Investigation of nonlinear optical properties of thin absorbing films near the critical angle of total reflection

The results of an experimental investigation of the nonlinear optical properties of thin absorbing films (solutions of cobalt chloride in ethanol) are presented. The dependence of the reflectivity of an absorbing plane-parallel film on the energy density of the incident radiation was obtained for different thicknesses of the film and different angles of incidence. It was found that near the critical angle of total reflection, the reflection coefficient of thin (of the order of a few microns) absorbing films varied fairly widely (approximately between 0 and 1). This was due to the establishment of a thermal mechanism of nonlinearity of the refractive index in the thin film, combined with interference. Q-switched laser action was achieved using a thin plane-parallel absorbing film as the Q-switch. The pulse energy was 1-1.5 J with half-width durations of 140-30 nsec, respectively.