Basic Mechanisms involved in the Off-resonant Nonlinear Refractive Index of PTS-Polydiacetylene

Polydiacetylene single crystal, PTS, is known to have the largest off-resonant nonlinear refractive index (n2). Recently we have demonstrated picosecond all-optical switching using PTS-polydiacetylene in a Fabry-Perot cavity1. This measurement was performed using a pump-probe technique with 90ps pulses at 1.06μm wavelength. In that measurement, besides demonstrating a large modulation depth, we showed that the sign of n2 was negative. In another work, however, a positive sign for n2 was reported from a z-scan measurement2 and two photon resonances were used to explain the positive sign and the off-resonant nonlinear optical processes in PTS. In this work we have made detailed measurements of the sign using Michelson interferometry and z-scan. These and previous measurements using pump-probe technique at different off-resonant wavelengths show that the sign of n2 of PTS is negative over the wavelength of 650 - 1064nm.