can be local-ized thereby giving rise to an inhomogeneous EMfield distribution that can be exploited in local EM enhancement[59].The inhomogeneity can also be used to specifically excite a givenfluorophore and constitutes one of the causes of large enhancement when coupled to SPs.All in all,localization of the EM near metal nanostructures and coupling to propagating modes can increasefluorescence by sev-eral orders of magnitude[49].Based on the previous discussion,it can be inferred that near-field coupling between the emitter and surface modes plays a crucial role in metal-enhancedfluorescence.The near-field components provide resonant coupling between thefluorophore and metallic surface,resulting in large enhance-ment in the vicinity of metallic particles when the size is much smaller than the detected wavelength. By means of near-field transmission imaging and near-field two-photon excitation imaging,investiga-tion of SPs on metal nanostructures can be conducted[60].It is noted that plasmons are the results of quantization of classical plasma oscillations,and so most of their properties can be derived from Maxwell’s equations.Thus it is possible to design specific surfaces with matching optical spectrum utilizing calculated results since the surface morphology of the materials determines the types of SPs[51].Hence,the main challenge of surface-enhancedfluorescence imaging is thus to control and design surface structures to maximize thefluorescence enhancement of afluorophore by adjust-ing the local electricfield.Q.Hao et al./Progress in Surface Science87(2012)23–45272.2.Surface-enhanced cellularfluorescence imagingFluorescence is a spontaneous emission process and the rate of relaxation is dictated by the cou-pling between the excited state of the molecule and vacuum oscillations in the surroundings.This can be reformulated in classical terms where the probability of photon emission is related to the pho-tonic mode density[33].As aforementioned,fluorophore emission can be modified by the EM bound-ary conditions near thefluorophore and so enhancement offluorescence partly relies on the ability to tailor the local environment of the molecule to maximize the radiative relaxation rate compared to the samefluorophore in free space.This is accomplished by minimizing competitive non-radiative pro-cesses[49].In most cases,it results in a reduction of thefluorescence lifetime and increase in thefluo-rescent intensity of thefluorophore.Several studies have been performed to study this issue[25,61–65]and it has been shown that the quantum yield is the most important characteristics of afluoro-phore.Good coupling between the localized EMfield and propagating modes enables successful com-petition with internal non-radiative decay of the emitter via emission of phonons into the materials immediately adjacent to the emitter.In this way,the low radiative quantum yield of emission may be enhanced[66].Thefluorescence quantum yield is defined as the ratio of the number of photons emitted to the number absorbed.Here,U describes the emission rate of thefluorophore or all the EM relaxation processes and k nr represents the sum of all possible non-radiative decay rates.The quantum yield Q and lifetime s are given byQ¼C C nrands¼1:The presence of a nearby metallic surface can modify the radiative rate of an excitedfluorophore. Supposing that afluorophore is located at a given distance from a metallic surface and the radiative rate increase is given by U m,the quantum yield(Q m)of thefluorophore is given byQ m¼CþC mCþC mþk nr:Accordinglys¼1CþC m:This equation suggests that high quantum yields can be accomplished when U m is comparable to k nr.Furthermore,these effects are larger forfluorophores with lower quantum yields and hence,low quantumfieldfluorophores can be enhanced by fabricating metallic particles with a suitable size and shape.Altering the distance is another important aspect to achieve maximum enhancement and it will be discussed in the next section.The process is termed radiative decay engineering be-cause the increased radiative decay rate is perhaps the most unusual effect of a metallic surface[25]. It is unusual because this intrinsic rate which is determined by the extinction coefficient and the local refractive index is typically constant in a givenfluorophore[67].In radiative decay engineering, an increase in the radiative decay rate provides a unique way to increase the quantum yield while the lifetime decreases.It results in useful emission from weaklyfluorescent molecules thereby offer-ing the potential to imagefluorophores with intrinsically low quantum yield in cells.By analyzing the equation shown above,the reduced lifetime is concurrent with increased decay rate.These changes increase the sensitivity and photostability while interference from unwanted background emission can be reduced.Relevant applications are thus being developed and the use of metal-en-hancedfluorescence in cellular imaging,so-called surface-enhanced cellularfluorescence imaging,is a good example.28Q.Hao et al./Progress in Surface Science87(2012)23–45Q.Hao et al./Progress in Surface Science87(2012)23–4529 3.Design of surfaces suitable for enhanced cellularfluorescence imaging3.1.Distance dependenceWe have mentioned thefirst study aboutfluorescence enhancement was in1970.However, researchers got little achievements in the next20years.The research on metal-enhancedfluorescence in this period was overshadowed by the large signal enhancements offered by surface-enhanced Ra-man scattering.Efficient Raman enhancement requires close contact between the molecules being studied and metallic surface and the typical distance is less than2–3nm.At this short distance,fluo-rescence of molecules is significantly quenched primarily by energy transfer to the metal surface. Fluorescence enhancements that are much smaller than those of the Raman signals have been ob-served[68].Pockrand et al.[69]used momentum-matching techniques to determine the distance dependence of the coupling between the emitters and SPs and found a maximum coupling distance of approximately20nm.Knobloch et al.[70]also observed an optimum coupling distance for SPs using gratings to scatter SPs thus allowing the SPs decay channel to be monitored.In1997,W.L. Barnes measured the decay time of a europium(Eu3+)complex positioned at various distances from a planar silver mirror[71].They investigated the distance dependence on the emission lifetime of Eu3+ions in front of silver mirrors with thicknesses from13to200nm.The mirrors were coated with spacer layers of22-tricosenoic acid using the Langmuir–Blodgett technique[72,73].By varying the number of Langmuir–Blodgett layers,the emitter surface separation could be varied in a controlled way.By conducting lifetime measurements,they found that the spontaneous emission rate of Eu3+ could oscillate with both the distance and thickness of the silver mirror but the lifetime of the thinnest film displayed a different tendency.Thefluorescence lifetime dropped dramatically for small metal-fluorophore distances[74],implying that spontaneous emission was quenched for small emitter-sur-face separations.They were also surprised tofind that coupling between the emitter and SP mode was maximum for a small butfinite separation between the emitter and surface.They attributed this phe-nomenon to the competition between the decay channels and surface waves.As the separation was reduced,the latter decay route became dominant.A systemic explanation was proposed to explain this phenomenon[33].Relaxation processes on flat metallic thinfilms have already been discussed earlier in this paper.The emitter interferes with the reflected EM waves and the spontaneous emission rate oscillates with increasing distances[75]. In the experiments conducted by W.L.Barnes,as the thickness of the Langmuir–Blodgett layers was increased,it wasfinally able to support a waveguide mode.The waveguide mode,like SPs,is a resonant optical mode of the system and may provide a new decay route for the excited molecule. As the thickness of the layer is increased,further waveguide modes may be supported by the structure thereby adding more decay channels.The coupling between excited molecules and waveguide modes is important to sensing applications[76],but in surface-enhanced cellularfluorescence imaging,this causes strong quenching of thefluorophores.Several processes can result influorescence quenching[77,78].There are three main physical quenching mechanisms,all involving excitation of an electron–hole pair[33].Process A arises from the bulk and the excitation energy of the molecule is absorbed by the creation of an exciton in the sub-strate.Process B arises from the surface and the excitation energy is absorbed by the creation of an exciton in the surface.Process C arises from the spatial variation in the nearfield,which is related to the roughness of the nanostructured surface.Besides these mechanisms,fluorophores can form nonfluorescent complexes with quenchers and can be quenched by attenuation of the incident light or other absorbing species.A classical model allows one to observe and evaluate quantitatively each relaxation channel[46].It implies that depending on the distance,metallic surfaces or particles can lead to either quenching or enhancement offluorescence[49].Commonly,strong quenching occurs at distances very close to the planar metallic surface,usually less than10nm,whereas enhancement generally occurs at tens of nanometers from the surface and is sensitive to the nanoscale roughness of the metal.However,it is difficult to predict precisely the optimal distance under a specific condition to obtain the maximum radiative decay rates because it varies with the type and surface roughness of the metal[79].Q.Hao et al./Progress in Surface Science87(2012)23–4531 Although these results provide better understanding of the distance dependence of metal-enhancementfluorescence,the spacing between thefluorophore and metallic surface is usually dif-ficult to control experimentally.Krishanu Ray adopted an improved method by using the same tech-nique together with inert amphiphilic stearic acid layers in order to more accurately control the distance[80].In this way,the distance between thefluorophore and surface islandfilms can be tai-lored by varying the number of inert amphiphilic stearic acid layers to a resolution of$2.5nm.They also used silver islandfilms as metallic materials and two long-chain nitrobenzoxadiazole derivatives (NBD-C16and NBD-C18)as probes.As shown in Fig.2,a maximum enhancement of32folds is accomplished in contact with the silver surface but the enhancement is reduced to4folds when the probe is90nm from the stearic acid layers.The maximum metal-enhancedfluorescence occurs when the probes are about10nm from the metallic structure.Thefindings suggest that silver nano-structures can be used to amplify thefluorescent signatures and increase the detection limits in cell imaging.Considering the distance dependence,the use offluorescence cellular imaging is extremely useful to the study of membrane proteins.The thickness of a cell membrane is typically about7nm,which is close to the optical distance forfluorescence enhancement.Therefore,fluorescence imaging of pro-teins on cell membranes is available without the worry of interference from signals inside the cell. It is known that membrane proteins play a key role in the physiology of living cells and carry out a number of important functions,for instance,energy metabolism,cell-cell interaction,or uptake of nutrients and ions[81].Investigation of membrane proteins in the natural environment can provide more comprehensive results considering that many intricate molecular reactions take place in the highly dynamic and complex plasma membranes.Considering good biological compatibility and low background of surface enhanced cellularfluorescence imaging,this technology,which can probe proteins in the membranes of living cells,has great potential.By comparing thefluorescent intensities of the cell nuclei and membranes,the distance-dependent phenomenon can also be further verified [82].Fig.3shows that the‘‘nucleus labeled’’cells can be imaged well by confocal microscopy on both the glass and silver islandfilms.However,in contrast to the‘‘membrane labeled’’cells,the‘‘nucleus labeled’’cells on the metal show insignificant differences in both the intensity and lifetime from those on the glass.It is clear that thefluorophores in the cell membranes are localized within,but thefluo-rophores in the cell nuclei are beyond the region of metal-enhancedfluorescence.Thus,the metal can be used to improve the detection sensitivity of intrinsicfluorescent proteins and target molecules on cell surfaces when they arefluorescently labeled.3.2.Surface patternsExperiments have confirmed the importance of overlapping between the localized SP resonance energy of surface configuration and emission energy[83].The type,shape,height,and density of the surface nanostructures determine the degree of enhancement.In order to explore the cause and mechanism,experiments have been conducted carefully often excluding other possible factors which may contribute to the enhancement such as reflection from the metallic nanostructures,emission from the metallic nanostructures themselves,increased absorption of light in photoluminescence enhancement,and quenching of defect emission.Although the exact mechanism is still debatable,it is generally agreed that the effects of metallic colloids interacting withfluorophores can be under-stood by the formation of metal interstitial sites.These interstitial sites,so-called‘‘hot spots’’or ‘‘hot junctions’’in the nanostructures,consist of two or more coupled particles or nanostructured sur-faces with closely spaced features,and there are highly concentrated EMfields associated with strong localized SP resonance[58].Metallic colloids play an important role in surface-enhanced cellularfluorescence imaging. Although spraying silver or gold colloids onto a substrate leads to a highfluorescence signal from some local‘‘hot spots’’,it is not easy to obtain a reliable,stable,and uniform signal with a wide dy-namic range due to particle aggregation.Popular approaches to remedy the problems such as poor control of the particle aggregation states include surface immobilization[84],entrapment in stable matrices[85],and fabrication of complex surface structures(e.g.by means of microfabrication) [86,87].For example,silver islandfilms have been produced on glass by reduction of silver nitrate。

