STUDIES OF NEAR-FIELD NONLINEAR OPTICAL IMAGING OF THIN FILMS AND TRANSIENT PHOTOCONDUCTIVITY IN CONJUGATED POLYMERS

Abstract

The dissertation is composed of two parts: 1) near-field nonlinear optical studies of ferroelectric and polymeric thin films and 2) transient phenomena in polymers. Ferroelectric thin films are imaged with both topology and surface optical second harmonic (SH) distribution. The lateral resolution is determined to be 80nm. The relation between local optical second harmonic intensity and polarization of the ferroelectric domains is explored both experimentally and theoretically. A polydomain ferroelectric film with grid domain structures is used as a standard sample to check the validity of this relationship. Second order nonlinear properties of nonlinear optical (NLO) polymers are studied both in the far-field and near-field.

Transient photoconductivity in conjugated polymers is studied systematically. Picosecond electric pulses generated in a photoconductive switch fabricated on polymer are used to derive the photoconductance of the device. The pulse width of electric pulse is measured by photoconductive sampling to be around 1-2ps, which is the best result in polymers up to now. Transient photoconductivity and mobility are calculated, the transient mobility in MEH-HPPV (poly[2-methoxy-5-(28-ethylhexyloxy)-1,4-phenylenevinylene]) is estimated to be ~800 cm2/Vs. A new method of photoconductive sampling is proposed using only one photoconductive switch, and transient reflectivity change of the polymer is also observed.

Optical rectification in poled nonlinear optical (NLO) polymers has been used to generate and detect THz radiation. Relation between THz radiation power and incident laser spot size are studied for the first time in polymer materials; an optimized pump laser spot size of close to THz wavelength is obtained. Application of THz to study organic samples is presented.