Novel lock-in waveform technique for signal-to-noise ratio and dynamic-range enhancement in highly noised photothermal experiments

Conventional frequency domain photothermal methods are basically single-ended techniques meaning that a 50% duty-cycle square wave, or a sinusoidal wave is used for modulating the heating beam intensity along with a lock-in amplifier (LIA) for signal processing. The implications of single-ended detection are that, if the signal contributions from sample inhomogeneities are much smaller than that from the homogeneous bulk of the material (background signal), then they cannot be easily detected. In a single-ended technique the sensitivity of the experiment is determined by the magnitude of the background signal. Without further conditioning the signal level is simply too high to probe variations of amplitude much smaller than this background. In this work, these very small variations are called "contrast signals". We introduce a lock-in common-mode rejection demodulation signal methodology as an alternative to the single-ended techniques. This signal generation scheme, when coupled to a photothermal detection system is, in principle, capable of detecting very weak inhomogeneities in materials that are not possible to detect with conventional techniques. Furthermore, some calibration measurements obtained on a homogeneous Zr alloy sample will be presented. These measurements will be further compared with that obtained by irradiating the sample with the conventional 50% duty-cycle square wave, in order to compare their noise characteristics. Finally, some preliminary measurements on Zr-2.5Nb shot-peened samples will be presented as a case study of weakly inhomogeneous solids and for comparison with that obtained with the conventional frequency scan.(1)