THE DESIGN OF A DUAL polarized vivaldi antenna

伯尔定英文版As an AI language model, I am here to assist you in creating a document on "Bert in English Version." Let's delve into the world of this powerful language model, its significance, and its applications.Bert, short for Bidirectional Encoder Representations from Transformers, is a groundbreaking deep learning model developed by Google's AI research team. First introduced in 2018, it revolutionized the field of natural language processing (NLP) by offering a new approach to understanding and processing text. Unlike traditional models that process text sequentially, Bert considers the context in both forward and backward directions, making it highly effective in tasks like language understanding, question answering, and text classification.In its English version, Bert has been trained on massive amounts of text data, including books, articles, and web content, which allows it to capture the nuances of English language. This multilingual model, when fine-tuned for specific tasks, can adapt to various English dialects and contexts, enhancing its versatility.One of Bert's key features is its pre-training, where it learns to represent words and sentences in a vector space. This enables it to understand relationships between words and phrases, making it particularly useful in tasks like sentiment analysis, where it can accurately determine the emotional tone of a text. Moreover, its ability to handle long sentences and complex structures is a significant improvement over previous models.In the realm of NLP, Bert has been instrumental in various applications. In the field of search engines, it helps improve the relevance of search results by understanding the user's intent better. In language translation, it can be fine-tuned to translate between languages, preserving the meaning and context. Educational platforms often leverage Bert for automated essay grading, as it can assess the coherence and structure of written work.However, it's important to note that while Bert has proven to be a game-changer, it's not a silver bullet. Its performance can be influenced by the quality and quantity of training data, and it may not always outperform task-specific models when dealing with specialized domains. Continuous research and improvements are necessary to enhance its capabilities.In conclusion, Bert in its English version represents a significant milestone in NLP, offering a powerful foundation for understanding and processing language. Its adaptability and effectiveness in various applications have made it a go-to tool for researchers and developers alike. As the field of AI continues to evolve, we can expect further advancements in language models like Bert, shaping the future of communication and understanding.。

CHAPTER VIP.A.M.DIRAC AND ANTI-PARTICLESThe Theory of Relativity and the Quantum Theory, which appeared almost simultaneously at the start of the present century, were two great explosions of the human mind and shook the very foundations of classical physics: relativity in the case of velocities approaching that of light; quantum in the case of motions confined to very small (atomic) dimensions. But for almost three decades these two great theories stood apart, more or less, from each other. Bohr’s original theory of quantized orbits, as well as Schrodinger’s wave equations into which it developed, were essentially non-relativistic; both were applicable only to particles moving with a velocity small as compared with that of light. But the velocities of electrons within the atoms are not that small. For example, the electron on the first hydrogen orbit, calculated on the basis of Bohr’s theory, has the velocity of 2.2 ×108cm/sec, which is only a little less than I per cent of the speed of light. The velocity of electrons inside heavier atoms is considerably larger. Of course, a few per cents is not too much, and the calculated value could be improved by introducing “relativistic corrections,”which could make the agreement with direct experimental measurement somewhat better. But this was only an improvement and not the completion of the theory.Another trouble arose in the case of the electron’s magnetic moment. In 1925, Goudsmit and Uhlenbeck showed that in order to explain certain details of atomic spectra it is necessary to ascribe to the electron certain angular and magnetic moment at commonly known as the electron spin. The naïve picture at that time was that the electron is a little charged sphere about 3×10-13cm in diameter. The rapid rotation of this sphere around its axis was supposed to produce a magnetic moment, resulting in additional interaction with its orbital motion and with the magnetic moments of other electrons. It turned out, however, that in order to produce the necessary magnetic field the electron would have to rotate so fast that the point on its equator would move at much higher velocities than light! Here again one encountered a conflict between quantum and relativistic physics. It was becoming clear that relativity and quantum physics could not just be added together. A more general theory which would contain both relativistic and quantum ideas in a harmoniously unified form was needed.See Francis Bitter, Magnets. Doubleday, Science Study Series(1959).The most important step in this direction was taken in 1928 by a British physicist, P. A. M. Dirac, who started his career as an electrical engineer but, finding it difficult to get satisfactory employment, applied for a fellowship in physics at Cambridge University. His application (which was accepted) now hangs, attractively farmed, in the University Library, side by side with the Nobel Prize certificate which he received not too many years after changing from electrical engineering to quantum physics.。

高增益宽带圆极化Vivaldi天线阵的设计许唐红;张弘;王东;朱海涛;兰敏【期刊名称】《强激光与粒子束》【年(卷),期】2013(025)003【摘要】提出了一种由新型Vivaldi天线单元构成的2×2十字交叉圆极化天线阵.Vivaldi天线单元采用边缘渐变对拓的锯齿结构,提高了天线在4.7～7.0 GHz频带内的增益,其反射系数低于-10 dB的带宽为2.4～11.0 GHz,具有超宽带线极化特性.圆极化天线阵测量结果显示,在4.5～7.0 GHz频带范围内,其轴比均低于3 dB,且整个频带范围内增益达11～13 dBi.%A circularly polarized (CP) crossed antenna array with 2X2 elements based on a novel antipodal Vivaldi antenna is proposed. The Vivaldi antenna with tapering serrated structure at the edges, which is a ultra-wideband (UWB) linearly polarized antenna, has a high gain at 4. 7-7. 0 GHz, and a wide impedance bandwidth of 2. 4-11. 0 GHz when the reflection coefficient is smaller than -10 dB. The measured data of the antenna array show that the axial ratio is lower than 3 dB between 4. 5 GHz and 7. 0 GHz. A high antenna gain from 11 dBi to 13 dBi is also achieved at the whole bandwidth of the array. The proposed antenna element and array both own UWB and high-gain characteristics.【总页数】4页(P685-688)【作者】许唐红;张弘;王东;朱海涛;兰敏【作者单位】四川大学电子信息学院,成都610064;四川大学电子信息学院,成都610064;四川大学电子信息学院,成都610064;四川大学电子信息学院,成都610064;四川大学电子信息学院,成都610064【正文语种】中文【中图分类】TN82【相关文献】1.高增益低副瓣X波段宽带圆极化Vivaldi天线阵设计 [J], 吴文鹤;韦高;赵文奇;李文廷2.高增益宽带圆极化微带天线阵研究 [J], 黄迎春;张涛;张福顺3.基于单层线-圆极化转换聚焦超表面的宽带高增益圆极化天线设计 [J], 李唐景;梁建刚;李海鹏;牛雪彬;刘亚峤4.一种宽带高增益圆极化天线阵 [J], 石小林;黄迎春;孙全国5.基于人工电磁结构的宽带宽波束高增益圆极化微带天线阵 [J], 林家栋;柴晋飞;苏周;巫勇因版权原因，仅展示原文概要，查看原文内容请购买。

