High-resolution spectroscopy with a femtosecond laser frequency comb

赵威等：多维傅里叶变换光谱 187 Optical Multidimensional Fourier-transform Spectroscopy Zhao Wei丨 Zhou Zhao-Yu, Yang Jin-Xin, Dai Xing-Can State Key Laboratory of Low-DimensionsI Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084 The concept of two-dimensional or multidimensional Fourier-transform (2DFT or MDFT spec- troscopy originated from nuclear magnetic resonance (NMR spectroscopy, the development of femtosecond laser technology allows the MDFT spectroscopy to be extensively implemented in the optical region in recent years, where the conventional radio frequency (RF field in the NMR is replaced by pulse lasers with wavelength ranging from infrared to visible domain. The opti- cal MDFT spectroscopy can be used to identify the population of each quantum state and the coupling between quantum states, thus to determine the evolutionary dynamics of quantum sys- tems, such as energy transfer in quantum systems and molecules interactions, in addition to the information obtained from one-dimensional spectroscopy. The optical MDFT spectroscopy has been successfully applied in the studies of a variety of quantum systems including alkali atoms, quantum wells, molecules, etc. Key words: Two-dimensional Fourier-transform spectroscopy; Four-wave mixing; Ultrafast pulse laser; Phase lock。

Xingchen Zhao,† Fang Hao,† Dawei Lu,† Wei Liu,‡ Qunfang Zhou,*,† and Guibin Jiang†
†
Downloaded by RES CENTER OF ECO ENVIR SCIENCE RCEES on September 15, 2015 | Publication Date (Web): August 13, 2015 | doi: 10.1021/acsami.5b05895
Received: July 1, 2015 Accepted: August 7, 2015 Published: August 7, 2015
18880
DOI: 10.1021/acsami.5b05895 ACS Appl. Mater. Interfaces 2015, 7, 18880−18890
ACS Applied Materials & Interfaces α-Chymotrypsin (α-ChT) is an important digestive enzyme component in pancreatic juice, and it performs proteolysis in the duodenum. It is commonly used as an excellent enzyme model for studying the nanomaterial-induced enzymatic activity inhibition because of its well-defined structure and extensively characterized enzymatic properties. In this work, we focused on the structure and activity changes of α-ChT when they were absorbed onto multiwalled carbon nanotubes (MWCNTs) with gradient functionalization densities. A combination of techniques like fluorescence, circular dichroism (CD) spectroscopy, ζ-potential analysis, atomic force microscopy (AFM), and bicinchoninic acid (BCA) assay was used to detail the interaction between CNTs and α-ChT in the hope of revealing the role of the CNT functional group density. The findings regarding the protein structural and activity alterations provide fundamental understandings for the toxicological effects of CNTs.

Time resolved photoluminescence investigations of polyacetylene derivativesHui Wang a)Department of Physics,Zhongshan University,Guangzhou510275,People’s Republic of ChinaKam Sam WongDepartment of Physics,Hong Kong University of Science and Technology,Clear Water Bay,Kowloon,Hong Kong͑Received11March1998;accepted for publication23June1998͒Time resolved photoluminescence dynamics of poly͑phenylacetylene͒͑PPA͒andpoly͑1-phenyl-1-butyl͒͑PPB͒were investigated by using femtosecond luminescence upconversionspectroscopy technique.An ultrafast initial luminescence decay͑ϳ5ps͒was observed for PPA,butit was absent for PPB.The transient luminescence spectrum of PPA exhibits a much larger red-shift ͑ϳ140meV͒than that of PPB͑ϳ60meV͒.PPB shows a much higher luminescence efficiency than that of PPA.The influence of the substitution on the photoluminescence behaviors of PPA and PPBis discussed.©1998American Institute of Physics.