低温物理学中的玻色爱因斯坦凝聚和冷原子物理低温物理学是研究在极低温条件下物质的性质和行为的学科。

在低温条件下，量子效应开始显著影响物质的行为，使得一些新的现象和现象变得显著。

玻色爱因斯坦凝聚和冷原子物理就是低温物理学中的两个重要研究领域。

一、玻色爱因斯坦凝聚玻色爱因斯坦凝聚（Bose-Einstein condensation，简称BEC）是低温物理学领域的一个重要现象，它预言了在极低温度下，玻色子（具有整数自旋的粒子）可以集体行为，形成一种新的物质状态。

这种集体行为是由波色子的波动性质所决定的。

玻色爱因斯坦凝聚的实验观察最早是在1995年由美国国家标准与技术研究院的卡尔·韦曼诺夫和埃里克·科尔林斯等科学家团队完成的。

他们通过使用激光冷却和磁力约束等技术，将亚稳态的低温铯原子气体冷却到非常接近绝对零度的温度，成功实现了玻色爱因斯坦凝聚的观测。

玻色爱因斯坦凝聚的形成是由于在极低温下，波色子的波长相互重叠，从而导致它们开始表现出集体行为。

在低温下，大量的波色子会互相吸引，并且趋向于占据相同的量子状态，形成一个大的波色爱因斯坦凝聚体。

这种凝聚态具有宏观量子性质，如相干性、超流性等，对于研究物质的量子行为和基本粒子的统计行为具有重要意义。

玻色爱因斯坦凝聚在冷原子物理领域得到了广泛的研究和应用。

利用不同原子的特性，科学家们可以通过调节外界条件，如温度、密度和与光的相互作用等因素，来研究玻色爱因斯坦凝聚体中的量子行为、相变和超流性等性质。

这使得玻色爱因斯坦凝聚成为探索量子性质和开展量子信息处理的重要平台。

二、冷原子物理冷原子物理是低温物理学中另一个重要的研究领域，它主要关注将原子冷却到极低温度，以便进一步研究和控制原子的性质和行为。

冷原子物理与玻色爱因斯坦凝聚有很多重叠之处，并且在实验技术和理论方法上有很大的交叉。

冷原子物理通过激光冷却、蒸发冷却、磁光陷阱等技术手段，将原子冷却到接近绝对零度的温度。

托福阅读语法点中的后置定语5大类型介绍店铺为大家带来“托福阅读语法点中的后置定语5大类型介绍”，希望对大家托福备考有所帮助。

更多精彩尽请关注店铺!托福阅读语法点中的后置定语5大类型介绍什么是托福阅读中的后置定语?托福阅读中后置定语，顾名思义分为后置和定语两个部分。

后置也就是此种短语出现的位置是在名词之后，定语就是起到修饰限定作用的短语，注意是短语而不是句子，本质上相当于形容词。

所以后置定语就是放在名词的后面起到限定修饰作用的短语。

托福阅读后置定语第一类形容词做后置定语。

如：fossil available.即为可用的化石。

托福阅读后置定语第二类介词短语做后置定语。

如lava on the surface，中on the surface介词短语修饰lava，表示为表面上的熔岩。

托福阅读后置定语第三类现在分词短语做后置定语。

如the blood vessels carrying cooled blood.中的carrying cooled blood就是现在分词短语用来修饰限定the blood vessels，理解为运载着凉的血液的血管。

