J . Phys. Chem. 1993, 97. 1474-1476
1474
Subpicosecond Photoinduced Switching of Second-Harmonic Generation from a Ruthenium Complex in Supported Langmuir-Blodgett Films Hiroshi Sakaguchi,**tLuis A. Gomez-Jahn, and Mark Prichard NSF Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627
Thomas L. Penner Corporate Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-21 10
David G. Whitten NSF Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York I4627
Toshihiko Nagamura Crystalline Films Laboratory, Research Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan Received: August 3, 1992; In Final Form: November 17, 1992
We demonstrate that ultrafast modulation of second-harmonic (SH)intensity from a Langmuir-Blodgett (LB) film consisting of a ruthenium complex is possible by laser irradiation of the films. The switching of the second-harmonic intensity is demonstrated to occur in less than 2 ps by a picosecond pumpprobe experiment. Results from time-resolved fluorescence studies of the LB film correlate quantitatively with those of the secondharmonic generation (SHG).The mechanism of the photoinduced SHG switching is ascribed to the change of molecular hyperpolarizability of the ruthenium complex on going from the ground state to the excited state.
Introduction Organic compoundsare expected to be basic materials in future optoelectronic and photonic devices because of their frequently large optical n~nlinearity.I-~Many studies of the second-order and third-order optical nonlinearity of organic compounds such as harmonic generation, electrcoptic effects, and photoinduced refractiveindex changehave been carried 0ut.5-~The development of optical phenomena and materials to control light by light is very important for making optical switches and phototransistors. Photobistability using third-order optical nonlinearity is one of the most promising phenomena, and many investigations into this area have been carried 0 ~ t . The l ~ basic ~ ~ concept of photobistability is to combine the photoinduced third-order refractive index change with light feedback in the cavity. Transmitted light intensity from the cavity is controlled by input light intensity and an ultrafast switching time is expected. We have investigated another optical switching phenomenon based on a different principle that the nonlinear susceptibility of the materials can be changed directly by the controlling light. We demonstrated that optical second-harmonic generation induced by nanosecond IR laser pulses (1064 nm) in LB films of a ruthenium bipyridyl complex where one of the ligands was substituted by amide groups was controlled by UV pulsed laser (355 nm) irradiation.I4-l7 These experiments suggested that the mechanism of this SHG switching phenomenon involved a change of molecular hyperpolarizability of the ruthenium complex on going from the ground state to the excited state by UV laser excitation. In the present letter, the response time and dynamics of the SHG modulation were investigated by a picosecond p u m p probe experiment. Other recent studies have shown fast time resolved changes in SHG to be a useful probe for investigating dynamics at the air-water interface and on solid surface~.'~-2l Experimental Section The sample is an assembly consisting of alternate LB film layers, which has the noncentrosymmetry necessary for SHG
' On leave from Research Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan. 0022-3654/93/2097-1474$04.00/0
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De x Figure 1. Structural formulas and symbolic abbreviations of compounds used, with a schematic diagram of LB film configuration.
andcontainsasdyelayersa 1:4mixtureof RuC18Band2C18NB and alternating (spacer) layers of 2C18NB as shown in Figure 1. The LB film is deposited at 20 mN m-I as a polyion complex with dextran (Dex, see structure in Figure l), on a glass slide with 30 bilayers on each side of the support. Under these conditions the surface concentration of RuC18B in each bilayer can be calculated from the pressure-area isotherms previously published to be ca. 4 X lOI3 molecules/cm2. LB films of this material prepared under the same conditions were estimated to have a molecular hyperpolarizability of 70 X esu.I4 Subpicosecondseedingdye laser pulsts of temporal width from 0.4 to 1 ps are generated by synchronous pumping of a dye laser cavity with the second-harmonic (527 nm) of a mode-locked Nd: YLF laser (1053 nm). The dye laser cavity consists of a rhodaminedG (Rh6G) dye jet, a 3,3'-diethyloxacarbocyanine iodide (DODCI) saturable absorber jet and a birefringence filter. The wavelength of the dye laser can be tuned by changing the angle of the birefringence filter in the range 580-630 nm. One portion of the Nd:YLF laser pulses at 1053 nm is injected into a Nd:YLF regenerative amplifier. Then the amplified IR pulses are passed through a @-bariumborate (BBO) crystal, and 0.5-mJ amplified pulses at 527 nm are obtained. In the present case, 590 nm is used to induce the second-harmonic generation from the LB film. The energy of the seeding dye laser pulses is 1 nJ, too 0 1993 American Chemical Society
