Subpicosecond transient absorption (TA) spectroscopy was used to identify excited states, and to measure their relaxation kinetics as a function of excitation wavelength in the halogen-bridged transition-metal (MX) compound PtCl. For pump photon energies E-pump between 1.3 and 1.8 eV, below the threshold for exciton formation, intrinsic polarons are photoexcited and decay within 1-3 ps. Excitation into the exciton absorption tail (2.0-2.3 eV) yields singlet excitons that rapidly decay into an even-parity state, which has a lifetime (similar to 100 ps) varying with the intial photon energy. Excitation (3.1 eV) well above the exciton threshold appears to generate more slowly decaying excitons, as well as polarons that are formed upon exciton dissociation. The TA spectra also indicate that relaxation of excitons for E-pump>2 eV is mediated by large-amplitude vibrations perpendicular to the chains. Using third-harmonic generation spectroscopy in addition to TA, we develop an energy-level diagram for the excited states, including polaron levels, and the lowest odd-and even-parity excitons.

We report the results of femtosecond (fs) and picosecond (ps) photoinduced absorption (PA) and ps resonant Raman scattering (RRS) measurements for the MX chain solid PtCl. Although a fast relaxation of the excited state population is observed in the PA, features appearing in the ps photoinduced RRS are primarily the products of long-lived processes.

We present a short survey of our results for a few interesting polymer systems. We begin with trans-(CH)x grown by the Shirakawa method, and we compare the transient photoconductivity (TP) results to those obtained from PDA-TS which currently is the only conjugated polymer in single crystal form. From this comparison, we are able to gauge the improvements in the material quality as was demonstrated by recent TP in trans-(CH)x grown by the Naarmann/Theophilou method. The direct measurement of the mobility in PDA-TS using the sweep-out method is described as well.

Along with the third-order nonlinear susceptibility, χ(3), the magnitude of the optical absorption in the transparent window below the principal absorption edge is an important parameter for conjugated polymers used in active integrated optical devices. Photothermal deflection spectroscopy (PDS) is an ideal technique for determining the absorption coefficients of thin films of ‘transparent’ materials. We have used PDS to measure the optical absorption spectra of the conjugated polymers, poly(1,4-phenylene-vinylene) (and derivatives) and polydiacetylene-4BCMU, in the spectral region from 0.55 to 3 eV. We find that the shape of the absorption edge varies considerably from polymer to polymer, with polydiacetylene-4BCMU having the steepest absorption edge. The minimum absorption coefficients measured varied somewhat with sample age and quality, but were typically in the range 1–10 cm−1. In the region below 1 eV, overtones of C-H stretching modes dominate the absorption behavior. We also observe that irradiation of all of these polymers with light above ∼ 2.5 eV produces enhanced absorption below the fundamental edge. In the absence of light, these excitations decay with characteristic times of 10–1000 s and in some cases they may determine the effective IR transparency in the energy range 1.0–1.8 eV.

The technique of time-resolved waveguide modulation is applied to optical waveguides of the conjugated polymers, polydiacetylene-(CH2)4OCONHOCOC4H9 (poly-4BCMU) and poly(3-hexylthiophene (P3HT). This method yields values of the sign and magnitude of the complex intensity-dependent refractive index, n2, for guided be agt 630 nm (poly-4BCMU) and at 1.06 μm (P3HT), upon pumping near the principal absorption band. In poly-4BCMU, n2 is real and negative, with a magnitude in the order of −10−7 (MW/cm2)−1. The decay of 2.5 ps closely matches the decay of the bleaching of the exiton absorption following resonant excitation. These results are consistent with phase-space filling by excitons as the principal nonlinear mechanism. In P3HT, fn2 has a negative real part and a positive imaginary part, with |n2|≌1×10−4 (MW/cm2)−1, for a pump photon energy of 2.06 eV. The positive imaginary part correllates with a photoinduced absorption at 1.06 μm (1.17 eV), indicating a shift of oscillator strength from the interband transition to localized absorptions in the infrared. The negative real part implies that the observed photoinduced absorption peaks at an energy below 1.17 eV.