托福阅读后置定语第四类过去分词短语做后置定语，the gradual drying of the soil caused by its diminished ability中的caused by its diminished ability就是过去分词短语做后置定语修饰the gradual drying of the soil，理解为减少的能力导致的土壤的干燥。

托福阅读后置定语第五类不定式短语做后置定语。

the ability to absorb water中to absorb water限定修饰 the ability，理解为吸收水的能力。

托福阅读TPO31第1篇:Speciation in Geographically Isolated Populations【1】Evolutionary biologists believe that speciation, theformation of a new species, often begins when some kind of physical barrier arises and divides a population of a single species into separate subpopulations. Physical separation between subpopulations promotes the formation of new species because once the members of one subpopulation can no longer mate with members of another subpopulation, they cannot exchange variant genes that arise in one of the subpopulations. In the absences of gene flow between the subpopulations, genetic differences between the groups begin to accumulate. Eventually the subpopulations become so genetically distinct that they cannot interbreed even if the physical barriers between them were removed. At this point the subpopulations have evolved into distinct species. This route to speciation is known as allopatry (“alio-” means “different”，and “patria” means “homeland”).【2】Allopatric speciation may be the main speciation route. This should not be surprising, since allopatry is pretty common. In general, the subpopulations of most species are separated from each other by some measurable distance. So even under normal situations the gene flow among the subpopulations is more of an intermittent trickle than a steady stream. In addition, barriers can rapidly arise and shut off the trickle. For example, in the 1800s a monstrous earthquake changed the course of the Mississippi River, a large river flowing in the central part of the United States of America. The change separated populations of insects now living along opposite shore, completely cutting off gene flow between them.【3】Geographic isolation also can proceed slowly, over great spans of time. We find evidence of such extended events in the fossil record, which affords glimpses into the breakup offormerly continuous environments. For example, during past ice ages, glaciers advanced down through North America and Europe and gradually cut off parts of populations from one another. When the glacier retreated, the separated populations of plants and animals came into contact again. Some groups that had descended from the same parent population were no longer reproductively compatible—they had evolved into separate species. In other groups, however, genetic divergences had not proceeded so far, and the descendants could still interbreed—for them, reproductive isolation was not completed, and so speciation had not occurred.【4】Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth’s surface. About 5 million years ago such geologic movements created the land bridge between North America and South America that we call the Isthmus of Panama. The formation of the isthmus had important consequences for global patterns of ocean water flow. While previously the gap between the continents had allowed a free flow of water, now the isthmus presented a barrier that divided the Atlantic Ocean from the Pacific Ocean. This division set the stage for allopatric speciation among populations of fishes and other marine species.【5】In the 1980s, John Graves studied two populations of closely related fishes, one population from the Atlantic side of isthmus, the other from the Pacific side. He compared four enzymes found in the muscles of each population. Graves found that all four Pacific enzymes function better at lower temperatures than the four Atlantic versions of the same enzymes. This is significant because Pacific seawater is typically 2 to 3 degrees cooler than seawater on the Atlantic side of isthmus.Analysis by gel electrophoresis revealed slight differences in amino acid sequence of the enzymes of two of the four pairs. This is significant because the amino acid sequence of an enzyme is determined by genes.【6】Graves drew two conclusions from these observations. First, at least some of the observed differences between the enzymes of the Atlantic and Pacific fish populations were not random but were the result of evolutionary adaption. Second, it appears that closely related populations of fishes on both sides of the isthmus are starting to genetically diverge from each other. Because Graves’s study of geographically isolated populations of isthmus fishes offers a glimpse of the beginning of a process of gradual accumulation of mutations that are neutral or adaptive, divergences here might be evidence of allopatric speciation in process.托福阅读TPO31试题第1篇:Speciation in Geographically Isolated Populations1.The word "promotes" in the passage is closest in meaning toA.describes.B.encourages.C.delays.D.requires.2.According to paragraph 1, allopatric speciation involves which of the following?A.The division of a population into subspecies.B.The reuniting of separated populations after they have become distinct species.C.The movement of a population to a new homeland.D.The absence of gene flow between subpopulations.3.Why does the author provide the information that "the subpopulations of most species are separated from each other by some measurable distance"?A.To indicate how scientists are able to determine whether subpopulations of a species are allopatric.B.To define what it means for a group of animals or plants to be a subpopulation.C.To suggest that allopatric speciation is not the only route to subpopulation.D.To help explain why allopatric speciation is a common way for new species to come about.4.The word "accumulate" in the passage is closest in meaning toA.Become more significant.B.Occur randomly.C.Gradually increase in number.D.Cause changes.5.In paragraph 2,why does the author mention that some insect populations were separated from each other by a change in the course of Mississippi River caused by an earthquake?A.To make the point that some kind of physical barrier separates the subpopulations of most species.B.To support the claim that the condition of allopatry can sometimes arise in a short time.C.To provide an example of a situation in which gene flow among the subpopulations of a species happens at a slow rate.D.To explain why insects living along opposite shores of the Mississippi River are very different from each other.6.According to paragraph 3，separation of subpopulations by glaciers resulted in speciation in those groups of plants andanimals thatA.were reproductively isolated even after the glaciers disappeared.B.had adjusted to the old conditions caused by the glaciers.C.were able to survive being separated from their parent population.D.had experienced some genetic divergences from their parent population.7.The word "colossal" in the passage is closet in meaning toA.consistent.B.gradual.C.enormous.D.effective.8.According to paragraph 4, which of the following is true of the geologic movements that brought about the Isthmus of Panama?A.The movements brought populations of certain fishes and marine organisms into contact with one another for the first time.B.The movements transferred populations of fishes and other marine animals between the Pacific and Atlantic Oceans.C.The movements created conditions that allowed water to flow more freely between the Pacific and Atlantic Oceans.D.The movements created conditions for the formation of new species of fishes and other marine animals.9.The word "sequence" in the passage is closet in meaning toA.quality.B.order.C.function.D.number.10.According to paragraph 5, by comparing the enzymesfrom two related groups of fishes on opposite sides of the isthmus, Graves found evidence thatA.there were slight genetic divergences between the two groups.B.the Atlantic group of fishes were descended from the Pacific group of fishes.C.the temperature of water on either side of the isthmus had changed.D.genetic changes in the Atlantic group of fishes were more rapid and frequent than in the Pacific group of fishes.11.It can be inferred from paragraph 5 and 6 that the reason Graves concluded that some of the differences between the Pacific and Atlantic enzymes were not random was thatA.each of the Pacific enzymes works better in cooler waters.B.the Enzymes of the Atlantic fish populations had not changed since the formation of the Isthmus of Panama.C.gel electrophoresis showed that the changes benefited both the Atlantic and the Pacific fish populations.D.the differences between the enzymes disappeared when the two fish populations were experimentally switched to other side of the isthmus.12.Which of the sentence below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.A.Graves's study provides evidence that isthmus fishes are in the process of becoming geographically isolated.B.Graves's study of mutating isthmus fishes yields results that differ from results of other studies involving allopatric speciation.C.Graves's study of isolated populations of isthmus fishesprovides some evidence that allopatric speciation might be beginningD.Grave's study indicates that when isolated, populations of isthmus fished register neutral or adaptive mutations.13. Look at the four squares [■] that indicate where the following sentence can be added to the passage.Where would the sentence best fit? The formation of the isthmus had important consequences for global patterns of ocean water flow.Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth's surface. ■【A】 About 5 million years ago such geologic movements created the land bridge between North America and South America that we call the Isthmus of Panama. The formation of the isthmus had important consequences for global patterns of ocean water flow. ■【B】While previously the gap between the continents had allowed a free flow of water, now the isthmus presented a barrier that divided the Atlantic Ocean from the Pacific Ocean. ■【C】This division set the stage for allopatric speciation among populations of fishes and other marine species. ■【D】14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passages or are minor ideas in the passage. This question is worth 2 points.Allopatric speciation takes place when physically separated populations of a single species gradually diverge genetically to the point of becoming unable to interbreedA.Allopatric speciation is common because the gene flow between subpopulations is generally limited and the barriers that completely separate subpopulations can arise in a variety of ways.B.During past ice ages, some, but not all, subpopulations separated by glaciers evolved into distinct species.C.Speciation does not need to take place through allopatry because subpopulations will form distinct species whenever there are adaptive advantages or notD.Physical barriers from glaciers and the movement of tectonic plates form so slowly that the subpopulations on either side of the barriers usually do not form distinct species.E.Graves's study of fish populations separated by the Isthmus of Panama may well provide a picture of the beginning stages of speciation.F.Graves's study of physically separated fish populations show that there must be large differences between the environments of the isolated populations if allopatric speciation is to take place.托福阅读TPO31答案第1篇:Speciation in Geographically Isolated Populations1.promote本身是促进的意思。

