In recent years, photoacoustic spectroscopy (PAS) has emerged as an attractive and powerful technique well suited for sensing applications. The development of high-power radiation sources and more sophisticated electronics, including sensitive microphones and digital lock-in amplifiers, have allowed for significant advances in PAS. Furthermore, photoacoustic (PA) detection of IR absorption spectra using modern tunable lasers offers several advantages, including simultaneous detection and discrimination of numerous molecules of interest. Successful applications of PAS in gases and condensed matter have made this a notable technique and it is now studied and employed by scientists and engineers in a variety of disciplines.

Simplified schematic diagram of the ARL laser-based photoacoustic gas sensor system

PAS is a detection technique under the umbrella of photothermal spectroscopy. Photothermal spectroscopy encompasses a group of highly sensitive methods that can be used to detect trace levels of optical absorption and subsequent thermal perturbations of the sample in gas, liquid, or solid phases. The underlying principle that connects these various spectroscopic methods is the measurement of physical changes (i.e., temperature, density, or pressure) as a result of a photo-induced change in the thermal state of the sample. Other photothermal techniques include photothermal interferometry (PTI), photothermal lensing (PTL), and photothermal deflection (PTD).

All photothermal processes consist of several linked steps that result in a change of the state of the sample. In general, the sample undergoes an optical excitation, which can take various forms of radiation, including laser radiation. This radiation is absorbed by the sample placing it in an excited state (i.e., increased internal energy). Some portion of this energy decays from the excited state in a nonradiative fashion.

This increase in local energy results in a temperature change in the sample or the coupling fluid (e.g., air). The increase in temperature can result in a density change and, if it occurs at a faster rate than the sample or coupling fluid can expand or contract, the temperature change will result in a pressure change. As mentioned, all photothermal methods attempt to key in on the changes in the thermal state of the sample by measuring the index of refraction change as with PTI, PTL, and PTD; temperature change as with photothermal calorimetry and photothermal radiometry; or pressure change as with PAS.

In order to generate acoustic waves in a sample, periodic heating and cooling of the sample is required to produce pressure fluctuations. This is accomplished using modulated or pulsed excitation sources. The pressure waves detected in PAS are generated directly by the absorbed fraction of the modulated or pulsed excitation beam. Therefore, the signal generated from a PA experiment is directly proportional to the absorbed incident power. However, depending on the type of excitation source (i.e., modulated or pulsed), the relationship between the generated acoustic signal and the absorbed power at a given wavelength will differ. There are 2 main categories of light sources used for PAS: broadband sources and narrowband laser sources. Although lamp-based PAS is still common, modern PAS research has been mainly performed using laser sources.

In 1994 the introduction of the quantum cascade laser (QCL) by Bell Labs changed the prospects of laser PA and, in general, IR spectroscopy. Since that time, continuing and aggressive evolution has been occurring. The QCL has matured to a level at which numerous companies can produce gain material for laser systems both in the United States and abroad. Along with this production, several companies have produced laser systems that are suitable for spectroscopic purposes, allowing for continuous wavelength tuning ranges of greater than or equal to 200 cm-1. The resolution of well-constructed systems can tune continuously and without mode-hopping over the whole tuning band with a nominal resolution of approximately 1 cm-1. Power output of spectroscopic sources has generally been moderate; 10s of milliwatt average power and on the order of 100s of milliwatt peak pulsed power. Furthermore, QCLs, operating in low duty cycles, have demonstrated that PAS based on lock-in amplification can still be performed and indeed shows great promise.

This work was done by Ellen L Holthoff and Paul M Pellegrino of the Army Research Laboratory. ARL-0201

This Brief includes a Technical Support Package (TSP).
Development of Photoacoustic Sensing Platforms at the US Army Research Laboratory

(reference ARL-0201) is currently available for download from the TSP library.

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