͓S0003-6951͑98͒02734-X͔In recent years,photoluminescence͑PL͒and electrolu-minescence͑EL͒properties of conjugated polymers have at-tracted much attentions because of their promising applica-tions in optical and optoelectronic devices.1,2Polyacetylenederivatives represent a big family of conjugated polymersand have substantially different properties from those ofpolyacetylene.For example,they are fairly stable in air andhighly transparent because of their large energy band gaps.3Recently,some polyacetylene derivatives such as poly ͑diphenylacetylene͒͑PDPA͒have shown high PL and EL ef-ficiencies,which are comparable to those of the well-studiedEL materials such as poly͑p-phenylene vinylene͒͑PPV͒and8-hydroxy quinoline aluminum͑Alq3͒.4Since various sub-stitutes can be attached onto the main backbone,a largenumber of polyacetylene derivatives can be derived for theapplications of optoelectronic devices.Preliminary studieshave shown that the substitutes have an important influenceon the PL and EL efficiencies of polyacetylene derivatives.Monosubstituted polyacetylenes such as poly͑phenylacetyle-ne͒͑PPA͒show low PL and EL efficiencies.However,disubstituted polyacetylenes such as PDPA usually exhibitmuch higher PL and EL efficiencies.4In order to explain thedifference of luminescence͑PL and EL͒properties observedin these polyacetylene derivatives,the understanding of therecombination mechanisms and their dependence on the sub-stitution of polyacetylene backbone is fundamentally impor-tant.We have therefore performed detail investigations on the time resolved photoluminescence of PPA and poly͑1-phenyl-1-butyl͒͑PPB͒.PPA and PPB powders were synthesized using a group of new organorhodium catalysts from actylene as a monomer.5,6The products of polymerization were rich in cis-isomer͑94%͒.The molecular structures of PPA and PPB are depicted in Figs.1͑a͒and1͑b͒,respectively.Both PPA and PPB molecules under study were dissolved in toluene with the concentration of about20mg/ml.The time resolved PL dynamics of PPA and PPB are studied by using femtosecond luminescence upconversion spectroscopy technique.7The laser source is a Kerr–Lens–Modelocked Ti-sapphire laser with chirped pulse amplifica-tion,which produces150fs laser pulse with the photon en-ergy of1.6eV and a repetition rate of1kHz.The sample solution is of1mm thick and excited by frequency-doubled laser pulses with the photon energy of about3.2eV and the excitation power is about300␮W.All the measurements were performed at roomtemperature.FIG.1.Molecular structure of the materials studied:͑a͒poly͑phenylacety-lene͒͑PPA͒;͑b͒poly͑1-phenyl-1-butyl͒͑PPB͒.a͒Electronic mail:wang-hui@FIG.2.Steady-state photoluminescence spectra of͑a͒PPA and͑b͒PPBexcited at3.2eV.APPLIED PHYSICS LETTERS VOLUME73,NUMBER1221SEPTEMBER199816370003-6951/98/73(12)/1637/3/$15.00©1998American Institute of PhysicsFigure 2shows the steady-state ͑PL ͒spectra of PPA and PPB excited by the photon energy of 3.2eV,which is coin-cident with the excitation photon energy in the transient ex-periments.PPB shows blue-green light emission with a peak around 2.7eV,which is much efficient than that of PPA at 2.5eV.The color change from blue-green to green is due to the decrease of the length of butyl side chain.Similar results have been reported in Ref.8.Time resolved PL spectra of PPA and PPB recorded at different time delays are presented in Figs.3͑a ͒and 3͑b ͒,respectively.Both PPA and PPB show the instantaneous lu-minescence,reflecting the ultrafast vibronic relaxation on a time scale much shorter than the time resolution of the setup.The transient spectra of PPA and PPB evolve rapidly to the shapes similar to the steady-state luminescence in the first few picoseconds,accompanying significant red-shifts of the transient luminescence spectra.For comparison,the peak po-sitions of the transient luminescence spectra versus time for both PPA and PPB are plotted as shown in Fig.4.The red-shifts ͓͗E ͘(t )Ϫ͗E 0͔͘of about 140and 60meV are mea-sured within the first 100ps for PPA and PPB,respectively.Figures 5͑a ͒and 5͑b ͒show the transient behaviors of the PL intensity at three spectral positions for PPA and PPB,respectively.The PL decay of PPA consists of two compo-nents,an initial ultrafast decay and a following slow relax-ation.By using a biexponential response function,the time constants of the initial ultrafast decay are fit to 5,5.7,and 6.7ps for the