D. McBranch

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.

Subpicosecond photoinduced absorption in poly[2,5-thienylene vinylene] and poly[3-methoxy-6-(2-ethyl-hexyloxy) phenylene vinylene]

Abstract The photoinduced absorption spectra of the conjugated polymers poly[3-methoxy-6-(2-ethyl-hexyloxy) phenylene vinylene] (MEH-PPV) and poly[2,5-thienylene vinylene] (PTV) have been measured using 100 fs laser pulses and white-light continuum generation. For MEH-PPV, a broad sub-gap absorption, peaking at ∼ 1.45 eV, appears within 1 ps of photoexcitation by a 2.05 eV pulse. With increasing time, the induced sub-gap absorption decays and sharpens slightly. These features are tentatively assigned to the photoproduction of triplet excitons. For PTV, excitation at 2.05 eV results in a sub-gap absorption whose maximum is somewhere below the low-energy cut-off of our experimental apparatus (1.3 eV). At early times, induced absorption and bleaching peaks are observed in the vicinity of the principal absorption edge. These features evolve with increasing time, and, by 100 ps, closely resemble the thermal modulation spectrum expected for complete transfer of the pump energy to the lattice due to the nonradiative recombination of the photoexcitations.

Linear and nonlinear optical studies of poly(p-phenylene vinylene) derivatives and polydiacetylene-4BCMU

Abstract 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) (3 ω ; ω , ω , ω ), where h ;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 PPVs are promising materials for optical applications, as they offer the advantages of durability, relatively high nonlinearity, high index, and ease of synthesis and processing.

Photothermal deflection spectroscopy of conjugated polymers

Abstract 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.

Fullerene doped glasses as solid state optical limiters

We discuss the optical limiting behavior of sol-gel glasses doped with fullerene derivatives. Comparisons to fullerene solutions and thin films are made.

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).

Subpicosecond Photoinduced Electron Transfer in Semiconducting Polymer - C60 Composites

Abstract We present the results of ultrafast photoinduced absorption studies of poly(3-octylthiophene), (P3OT) poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene)] (MEH-PPV) and poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylenel, (BCHA-PPV) in their pure form and mixed with C60. The results showa subpicosecond photoinduced electron transfer from the polymer host onto the C60 molecule.

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).

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.