Resonant nonlinear optical response of 1-D excitons

Abstract

The nonlinear optical properties of polymeric semiconductors have recently come under intensive study due to increasing interest in optical signal processing. The nonresonant nonlinear optical constants of polydiacetylene-PTS have previously been well characterized. In this paper we present results on the resonant nonlinear optical response of PTS. One measurement was performed using the transient grating technique. Two 1.97-eV pulses were overlapped at the sample to form an intensity grating. A reflectivity grating developed which was probed by a third time-delayed pulse. The magnitude of the reflectivity change per absorbed photon was extremely large, corresponding to a nonlinear refractive index, n2, of 3 × 10−2 (MW/cm2)−1. The recovery time of the system following pulsed excitation was 2.0 ± 0.2 ps. We interpret our results in terms of a model in which the nonlinear response is entirely due to the dynamics of the singlet exciton in PTS. We describe a straightforward extension of the ideas developed by Schmitt-Rink, Chemla, and Miller for a 2-D exciton system to 1-D.1 Using their "phase-space filling” model, the magnitude of n2 allows us to determine the Bohr radius of the exciton to be 30 Å, which is in excellent agreement with theoretical predictions. In addition we discuss a simple offshoot of this model which gives a good quantitative description of nonresonant response of PTS as well.