4Continuous-Wave Terahertz Sources and DetectorsThe technology behind continuous-wave(CW)THz emitters and sensors has a long history involving many different types of technical schemes unlike the methods of broadband THz radiation,mainly relying on ultrafast optical tech-nology.The efforts to integrate optical technology with electronics result in THz optoelectronic devices such as photomixers and difference frequency gen-erators.Optically pumped THz gas lasers produce high power THz radiation using the rotational transitions of heteropolar molecules in the gas phase.The implementation of actual devices requires compact and portable all-solid-state THz sources.Diode-based frequency multipliers,p-type germanium lasers,and quantum cascade lasers belong to this category.The practical demand has also been encouraging the development of solid-state sensors such as inter-subband detectors and Schottky diodes.Free-electron based sources include small scale devices such as backward wave oscillators and large facilities such as free-electron monly used THz sensors are thermal detectors such as bolometers,Golay cells,and pyroelectric devices.In this chapter,we shall briefly review various methods of generating and detecting CW THz ra-diation and look into the underlying mechanisms under which the devices are operating.4.1PhotomixingPhotomixing,also known as optical heterodyne downconversion,is a technique to generate CW THz radiation with a PC switch.LT-GaAs is the prevailing PC material for this technique because of its high mobility and short lifetime.A photomixer is a compact,solid-state device.The tuning range can be excep-tionally broad provided a high-quality,tunable,dual-frequency laser system is available.The primary disadvantage of this method is that the output power is relatively low compared with other techniques of CW THz generation.Its optical-to-THz conversion efficiency is10−6–10−5,and the typical output power is in the microwatt range.Carrier transport in LT-GaAs,described in1184Continuous-Wave Terahertz Sources and Detectorssection3.2.1,is an important factor governing photomixing processes.Because photomixing requires continuous optical excitation,the maximum THz output power is limited by the low thermal conductivity of LT-GaAs(∼15W/mK). The damage threshold of a1-µm LT-GaAs layer with biased electrodes is less than105W/cm2of optical excitation[77].diagram of photomixing.(a)THz radiation from a photomixerwith interdigitated electrodefingers.(Reprinted At a fundamental level,photomixing shares several essential features with the THz radiation from a PC emitter as discussed in section3.2.2.Figure4.1 illustrates the principle of CW THz generation from a photomixer.A typi-cal photomixer includes an antenna structure of metal on a LT-GaAs layer grown on a SI-GaAs substrate.A silicon hyper-hemispherical lens is attached to the back side of the substrate.A commonly used antenna structure for pho-tomixing is the logarithmic uniplanar spiral antenna(log-spiral antenna)with interdigitated electrodefingers shown in Fig.4.1(b)[78].It has the advantage of a broad tuning range:its radiation pattern,impedance,and polarization remain virtually unchanged below1THz.The optical excitation of photomixing utilizes a beat between two CW laser beams with slightly different frequencies.The most common light sources are diode lasers in the spectral range between800and850nm.The opticalfield at the antenna is expressed as:4.1Photomixing119E opt(t)=E1e−iω1t+E2e−iω2t,(4.1) where E1,E2,ω1,andω2are the electricfield amplitudes and the angular fre-quencies of the two CW laser beams,respectively.Thus,the optical intensity at the photomixer is given byI opt(t)=12c 0|E opt(t)|2=I0+I B cosωt,(4.2)where I0=I1+I2is the average intensity,I B=2√I1I2is the beat intensity,andω=ω1−ω2is the difference frequency.Due to the periodic modula-tion of the optical intensity,the induced photocurrent oscillates with the beat frequencyω.Therefore,the antenna,an oscillating dipole in this picture,radi-ates CW electromagnetic waves of the frequencyω.THz radiation is obtained when the difference frequency is tuned to the THz frequency range.The Drude-Lorentz model(see section3.2.2)provides a simple,yet accu-rate picture of the essential properties of THz radiation from a photomixer. According to Eqs.3.24,3.25,and3.27,the induced photocurrent is the con-volution of the optical intensity and the impulsive current density governed by the carrier lifetimeτc and the momentum relaxation timeτs:I P C(t)=I opt(t−t )[e n(t )v(t )]dt=µe E DC∞0{I0+I B cos[ω(t−t )]}e−t /τc1−e−t /τsdt .(4.3)The momentum relaxation time in LT-GaAs(20−30fs)is much shorter than the carrier lifetime(200−500fs),thus Eq.4.3reduces toI P C(t)=I D+I A1+ω2τ2ccos(ωt−φ),(4.4)where I D=τcµe E DC I0is the DC photocurrent,I A=τcµe E DC I B is the amplitude of the AC photocurrent at the frequencyω,andφ=tan−1(ωτc)is the phase delay induced by thefinite carrier lifetime.The THzfield radiated from the oscillating current isE T Hz(t)∝dI P C(t)dt=−ωI A1+ω2τ2csin(ωt−φ),(4.5)which leads to THz radiation power of frequencyωP T Hz(ω)=12I2AR A1+ω2τ2c=12R A·(E2DC I2B)·τ2cµ2e1+ω2τ2c,(4.6)where R A is the radiation resistance.The radiation power increases quadrat-ically with the optical intensity I B and the biasfield E DC.The last term in1204Continuous-Wave Terahertz Sources and DetectorsEq.4.6represents the effects of the intrinsic material properties,mobility and carrier lifetime,of the PC substrate.The radiation resistance of a Hertzian dipole is given by R A =197 d λ 2∝ω2,where d is the size of the dipole and λ=2πc/ωis the free-space wave-length.For a practical PC switch,the radiation resistance corresponds to the real part of the antenna load impedance Z R =R A /[1+(ωR A C A )2],where C A is the electrode capacitance.Consequently,the THz radiation power depends on the antenna circuit design [77]:P T Hz (ω)=12I 2A [1+(ωτc )]R A [1+(ωR A C A )].(4.7)The THz radiation power declines at high frequencies because the carrier response time decreases with an increase of the modulation frequency.Fig. 4.2.THz radiation power of the photomixers with log-spiral antennas ver-sus frequency.The antenna radiation resistance R A is 72Ω.Mixer 1comprises a 20×20µm 2active area with 1.8-µm gaps between 0.2-µm-wide metallic elec-trodes (C A =2.9fF).Mixer 2uses the same electrode geometry,but the area is only 8×8µm 2(C A =0.5fF).The normalized bandwidth curves were measured with a4.2-K bolometer.(Reprinted from [78].c1997IEEE)Figure 4.2shows the frequency-dependent output power of the two pho-tomixers with log-spiral antennas [78].The radiation resistance is R A =60π√ eff =72Ωfor a log-spiral antenna on a semi-infinite GaAs substrate.The carrier lifetime of LT-GaAs and the RC time constant of a typical log-spiral photomixer are a few hundred femtoseconds,thus the frequency roll-offstarts around 1THz.The radiation power is proportional to ω−4at higher frequencies.Mixer 2has broader bandwidth than Mixer 1because the RC time constant of Mixer 2is shorter.At the expense of a broad tuning range,the log-spiral antenna has low output power due to the relatively low radiation resistance.The radiation4.1Photomixing 121power can be enhanced by using resonant antenna structures.Figure 4.3shows the structure of a dipole PC antenna and the spectrum of its radiation power in the THz spectral range,where L is the distance between strip lines [79].The radiation resistance of a dipole antenna on a GaAs substrate peaks around L/λR =0.3[80],which corresponds to the resonant frequency νR ≈1.8THz for the 50-µm dipole.The measured output power is maximized near 1.2THz and extends throughout the broad spectral range from 0.5to 2THz.The peak radiation resistance is estimated as 360Ω,which is several times larger than that of the log-spiral antenna,72Ω.5×5µm 2gapL = 50 µm 20 mm(a)(b)V bFig. 4.3.(a)Schematic diagram of a dipole PC antenna.(b)Output radiation spectrum for the 50-µm dipole photomixer.The dashed curves show calculated ra-diation spectra for C A =0and 0.5fF.(Reprinted with permission from [79].c 1997,American Institute of Physics)Higher output powers are achievable using more sophisticated antenna de-signs.The dual dipole antennas illustrated in Fig.4.4(a)have several advan-(a)electrode dipoles(b)Fig.4.4.(a)Geometry of dual dipoles,with parameters given in the table below.dipoles D1,D2,D3,and D4and a spiral S1.