Although the fast dynamics of photoexcitations have been thoroughly explored in the conjugated polymer trans-polyacetylene, the experimental picture is much less complete for polymers in which the ground state is nondegerate. We report time-resolved photoinduced absorption data from films of one family of such polymers: the soluble polythiophene derivatives, poly(3-hexylthiophene) and poly(3-octylthiophene). Fast (<10−12s bleaching of the interband transition is observed at 2.06 eV, accompanied by subgap photoinduced aborption at 1.17 eV. The results indicate a shift of a fraction of the oscillator strength, consistent with rapid structural relaxation to form polarons with associated gap states. By varying the polarization of the optical probe relative to that of the pump, photoinduced dichroism is observed. The magnitude of the initial dichroism implies that in spin-cast films the polymers are (locally) chain extended and chain aligned. The time decay of the polarization memory is consistent with rapid trapping of the initially mobile charged excitations.

Third harmonic generation (THG) is used to probe the nonlinear susceptibility (χ(3)) of polyacetylene. The magnitude of χ||(3) (3ω; ω, ω, ω) is (4 ± 2) × 10-10 esu with ℏω = 1.17 eV; the only important component is that associated with π-electron motion along the backbone. Comparison of THG in cis- and trans-(CH)x shows that χ||(3)|trans is 15–20 times larger, implying a mechanism sensitive to the existence of a degenerate ground state. The results are consistent with calculations of χ||(3) based on virtual generation of solitons enabled by nonlinear zero point fluctuations (instantons).

Third harmonic generation (THG) is used to probe the nonlinear susceptibility (χ(3)) of polyacetylene. The magnitude of χ∥(3)(3ω;ω,ω,ω) is (4±2)×10−10esu withh̷ω=1.17eV; the only important component is that associated with π-electron motion along the backbone. Comparison of THG in cis- and trans-(CH)x shows that χ∥(3)|trans is 15–20 times larger, implying a mechanism sensitive to the existence of a degenerate ground state. The results are consistent with calculations of χ∥(3) based on virtual generation of solitons enabled by nonlinear zero point fluctuations (instantons).

We present the results of two types of measurements of the nonlinear optical properties of conjugated polymers. First, we have used third harmonic generation (THG) to probe the nonlinear susceptibility of polyacetylene. The magnitude of δ(3)(3ω;ω,ω,ω)is (4±2)× 10−10esu withh̵ω = 1.17 eV the only important component of χ(3) is that associated with π-electron motion along the polymer backbone. Comparison of cis- and trans-(CH)x shows that χ(3)|trans is 15–20 time larger than χ(3)|cis. Second, we have employed time-resolved waveguide modulation to measure the magnitude and sign of the nonlinear refractive index (n2) of polydiacetylene-4BCMU. We find that n2 is negative, with magnitude of approximately 10−7(MW/cm2)−1. The response time of this nonlinearity shows a fast (resolution-limited) decay, followed by a slower (∼2.5 ps) decay time. These results are consistent with bleaching of the excitonic absorption as the mechanism of the nonlinearity.

We present data for the index of refraction, absorption spectra, and the results of third harmonic generation (THG) experiments for the materials poly(p-phenylene vinylene) (PPV), poly(3,6 dimethoxy-p-phenylene vinylene) (PDMPV), poly(3,6 dihexyloxy-p-phenylene vinylene) (PDHPV), and polydiacetylene-4BCMU. The index of refraction was measured at 633 nm by Abeles' method. We used THG to determine the magnitude of χ(3)(3ω;ω,ω,ω), where h̵;ω=1.17 eV. We find the same value for the magnitudes of χ(3) of PPV and PDA-4BCMU: ∼ (2±1.5)×10−11 esu. We confirm that χ(3) is sensitive to the degree of π-electron delocalization, and that the dominant component of the χ(3) tensor is that with all indices parallel to the polymer chains. The PPV's are promising materials for optical applications, as they offer the advantages of durability, relatively high nonlinearity, high index, and ease of synthesis and processing.

