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Third-order Nonlinear Optical Properties of Conjugated Molecules by Coherent Raman Scattering
ELSEVIER
Third-order
Synth&M&ls84(1997)945-946
Nonlinear
Optical Properties of Conjugated
Molecules by Coherent Raman Scattering
K.J.Atherton, G.P.Keogh,andG.Rumbles. Departmentof Chemistry,ImperialCollegeof Science,TechnologyandMedicine, Exhibition Road, London,SW72AY, UK
Abstract
We have studiedCoherentAnti-Stokes RamanSpectra(CARS) of trans.-B-carotene in chloroformsolutionat variousconcentrations. We have determinedthe nonresonantmolecularthird-ordernonlinearoptical responseof this conjugatedmoleculein chloroformto be ~(-59lnm,65Omn,-721.4nm,65Onm)= (13W)x10”3esu. Keywords:Nonlinearopticalmethods;Conjugatedmolecules;CoherentRamanscattering
1. Introduction
3. Results and Analysis
Conjugatedmolecules and polymers exhibit large thirdorder nonlinearoptical propertiesdue to their delocalizedxelectron backbone. In order to use synthesis to develop molecular third-order materials with sufficiently high responses for deviceapplicationsit is necessaryto have a clear understandingof the microscopicorigin of the nonlinearity and thereforethe structure-propertyrelationship. Coherent Anti-Stokes Raman Spectroscopy of dilute solutions with concentration studies can elucidate the microscopicnonlinearity of moleculesin the absenceof any orderingeffects. This techniqueis sensitiveto resonancewith electronicone and two photon allowed statesin the medium. Therefore by variation of the electronic resonanceconditions whilst probing a single Ramanresonance,it is possibleto quantify the variouscontributionsto the nonlinearityandisolate the intrinsic responseof the materialunder study. This allows different chemical systemsto be meaningfully compared. CoherentRamanmay give complimentaryinformationto other third-ordertechniquesdueto the different resonances probed. We have studiedtrans+carotene, a conjugatedmolecule with elevendoublebonds,which hasbeenstudiedin the past due its relatively high third-order nonlinearity and the interest in using a simple polyene as a molecule for theoretical modelling.A variety of techniqueshavebeenusedpreviouslyto studythis molecule.1-4
CARS spectraof &ins-~-carotenein chloroform at three different concentrationsare shown in figure 1. The pump wavelengthwas650~1 andthe 1524cm-’Ramanbandhasbeen studied. The absorption maximum for trans-p-carotenein chloroformis at 46Onmwith the O-Otransitionat 487nm. The coherent Raman data has been analysedusing the following expressionfor CARS spectrallineshape5,6
2. Experimental
B = -2TrgIxnr + R2 + I2 + 2TrgL,R - 2TrglDt
The coherentRamanspectrometerusedtwo high repetition rate, picosecond dye lasersin a crossbeamgeometry.Variation in the intensities of the pump and Stokes beams were eliminatedusing a two compartmentspinningcell containing the sampleand &dmso as a nonresonantreference.The full detailshave beenpublishedelsewhere.’
The termshere are all macroscopic,as is the nature of thirdorder nonlinear measurements.& is the non-resonant backgroundterm due to the solvent, assumedhere to have no
0379-6779/971$17.008 PII
SO379-6779(96)04223-3
1997ElsevimScienceS.kAUrightsreserved
= x’, + D: + L; +2x,Dt
+ R2 +I2 S2+lY2 rg
+ 2WcnrR+DtR+W) _ 2Irg(IXm+Q-LtR) 82+r2rg g2+r2rg
(1)
which canbe simplifiedto 2&C
2 =A+B+82+r2 Tg
62
+Irg2
(2)
where
KJ. Atherton
946
et al. /Synthetic
one or two-photon resonance at these wavelengths. Dt and LI are the real and imaginary parts of the two-photon term for the sample. The real and imaginary parts of the Raman resonant contribution are R and I with S being the dettming from exact Raman resonance and Irg the damping constant obtained from the HWHM for the Raman transition. This term can be fixed by using the spontaneous Raman linewidth that we have measured previously. In equation (2) the initial term is a background term, which would be expected to be constant. The second term consists of two Lorentzians, a Raman resonant contribution which becomes dominant at high concentration and a cross-term that can add or subtract to the spectrum, depending upon the sign of I and becoming important where one-photon resonance occurs. The third term is a dispersive cross-term, increasing in importance as the concentration decreases as can be seen in figure 1.