(Reprinted from [81].c2001IEEE)1224Continuous-Wave Terahertz Sources and Detectorstages over simpler antenna designs:the radiation pattern is more symmetric and the radiation resistance is higher[81].The quintessential feature of the dual antenna design is that the electrode capacitance is cancelled out by the inductive tuning when the transmission line’s length is adjusted to the res-onant frequency.Thus,the radiation resistance is determined mainly by the carrier lifetime.The output spectra of the dual-dipole antennas in Fig.4.4(b) demonstrate significantly higher powers near their resonant frequencies when compared to the output from the log-spiral antenna[81].The maximum out-put powers of the dual-dipole antennas are3,2,0.8,and0.3µW at0.9, 1.0,1.6,and2.7THz,respectively.The peak power declines with increase in frequency asω−2,while the log-spiral output is proportional toω−4.The resonant frequencies are determined by the length of the dipole C and of the transmission line A+F.A few interesting ideas to improve radiation power are noteworthy.One way to avoid the limitation due to the low threshold for thermal damage of LT-GaAs,as well as to increase photocurrent,is to make the optical-excitation area large and to illuminate it with an extended beam from a high-power laser. If the dimensions of the illumination area are comparable to or larger than the wavelength of radiation,the optical excitation should be phase-matched with the THz radiation.A travelling-wave photomixer contains a long,thin active area between two electrodes.The active area is structured to maintain the coherent superposition between photocurrent and THz radiation[82].An-other approach to enhance photocurrent is to replace LT-GaAs substrates with semiconductor heterostructures such as p-i-n photodiodes.A uni-travelling-carrier photodiode(UTC-PD)contains a collection layer of InP which takes advantage of the exceptionally high electron mobility in the material and of the optical excitation at the optical communication wavelength,1.55µm[83].4.2Difference Frequency Generation and Parametric AmplificationDifference frequency generation(DFG)is a second-order nonlinear optical process which produces an electromagnetic wave of frequencyωT when two optical beams at frequenciesω1andω2are incident upon a nonlinear crystal, such that the output frequency is the difference between the two input fre-quencies:ωT=ω1−ω2.We have discussed the basic concepts and theoretical formulations of second-order nonlinear optical interactions in sections3.3.1 and3.3.2to describe optical rectification.Optical rectification can be inter-preted as DFG of the different frequency components within the broad band-width of ultrashort laser pulses.In this section,we shall explore two schemes of narrowband THz generation:DFG with two input beams and parametric generation with a single optical pump.4.2Difference Frequency Generation and Parametric Amplification 1234.2.1Principles of Difference Frequency GenerationE O (t )=E 1(t )+E 2(t )=E 0(sin ω1t +sin ω2t )=2E 0cos ωT 2t sin ωO t,(4.8)where ωO =(ω1+ω2)/2is the average optical frequency.The second-order nonlinear polarization of DFG is proportional to the beat intensity:P T (t )=χ(2)E 20 cos ωT 2t 2=12χ(2)E 20[1+cos (ωT t )].(4.9)Consequently,the THz radiation field induced by the nonlinear polarization is given byE T (t )∝∂2P T (t )∂t 2=−12χ(2)ω2T E 20cos (ωT t ).(4.10)The THz field oscillates at the difference frequency,ωT .We next discuss the generation and propagation of THz waves in a bulk medium.The wave equation formulated in section 3.3.3is still valid.We as-sume that a linearly polarized optical plane wave propagates in the z direction.1244Continuous-Wave Terahertz Sources and Detectors∂2E T(z,t)∂z2−n2Tc2∂2E T(z,t)∂t2=10c2∂2P(2)T(z,t)∂t2,(4.11)where E T(z,t)and P(2)T (z,t)are the THzfield and the nonlinear polarization.The THzfield is expressed asE T(z,t)=A T(z)e i(k T z−ωT t)+c.c.,(4.12)wherek T=n TωTc.(4.13)n T is the refractive index at the THz frequency.We assume that thefield amplitude A T(z)is a slowly varying function of z.The nonlinear polarization can be written asP(2)T(z,t)=P T(z)e−iωT t+c.c.(4.14) The amplitude of the nonlinear polarization induced by two monochro-matic optical waves is given byP(2) i (z,ωT)=j,k0χ(2)ijk(ωT,ω1,−ω2)E j(z,ω1)E∗k(z,ω2).(4.15)Forfixed propagation and polarization directions,this equation can be sim-plified as a scalar relation[84]:P T(z)=4 0d effE1(z)E∗2(z)=4 0d effA1A∗2e i(k1−k2)z,(4.16)where d eff=12χ(2)effis the effective nonlinear coefficient,and A1and A2arethe amplitudes of the opticalfields.Substituting Eqs.4.12,4.14,and4.16into the wave equation,Eq.4.11,we obtaind2A T dz2+2ik TdA Tdz=−4d effω2T0c2A1A∗2e i(k1−k2−k T)z.(4.17)The amplitude A T varies slowly,so that the change is negligible for the propa-gation distance of a wavelength,then we can neglect thefirst term in Eq.4.17. This approximation is called the Slowly Varying Envelope Approximation (SVEA).The wave equation is reduced todA T dz =2id effωT0n T cA1A∗2e i∆kz,(4.18)where∆k=k1−k2−k T is the momentum mismatch.The amplitudes of the optical waves also vary slowly and obey similar wave equations:dA1 dz =2id effω10n1cA2A T e−i∆kz,(4.19)and4.2Difference Frequency Generation and Parametric Amplification125dA2 dz =2id effω20n2cA1A∗T e i∆kz.(4.20)The momentum mismatch is closely related to the velocity mismatch dis-cussed in the section3.3.3.When the two optical beams are polarized in the same direction and the dispersion is negligible nearω1andω2,we can define the optical refractive index as n O≡n1(=n2).In this case,the momentum mismatch is proportional to the index mismatch,∆n=n O−n T,between the optical and THz waves:∆k=n O ω1c−n Oω2c−n TωTc=∆nωTc.(4.21)When the phase matching condition is satisfied,i.e.,∆k=0,the THz wave copropagates with the beat of the optical beams at the same velocity.Accord-ingly,the THzfield undergoes a coherent amplification.The ultimate upper limit of the optical-to-THz conversion efficiency is determined by the Manley-Rowe relations.The essence of the Manley-Rowe relations is that the creation and annihilation rates of photons should be equal at all frequencies involved in a nonlinear optical process.Imagine the initial intensities of the two optical beams and the THz wave are I1(0)= cN1¯hω1/n O,I2(0)=cN2¯hω2/n O,and I T(0)=0,where N1and N2are the initial photon number densities atω1andω2.A downconversion with100% quantum efficiency yields I1(L)=0,I2(L)=c(N1+N2)¯hω2/n O,and I T(L)= cN1¯hωT/n T.Therefore,the optical-to-THz conversion efficiency obeys the inequality relation,I T(L) I1(0)≤n OωTn Tω1∼10−3−10−2.(4.22)4.2.2Difference Frequency Generation with Two Pump Beams When the two optical input beams have similar intensities,we can assume that the optical beams are non-depleted,i.e.,A1and A2are constants.In practice,the optical-to-THz conversion efficiency is no more than10−4at best.With this non-depleted pump approximation,the integration of Eq.4.18 for a propagation distance L yieldsA T(L)=2id effωT0n T cA1A∗2Le i∆kz dz=2id effωT A1A∗20n T ce i∆kL−1i∆k,(4.23)which leads to the THz intensity,I T(L)=120cn T|A T(L)|2=8d2effω2TI1I230c n1n2n TL2sinc2∆kL2.(4.24)The sinc function peaks at the origin and is negligible beyond∆kL2>π.For a given∆k,I T(L)∝sin2∆kL2.Thus,the maximum THz intensity isobtained when the crystal length is equal to the coherence length l c=π∆k.1264Continuous-Wave Terahertz Sources and Detectors210.5Fig.4.6.Absorption spectra for an extraordinary wave in GaSe,GaAs,LiNbO3, GaP,CdSe,and LiTaO3.(Reprinted from[38].)Among many nonlinear crystals examined for DFG,GaSe is the most effi-cient for THz generation.Quartz,LiNbO3,GaP,and DAST(4-dimethylamino-N-methyl-4-stilbazolium-tosylate)are other nonlinear materials in which THz emission by DFG has been demonstrated.The generation efficiencies of these materials are substantially lower than GaSe,which has a few notable properties.First,its second-order nonlinear optical coefficient is very large: d22=54pm/V.Second,phase matching is attainable with optical pump beams in the infrared wavelength range.The output THz frequency is continuously tunable in the broad spectral range from0.2to5.3THz.Third,the linear ab-sorption in GaSe is relatively low in the THz frequency range.Figure4.6shows the absorption coefficient of GaSe,compared with those of other nonlinear op-tical crystals[38].The curves of GaSe,LiNbO3,and LiTaO3are theoretical calculations.Due to defects and second-order phonon processes,experimental values are higher than the theoretical predictions.The absorption coefficient of GaSe is approximately1cm−1in the sub-THz frequency range[85].Figure4.7illustrates the geometry for angle-tuned phase matching of type-II DFG in GaSe.The phase-matching angleθis the angle between the optic and the propagation axes.GaSe is birefringent due to its anisotropic crystal structure(hexagonal structure of¯6m2point group).The axis of anisotropy corresponds to the optic axisˆc.GaSe is a uniaxial crystal because it has only one optic axis.The uniaxial birefringence is quantified by two refractive in-dices:n o and n e are the refractive indices for polarizations perpendicular and parallel to the optic axis.GaSe is negative uniaxial because n e<n o.Type-II4.2Difference Frequency Generation and Parametric Amplification127a is n e (θ)2=(n e )2+(n o )2.(4.25)neand n o of GaSe are 2.46and 2.81at 1µm,respectively [86].θex (degrees)Fig. 4.8.Output frequency versus external phase-matching angle θex =sin −1(n O sin θ).Circles and solid curves,respectively,correspond to experimental and calculated results of refractive-index dispersion relations for GaSe.(Reprinted from [87].)1284Continuous-Wave Terahertz Sources and DetectorsThe phase matching condition,k T=k1−k2,is expressed as a function of the phase-matching angleθ,n e TωT=n o Oω1−n e O(θ)ω2.(4.26) Figure4.8shows the frequency tuning curve of GaSe as a function of the exter-nal phase-matching angle,θex=sin−1(n O sinθ),for the optical wavelength,λ1=2πc/ω1=1.064µm[87].The experimental data points(open circles)are compared with theoretical calculation(solid line).The coherent THz radiation output has a broad tuning range from0.2to5.3THz.Fig. 4.9.Peak output power versus output wavelength for three GaSe crystals with thicknesses(along the z axis)of4mm(triangles),7mm(circles),and15mm (squares).