Self-phase modulation in polydiacetylene single crystal measured at 720-1064 nm

Detailed measurement of spectral broadening in a poly-[2,4 hexadiyne-1,6 diol-bis-(p-toluene sulfonate)] (PTS) single crystal owing to self-phase modulation was performed as a function of wavelength by use of a Ti:sapphire laser producing 200-fs pulses at 720-920 nm and a Nd:YAG laser producing 50-ps pulses at 1064 nm. The nonlinear refractive index (nz) of PTS at these wavelengths was determined from the measured phase shift. Group-velocity dispersion was estimated and found to have a negligible effect on the observed spectral broadening. The two-photon absorption coefficient (alpha (2)) over this wavelength range was determined from nonlinear transmission measurements. The largest magnitude of nz observed at 720 nm was 3.9 X 10(-5) cm(2)/MW. The results show that the magnitude of nz monotonically decreases as wavelength is increased away from resonance, and two-photon absorption does not make a significant contribution to nz at off-resonant wavelengths up to 1064 nm. (C) 2001 Optical Society of America.