Diffraction effects in single- and two-laser photothermal lens spectroscopy.

A simple method for calculating the effects of optical geometry on photothermal lens signals is shown. This method is based on calculating cumulative electric-field phase shifts produced by a series of Gaussian refractive-index perturbations produced by the photothermal effect. Theoretical results are found for both pulsed-laser and continuous Gaussian laser excitation sources and both single- and two-laser apparatuses commonly employed in photothermal lens spectroscopy. The effects of apparatus geometry on the resulting signal are shown. Analytical time-dependent signal results are found for small signals. Analytical pump-probe focus geometry results allow direct optimization for certain conditions. The calculations indicate that the photothermal lens signal is, in general, optimized for near-field detection-plane geometries.