Abstract

In many third-order nonlinear materials, such as inorganic semiconductors or organic polymers, the imaginary part of the electronic third-order susceptibility χ³ is larger than the real part by an order of magnitude or more in certain wavelength regions. Thus, at these wavelengths, nonlinear absorption is expected to be much stronger than nonlinear refraction. However, in measurements with laser pulses that are of nanosecond or longer duration, thermal expansion of the material caused by absorption of light and subsequent heating gives rise to appreciable nonlinear refraction of the laser beam. Because both linear and two-photon absorption of the laser beam play a part in causing this nonlinear refraction effect, the nonlinearity cannot simply be characterized by a refractive-index change n₂I proportional to the laser intensity I; the order of the effective susceptibility is higher than 3.

© 1991 Optical Society of America