26
L’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ‘-I
24 22 20
Metals
84 (I 997) 945-946
literature value of 0.87 x 10‘i4esu for b of chloroforms,the nonresonantsolventterm, the varioussamplecontibutionscan be quantitativelydetermined.The valuesthat we haveobtained for this pump wavelength are shown in table 1. We have assumedthat two-photon resonanceis unimportant at this wavelengthandfixed Lt aszero. Table 1. Nonlinearparameters of trans-p-carotene 1033/esu 13 &4 390Lk40 170k 50 The instrument responsefunction, determined by the bandwidth of the two lasers was reconvoluted with the calculatedCARS spectrum.By fixing this term and also the Ramanlinewidth in our analysiswe are improving the fitting routineby reducingthe numberof variableparameters. The background for our CARS spectra have caused difficulties, insteadof a constantbackground,we have a curved background.This is a well known problemfor CARS as the phasematchingconditionsalter asa CARSspectrumis scanned due to the dispersivenature of the refractive index of the solution.Also the different concentrationsolutionswill have different refractive indices and therefore unique phase matching.For our presentinterpretationof the CARS data we haveapproximatedthe backgroundwith a lineartrend. The errorsare large due to a smallamountof correlation betweenthe parametersandalsothe dueto the phasematching problemmentionedabove. 4. Conclusions
420
1440 1460 1480 IS00 1520 1540 1560 1580 1600
Ramat? %ifi / cm” Fig 1. CARS spectraof tmm$carotene in chloroformat difIerent concentrations (i) lO%I, (ii) 5x10%, (iii) 10% The CARS spectrahave been fitted to this equationto extract A, B and C rather than the actual macroscopicterms directly becauseof the high degreeof correlationbetweenthese terms,By scanninga concentrationseriesfor the sameRaman resonancethe concentrationdependence of A, B and C can be determined.The Lorentz localfield correctionfactor xc3)= Nf‘+) canbe usedto considerA, B andC in termsof the microscopic nonlinerities and with the concentration dependencethe moleculartermscan be elucidated.The refractive index at the pumpwavelengthwastaken asthat of the solvent.7By usinga
Due to the nonresonantterm of the third-order optical nonlinearityeffectingthe coherentRamanlineshapewe canuse this resonanceto study the nonresonantnonlinearity which is experimentally easier. By studying the concentration dependence of the lineshapewe have beenable to separatethe one and two-photonmolecularcontributionsto the third-order nonlinearity.The valuethat we have obtainedfor the molecular nonresonant third-orderopticalnonlinearityfor tram+p-carotene is ~(-59lnm,65Onm,-721.4nrn,65Onm)= (13i4)x1r33esu. This valueis in goodagreement thoseobtainedpreviously.4 References [l.]F.Rohlfmg etal., Synth.Met., 76 (1996)35. [2.] J.B.vanBeek, FKajzar and A.C.Albrecht, J. Chem.Phys., 95 (9) (1991)6400. [3.]Z.G.Soos and D.Mukhopadhyay,J.Chem.Phys., 101 (7) (1994)5515. [4.] J.P.Hermann,DRicard and J.Ducing,Appl. Phys.Lett., 23 (4) (1973) 178. [5.] G.P.Keogh,Ph.D. Thesis,University of London,1996. [6.] W.Wemcke,MPfeiffer andA.Lau, Synth.Met., 51 (1992) 153. [7.] J.Timmermans, Physico-ChemicalConstants of Pure OrganicCompounds, Elsevier,(1950). [S.]R.T.LynchandH.Lotem,J.Chem.Phys.,66 (1977) 1905.
