Download Modern IR diode lasers enable novel photoacoustic sensors

10.1117/2.1201009.003209
Modern IR diode lasers enable
novel photoacoustic sensors
Ellen Holthoff, John Bender, and Paul Pellegrino
Using a single, continuously tunable laser provides better molecular discrimination and enables simultaneous detection of several
molecules.
The global war on terror and escalating environmental awareness have made rapid detection and identification of hazardous
materials a priority. Detecting a diverse range of chemical and
biological agents requires an adaptable sensor platform capable
of identifying threats before they cause harm. Research and development in hazardous-materials detection technology focuses
on increasing speed and sensitivity, while reducing size and cost.
Currently, the US army is using ion-mobility-spectrometrybased products to assess threats. While they are useful for certain
applications, they have limitations, including poor specificity
and little adaptability in combination with other chemical
systems. We are studying photoacoustic spectroscopy (PAS) to
improve current technologies and address the latest sensorsystem requirements.
PAS is a highly sensitive technique that can be used for detection of trace levels of gases. It uses optical absorption and
subsequent measurement of a pressure wave produced from
a photo-induced change in the sample’s thermal state (see
Figure 1). To generate acoustic waves in gases, the sample must
be heated periodically to produce pressure fluctuations. This is
accomplished using modulated or pulsed excitation sources. Although previous research has demonstrated the sensitivity of
photoacoustic sensors at parts per trillion,1, 2 the total system
size represents a large logistics burden in terms of bulk, cost,
and power consumption.
To date, limited research has been done to demonstrate the
feasibility of a miniaturized photoacoustic sensor.3–5 Initial examination of PAS scaling principles with respect to microelectromechanical-systems (MEMS) dimensions indicated that
photoacoustic signals would remain at similar sensitivities or
even surpass those commonly found in macro-scale devices.3–5
However, these investigations used relatively bulky carbon
dioxide laser sources. To realize the full advantage of evolving
Figure 1. Simplified photoacoustic gas-sensor system. Pressure waves
resulting from molecular changes in the sample are detected with a
microphone (mic). Its output is amplified and displayed as the photoacoustic signal. The latter, collected over a given wavelength range,
results in a photoacoustic vibrational spectrum of the sample.
Figure 2. Micro-electromechanical-systems-scale photoacoustic cell
used for data collection. The assembled cell has a microphone, tubing
for gas-sample entry and exit, and a connector for microphone power
and signal processing.
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Figure 3. (A) Laser photoacoustic (—-) and Fourier-transform IR
(FTIR) data (- - -) for vinyl acetate, acetic acid, acetone, and 1,4dioxane. The photoacoustic and FTIR spectra are in excellent agreement. (B) Partial least-squares (PLS) model data illustrating that the
photoacoustic spectral features for vinyl acetate, acetic acid, acetone,
and 1,4-dioxane are different. Data points for each chemical are encircled in colored lobes. PC 1, 2, 3: Principal components.
MEMS photoacoustic-cell designs, we are using quantumcascade lasers (QCLs) with small package sizes, increased
wavelength tunability, and room-temperature operation.
We have examined the efficacy of PAS applied to military and
environmental problems. Specifically, we have demonstrated
QCL-based, MEMS-scale photoacoustic sensors with detection
limits at parts-per-billion levels for the nerve-gas simulant
dimethyl methyl phosphonate (DMMP), as well as chlorobenzene, a propellant analog and an intermediate in the manufacture of pesticides such as dichlorodiphenyltrichloroethane
(better known as DDT) and hexafluoroethane (Freon 116), also
a propellant analog and a component in refrigerants.6, 7 These
analytes exhibit known absorbance features in the wavelength
tuning range of the sensor platform’s QCLs, which allowed
collection of their photoacoustic vibrational spectra. The latter agreed with the compounds’ Fourier-transform IR spectra.
These results suggest that combining a continuously tunable
QCL characterized by a broad tuning range with a MEMS-scale
photoacoustic device provides increased molecular discrimination and simultaneous detection of several molecules of interest.
We recently used a continuously tunable QCL with a tuning
range of 225cm 1 with a MEMS-scale photoacoustic-cell design
(see Figure 2) to detect and discriminate acetic acid, acetone,
1,4-dioxane, and vinyl acetate.8 Combined with a partial leastsquares (PLS) chemometrics identification model, we could distinguish among these species based on the laser photoacoustic
spectra, which exhibit known absorbance features in the QCL’s
wavelength tuning range. These results suggested that this approach can be used with specificity for numerous analytes or
even mixtures of analytes.
In summary, the next-generation PAS sensor using broadly
tunable QCL sources shows promise to meet the requirements
for environmental and military applications. We have successfully demonstrated a QCL-based, MEMS-scale photoacoustic
sensing platform capable of both trace-vapor detection and
molecular discrimination. We believe that this sensor platform
is an important step towards development of a portable prototype. Our next step will be to use multiphysics modeling to better understand the acoustics of MEMS-scale, photoacoustic gas
sensors. Other investigations will take advantage of the QCLbased photoacoustic motif and examine it for detection of solid
samples.
The authors thank Nancy Stoffel and Almon Fisher from Infotonics
Technology Center.
Author Information
Ellen Holthoff, John Bender, and Paul Pellegrino
US Army Research Laboratory (ARL)
Adelphi, MD
Ellen Holthof holds a PhD in chemistry from the University at
Buffalo (State University of New York).
John Bender participated in the US Army Student Temporary
Employment Program (STEP) at ARL from 2008 to 2010. He currently attends graduate school at the University of Maryland.
Paul Pellegrino, a physicist at ARL for more than 10 years, leads
the Optical Devices and Sensors Team in the Sensors and Electron Devices Directorate. He holds an MS and PhD in physics
from New Mexico State University. He has more than 20 years of
experience in optics, physics, and computational physics, with
an emphasis in the last 14 years on applying novel spectroscopy
and optical transduction for chemical, biological, and energetic
sensing.
References
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photoacoustic cell for trace-gas sensing, Appl. Phys. B 70, pp. 895–901, 2001.
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photoacoustic detection, J. Appl. Phys. 92, pp. 1555–1563, 2002. doi:10.1063/1.1489493
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7. E. L. Holthoff, D. A. Heaps, and P. M. Pellegrino, Development of a MEMS-scale
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c 2010 SPIE