Letters
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weak to be used for the present experiment, so the seeding pulses are amplified to 10 pJ by passing through two dye cells pumped by 0.5-mJ pulses of the second-harmonic from the regenerative amplifier. Due to saturation in the amplification stages, the amplified pulse at 590 nm has a width of 2 ps. The optical experimental setup is shown in Figure 2. UV laser pulses at 378 nm, 1 pJ, are used for the pump pulses that are generated by sum-frequency mixing of 590 nm and 1053 nm laser pulses passing through the rhodium dihydrogen phosphate (RDP) crystal. 590-nm dye laser pulses are used for SHG probe pulses. Pump pulses are introduced into the film to excite the ruthenium complex, and then probe pulses delayed for various times irradiate the film for SHG determination. Both pump and probe pulses are focused to a 50-pm spot in the LB films. The SH intensity from the LB film is detected while changing the delay time between the pump and probe pulses. The delay time is changed by a computer-controlled stepping motor. The SH signal is detected by a photomultiplier tube and 1000 pulses are averaged by boxcar integration followed by collection with a computer. Results and Discussion
The dynamics of the SH intensity after UV laser excitation of the LB film are shown in Figure 3. The SH intensity decreased to 70% of its initial value upon excitation and returned to almost the initial value within several hundred picoseconds. The fluorescence decay dynamics of RuC18B in LB films was investigated by single photon-counting, as shown in Figure 4. The fluorescence decay was best fitted with three lifetime componentsof 178 ps, 3.9 ns, and 63 ns in this time window. This result suggested that at least three kinds of molecular aggregates for RuC18B exist in the LB assemblies due to the high concentrationof RuC18B. The dashed linein Figure 3 shows the predicted time dependence of SHG resulting from the excitedstate decay described by the followingequations. The derivation of these equations is based on a kinetic analysis similar to that
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Figure 5. Temporal profile of SH intensity upon UV laser excitation of alternate LB film with 2-ps time resolution.
in ref 19 and is included as supplementary material: z~H(~x)/z~H= W
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defining C by
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and F(r) by F(t) = ~f(O/Zf(O)
(3) IsH(ex) and I s ~ ( 0 )indicate the SH intensity with and without UV pump pulses. r = (Bex)/(fig) is the ratio of orientationally averaged molecular hyperpolarizabilitia of RuCl8B in the excited state and ground state. u is the absorption cross section, and DIJV is the total photon dose from the UV laser pulse delivered within the probe pulse volume. F(t) is the temporal profile of theexcitedstate concentration defined by eq 3, where Zdt) is the fluorescence intensity at time t after the UV pump pulse. These equations are applicable in the case where there are no decay processes faster than the laser pulse width. The curve calculated from eq 1 using the experimentally measured F ( t ) from Figure 4 agrees well with the experimental time dependence of the SHG change. The best fit is obtained for the constant value of C = -0.16. This result suggests that the SHG change with UV laser excitation is caused by the change of molecular hyperpolarizability on going from the groundstate to theexcitedstate. Thenegativevalueof Cindicates that l(bex)lis less than [(&)I or they are of opposite sign. If there are some photophysical processes faster than the time of the laser pulse width, the constant value C i s inaccurate. The excitation laser intensity for SHG measurement is so much higher than that of the fluorescence measurement, that the possibility exists for ultrafast processes such as excited state-excited state annihilation to occur within the time of the laser pulse width. Additional investigations of the ultrafast dynamics of high-density excited state of RuC18B are necessary to determine the precise value of C and the molecular hyperpolarizability of the excited states.
1476 The Journal of Physical Chemistry. Vol. 97, No. 8. 1993 The temporal profile of photoinduced SHG switching with 2 - p time resolution is shown on an expanded scale in Figure 5 . The switching occurred in less than 2 ps. This demonstrates ultrafast switching of photoinduced SHG for the first time. From our results the most likely cause of this SHG change is due to the formation of the excited state, which is expected to occur on the femtosecond time scale.
Letters (2) Lipscomb, G. F.; Garito, A. F.; Narang, R. S. J. Chem. Phys. 1981, 75, 1509.
