Abstract

Transient photoinduced absorption (δα) measurements⁽¹⁾ have been useful for studying the generation and characteristics of nonlinear excitations in polyacetylene. Here we focus on the transport properties of these excitations as revealed through studies of the transient photoconductivity,⁽²⁾ σ_p(t), measured using the Auston switch techniques⁽³⁾ The switch consisted of a 50 Ω microstrip transmission line with a narrow gap (0.2 mm) on which a thin polymer sample was grown. The excitations were produced by uniformly illuminating the sample with 20 ps dye laser pulses (1 μJ/pulse) at 2.1 eV. The overall rise time in our measurements of σ_p(t) is ~ 50 ps. Fig. 1 shows σ_p(t) at a bias voltage of 300 V. The fast rise is followed by (approximately exponential) decay with τ ~ 300 ps which is the same magnitude as the rate of decay of δα(t). This indicates that mobile charged soliton excitations are produced within picoseconds, consistent with the Su-Schrieffer mechanism⁽⁴⁾, and that the decay of photocurrent is primarily due to decay of the number of charge carriers. The magnitude of σ_p at 50 ps is about 0.3 S/cm for a flux of 10¹⁵ cm⁻² per pulse, an increase of about six orders of magnitude over the dark σ at room temperature.

© 1986 Optical Society of America