(Reprinted from[87].)Figure4.9shows the THz peak output power as a function of output wavelength for4-mm,7-mm,and15-mm thick GaSe crystals[87].The optical pump sources are a Q-switched Nd:YAG laser(wavelength,1.064µm;pulse duration,10ns;pulse energy,6mJ;repetition rate,10Hz)and the tunable idler output(duration,5ns;pulse energy,3mJ)of an optical parametric oscillator(OPO)pumped by the same Nd:YAG laser.Accordingly,the pulse duration and the repetition rate of the THz pulses are5ns and10Hz,re-spectively.The maximum THz peak power of the15-mm crystal is69.4W at1.53THz,which corresponds to an optical-to-THz conversion efficiency of 1.8×10−4,a pulse energy of0.5µJ,and an average power of5µW.Sophisticated optical pumping schemes facilitate much higher THz out-put power.Figure4.10shows the schematic of the THz generation from a4.2Difference Frequency Generation and Parametric Amplification129 quasi-phase-matched(QPM)GaAs crystal placed inside the cavity of a syn-chronously pumped optical parametric oscillator(OPO)[88].The average THz output power is1mW at2.8THz with a bandwidth of0.3THz.The OPO is pumped by a mode-locked laser at1.064µm(7ps pulse duration, 50MHz repetition rate,and10W average output power).The gain medium of the OPO is a periodically-poled lithium niobate(PPLN)crystal.The OPO converts a1.064-µm photon to two-photons near the degeneracy wavelength 2.128-µm.The OPO output spectra are shown in Fig.4.10(b).The frequency splitting,which is in the THz frequency range,is tunable via temperature control of the PPLN crystal.Fig.4.10.(a)Schematic of a linear doubly resonant OPO with an“offset”cavity design.M1-M8,cavity mirrors;M9,off-axis parabolic mirror for THz outcoupling.(b)PPLN OPO line shapes near degeneracy for two different PPLN temperatures: T=90◦C(frequency splitting of2.05THz,black lines)and T=100◦C(frequency splitting of0.96THz,gray lines).The dotted line represents the degeneracy point 2.128µm.The inset shows OPO tuning curves as a function of PPLN crystal tem-perature.(Reprinted from[88].)4.2.3Optical Parametric AmplificationOptical parametric generation is a second-order nonlinear optical process where the photon of a pump pulse is converted into two photons with lower energies.The sum of the two photon energies is equal to the pump photon energy:ωp=ωi+ωT,where pump and idler photons,ωp andωi,are at op-tical frequencies and a signal photon,ωT,is at THz frequency.The idler and THz waves are amplified when the phase-matching condition,k p=k i+k T, is satisfied.The parametric process has been utilized to generate tunable narrowband THz waves in LiNbO3crystals.Figure4.11shows the momentum conservation of the pump,idler,and THz waves.The three wave vectors are noncollinearly phase matched.With a1.064-µm optical pump,THz frequency is continuously tunable from1to3THz by changing the angleφbetween the pump and the1304Continuous-Wave Terahertz Sources and Detectorsidler[89].The angle is changed between0.5◦and1.5◦for this tuning range. The angle between the THz wave and the idler wave hardly changes at65◦.d2A T dz =4d2effωTωi20n T n i c|A p|2A T≡g2T A T,(4.29)where the exponential gain coefficient g T is given asg T=4d2effωTωi20n T n i c21/2|A p|.(4.30)The gain coefficient is proportional to the pumpfield amplitude.Assuming the initial THzfield A T(0)=0,we get the solutionA T(z)=ie iφpn iωTn TωiA∗i(0)sinh g T z,(4.31)whereφp is the phase of the complex amplitude A p.The intensity of the THzwave isI T(z)=120cn T|A T(z)|2=ωTωiI i(0)sinh2g T z.(4.32)A notable implication of this equation is that the output THz intensity is proportional to the initial idler intensity.4.2Difference Frequency Generation and Parametric Amplification131We describe two methods to strengthen the idler intensity,shown in Fig.4.12:(a)injection-seeded THz-wave parametric generator(TPG)and(b) THz-wave parametric oscillator(TPO).A CW Yb-fiber laser(wavelength, 1.070-µm)injects the seed beam of the idler into the TPG.The optical pump source is a Q-switched Nd:YAG laser(wavelength,1.064mm;pulse energy, 45mJ/pulse;pulse duration,15ns;repetition rate,10Hz).The maximum THz pulse energy is0.6nJ with45-mJ pump pulses,which corresponds to an optical-to-THz conversion efficiency of1.3×10−8.In a TPO,the idler beam is confined within an optical cavity,which gives rise to a significant enhance-ment of the idler intensity.For the specific design depicted in Fig4.12(b),the LiNbO3crystal is placed inside the pump laser cavity.The maximum THz pulse energy is5nJ when the pump pulse energy is1.3mJ.Accordingly,the optical-to-THz conversion efficiency is3×10−6.(a)(b)Fig. 4.12.Schematic diagrams of(a)the injection-seeded TPG(Reprinted from[90].c 2001,American Institute of Physics)and(b)the non-collinear phase-matched TPO(Reprinted with permission from[91].c 2006,American Institute of Physics).1324Continuous-Wave Terahertz Sources and Detectors4.3Far-Infrared Gas LasersThe basic design of THz gas lasers is similar to that of the typical laser sys-tem shown in Fig.2.24.An extra component of importance is an intracavity waveguide used to confine the laser modes in the transverse direction.The gain media of THz gas lasers are molecular gases such as CH 3F,CH 3OH,NH 3and CH 2F 2.The THz radiation originates from the rotational transitions of the molecules (see section 2.2.3).The molecules have permanent dipole moments,hence their rotational transitions are directly coupled to electromagnetic ra-diation via dipole interactions.v = 1J -1J -2JOptical pumping with CO 2laser THz radiationRotational modes VibrationalmodesE ≈≈K K -1K +1v = 0J J +1Thermalpopulation (λ~ 10 µm)N ( E )Fig.4.13.Energy level diagram of optical excitation (v =0→1)and THz radiation (J +1→J for v =0,J →J −1and J −1→J −2for v =1)in an optically-pumped THz gas laser.Figure 4.13illustrates the lasing scheme of a typical THz gas laser.At room temperature,the molecules occupy the lowest vibrational mode (v =0)with a thermal populationN (J,K )∝g (J,K )e −E rot (J,K )/k B T (4.33)of rotational states,where E rot (J,K )is the rotational energy eigenvalues (Eq.2.177).Optical pumping with a CO 2laser excites some of the molecules4.4P-Type Germanium Lasers133 from the lowest to thefirst excited vibrational mode.For symmetric-top molecules,the vibrational-rotational transitions obey the selection rules∆v= 1,∆J=0or±1,and∆K=0(see section2.2.3).The optically induced pop-ulation inversions between(J+1)and J-levels for v=0and between J and (J−1)-levels for v=1give rise to emissions at THz frequencies.The cascade transition from(J−1)to(J−2)-level for v=1also contributes to the THz radiation.Many chemical species have been examined for lasing in the THz region, and several hundred THz laser emission lines have been observed.Table4.1 lists some of the stronger laser lines in the THz region[92].ser lines of optically pumped THz gas lasersFrequency(THz)Molecule Output Power(mW)8.0CH3OH∼107.1CH3OH∼104.68CH3OH>204.25CH3OH∼1003.68NH3∼1002.52CH3OH>1002.46CH2F2∼101.9615NH3∼2001.81CH2F2<1001.27CH2F2∼100.86CH3Cl∼100.59CH3I∼100.525CH3OH∼400.245CH3OH∼10Data from Ref.[92]4.4P-Type Germanium LasersP-type germanium THz lasers are electrically pumped all-solid-state lasers. The usual dopant is beryllium,which provides high optical gain.The lasing action is based on streaming motion and population inversion of hot-carriers in p-type Ge crystals submerged in crossed electric and magneticfields.THz photons are generated by stimulated transitions between two light-hole Landau levels.Classically,a charged particle in a magneticfield moves in circular motion with a cyclotron frequency proportional to thefield strength. Because of spatial confinement,the quantum mechanical energy levels of the charged particle can only have discrete values.The discrete energy levels are called Landau levels.1344Continuous-Wave Terahertz Sources and DetectorsA population inversion is accomplished by intricate intraband transitions among light hole and heavy hole states at low temperature(<40K).In this cryogenic condition,the predominant mechanism of hole scattering in p-type Ge is spontaneous emission of optical phonons(¯hωOP=37meV).If the ap-plied electricfield is strong enough,a hole is freely accelerated up to the optical phonon energy where it makes a transition to a lower energy state by emitting an optical phonon.This phenomenon is called streaming motion.A certain amount of heavy holes(HH)scatter into pseudo-stable Landau lev-els in the light-hole(LH)band.These pseudo-stable states are formed under the specific condition of crossed electric and magneticfields.The accumula-tion of streaming heavy holes in the pseudo-stable LH states results in the population inversion between the stable Landau levels and lower energy lev-els.Figure4.14illustrates the process of population inversion in a p-type Ge crystal when crossed electric and magneticfields are applied.dv x dt =emE+ωc v y anddv ydt=−ωc v x,(4.35)whereωc=eB/m is the cyclotron frequency.The solution to these equations is a circular trajectory in the velocity space centered at−v d e y:。