Third-order
Synth&M&ls84(1997)945-946
Nonlinear
Optical Properties of Conjugated
Molecules by Coherent Raman Scattering
K.J.Atherton, G.P.Keogh,andG.Rumbles. Departmentof Chemistry,ImperialCollegeof Science,TechnologyandMedicine, Exhibition Road, London,SW72AY, UK
Abstract
We have studiedCoherentAnti-Stokes RamanSpectra(CARS) of trans.-B-carotene in chloroformsolutionat variousconcentrations. We have determinedthe nonresonantmolecularthird-ordernonlinearoptical responseof this conjugatedmoleculein chloroformto be ~(-59lnm,65Omn,-721.4nm,65Onm)= (13W)x10”3esu. Keywords:Nonlinearopticalmethods;Conjugatedmolecules;CoherentRamanscattering
1. Introduction
3. Results and Analysis
Conjugatedmolecules and polymers exhibit large thirdorder nonlinearoptical propertiesdue to their delocalizedxelectron backbone. In order to use synthesis to develop molecular third-order materials with sufficiently high responses for deviceapplicationsit is necessaryto have a clear understandingof the microscopicorigin of the nonlinearity and thereforethe structure-propertyrelationship. Coherent Anti-Stokes Raman Spectroscopy of dilute solutions with concentration studies can elucidate the microscopicnonlinearity of moleculesin the absenceof any orderingeffects. This techniqueis sensitiveto resonancewith electronicone and two photon allowed statesin the medium. Therefore by variation of the electronic resonanceconditions whilst probing a single Ramanresonance,it is possibleto quantify the variouscontributionsto the nonlinearityandisolate the intrinsic responseof the materialunder study. This allows different chemical systemsto be meaningfully compared. CoherentRamanmay give complimentaryinformationto other third-ordertechniquesdueto the different resonances probed. We have studiedtrans+carotene, a conjugatedmolecule with elevendoublebonds,which hasbeenstudiedin the past due its relatively high third-order nonlinearity and the interest in using a simple polyene as a molecule for theoretical modelling.A variety of techniqueshavebeenusedpreviouslyto studythis molecule.1-4
CARS spectraof &ins-~-carotenein chloroform at three different concentrationsare shown in figure 1. The pump wavelengthwas650~1 andthe 1524cm-’Ramanbandhasbeen studied. The absorption maximum for trans-p-carotenein chloroformis at 46Onmwith the O-Otransitionat 487nm. The coherent Raman data has been analysedusing the following expressionfor CARS spectrallineshape5,6
2. Experimental
B = -2TrgIxnr + R2 + I2 + 2TrgL,R - 2TrglDt
The coherentRamanspectrometerusedtwo high repetition rate, picosecond dye lasersin a crossbeamgeometry.Variation in the intensities of the pump and Stokes beams were eliminatedusing a two compartmentspinningcell containing the sampleand &dmso as a nonresonantreference.The full detailshave beenpublishedelsewhere.’
The termshere are all macroscopic,as is the nature of thirdorder nonlinear measurements.& is the non-resonant backgroundterm due to the solvent, assumedhere to have no
0379-6779/971$17.008 PII
SO379-6779(96)04223-3
1997ElsevimScienceS.kAUrightsreserved
= x’, + D: + L; +2x,Dt
+ R2 +I2 S2+lY2 rg
+ 2WcnrR+DtR+W) _ 2Irg(IXm+Q-LtR) 82+r2rg g2+r2rg
(1)
which canbe simplifiedto 2&C
2 =A+B+82+r2 Tg
62
+Irg2
(2)
where
KJ. Atherton
946
et al. /Synthetic
one or two-photon resonance at these wavelengths. Dt and LI are the real and imaginary parts of the two-photon term for the sample. The real and imaginary parts of the Raman resonant contribution are R and I with S being the dettming from exact Raman resonance and Irg the damping constant obtained from the HWHM for the Raman transition. This term can be fixed by using the spontaneous Raman linewidth that we have measured previously. In equation (2) the initial term is a background term, which would be expected to be constant. The second term consists of two Lorentzians, a Raman resonant contribution which becomes dominant at high concentration and a cross-term that can add or subtract to the spectrum, depending upon the sign of I and becoming important where one-photon resonance occurs. The third term is a dispersive cross-term, increasing in importance as the concentration decreases as can be seen in figure 1.
