Abstract

We observe large self-refraction in CS₂ in the 9-11-µm wavelength range using 50-250-ns (FWHM) pulses. At input energies as small as 100 µJ the effects of self-lensing are observed in an optical limiting geometry where the tightly focused pulses are observed in the far field through a small aperture. The energy transmitted through this aperture depends on the position of the focus with respect to the thin CS₂ filled cell. A focal position prior to the cell results in a transmission decrease, whereas focusing behind the cell results in increased transmission. These results are indicative of the creation of an irradiance or fluence-dependent negative lens in the CS₂. In addition, by operating the laser at another wavelength, where the linear absorption is different, we see the same transmission change if we keep αE constant where α is the linear absorption coefficient and E is the laser energy. That is, if the total energy absorbed is the same for the two different wavelengths, the nonlinear refraction is the same. This is consistent with a negative thermal nonlinearity. In fact, using the simple model of a thin fluence-dependent negative lens at the position of the sample we can calculate the thermooptic coefficient and obtain dn/dt~ 5 × 10⁻⁴°C⁻¹, while the literature value is ~8 × 10⁻⁴°C⁻¹.

© 1988 Optical Society of America