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This article presents new spectroscopic results in standoff chemical detection that are enabled by monolithic arrays of Distributed Feedback (DFB) Quantum Cascade Lasers (QCLs), with each array element at a slightly different wavelength than its neighbor. The standoff analysis of analyte/substrate pairs requires a laser source with characteristics offered uniquely by a QCL Array. This is particularly true for time-evolving liquid chemical warfare agent (CWA) analysis. In addition to describing the QCL array source developed for long wave infrared coverage, a description of an integrated prototype standoff detection system is provided. Experimental standoff detection results using the man-portable system for droplet examination from 1.3 meters are presented using the CWAs VX and T-mustard as test cases. Finally, we consider three significant challenges to working with droplets and liquid films in standoff spectroscopy: substrate uptake of the analyte, time-dependent droplet spread of the analyte, and variable substrate contributions to retrieved signals.

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We report a portable broadband photoacoustic spectroscopic system for trace gas detection using distributed feedback quantum cascade laser arrays. By sequentially firing 128 lasers, our system acquires a photoacoustic spectrum covering 565 c m − 1 ( 935 − 1500 c m − 1 ) with a normalized-noise-equivalent-absorption coefficient of 2.5 × 10 − 9 c m − 1 W H z − 1 / 2 . The firing sequence that determines when and which laser to activate is programmable, which enables frequency-multiplexing excitation. For demonstration, 12 lasers are modulated simultaneously at distinct frequencies, and a photoacoustic spectrum is acquired within 13 ms. The compactness ( 28 c m × 17 c m × 13 c m , 3.5 kg) and low power consumption enable convenient installation for on-site monitoring.

We describe tunable vertical cavity surface emitting lasers with a half-symmetric cavity structure. The cavity is realized by inducing a curvature (R∼300 μm) in the top movable dielectric mirror. This half-symmetric microcavity forces lasing oscillations in a single fundamental spatial mode with a side-mode suppression ratio of >25 dB. Continuous wavelength tuning of 44 nm was achieved microelectromechanically with a tuning voltage of 14 V. The device operates near 950 nm with maximum power output of ∼0.9 mW.