Nonlinear refractive index of CS2 at 10 μm

The reported values of the nonlinear refractive index n(l) of CS2 near 10 μm differ by 3 orders of magnitude, and the mechanisms responsible for the nonlinearity are not understood.1 We measure n(l) for CS2 directly by observing the deflection of a CO2 laser beam which traverses a prismatic cell filled with CS2 as a function of intensity up to 10 MW/cm2 at different frequencies. At 10.6 μm ∂n/∂l is positive, while at 9.5 μm ∂n/∂l is negative. Based on our results and the known CS2 absorption and linear dispersion curves, we conclude that three mechanisms contribute to the nonlinearity: (1) the molecular reorientation Kerr effect resulting in frequency-independent relatively small positive n2 values1; (2) thermal effects with negative n2 responsible for self-defocusing; (3) near two-photon resonance with vibronic states2 at 6.7 and 4.7 μm. The sign of the third contribution to n(l) depends on which side of the corresponding resonance the 2v L frequency is situated, while its magnitude is proportional to the laser intensity squared.