We report high-sensitivity detection of 2,4,6-trinitrotoluene (TNT) by using laser photoacoustic spectroscopy where the laser radiation is obtained from a continuous-wave room temperature high-power quantum cascade laser in an external grating cavity geometry. The external grating cavity quantum cascade laser is continuously tunable over Ϸ400 nm around 7.3 m and produces a maximum continuouswave power of Ϸ200 mW. The IR spectroscopic signature of TNT is sufficiently different from that of nitroglycerine so that unambiguous detection of TNT without false positives from traces of nitroglycerine is possible. We also report the results of spectroscopy of acetylene in the 7.3-m region to demonstrate continuous tunability of the IR source.quantum cascade lasers ͉ high-power lasers ͉ continuous-wave operation ͉ room temperature operation ͉ TNT detection D etection of illegally transported explosives has become important since the global rise in terrorism subsequent to the events of September 11, 2001. Although not a choice of suicide bombers, 2,4,6-trinitrotoluene (TNT) is a potent explosive for which techniques for detection on a person's body or in one's baggage is considered important for assuring safety of airports and air travel. As with detection of other similar compounds, such as chemical warfare agents, any detection scheme that claims to detect these targets must exhibit acceptable receiver operational characteristic (ROC) that assures detection at very low levels without an unacceptable level of false alarms (1, 2). The molecular mass of TNT (C 7 H 5 N 3 O 6 ) is almost exactly identical to the molecular mass of nitroglycerine (C 3 H 5 N 3 O 9 ) even though the chemical compositions of the two molecules are very different (TNT, 227.131 Da vs. nitroglycerine, 227.0872 Da). The nearly same molecular masses often lead to problems for unambiguous detection of TNT using techniques that rely on measuring the molecular mass of the species. On the other hand, the differences in the chemical structure between TNT and nitroglycerine lead to noticeably different infrared (IR) absorption signatures (3), making it possible to distinguish between the two. However, the detection of TNT in vapor phase is hampered by its low vapor pressure of Ϸ2 ϫ 10 Ϫ4 torr at 25°C. In this work, we report on studies of detection of TNT by using room-temperature (RT) quantum cascade laser (QCL)-based photoacoustic spectroscopy (QCL-PAS). The high sensitivity afforded by laser-based photoacoustic spectroscopy (L-PAS) (4) shows that the vapor-phase detection of TNT at an ambient temperature of Ϸ25°C is possible.Previously, CO and CO 2 lasers have been used for photoacoustic (PA) spectroscopic detection (3, 5) of vapors of explosives. However, both of these laser sources are step tunable, and neither of the lasers is able to access the strong absorption features of TNT that lie in the 6.0-7.5 m region. Quantum cascade lasers (QCLs), with their continuous tunability, should be the right sources for the detection of TNT and other species that do...

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We report the sub-parts-per-billion-level detection of NO2 using tunable laser-based photoacoustic spectroscopy where the laser radiation is obtained from a room-temperature continuous-wave high-power quantum cascade laser operating in an external grating cavity configuration. The continuously tunable external grating cavity quantum cascade laser produces maximum single-frequency output of Ϸ300 mW tunable over Ϸ350 nm centered at 6.25 m. We demonstrate minimum detection level of Ϸ0.5 parts per billion of NO2 in the presence of humidified air.IR lasers ͉ NO2 spectroscopy ͉ photoacoustic spectroscopy ͉ sub-parts-per-billion detection of gases R eal-time trace-level gas detection is an area of fast growth with applications in such diverse fields as medical diagnostics, process control, national security, and environmental airquality monitoring. Techniques based on the measurement of optical absorption using tunable laser sources are attractive detection methods because of their intrinsic sensitivity and selectivity, i.e., an ability for discriminating against interferents (1, 2). The most recent advances, intercomparison of spectroscopic techniques, and status of available laser sources are summarized in a recent review (3). Furthermore, photoacoustic (PA) spectroscopy is seen to be well suited for optical absorption measurements because it combines high sensitivity with the ruggedness needed for field-deployable instrumentation. However, laser PA spectroscopy (L-PAS) requires multihundredmilliwatt-level laser sources to achieve a sub-parts-per-billion (sub-ppb) level of sensitivity for many of the environmentally and industrially important gases. As a result, heretofore most of the sensitive L-PAS detection schemes have used continuouswave (CW) molecular gas lasers (4). Results and DiscussionHigh-Power Continuously Tunable Laser Source. The tuning characteristics of our high-power single-frequency source are seen in Fig. 1, which shows the single-mode laser power available from the CW room temperature (RT) external grating cavity (EGC) quantum cascade laser (QCL) (near 6.3 m) at several grating settings. Fig. 1 also shows the overall tuning of the output that covers Ͼ350 nm with CW RT laser power in excess of Ϸ200 mW with maximum CW RT laser power of 300 mW near the center of the tuning curve. Details are given in Materials and Methods.PA Spectroscopy of NO2. We have used the broadly and continuously tunable, high-power CW RT EGC-QCL source for PA spectroscopy of NO 2 , which exhibits strong absorption features near Ϸ1,600 cm Ϫ1 . NO 2 is a smog and particulate matter precursor and one of the key pollutants that is routinely monitored. Ambient concentrations of NO 2 typically are in single digits to tens of parts per billion (ppb) levels (5). The national ambient air quality standard (NAAQS) level for NO 2 is 53 ppb for arithmetic mean average (6). Consequently, an acceptable ambient NO 2 sensor should have single-digit ppb or, better, sub-ppb, sensitivity. Earlier, cryogenically cooled lead salt lasers (7) operat...

We report high-throughput, nondispersive optical multiplexing of laser beams using a scanning galvanometer. We have utilized this technique for multispecies trace-gas detection using multiple quantum cascade laser photoacoustic spectroscopy. We demonstrate switching from one laser to another in less than 1 s, a performance level needed for a comprehensive multispecies sensor, and a high signal-to-noise ratio detection of five gaseous components, NH(3), NO(2), dimethyl methyl phosphonate (DMMP, a simulant for nerve agents), acetone, and ethylene glycol, in a room air gas mixture containing approximately 3 ppb of NH(3), approximately 8 ppb of NO(2), approximately 20 ppb of DMMP, approximately 30 ppb of acetone, and approximately 40 ppb of ethylene glycol.

We report operation of tunable external cavity quantum cascade lasers with emission wavelength controlled by an acousto-optic modulator (AOM). A long-wave infrared quantum cascade laser wavelength tuned from ∼8.5 μm to ∼9.8 μm when the AOM frequency was changed from ∼41MHz to ∼49 MHz. The laser delivered over 350 mW of average power at the center of the tuning curve in a linewidth of ∼4.7 cm−1. Measured wavelength switching time between any two wavelengths within the tuning range of the QCL was less than 1 μs. Spectral measurements of infrared absorption features of Freon demonstrated a capability of obtaining complete spectral data in less than 20 μs.