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

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 (n(2)) 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 n(2) observed at 720 nm was 3.9x10(-5)cm (2)/MW . The results show that the magnitude of n(2) monotonically decreases as wavelength is increased away from resonance, and two-photon absorption does not make a significant contribution to n(2) at off-resonant wavelengths up to 1064 nm.