第11期 肀螬f SM 龛找*f MVol .15N o.il2020 年 11 月Journal of CAEIT Nov . 2020doi ： 10. 3969/j . issn . 1673-5692. 2020. 11.006超宽带Vivaldi 天线单元及阵列设计史信荣、史劼2,熊洋洋\柯进、罗旭东1(1.广东省计量科学研究院广东省现代几何与力学计量技术重点实验室，广东广州51〇4〇5;2.中国工业互联网研究院，北京100110)摘要：文中设计了一种新型超宽带平衡对跖Vivaldi 天线单元和阵列。

研究分析了主要结构参数 对天线性能的影响，通过增加金属隔板、接地柱、减小天线剖面高度等方式，将天线单元的阻抗带宽由1.7个倍频程提升至5个倍频程。

该新型天线单元具有阻抗带宽较宽、结构尺寸小的特点，是一 种较为理想的超宽带阵列天线单元。

在单元优化的基础上，文中对8 x 8的超宽带天线阵列性能进 行了研究，结果表明该天线阵列具有良好的阻抗带宽和辐射性能。

关键词：超宽带；Vivaldi 天线；平衡对跖中图分类号:TN 98文献标志码：A文章编号：1673-5692(2020) 11-10654)5Design of Ultra-Wideband Wide-angle Scanning VivaldiAntenna and ArraySHI Xin-rong 1 , SHI Jie 2 , XIONG Yang -yang 1 ,KE Jin ' , LUO Xu -dong 1(1. Guangdong Institute of Metrology , Guangdong Provincial Key Laboratory of Modem Geometric and MechanicalMetrology Technology , Guangzhou 510405 ,China ；2. China Academy of Industrial Internet , Beijing 100036,China )Abstract ： A novel ultra-wideband (UWB ) balanced antipodal Vivaldi antenna element and array withwide-angle scanning is designed . The influence of the main structural parameters on the antenna perform ­ance is analyzed . The impedance bandwidth of the antenna element is improved from 1. 7 octaves to 5 oc ­taves by adding metallic partitions , metallic poles and reducing the height of the antenna . The novel an ­tenna element not only has a wide impedance bandwidth , but also a smaller structure size . The length and width of the antenna element is only half of the wavelength corresponding to the highest frequency . It is an ideal UWB wide-angle scanning array antenna element . On the basis of element optimization , the performance of 8 x 8 UWB array is studied . The results show that the aiTay has a wide impedance band ­width and good radiation performance .Key words ： ultra-wideband (UWB ) ； vivaldi antenna ； balanced antipodal〇引言阵列天线具有快速扫描、波束形状捷变、空间功 率合成的能力，广泛应用在卫星通信、遥感遥测等领域。

一种低剖面Vivaldi天线的设计与仿真作者：郑灵曹军陈嗣乔张小刚来源：《电子科学技术》2017年第03期摘要：Vivaldi天線是目前应用最广泛、研究最多的一种超宽带阵列天线单元，该天线的基本原理是通过传输线的结构渐变实现电磁波从射频传输线到自由空间的模式转换及相应的阻抗变换。

Vivaldi天线的优点是带宽较宽、宽角扫描潜力大，缺点是剖面较高，一般为低频波长的1/4。

本文设计了一种低剖面的Vivaldi天线，将高度降低到了低频波长的1/6。

关键词：低剖面；Vivaldi天线中图分类号：TN822+.8 文献标识码：A 文章编号： 2095-8595 （2017） 03-061-003电子科学技术 URL： http：// DOI： 10.16453/j.issn.2095-8595.2017.03.015引言随着技术的不断发展，雷达、电子对抗、通信等系统对天线提出了越来越多的要求[1-3]。

为了减小天线对飞机、卫星等平台机动性能的影响，需要天线具有低剖面特性；同时，由于机载、星载等平台的空间有限，希望用一副天线实现雷达、电子对抗、通信等多种功能，这就需要天线具有较宽的工作带宽。

近年来，低剖面、超宽带天线引起了越来越多的关注[5-8]。

Vivaldi天线是目前应用最广泛、研究最多的一种超宽带阵列天线单元。

Vivaldi天线是一种行波天线，没有固定的相位中心，随着频率变化，天线的相位中心也随之改变[2]。

对于Vivaldi天线来说，缝隙的初始端辐射高频电磁能量，缝隙的末端辐射低频电磁能量，从而实现较宽的工作带宽。

Vivaldi天线的优点是带宽较宽、宽角扫描潜力大，缺点是剖面较高，一般为低频波长的1/4。

本文设计了一种低剖面的Vivaldi天线，将高度降低到了低频波长的1/6。

并对这种天线的辐射特性进行了研究，得到了天线单元的驻波和增益特性，进而对阵列天线的扫描特性进行了分析。

1 天线模型本文设计的Vivaldi天线宽度W=75mm，高度L=50mm，天线采用Rogers RT/duroid 5880材料作为基板，基板两面覆铜，一面为含渐变缝隙的辐射部分，一面为馈电部分，如图1所示。

