Yang Pang

Poly(p-phenylene vinylene)-silica composite: a novel sol-gel processed non-linear optical material for optical waveguides

Abstract We report here the first case (to our knowledge) of a compatible blend of an inorganic polymer, silica glass, and a π-conjugated non-linear optical polymer, poly( p -phenylene vinylene), homogeneously miscible over wide composition ranges. This composite material was prepared by combining sol-gel processing techniques applicable to the silica glass with those used for the synthesis of the organic polymer from a water/alcohol-soluble sulphonium salt precursor. In this preparation the organic polymer precursor and the inorganic sol were mixed in a common solvent and converted to the final composite material. The thermal conversion of the precursor polymer released HCl, which catalysed the gelation of the inorganic sol. The resulting composite material has been characterized by infra-red and ultra-violet/visible spectral analysis and thermogravimetric and differential scanning calorimetric analysis. The material can be cast into various forms. Thin films cast by the doctor-blading technique exhibit good optical quality and show promise for application as optical waveguides.

Third‐order nonlinearity and two‐photon‐induced molecular dynamics: Femtosecond time‐resolved transient absorption, Kerr gate, and degenerate four‐wave mixing studies in poly (p‐phenylene vinylene)/sol‐gel silica film

Femtosecond response and relaxation of the third‐order optical nonlinearity in a newly developed poly (p‐phenylene vinylene)/sol‐gel silica composite are investigated by time‐resolved forward wave degenerate four‐wave mixing, Kerr gate, and transient absorption techniques using 60 fs pulses at 620 nm. Using a theoretical description of two‐ and four‐wave mixing in optically nonlinear media, it is shown that the results obtained from simultaneous use of these techniques yield valuable information on the real and imaginary components of the third‐order susceptibility. In the composite material investigated here, the imaginary component is derived from the presence of a two‐photon resonance at the wavelength of 620 nm used for the present study. This two‐photon resonance is observed as transient absorption of the probe beam induced by the presence of a strong pump beam. It also provides fifth‐order nonlinear response both in transient absorption and in degenerate four‐wave mixing. The fifth‐order contributio...

Photoinduced processes and resonant third‐order nonlinearity in poly (3‐dodecylthiophene) studied by femtosecond time resolved degenerate four wave mixing

We have investigated the dynamics of resonant third‐order optical nonlinearity of chemically prepared poly(3‐dodecylthiophene) by the degenerate four wave mixing technique using 60 fs pulses at 620 nm. The measured effective value of χ(3) is 5.5×10−11 esu, sixfold smaller than that obtained with 400 fs pulses, emphasizing the pulse width dependence of effective χ(3) when the relaxation time of the photogenerated excitation responsible for the optical nonlinearity is comparable to the pulse width. Within the resolution of the optical pulse, the rise time of the nonlinear response is instantaneous and the dominant decay occurs within 200 fs, revealing that the short time, nonlinear response is derived from the initially photogenerated excitons. A detailed analysis of the total decay behavior is consistent with the polaron dynamics of the conformational deformation model proposed by Su, Schrieffer, and Heeger for a conjugated linear polymer with bond alternation.

Temporal behavior of stimulated Kerr scattering in a CS 2 liquid-core hollow-fiber system

The temporal behavior of the stimulated Kerr scattering (SKS) from a 1.5-m-long CS2 liquid-core hollow-fiber system is studied by using nanosecond and subpicosecond pump-laser pulses. In both situations the duration of the SKS pulses is much shorter than that of the corresponding pump pulses. With ~20-ns pump pulses, the pulse width of a SKS component that is located within a narrow (~1.1 nm) spectral interval and is generated by the second- or third-order stimulated Raman scattering can be shorter than 2–3 ns. In order to measure the temporal behavior of SKS obtained with ultrashort (0.5 ps) pump pulses, a Michelson-interferometer-grating device is used. The pulse duration of the SKS component within a narrow (~1.2 nm) spectral interval is approximately 260–270 fs with ~0.5-ps pump pulses. Our experimental results are qualitatively consistent with the theoretical consideration of the SKS processes.

Study of anisotropy of acoustic wave propagation in stretched poly(vinylidene difluoride) film using the picosecond transient grating technique

Abstract The picosecond transient grating technique has been used to obtain acoustic speed as a function of film orientation in a uniaxially stretch-oriented film of poly(vinylidene difluoride) at liquid nitrogen temperature. By the analyses of acoustic speed anisotropy using the general Christoffel equation, we have obtained both the longitudinal and shear components of the elastic moduli of the film. Our results are in qualitative agreement with those of a published Brillouin scattering study.

Degenerate four wave mixing study of conformational transition of a polydiacetylene, poly‐4‐BCMU, in solution

The conformational change in the poly‐4‐BCMU polydiacetylene was investigated by using nanosecond time‐resolution degenerate four wave mixing. Polarization data along with the time response of the signal indicated the signal to be from a thermal grating. The degenerate four wave mixing signal changes dramatically during the rod‐to‐coil transition and fits a theoretical model of single chain conformational transition.

Speed of optically controlled superconducting devices

The speed of optically controlled superconducting devices is discussed. The results of experimental measurements on the speed of nonequilibrium carrier recombination are reviewed. These results indicate that a voltage change can be induced in YBCO samples by a short laser pulse. It is shown that high-Tc superconducting devices can potentially operate in the subpicosecond regime.