M. Sinclair

Nonlinear excitations and nonlinear phenomena in conductive polymers

Abstract 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.

Semiconducting polymers: Fast response non-linear optical materials

Abstract Semiconductor polymers such as polyacetylene and polythiophene have experimentally demonstrated non-linear optical processes (photoinduced absorption, photoinduced bleaching and photoluminescence), with characteristic time scales in the picosecond range or faster. These phenomena are intrinsic and originate from the instability of these conjugated polymers toward structural distortion. The major shifts in oscillator strength due to photoexcitation of solitons, polarons and bipolarons lead to relatively large third-order non-linear optical processes (χ (3) ) on time scales of order 10 −13 s. These novel photoexcitations, largely overlooked in earlier analyses, are the key to understanding the non-linear optical properties of this growing class of semiconducting (conjugated) polymers.

Nonlinear optics of conjugated polymers

Abstract 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 −10 esu with h ω = 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 (n 2 ) of polydiacetylene-4BCMU. We find that n 2 is negative, with magnitude of approximately 10 −7 (MW/cm 2 ) −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.

Time‐resolved waveguide modulation of a conjugated polymer

We present a new pump and probe technique for measuring intensity‐dependent refractive indices (n2) of waveguide quality thin films, and we apply it to films of polydiacetylene‐(CH2)4OCONHOCOC4H9 (PDA‐4BCMU). Just below the exciton absorption band, the real part of n2 is negative with magnitude ≊10−7(MW/cm2)−1. The initial fast response of n2 is followed by a slower (∼2.5 ps) decay, in close agreement with the decay of the bleaching of the exciton absorption following resonant excitation. These results are consistent with phase space filling by excitons as the mechanism for the nonlinear index.

Measurement of the third order susceptibility of trans-polyacetylene by third harmonic generation

Abstract 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.

Nonlinear ground state fluctuations (zero point motion) as the source of the nonlinear optical properties of polyacetylene

Abstract 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 h ω=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).

Instantons as the origin of the nonlinear optical properties of polyacetylene

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 ω = 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).

Transient photoconductivity in polyacetylene and polydiacetylene

Abstract 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.

Picosecond waveguide modulation in conjugated polymers

Abstract The technique of time-resolved waveguide modulation is applied to optical waveguides of the conjugated polymers, polydiacetylene-(CH 2 ) 4 OCONHOCOC 4 H 9 (poly-4BCMU) and poly(3-hexylthiophene (P3HT). This method yields values of the sign and magnitude of the complex intensity-dependent refractive index, n 2 , for guided be agt 630 nm ( poly -4 BCMU ) and at 1.06 μm (P3HT), upon pumping near the principal absorption band. In poly-4BCMU, n 2 is real and negative, with a magnitude in the order of −10 −7 (MW/cm 2 ) −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, fn 2 has a negative real part and a positive imaginary part, with |n 2 |≌1×10 −4 ( MW/cm 2 ) −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.

Picosecond photoconductivity in trans-polyacetylene

Abstract Fast transient photoconductivity measurements of trans -polyacetylene as a function of temperature and photon energy indicate a relatively high quantum efficiency for the photoproduction of mobile, charged, nonlinear excitations. Excitation by a 20ps pulse at 590 nm with 10 15 photons/cm 2 results in a transient photoconductivity of ∼0.3 S /cm (at 50 ps) with time scale for decay similar to that measured in picosecond photoinduced absorption experiments. The temperature independence of the fast photoconductivity is interpreted in terms of the photoproduction of “hot” soliton excitations.