(3) Oudar, J. L.; Chemla, D. S. J . Chcm. f h y s . 1977, 66, 2664. (4) Lalama, S. J.; Garito, A. F. Phys. Rcu. A 1979, 20, 1179. (5) Carter, G.;Zyss, J. J. Opt. Soc. A m . B 1987, 4, 945. (6) Williams, D. J. Angew. Chem., fnt. Ed. Engl. 1984, 23, 690. (7) Williams, D. J., Ed. Nonlinear Optical Properties of Organic and Polymeric Materials; American Chemical Society: Washington, DC, 1983. (8) Chemla, D. S., ZYSS,J., E&. Nonlinear Optical fropertiesof Organic Conclusions Molecules und Crysrals; Academic: New York, 1987; Vols. 1, 2. In mclusion, the temporal profile of the photoinduced SHG (9) Zyss, J. Mol. Electron. 1985, I, 25. switching phenomenon in supported LB films was investigated (10) Auston, D. H.; Ballman, A. A.; Bhattacharya, P.; Bjorkland, G.J.; Bowden, C.; Boyd, R.W.; Brody, P. S.; Burnham, R.;Byer, R. L.; Carter, by a subpicusecond pumpprobe experiment. The SHG switching G.;Chemla, D.; Dagenais, M.; Dohler, G.; Efron, U.;Eimerl. D.; Feigelson, effect is ascribed to a change of molecular hyperpolarizability on R.S.; Feinberg, J.; Feldman, B. J.; Garito, A. F.; Garmire, E. M.; Gibba, H. going from ground state to excited state. This was also supported M.;Glass,A. M.;Goldberg,L.S.;Gunshor,R.L.;Gustafson,T. K.; Hellworth, R.W.; Kaplan, A. E.; Kelley, P. L.; Leonbcrger, F. J.; Lytel, R.S.; Majerfeld, by a time resolved fluorescence experiment. The switch-on time A.; Menyuk, N.; Meredith, G.R.;Neurgaonkar, R. R.; Peyghambarian, N. is less than 2 p and the switch-off time is controlled by the G.; Prasad, P.; Rakuljic,G.;Shen, Y.-R.; Smith,P. W.;Stamatoff, J.;Stegeman, relaxation timeof the excited stateof RuC18B. This phenomenon G.I.;Stillman,G.;Tang,C.L.;Temkin,H.;Thakur,M.;Valley,G.C.; Wolff, P. A.; Woods, C . Appl. Opt. 1987, 26, 211. is expected to be useful for optical switching based on the concept (11) Degenais, M.; Sharfin, W. F. J. Opt. SOC.Am. B 1985, 2, 1179. of directly changing the nonlinear susceptibility of materials in (12) Gibbs, H. M.; Tarng, S. S.; Jewell, J. L.; Weinberger, D. A.; Tai, K.; a reversible manner. Gossard, A. C.; McCall, S. L.; Passner, A.; Wiegman, W. Appl. Phys. h i t . 1982, 41, 221. Acknowledgment. This work was carried out under the Japan(13) Hanamura, E. Phys. Reo. B 1988, 37, 1273. U S . Cooperative Photoconversion and Photosynthesis Research (14) Sakaguchi, H.; Nakamura, H.; Nagamura, T.; Ogawa, T.; Matsuo, Program and also as a research project by the NSF Center for T.Chem. Lett. 1989, 1715. Photoinduced Charge Transfer (Grant No. CHE912001). The ( I 5) Sakaguchi, H.; Nagamura. T.; Matsuo, T. Jpn. J. Appl. Phys. 1991, authors are grateful to Professor T. Matsuo, Kyushu University 30, L377. for providing the ruthenium complex. (16) Nagamura, T.; Sakaguchi, H.; Matsuo. T. Thin Solid Films 1992, 210, 160. Supplementary Mntertl Available: Derivation of eq 1 cor(17) Experiments on crystalline powders demonstrated that the amidesubstituted ruthenium(I1)tris-bipyridyl complex was SHG-active while the relating the time dependence of SHG with excited-state decay unsubstituted ruthenium complex was not, supporting a mechanismofvectorial (2 pages). Ordering informationis given on any current masthead intramolecular charge transfer as the source of the nonlinear optical activity." page. (18) Sakaguchi, H.; Nagamura, T.; Matsuo, T. Appl. Orgammer. Chem. 1991, 5, 257. References and Notes (19) Sitzmann, E. V.; Eisenthal, K. B. J. f h y s . Chcm. 1988, 92, 4579. (1) Levin,B.F.;Bethea,C.G.;Thurmond,C.D.;Lynch,R.T.;Bernstein,(20) Castro, A.; Sitzmann, E. V.;Zhang, D.; Eisenthal, K. B. f h y s . Chcm. 1991, 95, 6752. J. L. 1. Appl. Phys. 1979, 50, 2523. Levin. B. F. Chem. Phys. Lett. 1976, 37, 516. Levin, 8. F.; Betha, C. G. J. Chem. Phys. 1975, 63, 2666. (21) Meech, S. R.; Yoshihara, K. J. Phys. Chem. 1990, 94, 4913.