26
L’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ‘-I
24 22 20
Metals
84 (I 997) 945-946
literature value of 0.87 x 10‘i4esu for b of chloroforms,the nonresonantsolventterm, the varioussamplecontibutionscan be quantitativelydetermined.The valuesthat we haveobtained for this pump wavelength are shown in table 1. We have assumedthat two-photon resonanceis unimportant at this wavelengthandfixed Lt aszero. Table 1. Nonlinearparameters of trans-p-carotene 1033/esu 13 &4 390Lk40 170k 50 The instrument responsefunction, determined by the bandwidth of the two lasers was reconvoluted with the calculatedCARS spectrum.By fixing this term and also the Ramanlinewidth in our analysiswe are improving the fitting routineby reducingthe numberof variableparameters. The background for our CARS spectra have caused difficulties, insteadof a constantbackground,we have a curved background.This is a well known problemfor CARS as the phasematchingconditionsalter asa CARSspectrumis scanned due to the dispersivenature of the refractive index of the solution.Also the different concentrationsolutionswill have different refractive indices and therefore unique phase matching.For our presentinterpretationof the CARS data we haveapproximatedthe backgroundwith a lineartrend. The errorsare large due to a smallamountof correlation betweenthe parametersandalsothe dueto the phasematching problemmentionedabove. 4. Conclusions
420
1440 1460 1480 IS00 1520 1540 1560 1580 1600
Ramat? %ifi / cm” Fig 1. CARS spectraof tmm$carotene in chloroformat difIerent concentrations (i) lO%I, (ii) 5x10%, (iii) 10% The CARS spectrahave been fitted to this equationto extract A, B and C rather than the actual macroscopicterms directly becauseof the high degreeof correlationbetweenthese terms,By scanninga concentrationseriesfor the sameRaman resonancethe concentrationdependence of A, B and C can be determined.The Lorentz localfield correctionfactor xc3)= Nf‘+) canbe usedto considerA, B andC in termsof the microscopic nonlinerities and with the concentration dependencethe moleculartermscan be elucidated.The refractive index at the pumpwavelengthwastaken asthat of the solvent.7By usinga
Due to the nonresonantterm of the third-order optical nonlinearityeffectingthe coherentRamanlineshapewe canuse this resonanceto study the nonresonantnonlinearity which is experimentally easier. By studying the concentration dependence of the lineshapewe have beenable to separatethe one and two-photonmolecularcontributionsto the third-order nonlinearity.The valuethat we have obtainedfor the molecular nonresonant third-orderopticalnonlinearityfor tram+p-carotene is ~(-59lnm,65Onm,-721.4nrn,65Onm)= (13i4)x1r33esu. This valueis in goodagreement thoseobtainedpreviously.4 References [l.]F.Rohlfmg etal., Synth.Met., 76 (1996)35. [2.] J.B.vanBeek, FKajzar and A.C.Albrecht, J. Chem.Phys., 95 (9) (1991)6400. [3.]Z.G.Soos and D.Mukhopadhyay,J.Chem.Phys., 101 (7) (1994)5515. [4.] J.P.Hermann,DRicard and J.Ducing,Appl. Phys.Lett., 23 (4) (1973) 178. [5.] G.P.Keogh,Ph.D. Thesis,University of London,1996. [6.] W.Wemcke,MPfeiffer andA.Lau, Synth.Met., 51 (1992) 153. [7.] J.Timmermans, Physico-ChemicalConstants of Pure OrganicCompounds, Elsevier,(1950). [S.]R.T.LynchandH.Lotem,J.Chem.Phys.,66 (1977) 1905.