February 1, 1995 / Vol. 20, No. 3 / OPTICS LETTERS 255

Excited-state enhancement of the third-order nonlinearoptical susceptibility of nonether polyphenylquinoxaline

Jun Yan,* Jianyao Wu, Heyuan Zhu, Xiaotian Zhang, Diechi Sun, Yimei Hu, and Fuming Li

State Key Joint Laboratory for Materials Modification by Laser, Ion and Electron Beams, Fudan University Branch,Department of Physics, Fudan University, Shanghai 200433, China

Meng Sun

Department of Macromolecular Science, Fudan University, Shanghai 200433, China

Received July 25, 1994

The microscopic third-order nonlinear-optical susceptibility gs2v; v, v, 2vd of a conjugated polymer, nonetherpolyphenylquinoxaline, was observed to be enhanced by two times from 355-nm optically pumped excited-statepopulations under the condition of unsaturated pumping. We studied the time evolution of the enhancement andverified by a fluorescence measurement that the excited-state enhancement of gs2v; v, v, 2vd is proportionalto the population of the excited state.

Conjugated polymers play an important role asnonlinear-optical (NLO) materials in communica-tion, optical computing, and other devices.1 Todate all kinds of polymers studied, such as poly-acetylenes,2 polydiacetylenes,3 polythiophenes,4 andpolyparaphenylenevinylenes,5 exhibited third-orderNLO susceptibility x s3d of the order 10210 –1028 esu.This value is close to the requirement for serial all-optical processing; unfortunately it is 4 or 5 ordersof magnitude smaller than the criterion for parallelall-optical processing.6

Recently Garito and colleagues found a new wayto achieve larger x s3d: theoretical calculation showedthat the microscopic third-order NLO susceptibilitygs2v4; v1, v2, v3d of the excited state could be en-hanced by orders of magnitude, or even change sign,compared with that of the ground state7; the en-hanced NLO process from optically pumped electronicexcited states was observed in the two-dimensionalconjugated macromolecule silicon naphthalocyanine(SINC)8 and conjugated linear chain diphenylhexa-triene (DPH).9 This enhancement should prove use-ful generally in the NLO spectroscopy of excitedstates and their dynamics.10

In this Letter we present an experimental studyof the excited-state enhancement of gs2v; v, v, 2vdof a heterocyclic ladder polymer, nonether poly-phenylquinoxaline (PPQ). The molecular structureof nonether PPQ is shown in Fig. 1. The quinox-aline rings, each linked with a benzene ring, areconnected to each other by either a single bond or abenzene ring, forming a delocalized p-electron sys-tem. The optical absorption spectrum of a nonetherPPQychloroform solution was studied: the absorp-tion peak is at 395 nm, the low-energy edge of theabsorption peak is at 420 nm, and the near-resonancewavelength is 532 nm. We used retroreflection de-generate four-wave mixing (DFWM) to measure thenonether PPQ’s ground-state x s3ds2v, v, v, 2vd.The x s3ds2v, v, v, 2vd of PPQychloroform solutions

0146-9592/95/030255-03$6.00/0

of different concentrations were measured, and thex s3ds2v, v, v, 2vd of neat nonether PPQ was calcu-lated to be 3.6 3 10210 esu (the corresponding g is8.2 3 10230 esu). The near-resonance enhancementfactor can be written11 as jvn 2 vng 1 iGngyvr 2

vng 1 iGngj, where vn and vr are the frequenciesof nonresonance and the resonant wavelength, re-spectively, vng is the frequency of the absorptionpeak, and Gng is the corresponding bandwidth. Letln ­ 700 nm, lr ­ 532 nm, and lng ­ 395 nm; theenhancement factor is 1.7 at near-resonance wave-length, so the x s3d of nonether PPQ will still be ofthe order of 10210 esu in the nonresonance band.The response times of the x s3ds2v, v, v, 2vd ofPPQychloroform solutions were found to be muchfaster than 35 ps (limited by the pulse width of thelaser used). These parameters and the material’sease of processing, low absorption, and high thermalstability suggest prospects of useful applications ofnonether PPQ. Details of the ground-state third-order NLO properties of nonether PPQ are describedin another publication.12 Because the ground-stategS0 of the previously studied materials (SINC andDPH) is quite small s,5 3 10235 esud, we believe thatit is more meaningful to study the excited-state NLOprocess of polymers whose enhancement is based onlarger gS0 .

The experiment is similar to that described inRef. 9. A DFWM probe beam is placed at eitherside of a normal-incidence pump beam; the probebeams form an angle of 2.3±. The 532-nm probebeams and the 355-nm pump beam are produced bya mode-locked Nd:YAG laser (the single-pulse energyof 1.06 mm is ,2 mJ ) at a repetition rate of 10 Hzthrough two KD*P crystals and divided by a prism.Their pulse width is ,35 ps. The maximum inten-sity of the incident 355-nm beam upon the samplecell is ,20 mJ . The intensity of every 532-nm beamis ,100 mJ . The diameter of the 355-nm beam is,1 mm, whereas those of the 532-nm beams are

1995 Optical Society of America

256 OPTICS LETTERS / Vol. 20, No. 3 / February 1, 1995

Fig. 1. Molecular structure of nonether PPQ.

