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Development of Photoacoustic Sensing Platforms

Research focuses on sensor miniaturization and detection of chemical targets both proximally and at range.

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

Topics:
Aerospace Amplifiers Data management Electronic equipment Gases Hazardous materials Hazards and emergency operations Lasers Optics Photonics Pressure Radiation Research and development Sensors and actuators Spectroscopy Technical review
This Brief includes a Technical Support Package (TSP).
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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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    Aerospace & Defense Technology Magazine

    This article first appeared in the June, 2017 issue of Aerospace & Defense Technology Magazine (Vol. 2 No. 4).

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    Overview

    The document titled "Development of Photoacoustic Sensing Platforms at the US Army Research Laboratory" is a technical report authored by Ellen L. Holthoff and Paul M. Pellegrino, published in September 2016. It details the advancements in photoacoustic sensing technologies developed by the US Army Research Laboratory (ARL). The report is approved for public release, indicating its accessibility for broader audiences.

    Photoacoustic sensing is a technique that utilizes the absorption of light by materials, leading to the generation of sound waves. This method is particularly valuable for detecting and analyzing various substances, including explosives and biological agents. The report outlines the development of new sensing platforms that leverage this technology to improve detection capabilities.

    One of the key highlights of the research is the use of drop-on-demand inkjet printing for preparing explosive samples. This method was chosen to maximize the uniformity of the analyte, which is crucial for accurate detection and analysis. The report emphasizes the advantages of this printing technique over traditional methods such as spray deposition or dropcasting, showcasing its potential for enhancing the reliability of sensor performance.

    The document also acknowledges the contributions of various individuals involved in the research, including David Heaps, John Schill, and Logan Marcus, among others. Their collaborative efforts were instrumental in advancing the project and achieving significant milestones in the development of photoacoustic sensing platforms.

    In addition to the technical aspects, the report includes standard disclaimers and notices, clarifying that the findings should not be construed as an official position of the Department of the Army unless specified otherwise. It also emphasizes the importance of proper handling and disposal of the report once it is no longer needed.

    Overall, this technical report serves as a comprehensive overview of the innovative work being done at the US Army Research Laboratory in the field of photoacoustic sensing. It highlights the potential applications of these technologies in military and civilian contexts, particularly in enhancing detection methods for hazardous materials. The findings contribute to the ongoing efforts to improve safety and security through advanced sensing capabilities.

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