Stimulated by the initial proposal that molecules could be used as the functional building blocks in electronic devices 1, researchers around the world have been probing transport phenomena at the single-molecule level both experimentally and theoretically 2–11. Recent experimental advances include the demonstration of conductance switching 12–16, rectification 17–21, and illustrations on how quantum interference effects 22–26 play a critical role in the electronic properties of single metal–molecule–metal junctions. The focus of these experiments has been to both provide a fundamental understanding of transport phenomena in nanoscale devices as well as to demonstrate the engineering of functionality from rational chemical design in single-molecule junctions. Although so far there are no ‘molecular electronics’ devices manufactured commercially, basic research in this area has advanced significantly. Specifically, the drive to create functional molecular devices has pushed the frontiers of both measurement capabilities and our fundamental understanding of varied physi-cal phenomena at the single-molecule level, including mechan-ics, thermoelectrics, optoelectronics and spintronics in addition to electronic transport characterizations. Metal–molecule–metal junctions thus represent a powerful template for understanding and controlling these physical and chemical properties at the atomic- and molecular-length scales. I n this realm, molecular devices have atomically defined precision that is beyond what is achievable at present with quantum dots. Combined with the vast toolkit afforded by rational molecular design 27, these techniques hold a significant promise towards the development of actual devices that can transduce a variety of physical stimuli, beyond their proposed utility as electronic elements 28.n this Review we discuss recent measurements of physi-cal properties of single metal–molecule–metal junctions that go beyond electronic transport characterizations (Fig. 1). We present insights into experimental investigations of single-molecule junc-tions under different stimuli: mechanical force, optical illumina-tion and thermal gradients. We then review recent progress in spin- and quantum interference-based phenomena in molecular devices. I n what follows, we discuss the emerging experimentalSingle-molecule junctions beyond electronic transportSriharsha V. Aradhya and Latha Venkataraman*The id ea of using ind ivid ual molecules as active electronic components provid ed the impetus to d evelop a variety of experimental platforms to probe their electronic transport properties. Among these, single-molecule junctions in a metal–molecule–metal motif have contributed significantly to our fundamental understanding of the principles required to realize molecular-scale electronic components from resistive wires to reversible switches. The success of these techniques and the growing interest of other disciplines in single-molecule-level characterization are prompting new approaches to investigate metal–molecule–metal junctions with multiple probes. Going beyond electronic transport characterization, these new studies are highlighting both the fundamental and applied aspects of mechanical, optical and thermoelectric properties at the atomic and molecular scales. Furthermore, experimental demonstrations of quantum interference and manipulation of electronic and nuclear spins in single-molecule circuits are heralding new device concepts with no classical analogues. In this Review, we present the emerging methods being used to interrogate multiple properties in single molecule-based devices, detail how these measurements have advanced our understanding of the structure–function relationships in molecular junctions, and discuss the potential for future research and applications.methods, focusing on the scientific significance of investigations enabled by these methods, and their potential for future scientific and technological progress. The details and comparisons of the dif-ferent experimental platforms used for electronic transport char-acterization of single-molecule junctions can be found in ref. 29. Together, these varied investigations underscore the importance of single-molecule junctions in current and future research aimed at understanding and controlling a variety of physical interactions at the atomic- and molecular-length scale.Structure–function correlations using mechanicsMeasurements of electronic properties of nanoscale and molecu-lar junctions do not, in general, provide direct structural informa-tion about the junction. Direct imaging with atomic resolution as demonstrated by Ohnishi et al.30 for monoatomic Au wires can be used to correlate structure with electronic properties, however this has not proved feasible for investigating metal–molecule–metal junctions in which carbon-based organic molecules are used. Simultaneous mechanical and electronic measurements provide an alternate method to address questions relating to the struc-ture of atomic-size junctions 31. Specifically, the measurements of forces across single metal–molecule–metal junctions and of metal point contacts provide independent mechanical information, which can be used to: (1) relate junction structure to conduct-ance, (2) quantify bonding at the molecular scale, and (3) provide a mechanical ‘knob’ that can be used to control transport through nanoscale devices. The first simultaneous measurements of force and conductance in nanoscale junctions were carried out for Au point contacts by Rubio et al.32, where it was shown that the force data was unambiguously correlated to the quantized changes in conductance. Using a conducting atomic force microscope (AFM) set-up, Tao and coworkers 33 demonstrated simultaneous force and conductance measurements on Au metal–molecule–metal junc-tions; these experiments were performed at room temperature in a solution of molecules, analogous to the scanning tunnelling microscope (STM)-based break-junction scheme 8 that has now been widely adopted to perform conductance measurements.Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA. *e-mail: lv2117@DOI: 10.1038/NNANO.2013.91These initial experiments relied on the so-called static mode of AFM-based force spectroscopy, where the force on the canti-lever is monitored as a function of junction elongation. I n this method the deflection of the AFM cantilever is directly related to the force on the junction by Hooke’s law (force = cantilever stiff-ness × cantilever deflection). Concurrently, advances in dynamic force spectroscopy — particularly the introduction of the ‘q-Plus’ configuration 34 that utilizes a very stiff tuning fork as a force sen-sor — are enabling high-resolution measurements of atomic-size junctions. In this technique, the frequency shift of an AFM cantilever under forced near-resonance oscillation is measuredas a function of junction elongation. This frequency shift can be related to the gradient of the tip–sample force. The underlying advantage of this approach is that frequency-domain measure-ments of high-Q resonators is significantly easier to carry out with high precision. However, in contrast to the static mode, recover-ing the junction force from frequency shifts — especially in the presence of dissipation and dynamic structural changes during junction elongation experiments — is non-trivial and a detailed understanding remains to be developed 35.The most basic information that can be determined throughsimultaneous measurement of force and conductance in metalThermoelectricsSpintronics andMechanicsOptoelectronicsHotColdFigure 1 | Probing multiple properties of single-molecule junctions. phenomena in addition to demonstrations of quantum mechanical spin- and interference-dependent transport concepts for which there are no analogues in conventional electronics.contacts is the relation between the measured current and force. An experimental study by Ternes et al.36 attempted to resolve a long-standing theoretical prediction 37 that indicated that both the tunnelling current and force between two atomic-scale metal contacts scale similarly with distance (recently revisited by Jelinek et al.38). Using the dynamic force microscopy technique, Ternes et al. effectively probed the interplay between short-range forces and conductance under ultrahigh-vacuum conditions at liquid helium temperatures. As illustrated in Fig. 2a, the tunnel-ling current through the gap between the metallic AFM probe and the substrate, and the force on the cantilever were recorded, and both were found to decay exponentially with increasing distance with nearly the same decay constant. Although an exponential decay in current with distance is easily explained by considering an orbital overlap of the tip and sample wavefunctions through a tunnel barrier using Simmons’ model 39, the exponential decay in the short-range forces indicated that perhaps the same orbital controlled the interatomic short-range forces (Fig. 2b).Using such dynamic force microscopy techniques, research-ers have also studied, under ultrahigh-vacuum conditions, forces and conductance across junctions with diatomic adsorbates such as CO (refs 40,41) and more recently with fullerenes 42, address-ing the interplay between electronic transport, binding ener-getics and structural evolution. I n one such experiment, Tautz and coworkers 43 have demonstrated simultaneous conduct-ance and stiffness measurements during the lifting of a PTCDA (3,4,9,10-perylene-tetracarboxylicacid-dianhydride) molecule from a Ag(111) substrate using the dynamic mode method with an Ag-covered tungsten AFM tip. The authors were able to follow the lifting process (Fig. 2c,d) monitoring the junction stiffness as the molecule was peeled off the surface to yield a vertically bound molecule, which could also be characterized electronically to determine the conductance through the vertical metal–molecule–metal junction with an idealized geometry. These measurements were supported by force field-based model calculations (Fig. 2c and dashed black line in Fig. 2d), presenting a way to correlate local geometry to the electronic transport.Extending the work from metal point contacts, ambient meas-urements of force and conductance across single-molecule junc-tions have been carried out using the static AFM mode 33. These measurements allow correlation of the bond rupture forces with the chemistry of the linker group and molecular backbone. Single-molecule junctions are formed between a Au-metal sub-strate and a Au-coated cantilever in an environment of molecules. Measurements of current through the junction under an applied bias determine conductance, while simultaneous measurements of cantilever deflection relate to the force applied across the junction as shown in Fig. 2e. Although measurements of current throughzF zyxCantileverIVabConductance G (G 0)1 2 3Tip–sample distance d (Å)S h o r t -r a n g e f o r c e F z (n N )10−310−210−11110−110−210−3e10−410−210C o n d u c t a n c e (G 0)Displacement86420Force (nN)0.5 nm420−2F o r c e (n N )−0.4−0.200.20.4Displacement (nm)SSfIncreasing rupture forcegc(iv)(i)(iii)(ii)Low HighCounts d9630−3d F /d z (n N n m −1)(i)(iv)(iii)(ii)A p p r o a chL i ft i n g110−210−4G (2e 2/h )2051510z (Å)H 2NNH 2H 2NNH 2NNFigure 2 | Simultaneous measurements of electronic transport and mechanics. a , A conducting AFM set-up with a stiff probe (shown schematically) enabled the atomic-resolution imaging of a Pt adsorbate on a Pt(111) surface (tan colour topography), before the simultaneous measurement of interatomic forces and currents. F z , short-range force. b , Semilogarithmic plot of tunnelling conductance and F z measured over the Pt atom. A similar decay constant for current and force as a function of interatomic distance is seen. The blue dashed lines are exponential fits to the data. c , Structural snapshots showing a molecular mechanics simulation of a PTCDA molecule held between a Ag substrate and tip (read right to left). It shows the evolution of the Ag–PTCDA–Ag molecular junction as a function of tip–surface distance. d , Upper panel shows experimental stiffness (d F /d z ) measurements during the lifting process performed with a conducting AFM. The calculated values from the simulation are overlaid (dashed black line). Lower panel shows simultaneously measured conductance (G ). e , Simultaneously measured conductance (red) and force (blue) measurements showing evolution of a molecular junction as a function of junction elongation. A Au point contact is first formed, followed by the formation of a single-molecule junction, which then ruptures on further elongation. f , A two-dimensional histogram of thousands of single-molecule junctionrupture events (for 1,4-bis(methyl sulphide) butane; inset), constructed by redefining the rupture location as the zero displacement point. The most frequently measured rupture force is the drop in force (shown by the double-headed arrow) at the rupture location in the statistically averaged force trace (overlaid black curve). g , Beyond the expected dependence on the terminal group, the rupture force is also sensitive to the molecular backbone, highlighting the interplay between chemical structure and mechanics. In the case of nitrogen-terminated molecules, rupture force increases fromaromatic amines to aliphatic amines and the highest rupture force is for molecules with pyridyl moieties. Figure reproduced with permission from: a ,b , ref. 36, © 2011 APS; c ,d , ref. 43, © 2011 APS.DOI: 10.1038/NNANO.2013.91such junctions are easily accomplished using standard instru-mentation, measurements of forces with high resolution are not straightforward. This is because a rather stiff cantilever (with a typical spring constant of ~50 N m−1) is typically required to break the Au point contact that is first formed between the tip and sub-strate, before the molecular junctions are created. The force reso-lution is then limited by the smallest deflection of the cantilever that can be measured. With a custom-designed system24 our group has achieved a cantilever displacement resolution of ~2 pm (com-pare with Au atomic diameter of ~280 pm) using an optical detec-tion scheme, allowing the force noise floor of the AFM set-up to be as low as 0.1 nN even with these stiff cantilevers (Fig. 2e). With this system, and a novel analysis technique using two-dimensional force–displacement histograms as illustrated in Fig. 2f, we have been able to systematically probe the influence of the chemical linker group44,45 and the molecular backbone46 on single-molecule junction rupture force as illustrated in Fig. 2g.Significant future opportunities with force measurements exist for investigations that go beyond characterizations of the junc-tion rupture force. In two independent reports, one by our group47 and another by Wagner et al.48, force measurements were used to quantitatively measure the contribution of van der Waals interac-tions at the single-molecule level. Wagner et al. used the stiffness data from the lifting of PTCDA molecules on a Au(111) surface, and fitted it to the stiffness calculated from model potentials to estimate the contribution of the various interactions between the molecule and the surface48. By measuring force and conductance across single 4,4’-bipyridine molecules attached to Au electrodes, we were able to directly quantify the contribution of van der Waals interactions to single-molecule-junction stiffness and rupture force47. These experimental measurements can help benchmark the several theoretical frameworks currently under development aiming to reliably capture van der Waals interactions at metal/ organic interfaces due to their importance in diverse areas includ-ing catalysis, electronic devices and self-assembly.In most of the experiments mentioned thus far, the measured forces were typically used as a secondary probe of junction prop-erties, instead relying on the junction conductance as the primary signature for the formation of the junction. However, as is the case in large biological molecules49, forces measured across single-mol-ecule junctions can also provide the primary signature, thereby making it possible to characterize non-conducting molecules that nonetheless do form junctions. Furthermore, molecules pos-sess many internal degrees of motion (including vibrations and rotations) that can directly influence the electronic transport50, and the measurement of forces with such molecules can open up new avenues for mechanochemistry51. This potential of using force measurements to elucidate the fundamentals of electronic transport and binding interactions at the single-molecule level is prompting new activity in this area of research52–54. Optoelectronics and optical spectroscopyAddressing optical properties and understanding their influence on electronic transport in individual molecular-scale devices, col-lectively referred to as ‘molecular optoelectronics’, is an area with potentially important applications55. However, the fundamental mismatch between the optical (typically, approximately at the micrometre scale) and molecular-length scales has historically presented a barrier to experimental investigations. The motiva-tions for single-molecule