Optimization of the optical path length amplitude for interferometric photothermal gas and aerosol sensing considering advection: A theoretical study

Abstract

Photothermal spectroscopy, and more specifically photothermal interferometry (PTI), is a highly sensitive technique for measuring gas and aerosol concentrations. Numerous implementations of different PTI configurations have demonstrated the versatility of the technique. This theoretical study presents a comprehensive analysis and an optimization of the PTI optical path length (OPL) amplitude using characteristic times. We investigate how the OPL amplitude depends on the dimensions and orientations of the interferometer laser beam and the continuous-wave excitation laser beam. This analysis quantifies the impact of advection on the OPL amplitude based on the relative orientation of the two laser beams and the gas flow direction. It is analytically shown that the possibilities for photothermal OPL amplitude optimization are limited when thermal diffusion is dominant. Theoretically, advection has the potential to double or cancel the OPL amplitude, depending on the specific configurations. In summary, we provide an in-depth understanding of the design and parameter considerations required when tailoring and optimizing a PTI sensor for different fields of applications.