Simple model for the time dependence of the periodically excited crossed-beam thermal lens

A time-dependent model is presented for the periodically excited crossed-beam thermal lens. In order to obtain good agreement between the theory and data for chopped excitation, it is necessary to consider the finite time required to block and unblock the pump beam. Unfortunately, a closed form expression has not been developed which considers the excitation waveform for a chopped Gaussian beam. On the other hand, a useful approximation has been developed based on a trapezoidal excitation function. This model is verified with respect to time, absorbance, alignment, and thermal diffusivity.