散发光成为光英语作文Radiating Light: The Luminary of the Universe.In the vast expanse of the cosmos, countless celestial bodies emit radiant energy, illuminating the darkness and illuminating our understanding of the universe. Among these luminous celestial objects, stars reign supreme as the primary source of light, energy, and awe for observers both on Earth and beyond.Stars, the building blocks of galaxies, are incandescent beacons of plasma held together by their own gravitational forces. Within their nuclear furnaces, the fusion of hydrogen atoms into helium releases prodigious amounts of energy, a process that sustains their brilliance for billions of years. This energy manifests as electromagnetic radiation, which travels through space as a spectrum of light waves.The light emitted by stars encompasses a vast range ofwavelengths, from short-wavelength gamma rays to long-wavelength radio waves. However, the human eye is only capable of perceiving a narrow band within this spectrum, known as visible light. Visible light ranges from violet to red, with each wavelength corresponding to a different color.Stars exhibit a remarkable diversity in their light output, ranging from faint and barely visible to dazzling and brilliant. The brightness of a star, as perceived by an observer on Earth, depends on several factors, includingits size, temperature, and distance from Earth.Large stars, with greater masses and hence more fuel to burn, typically emit more light than smaller stars. Temperature also plays a crucial role in determining astar's luminosity. Hotter stars emit blue and white light, while cooler stars radiate yellow or red light.The distance between a star and Earth also influences its apparent brightness. Stars that are closer to Earth appear brighter than those that are farther away. This isbecause the inverse square law of light dictates that the intensity of light decreases with the square of the distance from the source.The light of stars serves as a valuable tool for astronomers and astrophysicists. By analyzing the spectrum of light emitted by stars, scientists can determine their temperature, chemical composition, and other physical characteristics. This information helps us understand the evolution of stars, the nature of stellar populationswithin galaxies, and the history of the universe itself.Moreover, the light of stars provides a celestial beacon for navigators and explorers. For centuries, seafarers relied on the positions of stars to guide their ships across vast oceans. Even today, spacecraft venturing into the depths of space utilize star charts and celestial navigation to determine their location and trajectory.Beyond its practical applications, the light of stars also holds profound aesthetic and philosophical significance. Throughout human history, stars have capturedthe imagination of poets, artists, and philosophers. Their twinkling radiance has inspired countless works of art, literature, and music. Stars have also been associated with spirituality, divinity, and the pursuit of knowledge and enlightenment.In conclusion, the light of stars permeates our existence, providing both practical and profound benefits. It illuminates the darkness, guides our paths, and fuels our understanding of the universe. As we continue to explore the cosmos and unravel its mysteries, the light of stars will forever remain a constant and awe-inspiring source of wonder and inspiration.。