The promise of conducting polymers as fast response nonlinear optical materials has been recently emphasized [1,3]. Polymers such as polyacetylene, polythiophene and the soluble (and processible) poly(3-alkylthienylenes) contain a high density of x-electrons, and they are known to exhibit photoinduced absorption and photoinduced bleaching, indicating major shifts of oscillator strength upon photoexcitation [2,4]. For polyacetylene, these nonlinear effects have been studied in detail in the picosecond [5a,b] and sub-picosecond [5c] time regime and correlated with the photoproduction of charge carriers through fast photoconductivity measurements [6]. The data have demonstrated ultra-fast response with nonlinear shifts in oscillator strength occurring at times of the order of 10-13 seconds. These resonant nonlinear optical properties are intrinsic; they originate from the nonlinearity of the self-localized photoexcitations [7] which characterize this class of polymers: solitons, polarons and bipolarons [4].

Most applications of conjugated polymers in nonlinear optical devices will require knowledge of the intensity-dependent refractive index, defined by where n0 is the linear index, and I is the light intensity. We have developed a technique1 which allows for time-resolved measurements (to distinguish between thermal and electronic nonlinearities) of n2.

For a deeper understanding of the nonlinear optical response of conjugated polymers, the connection between the nonresonant response to pumping well below the absorption edge and the resonant response to pumping into the absorption band must be developed. For trans -(CH)X, resonant pumping causes photogeneration of charged solitons; the resulting shifts in oscillator strength lead to relatively large changes in the optical constants. We generalize to the nonresonant regime where the nonlinear response arises from virtual soliton-antisoliton (SS) pairs and is enabled by nonlinear ground state fluctuations, or "instantons".

Transient photoconductivity experiments have been carried out on single crystals of polydiacetylene-(bis p-toluene sulfonate), PDA-TS. The low--electric-field photocurrent decay consists of a temperature-independent fast (picosecond) initial component and a longer-time (nanosecond) component with magnitude that is strongly temperature dependent. Using very small spacings between electrodes on the sample, we have succeeded in achieving sweepout for the longer-lived carriers; the data yield a mobility of \ensuremath{\approxeq}5 ${\mathrm{cm}}^{2}$/V\ensuremath{\cdot}s at room temperature in the nanosecond regime. These results demonstrate that the Onsager geminate recombination model, previously used extensively for the polydiacetylenes, is not applicable to PDA-TS.

Semiconductor polymers such as polyacetylene and polythiophene have experimentally demonstrated nonlinear optical processes with characteristic time scales in the sub-picosecond range. Fast transient photoconductivity measurements on trans-(CH)x as a function of temperature and photon energy indicate a relatively high quantum efficiency for the photoproduction of mobile, charged, nonlinear excitations, consistent with the Su-Schrieffer mechanism for the photogeneration of charged solitons. The major shifts in oscillator strength due to these nonlinear photoexcitations lead to relatively large resonant third-order nonlinear optical processes (χ(3)) on time scales of order 10−13 s. A direct measurement of χ(3) in polyacetylene has been carried out by third harmonic generation. The measured nonresonant value of χ(3)(3ω = ω+ω+ω) = 4 × 10−10 esu. The implied value for χ‖(3) is an order of magnitude greater than the corresponding value for polydiacetylene.

We present the results of a series of measurements of the polarization dependence of the transient photoconductive response in both oriented and nonoriented trans-polyacetylene. Our results indicate that in nonoriented samples, the short-time photoconductivity is dominated by intrachain absorption and intrachain transport, while in oriented samples both interchain and intrachain photogeneration (with different absorption depths) of charge carriers are important. In oriented samples the photoconduction due to interchain excitation is slightly larger than that due to intrachain excitation.

We report a measurement of the third order nonlinear optical susceptibility of trans-polyacetylene by third harmonic generation in thin films. The measured susceptibility is X(3)(3ω = ω + ω + ω) = 5 x10-10 esu, which is comparable to the magnitude of the large nonlinear susceptibilities measured in the polydiacetylenes.

Transient photoconductivity experiments have been carried out on single crystals of polydiacetylene-(bis p-toluene sulfonate), PDA-TS. The low electric field photocurrent decay consists of a temperature-independent fast (picosecond) initial component and a longer time (nanosecond) component with magnitude that is strongly temperature dependent. Using very small spacings between electrodes on the sample, we have succeeded in achieving sweep-out for the longer-lived carriers; the data yield a mobility of H 5 cm2/Vs at room temperature in the ns regime. These results demonstrate that the Onsager geminate recombination model, previously used extensively for the polydiacetylenes, is not applicable to PDA-TS.