Frequency modulation time delay thermal lens effect spectrometry: a new technique of transient photothermal calorimetry.
A new type of laser-induced thermal lens effect measurement is demonstrated in which the thermooptic impulse response of a weakly absorbing sample is recovered in the time delay domain through excitation with a fast linear frequency sweep. The autocorrelation function of the excitation sweep approximates to a Dirac delta function to a time resolution limited by the modulation bandwidth of the sweep, thereby permitting the fast recovery of high quality frequency and impulse response information. Impulse response data recovered in the time delay domain showed good agreement with the results predicted from Fresnel diffraction theory, indicating an equivalence to the response recovered in a typical pulsed measurement. The FM time delay technique, however provides enhanced measurement dynamic range and an overall reduced peak excitation power when compared with the pulsed measurement.