Optical ray tracing for crossed beam photothermal deflection spectroscopy

Area of Science:

• Optical Physics
• Spectroscopy
• Laser-Matter Interactions

Background:

• Pulsed photothermal deflection spectroscopy (PDS) is a sensitive technique for analyzing gas properties.
• Accurate modeling of probe beam behavior is crucial for interpreting PDS signals.
• Previous analytical models may not fully account for thermal diffusion effects in PDS.

Purpose of the Study:

• To develop and apply a geometrical algorithm for tracing probe laser beam trajectories in pulsed PDS.
• To investigate the influence of various parameters on ray trajectories and signal deflection.
• To compare geometrical ray tracing results with analytical calculations and experimental observations.

Main Methods:

• Implementation of a geometrical algorithm for optical ray tracing of probe laser beams.
• Simulation of ray trajectories under varying conditions: time, pump beam energy, radius, and pulse duration.
• Computation and comparison of probe beam exit angles with analytical deflection angles.

Main Results:

• Excellent agreement between geometrical ray tracing and analytical calculations for short pump pulse durations (shorter than thermal diffusion time).
• Analytical model inaccuracy for long pump pulse durations due to neglected thermal diffusion.
• Observed signal saturation at high absorbed pump beam energy attributed to excessive absorber heating.
• Maximum probe beam deflection occurs at the end of the pump pulse duration.

Conclusions:

• Geometrical ray tracing provides an accurate method for modeling probe beam behavior in pulsed PDS.
• The timing of deflection measurement is critical, with the end of the pump pulse yielding maximum deflection.
• Understanding thermal diffusion effects is essential for accurate analytical modeling in PDS, especially for longer pulses.