optoelectronic studies are twofold: first, optical spectroscopies (especially Raman spectroscopy) could lead to a significantly better characterization of the local junction structure. The nanostructured metallic electrodes used to real-ize single-molecule junctions are coincidentally some of the best candidates for local field enhancement due to plasmons (coupled excitations of surface electrons and incident photons). This there-fore provides an excellent opportunity for understanding the interaction of plasmons with molecules at the nanoscale. Second, controlling the electronic transport properties using light as an external stimulus has long been sought as an attractive alternative to a molecular-scale field-effect transistor.Two independent groups have recently demonstrated simulta-neous optical and electrical measurements on molecular junctions with the aim of providing structural information using an optical probe. First, Ward et al.56 used Au nanogaps formed by electromi-gration57 to create molecular junctions with a few molecules. They then irradiated these junctions with a laser operating at a wavelength that is close to the plasmon resonance of these Au nanogaps to observe a Raman signal attributable to the molecules58 (Fig. 3a). As shown in Fig. 3b, they observed correlations between the intensity of the Raman features and magnitude of the junction conductance, providing direct evidence that Raman signatures could be used to identify junction structures. They later extended this experimental approach to estimate vibrational and electronic heating in molecu-lar junctions59. For this work, they measured the ratio of the Raman Stokes and anti-Stokes intensities, which were then related to the junction temperature as a function of the applied bias voltage. They found that the anti-Stokes intensity changed with bias voltage while the Stokes intensity remained constant, indicating that the effective temperature of the Raman-active mode was affected by passing cur-rent through the junction60. Interestingly, Ward et al. found that the vibrational mode temperatures exceeded several hundred kelvin, whereas earlier work by Tao and co-workers, who used models for junction rupture derived from biomolecule research, had indicated a much smaller value (~10 K) for electronic heating61. Whether this high temperature determined from the ratio of the anti-Stokes to Stokes intensities indicates that the electronic temperature is also similarly elevated is still being debated55, however, one can definitely conclude that such measurements under a high bias (few hundred millivolts) are clearly in a non-equilibrium transport regime, and much more research needs to be performed to understand the details of electronic heating.Concurrently, Liu et al.62 used the STM-based break-junction technique8 and combined this with Raman spectroscopy to per-form simultaneous conductance and Raman measurements on single-molecule junctions formed between a Au STM tip and a Au(111) substrate. They coupled a laser to a molecular junction as shown in Fig. 3c with a 4,4’-bipyridine molecule bridging the STM tip (top) and the substrate (bottom). Pyridines show clear surface-enhanced Raman signatures on metal58, and 4,4’-bipy-ridine is known to form single-molecule junctions in the STM break-junction set-up8,15. Similar to the study of Ward et al.56, Liu et al.62 found that conducting molecular junctions had a Raman signature that was distinct from the broken molecu-lar junctions. Furthermore, the authors studied the spectra of 4,4’-bipyridine at different bias voltages, ranging from 10 to 800 mV, and reported a reversible splitting of the 1,609 cm–1 peak (Fig. 3d). Because this Raman signature is due to a ring-stretching mode, they interpreted this splitting as arising from the break-ing of the degeneracy between the rings connected to the source and drain electrodes at high biases (Fig. 3c). Innovative experi-ments such as these have demonstrated that there is new physics to be learned through optical probing of molecular junctions, and are initiating further interest in understanding the effect of local structure and vibrational effects on electronic transport63. Experiments that probe electroluminescence — photon emis-sion induced by a tunnelling current — in these types of molec-ular junction can also offer insight into structure–conductance correlations. Ho and co-workers have demonstrated simultaneous measurement of differential conductance and photon emissionDOI: 10.1038/NNANO.2013.91from individual molecules at a submolecular-length scale using an STM 64,65. Instead of depositing molecules directly on a metal sur-face, they used an insulating layer to decouple the molecule from the metal 64,65 (Fig. 3e). This critical factor, combined with the vac-uum gap with the STM tip, ensures that the metal electrodes do not quench the radiated photons, and therefore the emitted photons carry molecular fingerprints. Indeed, the experimental observation of molecular electroluminescence of C 60 monolayers on Au(110) by Berndt et al.66 was later attributed to plasmon-mediated emission of the metallic electrodes, indirectly modulated by the molecule 67. The challenge of finding the correct insulator–molecule combination and performing the experiments at low temperature makes electro-luminescence relatively uncommon compared with the numerous Raman studies; however, progress is being made on both theoretical and experimental fronts to understand and exploit emission pro-cesses in single-molecule junctions 68.Beyond measurements of the Raman spectra of molecular junctions, light could be used to control transport in junctions formed with photochromic molecular backbones that occur in two (or more) stable and optically accessible states. Some common examples include azobenzene derivatives, which occur in a cis or trans form, as well as diarylene compounds that can be switched between a conducting conjugated form and a non-conducting cross-conjugated form 69. Experiments probing the conductance changes in molecular devices formed with such compounds have been reviewed in depth elsewhere 70,71. However, in the single-mol-ecule context, there are relatively few examples of optical modula-tion of conductance. To a large extent, this is due to the fact that although many molecular systems are known to switch reliably in solution, contact to metallic electrodes can dramatically alter switching properties, presenting a significant challenge to experi-ments at the single-molecule level.Two recent experiments have attempted to overcome this chal-lenge and have probed conductance changes in single-molecule junctions while simultaneously illuminating the junctions with visible light 72,73. Battacharyya et al.72 used a porphyrin-C 60 ‘dyad’ molecule deposited on an indium tin oxide (I TO) substrate to demonstrate the light-induced creation of an excited-state mol-ecule with a different conductance. The unconventional transpar-ent ITO electrode was chosen to provide optical access while also acting as a conducting electrode. The porphyrin segment of the molecule was the chromophore, whereas the C 60 segment served as the electron acceptor. The authors found, surprisingly, that the charge-separated molecule had a much longer lifetime on ITO than in solution. I n the break-junction experiments, the illuminated junctions showed a conductance feature that was absent without1 μm Raman shift (cm –1)1,609 cm –1(–)Source 1,609 cm–1Drain (+)Low voltage High voltageMgPNiAl(110)STM tip (Ag)VacuumThin alumina 1.4 1.5 1.6 1.701020 3040200400Photon energy (eV)3.00 V 2.90 V 2.80 V 2.70 V 2.60 V2.55 V 2.50 VP h o t o n c o u n t s (a .u .)888 829 777731Wavelength (nm)Oxideacebd f Raman intensity (CCD counts)1,5001,00050000.40.30.20.10.01,590 cm −11,498 cm −1d I /d V (μA V –1)1,609 cm –11,631 cm–11 μm1 μmTime (s)Figure 3 | Simultaneous studies of optical effects and transport. a , A scanning electron micrograph (left) of an electromigrated Au junction (light contrast) lithographically defined on a Si substrate (darker contrast). The nanoscale gap results in a ‘hot spot’ where Raman signals are enhanced, as seen in the optical image (right). b , Simultaneously measured differential conductance (black, bottom) and amplitudes of two molecular Raman features (blue traces, middle and top) as a function of time in a p-mercaptoaniline junction. c , Schematic representation of a bipyridine junction formed between a Au STM tip and a Au(111) substrate, where the tip enhancement from the atomically sharp STM tip results in a large enhancement of the Raman signal. d , The measured Raman spectra as a function of applied bias indicate breaking of symmetry in the bound molecule. e , Schematic representation of a Mg-porphyrin (MgP) molecule sandwiched between a Ag STM tip and a NiAl(110) substrate. A subnanometre alumina insulating layer is a key factor in measuring the molecular electroluminescence, which would otherwise be overshadowed by the metallic substrate. f , Emission spectra of a single Mg-porphyrin molecule as a function of bias voltage (data is vertically offset for clarity). At high biases, individual vibronic peaks become apparent. The spectra from a bare oxide layer (grey) is shown for reference. Figure reproduced with permission from: a ,b , ref. 56, © 2008 ACS; c ,d , ref. 62, © 2011 NPG; e ,f , ref. 65, © 2008 APS.DOI: 10.1038/NNANO.2013.91light, which the authors assigned to the charge-separated state. In another approach, Lara-Avila et al.73 have reported investigations of a dihydroazulene (DHA)/vinylheptafulvene (VHF) molecule switch, utilizing nanofabricated gaps to perform measurements of Au–DHA–Au single-molecule junctions. Based on the early work by Daub et al.74, DHA was known to switch to VHF under illumina-tion by 353-nm light and switch back to DHA thermally. In three of four devices, the authors observed a conductance increase after irradiating for a period of 10–20 min. In one of those three devices, they also reported reversible switching after a few hours. Although much more detailed studies are needed to establish the reliability of optical single-molecule switches, these experiments provide new platforms to perform in situ investigations of single-molecule con-ductance under illumination.We conclude this section by briefly pointing to the rapid pro-gress occurring in the development of optical probes at the single-molecule scale, which is also motivated by the tremendous interest in plasmonics and nano-optics. As mentioned previously, light can be coupled into nanoscale gaps, overcoming experimental chal-lenges such as local heating. Banerjee et al.75 have exploited these concepts to demonstrate plasmon-induced electrical conduction in a network of Au nanoparticles that form metal–molecule–metal junctions between them (Fig. 3f). Although not a single-molecule measurement, the control of molecular conductance through plas-monic coupling can benefit tremendously from the diverse set of new concepts under development in this area, such as nanofabri-cated transmission lines 76, adiabatic focusing of surface plasmons, electrical excitation of surface plasmons and nanoparticle optical antennas. The convergence of plasmonics and electronics at the fundamental atomic- and molecular-length scales can be expected to provide significant opportunities for new studies of light–mat-ter interaction 77–79.Thermoelectric characterization of single-molecule junctions Understanding the electronic response to heating in a single-mole-cule junction is not only of basic scientific interest; it can have a tech-nological impact by improving our ability to convert wasted heat into usable electricity through the thermoelectric effect, where a temper-ature difference between two sides of a device induces a voltage drop across it. The efficiency of such a device depends on its thermopower (S ; also known as the Seebeck coefficient), its electric and thermal conductivity 80. Strategies for increasing the efficiency of thermoelec-tric devices turned to nanoscale devices a decade ago 81, where one could, in principle, increase the electronic conductivity and ther-mopower while independently minimizing the thermal conductiv-ity 82. This has motivated the need for a fundamental understandingof thermoelectrics at the single-molecule level 83, and in particular, the measurement of the Seebeck coefficient in such junctions. The Seebeck coefficient, S = −(ΔV /ΔT )|I = 0, determines the magnitude of the voltage developed across the junction when a temperature dif-ference ΔT is applied, as illustrated in Fig. 4a; this definition holds both for bulk devices and for single-molecule junctions. If an addi-tional external voltage ΔV exists across the junction, then the cur-rent I through the junction is given by I = G ΔV + GS ΔT where G is the junction conductance 83. Transport through molecular junctions is typically in the coherent regime where conductance, which is pro-portional to the electronic transmission probability, is given by the Landauer formula 84. The Seebeck coefficient at zero applied voltage is then related to the derivative of the transmission probability at the metal Fermi energy (in the off-resonance limit), with, S = −∂E ∂ln( (E ))π2k 2B T E 3ewhere k B is the Boltzmann constant, e is the charge of the electron, T (E ) is the energy-dependent transmission function and E F is the Fermi energy. When the transmission function for the junction takes on a simple Lorentzian form 85, and transport is in the off-resonance limit, the sign of S can be used to deduce the nature of charge carriers in molecular junctions. In such cases, a positive S results from hole transport through the highest occupied molecu-lar orbital (HOMO) whereas a negative S indicates electron trans-port through the lowest unoccupied molecular orbital (LUMO). Much work has been performed on investigating the low-bias con-ductance of molecular junctions using a variety of chemical linker groups 86–89, which, in principle, can change the nature of charge carriers through the junction. Molecular junction thermopower measurements can thus be used to determine the nature of charge carriers, correlating the backbone and linker chemistry with elec-tronic aspects of conduction.Experimental measurements of S and conductance were first reported by Ludoph and Ruitenbeek 90 in Au point contacts at liquid helium temperatures. This work provided a method to carry out thermoelectric measurements on molecular junctions. Reddy et al.91 implemented a similar technique in the STM geome-try to measure S of molecular junctions, although due to electronic limitations, they could not simultaneously measure conductance. They used thiol-terminated oligophenyls with 1-3-benzene units and found a positive S that increased with increasing molecular length (Fig. 4b). These pioneering experiments allowed the iden-tification of hole transport through thiol-terminated molecular junctions, while also introducing a method to quantify S from statistically significant datasets. Following this work, our group measured the thermoelectric current through a molecular junction held under zero external bias voltage to determine S and the con-ductance through the same junction at a finite bias to determine G (ref. 92). Our measurements showed that amine-terminated mol-ecules conduct through the HOMO whereas pyridine-terminatedmolecules conduct through the LUMO (Fig. 4b) in good agree-ment with calculations.S has now been measured on a variety of molecular junctionsdemonstrating both hole and electron transport 91–95. Although the magnitude of S measured for molecular junctions is small, the fact that it can be tuned by changing the molecule makes these experiments interesting from a scientific perspective. Future work on the measurements of the thermal conductance at the molecu-lar level can be expected to establish a relation between chemical structure and the figure of merit, which defines the thermoelec-tric efficiencies of such devices and determines their viability for practical applications.SpintronicsWhereas most of the explorations of metal–molecule–metal junc-tions have been motivated by the quest for the ultimate minia-turization of electronic components, the quantum-mechanical aspects that are inherent to single-molecule junctions are inspir-ing entirely new device concepts with no classical analogues. In this section, we review recent experiments that demonstrate the capability of controlling spin (both electronic and nuclear) in single-molecule devices 96. The early experiments by the groups of McEuen and Ralph 97, and Park 98 in 2002 explored spin-depend-ent transport and the Kondo effect in single-molecule devices, and this topic has recently been reviewed in detail by Scott and Natelson 99. Here, we focus on new types of experiment that are attempting to control the spin state of a molecule or of the elec-trons flowing through the molecular junction. These studies aremotivated by the appeal of miniaturization and coherent trans-port afforded by molecular electronics, combined with the great potential of spintronics to create devices for data storage and quan-tum computation 100. The experimental platforms for conducting DOI: 10.1038/NNANO.2013.91。

