Pump/Probe Thermal Lens Spectroscopy with Polarization-Encoded Laser Beams

A new detection system for pump-and-probe thermal lens spectroscopy has been developed. Two orthogonally polarized beams derived from the same CW laser serve as pump and probe. The pump beam is chopped and recombined with the probe laser. A Glan prism located after the absorption cell separates pump-and-probe beams. The thermal lens is sensed as the probe beam overflows a limiting aperture and is processed through a lock-in amplifier. The thermal lens response is linear in analyte concentration. The system is applied to liquid chromatography detection. The noise level is about 4 microabsorbance rms. Once aligned, the system remains in alignment indefinitely, since laser pointing instabilities cancel.