Resonant nonlinear magneto-optical effects in atoms∗D.Budker†Department of Physics,University of California,Berkeley,CA94720-7300andNuclear Science Division,Lawrence Berkeley National Laboratory,Berkeley CA94720W.Gawlik‡Instytut Fizyki im.M.Smoluchowskiego,Uniwersytet Jagiello´n ski,Reymonta4,30-059Krakow,PolandD.F.Kimball,S.M.Rochester,and V.V.YashchukDepartment of Physics,University of California,Berkeley,CA94720-7300A.Weis§Depart´e ment de Physique,Universit´e de Fribourg,Chemin du Mus´e e3,CH-1700Fribourg,Switzerland (Dated:May20,2002)In this article,we review the history,current status,physical mechanisms,experimental methods,and applications of nonlinear magneto-optical effects in atomic vapors.We begin by describingthe pioneering work of Macaluso and Corbino over a century ago on linear magneto-optical effects(in which the properties of the medium do not depend on the light power)in the vicinity ofatomic resonances,and contrast these effects with various nonlinear magneto-optical phenomenathat have been studied both theoretically and experimentally since the late1960s.In recentyears,thefield of nonlinear magneto-optics has experienced a revival of interest that has ledto a number of developments,including the observation of ultra-narrow(1-Hz)magneto-opticalresonances,applications in sensitive magnetometry,nonlinear magneto-optical tomography,andthe possibility of a search for parity-and time-reversal-invariance violation in atoms.ContentsI.Introduction2II.Linear magneto-optics3A.Mechanisms of the linear magneto-optical effects3B.Forward scattering and line-crossing4C.Applications in spectroscopy61.Analytical spectroscopy and trace analysis,investigation of weak transitions62.Measurement of oscillator strengths73.Investigation of interatomic collisions84.Gas lasers95.Line identification in complex spectra and searchfor“new”energy levels96.Applications in parity violation experiments97.Investigations with synchrotron radiation sources10D.Related phenomena101.Magnetic depolarization offluorescence:Hanle-effect and level-crossing102.Magnetic deflection of light113.The mechanical Faraday effect11 III.Linear vs.nonlinear light-atom interactions12A.Perturbative approach12B.Saturation parameters13 IV.Early studies of nonlinear magneto-optical effects14∗This paper is dedicated to Professor Eugene mins on the occasion of his70th birthday.†Electronic address:budker@‡Electronic address:gawlik@.pl§Electronic address:antoine.weis@unifr.chA.Optical pumping14B.Nonlinear magneto-optical effects in gas lasers15C.Nonlinear effects in forward scattering16D.“Rediscoveries”of the nonlinear magneto-opticaleffects17 V.Physical mechanisms of nonlinearmagneto-optical effects18A.Bennett-structure effects18B.Coherence effects19C.Alignment-to-orientation conversion20 VI.Symmetry considerations in linear and nonlinear magneto-optical effects21 VII.Theoretical models22A.Kanorsky-Weis approach to low-power nonlinearmagneto-optical rotation22B.Density matrix calculations23 VIII.Nonlinear magneto-optical effects in specific situations24A.Buffer-gas-free uncoated vapor cells241.Basic features242.Peculiarities in the magneticfield dependence253.NMOE in optically thick vapors25B.Time-domain experiments26C.Atomic beams and separated lightfields,Faraday-Ramsey Spectroscopy261.Overview of experiments262.Line shape,applications273.Connection with the Ramsey separated oscillatoryfield method28D.Experiments with buffer-gas cells281.Warm buffer gas282.Cryogenic buffer gas28E.Antirelaxation-coated cells292 1.Experiments292.Theoretical analysis30F.Gas discharge30G.Atoms trapped in solid and liquid helium30ser-cooled and trapped atoms31 IX.Magneto-optical effects in selective reflection311.Linear effects312.Nonlinear effects32X.Linear and nonlinear electro-optical effects32 XI.Experimental techniques33A.A typical NMOE experiment33B.Polarimetry34C.Nonlinear magneto-optical rotation withfrequency-modulated light34D.Magnetic shielding35ser-frequency stabilization using magneto-opticaleffects35 XII.Applications36A.Magnetometry361.Quantum noise limits362.Experiments37B.Electric-dipole moment searches38C.The Aharonov-Casher phase shift39D.Measurement of tensor electric polarizabilities39E.Electromagneticfield tomography40F.Parity violation in atoms40 XIII.Closely-related phenomena and techniques41A.Dark and bright resonances41B.“Slow”and“fast”light41C.Self-rotation42 XIV.Conclusion42 Acknowledgments42 Appendices42A.Description of light polarization in terms of theStokes parameters42B.Description of atomic polarization431.State multipoles432.Visualization of atomic polarization43C.Abbreviations44References44 I.INTRODUCTIONMagneto-optical effects arise when light interacts with a medium in the presence of a magneticfield.These ef-fects have been studied and used since the dawn of mod-ern physics and have had a profound impact on its devel-opment.1Most prominent among the magneto-optical effects are the Faraday(1846a,b,1855)and the Voigt1Magneto-optics were listed among the most important topics in Physics at the World Congress of Physics in Paris in1900(Guil-laume and Poincar´e,1900).FIG.1The Faraday rotation effect.Light,after passing through a linear polarizer,enters a medium subjected to a longitudinal magneticfield B .Left-and right-circularly po-larized components of the light(equal in amplitude for lin-early polarized light)acquire different phase shifts,leading to optical rotation.A difference in absorption between the two components induces ellipticity in the output light.A particular polarization of the transmitted light,depending on the orientation of the analyzer relative to the polarizer,is detected.Analyzer orientation varies with the type of exper-iment being performed.In forward-scattering experiments (Sec.II.B),the analyzer is crossed with the input polarizer, so that only light of the orthogonal polarization is detected. In the“balanced polarimeter”arrangement(Sec.XI.B),a po-larizing beam splitter oriented atπ/4to the input polarizer is used as an analyzer.The normalized differential signal between the two channels of the analyzer depends on the ro-tation of light polarization while being insensitive to induced ellipticity.The Voigt effect is similar except that instead of a longitudinal magneticfield,a transversefield B⊥is applied. Here optical rotation and induced ellipticity are due to dif-ferential absorption and phase shifts of orthogonal linearly polarized components of the input light(Sec.VI). (1901)effects,i.e.,rotation of light’s polarization plane as it propagates through a medium placed in a longitudinal or transverse magneticfield,respectively(Fig.I).The linear(Secs.II,III)near-resonance Faraday effect is also known as the Macaluso-Corbino(1898a;1899;1898b)ef-fect.The Voigt effect is sometimes called the Cotton-Mouton(1907;1911)effect,particularly in condensed-matter physics.The remarkable properties of resonant(and,par-ticularly,nonlinear)magneto-optical systems—as com-pared to the well-known transparent condensed-matter magneto-optical materials such as glasses and liquids—can be illustrated with the Faraday effect.The mag-nitude of optical rotation per unit magneticfield and unit length is characterized by the Verdet constant V. For typical denseflint glasses that are used in commer-cial Faraday polarization rotators and optical isolators, V 3×10−5rad G−1cm−1.In subsequent sections,we will describe experiments in which nonlinear magneto-optical rotation corresponding to V 104rad G−1cm−1 is observed in resonant rubidium vapor(whose density,∼3×109cm−3,satisfies the definition of very high vac-uum).Taking into account the difference in density be-tween glass and the rarified atomic vapor,the latter can be thought of as a magneto-optical material with some3gΜB@ΩFIG.2An F=1→F =0atomic transition.In the pres-ence of a longitudinal magneticfield,the Zeeman sublevelsof the ground state are shifted in energy by gµBM.Thisleads to a difference in resonance frequencies for left-(σ+)and right-(σ−)circularly polarized light.1020greater rotation“per atom”than heavyflint.In this paper,we briefly review the physics and applica-tions of resonant linear magneto-optical effects,and thenturn to our main focus—the nonlinear effects(i.e.,effectsin which optical properties of the medium are modifiedby interaction with light).We will also discuss variousapplications of nonlinear magneto-optics in atomic va-pors,and the relation between nonlinear magneto-opticsand a variety of other phenomena and techniques,suchas coherent population trapping,electromagnetically in-duced transparency,nonlinear electro-optics effects,andself-rotation.II.LINEAR MAGNETO-OPTICSIn order to provide essential background for under-standing nonlinear magneto-optical effects(NMOE),wefirst review linear magneto-optics of atoms and moleculesnear resonance.(See Sec.III for a discussion of the dif-ference between the linear and nonlinear magneto-opticaleffects.)A.Mechanisms of the linear magneto-optical effectsAt the conclusion of the19th century,Macaluso andCorbino(1898a,1899,1898b),studying absorption spec-tra of the alkali atoms in the presence of magneticfields,discovered that the Faraday effect(magneto-optical ac-tivity)in the vicinity of resonance absorption lines alsohas a distinct resonant character(see also work by Forkand Bradley III,1964).The principal mechanism of the linear Macaluso-Corbino effect can be illustrated by the case of an F=1→F =0transition(Fig.II.A),where F,F are thetotal angular momenta.2Linearly polarized light inci-2Throughout this article,we designate as F,F total angular mo-menta of the lower and the upper states of the transition,respec-tively.For atoms with zero nuclear spin,F,F coincide with thetotal electronic angular momenta J,J.Light detuning (1RefractiveindexFIG.3The dependence of the refractive index on light fre-quency detuning∆in the absence(n)and in the presence(n±)of a magneticfield.Shown is the case of2gµB=¯hΓand a Lorentzian model for line broadening.The lower curveshows the difference of refractive indices for the two circularpolarization components.This is the characteristic spectralprofile of linear optical rotation.dent on the sample can be decomposed into two counter-rotating circular componentsσ±.In the absence of amagneticfield,the M=±1sublevels are degenerate andthe optical resonance frequencies forσ+andσ−coincide.The real part of the refractive index n associated with theatomic medium is shown in Fig.II.A as a function of thelight frequency detuning∆(the dispersion curve).Therefractive index is the same for the two circular compo-nents.When a magneticfield is applied,however,the Zeemanshifts3lead to a difference between the resonance fre-quencies for the two circular polarizations.This displacesthe dispersion curves for the two polarizations as shownin Fig.II.A.A characteristic width of these dispersioncurves,Γ,corresponds to the spectral width(FWHM)of an absorption line.Under typical experimental con-ditions in a vapor cell this width is dominated by theDoppler width and is on the order of1GHz for opti-cal transitions.The difference between n+and n−(Fig.II.A)signifies a difference in phase velocities of the twocircular components of light and,as a result,the planeof polarization rotates through an angleϕ=π(n+−n−)lλ.