photon energy of 2.7,2.5,and 2.3eV,respectively,which indicates the dependence of the initial PL decay on the photon energy.The following slow decay processes for the three photon energies ͑2.7,2.5,and 2.3eV ͒are almost the same of about 180ps.However,the PL decay of PPB exhib-its a single exponential process with time constants of 112,125,and 210ps for the photon energy of 2.8,2.7,and 2.5eV,respectively.The dependence of the decay times on pho-ton energy for PPB becomes significant in the slow decay process.The difference of the PL behaviors observed in PPA and PPB can be explained by their different substitutes attached on the main backbones.With double substitutions of phenyl and butyl,PPB molecules possess longer and more bulky side chains than that of PPA molecules with single phenyl substituent.As a result,the interchain transport of the excited states to the longer-conjugation-length segments 9is facili-tated between PPA molecules.The interchain migration,oc-curring following photoexcitation and prior to emission,is proposed as a nonradiative process,which is extremely rapid and can reduce the luminescence effectively.4,10This results in an ultrafast initial relaxation and a large red shift of tran-sient PL spectrum for PPA.Similar results have been ob-served in other EL polymers.2However,the longerdistanceFIG.3.Time resolved photoluminescence spectra of ͑a ͒PPA and ͑b ͒PPB after excitation with 150fs laser pulses at 3.2eV.FIG.4.Energetic positions of the main emission bands in ͑a ͒PPA and ͑b ͒PPB vs delay time.between adjacent PPB molecule main chains hampers the interchain transport and confines the relaxation process on the intrachain,4,8which lead to the absence of an initial ul-trafast PL decay in PPB.The red-shift of transient PL spec-trum and the dependence of decay time on photon energyobserved in PPB are probably due to chain distortion towards a more planar conformation following photoexcitation.3The bulky side groups attached on PPB molecular main chains may have a tendency to decrease the torsion angle of the repeat unit surface on the main chain,4which therefore lead to a small red-shift of the spectrum and slow PL decay.Con-sequently higher PL efficiency was observed for PPB.In conclusion,femtosecond PL studies of PPA and PPB demonstrate that side groups of substituted polyactylene molecule play an important role in the transient PL behavior.PPB molecules have longer and more bulky side groups than PPA molecule.As a result,PPB exhibits a much slower PL decay and a higher PL efficiency.Since EL and PL observed in conjugated polymers originate from the same recombina-tion processes,2,4we believe that our results are of particular interest for the design of new EL devices.The authors gratefully acknowledge technical assistance during the experiments by Dr.H.Wang and providing PPA and PPB samples by Dr.B.Z.Tang.They also thank Pro-fessor W.Z.Lin for careful reading of the manuscript and helpful suggestions.1R.Kersting,V.Lemmer,R.F.Mahrt,K.Leo,H.Kurz,H.Bassler,and E.O.Gobel,Phys.Rev.Lett.70,3820͑1993͒.2U.Lemmer,R.F.Mahrt,Y.Wada,A.Greiner,H.Bassler,and E.O.Gobel,Appl.Phys.Lett.62,2827͑1993͒.3uchlan,S.Etemad,T.C.Chung,A.J.Heeger,and A.G.Macdi-armid,Phys.Rev.B 24,3701͑1981͒.4R.Sun,T.Masuda,and T.Kobayashi,Jpn.J.Appl.Phys.,Part 235,L1434͑1996͒.5P.W.Hong,Thesis,Hong Kong University of Science and Technology,Thesis CHEM,C.2͑1996͒.6B.Z.Tang,W.H.Poon,S.M.Leung,W.H.Leung,and H.Peng,Mac-romolecules 38,1573͑1996͒.7J.Shah,T.C.Damen,B.Deveud,and Dominigue Block,Appl.Phys.Lett.50,1307͑1987͒.8R.Sun,T.Masuda,and T.Kobayashi,Jpn.J.Appl.Phys.,Part 235,L1673͑1996͒.9H.S.Woo,S.C.Graham,D.A.Halliday,D.D.C.Bradley,R.H.Friend,P.L.Burn,and A.B.Holmes,Phys.Rev.B 46,7379͑1992͒.10R.H.Friend,D.D.C.Bradley,and P.D.Townsend,J.Phys.D 20,1376͑1987͒.FIG.5.Time resolved photoluminescence decay of ͑a ͒PPA and ͑b ͒PPB,detected at different luminescence energies.。

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文章 编 号 １００４～９２４Ｘ（２０１１）０２—０２２１—０７
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