RAPID RECOVERY OF WIDE-BANDWIDTH PHOTOTHERMAL SIGNALS VIA HOMODYNE PHOTOTHERMAL SPECTROMETRY - THEORY AND METHODOLOGY

Homodyne photothermal spectrometry (HPS) is a very wide bandwidth sign al recovery technique which uses many of the elements of lock-in detec tion at very low cost. The method uses a frequency sweep, with a high- frequency bandwidth of up to 10 MHz, to excite a linear photothermal s ystem. The response sweep of the photothermal system is down-shifted i nto a bandwidth of a few kilohertz by means of in-phase mixing with th e excitation sweep with the use of a four-quadrant double-balanced mix er and a low-pass filter. Under conditions derived from theory, the fi lter output gives a good approximation to the real part of the phototh ermal system's frequency response, dispersed as a function of time. Fr om a recording of this signal, the frequency and impulse response of t he photothermal system are rapidly recovered at very high resolution. The method has been tested with the use of laser photopyroelectric eff ect spectrometry and provides an inexpensive, convenient method for th e recovery of high-frequency photothermal signals.