(1)Here l is the length of the sample,andλis the wavelengthof light.In addition to the difference in refraction for thetwo circular polarizations(circular birefringence),there3The connection between the Faraday and the Zeeman effects wasfirst established by Voigt(1898b),who also explained the obser-vations of Macaluso and Corbino(Voigt,1898a).4 also arises a difference in absorption(circular dichroism).Thus linear light polarization before the sample gener-ally evolves into elliptical polarization after the sample.For nearly monochromatic light(i.e.,light with spectralwidth much smaller than the transition width),and forzero frequency detuning from the resonance,the opticalrotation in the sample as a function of magneticfield Bcan be estimated from Eq.(1)as4ϕ2gµB/¯hΓ1+(2gµB/¯hΓ)2ll0.(2)Here g is the Land´e factor,µis the Bohr magneton,and l0is the absorption length.This estimate uses for the amplitude of each dispersion curve(Fig.II.A)the reso-nance value of the imaginary part of the refractive in-dex(responsible for absorption).The Lorentzian model for line broadening is assumed.The Voigt model(dis-cussed by,for example,Demtr¨o der,1996),which most accurately describes a Doppler-and pressure-broadened line,and the Gaussian model both lead to qualitatively similar results.The dependence of the optical rotation on the magnitude of the magneticfield[Eq.(2)]has a characteristic dispersion-like shape:ϕis linear with B at small values of thefield,peaks at2gµB∼¯hΓ,and falls offin the limit of largefields.For atoms with nonzero nuclear spin,mixing of differ-ent hyperfine components(states of the same M but dif-ferent F)by a magneticfield also leads to linear magneto-optical effects(Khriplovich,1991;Novikov et al.,1977; Papageorgiou et al.,1994;Roberts et al.,1980).The con-tribution of this mechanism is comparable to that of the level-shift effect discussed above in many practical situa-tions,e.g.,linear magneto-optical rotation in the vicinity of the alkali D-lines(Chen et al.,1987).For the Faraday geometry and when gµB ¯hΓ ∆hfs,the amplitude of the rotation can be estimated asϕ gµB∆hfsll0,(3)where∆hfs is the separation between hyperfine levels. Since hyperfine mixing leads to a difference in the mag-nitude of n+and n−(and not the difference in resonance frequencies as in the level-shift effect),the spectral pro-file of the rotation for the hyperfine-mixing effect corre-sponds to dispersion-shaped curves centered on the hy-perfine components of the transition.There exists yet another mechanism in linear magneto-optics called the paramagnetic effect.The populations of the ground-state Zeeman sublevels that are split by a magneticfield are generally different according to the Boltzmann distribution.This leads to a difference in4Explicit formulae for n±are given,for example,in Mitchell and Zemansky(1971,Appendix VII);analogous expressions can also be obtained for induced ellipticity.refractive indices for the corresponding light polariza-tion components.For gaseous media,this effect is usu-ally relatively small compared to the other mechanisms. However,it can be dramatically enhanced by creating a nonequilibrium population distribution between Zeeman sublevels.This can be accomplished by optical pumping, a nonlinear effect that will be discussed in detail in Sec. IV.A.B.Forward scattering and line-crossingMagneto-optics plays an important role in the study of resonant light scattering in the direction of the in-cident light(forward scattering,FS)whose detection is normally hindered experimentally by the presence of a strong incident light beam that has identical properties (frequency,polarization,direction of propagation)as the forward-scattered light.In order to investigate this ef-fect,Corney,Kibble,and Series(1966)used two crossed polarizers(Fig.I).In this arrangement,both the direct unscattered beam and the scattered light of unchanged polarization are blocked by the analyzer.Only the light that undergoes some polarization change during scatter-ing is detected.If the medium is isotropic and homoge-neous and there is no additional external perturbation or magneticfield,the forward-scattered light has the same polarization as the primary light and cannot be detected. The situation changes,however,when an external mag-neticfield B is applied.Such afield breaks the symmetry of theσ+andσ−components of the propagating light in the case of B k(andπandσcomponents when B⊥k) and results in a nonzero component of light with opposite polarization that is transmitted by the analyzer.In the late1950s,it was determined(Colegrove et al., 1959;Franken,1961)that coherence between atomic sub-levels(represented by off-diagonal elements of the den-sity matrix,see Sec.VII.B)affects lateral light scattering. For example,in the Hanle(1924)or level-crossing effects (Colegrove et al.,1959;Franken,1961),the polarization or spatial distribution offluorescent light changes as a function of relative energies of coherently excited atomic states.Another example is that of resonance narrow-ing in lateral scattering(Barrat,1959;Guichon et al., 1957).In this striking phenomenon the width of reso-nance features(observed in the dependence of the inten-sity of scattered light on an applied dc magneticfield or the frequency of an rffield)was seen to decrease with the increase of the density of the sample;this appeared as an effective increase in the upper state lifetime despite collisional broadening that usually results from elevated density.This effect is due to multiple light scattering(ra-diation trapping)which transfers excitation from atom to atom,each of the atoms experiencing identical evolution in the magneticfield(a more detailed discussion is given by,for example,Corney,1988).In lateral scattering,the resonance features of inter-est are usually signatures of interference between various5sublevels in each individual atom.In forward scattering, the amplitudes of individual scatterers add in the scat-tered light(Corney et al.,1966;Durrant,1972).Thus forward scattering is coherent,and interference can be observed between sublevels belonging to different atoms. The forward-scattered light has the same frequency as the incident light.However,the phase of the scattered light depends on the relative detuning between the incident light and the atom’s effective resonance frequency.This leads to inhomogeneous broadening of the FS resonance features;at low optical density,their width(in the mag-neticfield domain)is determined by the Doppler width of the spectral line.5For this reason,the corresponding FS signals associated with the linear magneto-optical ef-fects can be regarded as Doppler-broadened,multi-atom Hanle resonances.Here the Hanle effect is regarded ei-ther as a manifestation of quantum mechanical interfer-ence or atomic coherence,depending on whether it ap-pears in emissive or dispersive properties of the medium (see Sec.II.D.1).If the optical density of the sample increases to the extent that multiple scattering becomes important,substantial narrowing of the observed signals results,interpreted by Corney et al.(1966)as coherence narrowing.Forward-scattering signal narrowing was ob-served in Hg by Corney et al.(1966)and in Na by Kro-las and Winiarczyk(1972).Further exploring the re-lation between manifestations of single-and multi-atom coherence,Corney,Kibble,and Series analyzed the phe-nomenon of double resonance in the context of FS.This is a two-step process in whichfirst optical excitation by appropriately polarized resonant light populates atomic states.Subsequently,transitions are induced among the excited states by a resonant radio-frequency(rf)field. Corney,Kibble,and Series also studied double resonance in the limiting case in which the upper states have the same energy,so that the second resonance occurs at zero frequency.Such a zero-frequency resonantfield is simply a constant,transverse magneticfield.Thus the double-resonance approach is applied to the Voigt effect in an unorthodox manner.The fact that in forward scattering light scattered by different atoms is coherent makes it possible to study the phenomenon of line crossing.Whereas in level cross-ing(Colegrove et al.,1959;Franken,1961),signals in lateral light scattering are observed when different sub-levels of single atoms cross(for example,in an external magneticfield),in a line-crossing experiment interference occurs due to crossing of sublevels of different atoms. This effect wasfirst demonstrated by Hackett and Se-ries(1970).These authors observed interference in the FS signals due to crossing of Zeeman sublevels of dif-ferent Hg isotopes contained in one cell.Church and Hadeishi(1973)showed that line crossing occurs even 5Obviously,homogeneous broadening(e.g.,pressure broadening) also affects these widths.when atoms of different kinds are contained in separate cells.Hackett and Series(1970),Church and Hadeishi (1973),Stanzel(1974a),and Siegmund and Scharmann (1976)investigated the possibility of applying the line-crossing effect to precise measurements of isotope shifts. This idea is based on the fact that line crossings occur when the applied magneticfield is such that the Zeeman shifts compensate for the initialfield-free isotope shifts. It was hoped that strong coherence narrowing of the line-crossing resonance would significantly increase precision of such measurements.However,these authors found that various complications(arising,for example,from pressure broadening of the signals),render this method impractical for isotope shift measurements.Forward-scattering signals for weak-intensity light can be written asI F S=14ξ(ω)e−A+ωlc−e−A−ωlc2dω+ξ(ω)sin2(n+−n−)ωl2ce−(A++A−)ωlc dω,(4)whereξ(ω)is the spectral density of the incident light, A±and n±denote amplitude absorption coefficients and refractive indices for theσ±components of the incident light beam,respectively,and l is the sample length.We assume ideal polarizers here.In general,there are two comparable contributions to the FS signal,represented by the two terms in Eq.