,0.3 mm to ensure their overlap. The length of thecoincident path of the three beams is 5 mm. Wemeasured the DFWM signal with a photomultiplierand a boxcar interfaced to a microcomputer. Theconcentration of the nonether PPQychloroform so-lution is ,0.6 mgymL, and the absorptance a at532 nm of the solution is 0.015 cm21. Thus the ab-sorptive grating of the 532-nm beams cannot make ameasurable contribution to the DFWM signal. Thecontribution of the solvent is also negligible becauseboth ground-state x s3d at 532 nm and the absorptionat 355 nm of chloroform are so small.

In the optically pumped third-order NLO processthe material exhibits an effective g, geff , that is theweighted average over all the populated states:

geffijkl ­

PSn

rSnSn gSnijkl , (1)

where rSnSn is the density matrix. In our experimentthe signal intensity is proportional to the square ofgeff .

With the relative delay between the probe beamsset to zero and the pump beam on, geff

xxxx was ob-served to be 2.2 times gS0

xxxx. When the probe po-larizations were orthogonal the enhancement factorof gyxxy was observed to be 2.0, and the value ofgS0

xxxxygS0yxxy was measured to be 2.9 when the pump

beam was off, so geffxxxxygeff

yxxy retains approximately thesame value, ensuring that the observed geff is duemainly to the electronic third-order susceptibility ofthe excited state rather than to a thermal grating(gxxxx must be much greater than gyxxy if the thermalmechanism dominates13). We also observed that theDFWM signals both with and without the pump beamhave a cubic dependence on the probe intensities,which confirms that higher dependence such as two-photon absorption is not detected.

The fluorescence spectrum of a nonether PPQychloroform solution pumped by a 355-nm laser beamis shown in Fig. 2. The main peak of the nonetherPPQ is at 550 nm, and the high-energy edge of thefluorescence peak is at 500 nm, which is not coin-cident with the low-energy edge of the absorptionpeak. Thus a three-level system is assumed: Theoptical pumping initially populates a certain excitedstate sS2d, then a nonradiative relaxation from S2

to a lower excited state sS1d occurs, and finally thepopulation of S1 recovers to the ground state S0

through fluorescence emission.Varying the delay time of the 355-nm pump beam

while the probe beams were set to zero delay, wemeasured the time evolution of the excited DFWMsignals. As Fig. 3 shows, the signal exhibits a risetime that is faster than the pulse widths, and it is

followed by two exponential decays: the fast one(also faster than the pulse widths) corresponds tothe relaxation from S2 to S1; the slow one (nanosec-ond time scale) corresponds to the relaxation from S1

to S0.To confirm our explanation, we measured the in-

tensity of the excited DFWM signal when the pumppulse was set to precede the probes by 230 ps. Thesignal is proportional to the square of

geff ­ hgS1 1 s1 2 hdgS0 ­ sgS1 2 gS0 dh 1 gS0 , (2)

where h is the percentage of the population of S1.We also measured the fluorescence intensity at550 nm; it is proportional to h. Changing the pumpintensity, we observed the square root of the excitedDFWM signal as a function of the excited fluores-cence as shown in Fig. 4(a); it is in good agreementwith a linear correlation. Hence it is verified thatthe excited-state enhanced third-order NLO suscep-tibility of nonether PPQ is proportional to the popu-lation of the excited state. Figure 4(b) shows thecorrelation of the fluorescence to the 355-nm pumpintensity; from it we know that the pump intensitydid not saturate the absorption and that one can

Fig. 2. Fluorescence spectrum of nonether PPQychloroform solution pumped by a 355-nm laser.

Fig. 3. Time evolution of the excited-state enhancementof the third-order NLO process of nonether PPQ.

bruary 1, 1995 / Vol. 20, No. 3 / OPTICS LETTERS 257

Fe

Fig. 4. (a) Excited-state enhanced gs2v; v, v, 2vd ofnonether PPQ as a function of the 355-nm pumpedfluorescence. (b) The 355-nm pumped fluorescence asa function of the pump intensity.

reach greater geff by using a higher pump intensity,and a larger enhancement could also be expected.

In conclusion, we have observed that the micro-scopic third-order NLO susceptibility gs2v; v, v,2vd of a conjugated polymer, nonether PPQ, wasenhanced by two times from the 355-nm opticallypumped excited-state under an unsaturated pump-ing condition. We also measured the fluorescence ofthe polymer pumped by a 355-nm beam and useda three-level model to explain the time evolution ofthe excited-state enhancement. When we changedthe pump intensity we verified that the excited-stateenhancement of nonether PPQ’s gs2v; v, v, 2vd is

proportional to the population of the excited state.To our knowledge this is the first observation ofexcited-state enhanced third-order NLO susceptibil-ity in conjugated polymers.

The authors thank Longgen Luo and Zhifeng Yingfor their assistance. This research was partly sup-ported by the National Nature Science Foundation ofChina.

*Present address, Laboratory for Quantum Optics,Department of Physics, East China Normal Univer-sity, Shanghai 20062, China.

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