Photoacoustic chemical sensing: layered systems and excitation source analysis

Abstract

Photoacoustic spectroscopy (PAS) is a versatile tool that is well suited for the ranged interrogation of layered samples. We have previously demonstrated standoff photoacoustic (PA) chemical detection of condensed phase samples at one meter distance using an interferometric sensing platform. Current research investigates layered solid samples constructed from a thin layer of energetic material deposited on a substrate. The PA signal from the system, as measured by the interferometer, changes based on the differing optical and mechanical properties of the substrate. This signal variance must be understood in order to develop a sensor capable of detecting trace quantities of hazardous materials independent of the surface. Optical absorption and modal excitation are the two biggest sources of PA signal generated in the sample/substrate system. Finally, the mode of operation of the excitation source is investigated. Most PA sensing paradigms use a quantum cascade laser (QCL) operating in either pulsed or modulated CW mode. We will discuss photoacoustic signal generation with respect to these different operating modes.

Standoff Photoacoustic Spectroscopy of Explosives

“…Investigations in our laboratory into substrate response in standoff PAS experiments have shown that there is a contribution to the signal. 43 Absorption of the excitation source occurs in both the analyte and the substrate, generating PA signal when there is no analyte and amplifying the signal from the system when the analyte is present. There are beneficial experimental contributions from substrate absorption when performing PA depth profiling experiments.…”