(4). Thefirst term is due to differential absorption of theσ+ andσ−components of the incident light(circular dichro-ism),and the second term is due to differential disper-sion(circular birefringence).The two contributions have different frequency dependence.This can be illustrated with the simple case of the F=0→F =1transition, for which theσ±resonance frequencies are split by a lon-gitudinal magneticfield(Fig.II.B).While the function in thefirst integral goes through zero atω=ω0(since the function in the square brackets is anti-symmetric with respect to detuning),the second(birefringence)term is maximal at zero detuning(for small magneticfields).For a narrow-band light source,it is possible to eliminate the dichroic contribution by tuning to the center of the res-onance.One is then left with only the second integral, representing Malus’s law,whereϕ=(n+−n−)ωl/(2c)is the Faraday rotation angle[Eq.(1)].When the density-length product for the medium is sufficiently high,the range of variation ofϕcan easily exceedπ,and intensity transmitted through the appara-tus shown in Fig.I oscillates as a function of the magnetic field(Fig.II.B).These oscillations are clearly seen de-spite the proximity of the absorption line because the refractive indices drop with detuning slower than the ab-sorption coefficients.Whenξ(ω)represents a narrow spectral profile,the modulation contrast can be high, particularly when the magneticfield is strong enough to split the A±profiles completely.Such case is shown schematically in Fig.II.B(c).6n n(a)(b)(c)gΜB/@gΜB/@c y (Ω)BirefringencenDichroismFIG.4Spectral dependences of the circular dichroic(A+−A−)and birefringent(n+−n−)anisotropies that determinethe forward-scattering signal for a F=0→F =1transition.Theσ+andσ−resonance frequencies are split by a longitu-dinal magneticfield B.For curves(a)and(b)the magnitudeof the splitting is equal to the resonance widthΓ.For thecurves(c),it isfive times larger.At the center of a Zeeman-split resonance,absorptiondrops offwith the magnitude of the splitting much fasterthan optical rotation does.Thus,using a dense atomicvapor in a magneticfield and a pair of polarizers,it ispossible to construct a transmissionfilter for resonant ra-diation,which can be turned into an intensity modulatorby modulating the magneticfield(Aleksandrov,1965).C.Applications in spectroscopyMagneto-optical effects can be used to provide usefulspectroscopic information.For example,given a knownatomic density and sample length,a measurement ofFaraday rotation for a given transition can be used todetermine the oscillator strength f for that transition.Conversely,when f is known,Faraday rotation may beused to determine vapor density.Vliegen et al.(2001)found that Faraday rotation measurements with high al-kali metal densities(∼1015–1016cm−3)are free from cer-tain systematic effects associated with measurement ofabsorption.An earlier review of applications of magneto-optical rotation was given by Stephens(1989).1.Analytical spectroscopy and trace analysis,investigation ofweak transitionsAn example of an application of magneto-optical rota-tion to molecular spectroscopy is the work of Aubel andHause(1966)who,using a multi-pass cell,demonstratedFIG.5Forward-scattering signals I(B)observed on thesodium D1[(a),(c)]and D2[(b),(d)]lines by Gawlik(1975).Curves(a)and(b)are signals obtained with a conventionalspectral lamp(single D line selected by a Lyotfilter).Curves(c)and(d)are signals obtained with single-mode cw dye-laserexcitation.The laser is tuned to the atomic transition in allcases except the one marked(×).Curve(×)was recordedwith the laser detuned by about600MHz to demonstrate theinfluence of residual dichroism.In plot(a),flattening of somecurves at highfields is due to detector saturation—the dashedlines represent calculated signals.that the magneto-optical rotation spectrum of NO is eas-ier to interpret than the absorption spectrum.Molecu-lar magneto-optical spectra are in general much simplerthan the absorption spectra(see discussion by Herzberg,1989,Ch.V,5).This is because significant magneto-optical effects are only present for transitions betweenmolecular states of which at least one has nonzero elec-tronic angular momentum.In addition,since molecularg-values decrease rapidly with the increase of the rota-tional quantum number J(see discussion by Khriplovich,1991,Ch.7.2),only a small part of the rotational bandproduces magneto-optical effects.Magneto-optical rota-tion has been used to identify atomic resonance lines inBi against a complex background of molecular transitions7(Barkov and Zolotorev,1980;Roberts et al.,1980). Magneto-optical rotation,in particular,the concept of forward scattering,was also applied in analytical spec-troscopy for trace element detection.This wasfirst done by Church and Hadeishi(1974),who,using FS signals, showed sensitivity an order of magnitude higher than could be obtained with absorption measurements.The improved sensitivity of magneto-optical rotation com-pared to absorption is due to almost complete elimination of background light and a corresponding reduction of its influence on the signal noise.Such noise is the main limi-tation of the absorption techniques.Following this work, Ito et al.(1977)also employed both Faraday and Voigt effects for trace analysis of various elements.The detection of weak transitions by magneto-optical rotation(with applications to molecular and analytical spectroscopy)was spectacularly advanced by employ-ment of ing tunable color-center lasers,Litfin et al.(1980)demonstrated50times better sensitivity in detection of NO transitions in the vicinity of2.7µm with magneto-optical rotation compared to absorption spec-troscopy.Similar results were obtained by Yamamoto et al.(1986)who obtained200-fold enhancement in sen-sitivity over absorption spectroscopy in their work with a pulsed dye laser and the sodium D2line.Another in-teresting result was reported by Hinz et al.(1982)who worked in the mid-infrared range with NO molecules and a CO laser.These authors also demonstrated that magneto-optical rotation improves sensitivity in either of the basic configurations,i.e.,in the Faraday as well as the Voigt geometry.The advent of high-sensitivity laser spectropolarime-ters,allowing measurement of optical rotation at the level of10−8rad and smaller(Sec.XI.B),made possible the sensitive detection of species with low concentration.6It is important to note that,while it is beneficial from the point of view of the photon shot noise of the polarime-ter to operate at a high light power,great care should be taken to make sure that the atoms of interest are not bleached by nonlinear saturation effects(Sec.III.B).As a practical way to optimize a trace analysis setup,we suggest the use of a buffer gas to pressure-broaden the homogenous width of the transition up to the point when this width becomes comparable to the Doppler width. This way,the linear absorption and Faraday rotation are not compromised,but the light intensity constraints due to nonlinearities are relaxed by many orders of magni-tude.Due to its high sensitivity,the magneto-optical ro-tation method can also be applied to the study of weak transitions,such as magnetic dipole transitions with small transition magnetic moments(Barkov et al., 1989b).6Detection of on the order of hundreds of particles per cubic cen-timeter was reported by Vasilenko et al.(1978).2.Measurement of oscillator strengthsFork and Bradley III(1964)performed some of the earliest work in which resonant magneto-optical rota-tion was used to measure oscillator strengths.7They measured dispersion of Hg vapor at aboutfive Doppler widths from the center of the253.7-nm line.They used an electrodeless discharge198Hg lamp placed in a solenoid as a light source tunable over eight Doppler widths and measured the vapor dispersion using a Mach-Zehnder in-terferometer.In addition to the observation of Faraday rotation in excess of7rad,they also demonstrated the inversion of the sign of the dispersion of a vapor with inverted population,and the feasibility of using Faraday rotation for narrow-band modulatable opticalfilters. The advent of tunable lasers has enabled significant im-provement of magneto-optical-rotation methods for mea-suring absolute and relative oscillator strengths.This is illustrated by the results,shown in Fig.II.B,of an exper-iment performed by Gawlik(1975).Here curves(a)and (b)refer to a FS experiment performed with a sodium spectral lamp and a Lyotfilter that selected one of the Na D lines.The light from the lamp had an asymmet-ric spectral profile with some self-reversal(a line-profile perturbation due to re-absorption of resonance light at the line center)and FWHM of about8GHz.From the results for the D1and D2lines,one can see how the mod-ulation period depends on the product of atomic density and oscillator strength of the investigated transitions and how the oscillatory birefringent contribution(Fig.II.C.2) becomes overwhelmed by the structureless dichroic one, particularly at small magneticfields.Figures II.B(c)and (d)represent results from the same experiment but with, in place of a lamp,a narrow-band cw dye laser tuned to the centers of gravity of the Na D1and D2lines,respec-tively(Gawlik,1977;Gawlik et al.,1979).Several impor-tant changes are seen:first,the modulation depth is now 100%—this allows accurate determination ofϕ(nl,ω,B), whereωis the light frequency(Fig.II.C.2);second,the dichroic contribution can be fully eliminated by appropri-ately tuning the laser(this is because the dichroic effect has a nearly dispersive spectral dependence which goes through zero when the Faraday rotation contribution is nearly maximal);third,as seen by the envelope of the os-cillatory pattern,total absorption exp[−(A++A−)ωl/c] plays a role only for small B,in agreement with the above considerations[see Figs.II.B(c)and(d)].Fast oscilla-tion in the region of low total absorption makes it possible to determine the absorption profile(envelope)simultane-ously with the measurement ofϕ.This technique allows one to simultaneously determine the real and imaginary parts of the complex index of refraction,which is useful for measuring collisional parameters,for example.7Early measurements of oscillator strength are described in a monograph by Mitchell and Zemansky(1971).。