托福阅读tpo54全套解析阅读-1 (2)原文 (2)译文 (4)题目 (5)答案 (9)背景知识 (10)阅读-2 (10)原文 (10)译文 (12)题目 (13)答案 (18)背景知识 (20)阅读-3 (25)原文 (26)译文 (27)题目 (28)答案 (33)背景知识 (35)阅读-1原文The Commercialization of Lumber①In nineteenth-century America, practically everything that was built involved wood.Pine was especially attractive for building purposes.It is durable and strong, yet soft enough to be easily worked with even the simplest of hand tools.It also floats nicely on water, which allowed it to be transported to distant markets across the nation.The central and northern reaches of the Great Lakes states—Michigan, Wisconsin, and Minnesota—all contained extensive pine forests as well as many large rivers for floating logs into the Great Lakes, from where they were transported nationwide.②By 1860, the settlement of the American West along with timber shortages in the East converged with ever-widening impact on the pine forests of the Great Lakes states. Over the next 30 years, lumbering became a full-fledged enterprise in Michigan, Wisconsin, and Minnesota. Newly formed lumbering corporations bought up huge tracts of pineland and set about systematically cutting the trees. Both the colonists and the later industrialists saw timber as a commodity, but the latter group adopted a far more thorough and calculating approach to removing trees. In this sense, what happened between 1860 and 1890 represented a significant break with the past. No longer were farmers in search of extra income the main source for shingles, firewood, and other wood products. By the 1870s, farmers and city dwellers alike purchased forest products from large manufacturingcompanies located in the Great Lakes states rather than chopping wood themselves or buying it locally.③The commercialization of lumbering was in part the product of technological change. The early, thick saw blades tended to waste a large quantity of wood, with perhaps as much as a third of the log left behind on the floor as sawdust or scrap. In the 1870s, however, the British-invented band saw, with its thinner blade, became standard issue in the Great Lakes states' lumber factories.Meanwhile, the rise of steam-powered mills streamlined production by allowing for the more efficient, centralized, and continuous cutting of lumber. Steam helped to automate a variety of tasks, from cutting to the carrying away of waste. Mills also employed steam to heat log ponds, preventing them from freezing and making possible year-round lumber production.④For industrial lumbering to succeed, a way had to be found to neutralize the effects of the seasons on production. Traditionally, cutting took place in the winter, when snow and ice made it easier to drag logs on sleds or sleighs to the banks of streams. Once the streams and lakes thawed, workers rafted the logs to mills, where they were cut into lumber in the summer. If nature did not cooperate—if the winter proved dry and warm, if the spring thaw was delayed—production would suffer. To counter the effects of climate on lumber production, loggers experimented with a variety of techniques for transporting trees out of the woods. In the 1870s, loggers in the Great Lakes states began sprinkling water on sleigh roads, giving them an artificial ice coating to facilitate travel. The ice reduced the friction and allowed workers to move larger and heavier loads.⑤But all the sprinkling in the world would not save a logger from the threat of a warm winter. Without snow the sleigh roads turned to mud. In the 1870s, a set of snowless winters left lumber companies to ponder ways of liberating themselves from the seasons. Railroads were one possibility.At first, the remoteness of the pine forests discouraged common carriers from laying track.But increasing lumber prices in the late 1870s combined with periodic warm, dry winters compelled loggers to turn to iron rails. By 1887, 89 logging railroads crisscrossed Michigan, transforming logging from a winter activity into a year-round one.⑥Once the logs arrived at a river, the trip downstream to a mill could be a long and tortuous one.Logjams (buildups of logs that prevent logs from moving downstream) were common—at times stretching for 10 miles—and became even more frequent as pressure on the northern Midwest pinelands increased in the 1860s. To help keep the logs moving efficiently, barriers called booms (essentially a chain of floating logs) were constructed to control the direction of the timber. By the 1870s, lumber companies existed in all the major logging areas of the northern Midwest.译文木材的商业化①在19世纪的美国，几乎所有建筑材料都含有木材。