第35卷第2期2009年3月 光学技术OP T ICA L T ECHN IQ U EV ol.35No.2M arch2009 文章编号:1002-1582(2009)02-0163-09基于金属表面等离子激元控制光束的新进展王庆艳,王佳,张书练(清华大学精密测试技术及仪器国家重点实验室,北京　100084)摘　要:表面等离子激元(Surface plasmon polaritons,SPPs)是一种在金属-介质界面上激发并耦合电荷密度起伏的电磁振荡,具有近场增强、表面受限、短波长等特性,在纳米光子学的研究中扮演着重要角色。

近年来表面等离子光学和基于SPPs的纳米光子器件的研究引起了国际上科学家们的广泛关注。

讨论了SP Ps的基本原理和在亚波长结构下的光学特性,介绍了基于亚波长金属结构的表面等离子激元在空间光束准直与聚焦、平面内光束聚焦与传导和在近场纳米光束的控制等方面的研究情况,以及在纳米光子学器件中的潜在应用。

关键词:近场光学;纳米光子学器件;表面等离子;亚波长金属结构中图分类号:O43;TN491 文献标识码:AProgress of beam control based on metal surface plasmon polaritonsWAN G Qin g-yan,WANG Jia,Z HANG Sh u-lian(State Key Laboratory of P recision M easurement Technology and I nstruments,Department of P recisio n I nstruments,T singhua U niversity,Beijing　100084,China) Abstract:Surface plasmon polaritons(SPP s)is a kind of electromagnetic oscillation coupled w ith the undulation of charge intensity,w hich is ex cited at the interface betw een metal and dielectric.With the properties of near-field enhancement,surface co nfinement and shor t w avelength,SPPs play an important role in nanophotonics.Recently the researches of surface plasmo n optics and nano-photonic devices based on SPPs have drawn g reat attention of international scientists.T he principles of SPPs and their optical properties in subw avelength structures are discussed.T he applications of SPPs based on subwaveleng th metal structures in beaming,focusing,guiding and control of near-field beam are intro duced,and the potential applications in nano-photonic devices are reviewed.Key words:near-field optics;nano-pho tonic devices;surface plasmon;subw avelength metal structure1　引　言光学和光子学进入纳米领域是21世纪最重要的发展趋势之一。

Spontaneous emission(自发发射) of electromagnetic RadiationOne of the basic physical principles is that: Every system in nature "prefers" to be in the lowest energy state. This state is called the Ground state. When energy is applied to a system, the atoms(electrons) in the material are excited, and raised to a higher energy level. These electrons will remain in the excited state for a certain period of time, and then will return to lower energy states while emitting energy in the exact amount of the difference between the energy levels( E). The emission of the individual photon is random, being done individually by each excited atom, with no relation to photons emitted by other atoms. When photons are randomly emitted from different atoms at different times, the process is called Spontaneous Emission.译文；自发的电磁辐射有一个基础物理定律是这样说的：每一个系统都趋向于最低的能量状态。