Contributed Review: Quantum cascade laser based photoacoustic detection of explosives J. S. Li, B. Yu, H. Fischer, W. Chen, and A. P. Yalin Citation: Review of Scientific Instruments 86, 031501 (2015); doi: 10.1063/1.4916105 View online: http://dx.doi.org/10.1063/1.4916105 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/86/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Short-lived species detection of nitrous acid by external-cavity quantum cascade laser based quartzenhanced photoacoustic absorption spectroscopy Appl. Phys. Lett. 106, 101109 (2015); 10.1063/1.4914896 Intracavity quartz-enhanced photoacoustic sensor Appl. Phys. Lett. 104, 091114 (2014); 10.1063/1.4867268 Hydrogen peroxide detection with quartz-enhanced photoacoustic spectroscopy using a distributed-feedback quantum cascade laser Appl. Phys. Lett. 104, 041117 (2014); 10.1063/1.4863955 Detection of ultrafast laser ablation using quantum cascade laser-based sensing Appl. Phys. Lett. 101, 171107 (2012); 10.1063/1.4764115 Demonstration of a self-mixing displacement sensor based on terahertz quantum cascade lasers Appl. Phys. Lett. 99, 081108 (2011); 10.1063/1.3629991

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REVIEW OF SCIENTIFIC INSTRUMENTS 86, 031501 (2015)

Contributed Review: Quantum cascade laser based photoacoustic detection of explosives J. S. Li,1,a) B. Yu,1 H. Fischer,2 W. Chen,3 and A. P. Yalin4 1

Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of Education, Anhui University, Hefei, China 2 Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany 3 Laboratoire de Physicochimie de l’Atmosphére, Université du Littoral Côte d’Opale, Dunkerque, France 4 Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523-1374, USA

(Received 21 August 2014; accepted 8 March 2015; published online 30 March 2015) Detecting trace explosives and explosive-related compounds has recently become a topic of utmost importance for increasing public security around the world. A wide variety of detection methods and an even wider range of physical chemistry issues are involved in this very challenging area. Optical sensing methods, in particular mid-infrared spectrometry techniques, have a great potential to become a more desirable tools for the detection of explosives. The small size, simplicity, high output power, long-term reliability make external cavity quantum cascade lasers (EC-QCLs) the promising spectroscopic sources for developing analytical instrumentation. This work reviews the current technical progress in EC-QCL-based photoacoustic spectroscopy for explosives detection. The potential for both close-contact and standoff configurations using this technique is completely presented over the course of approximately the last one decade. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916105]

TABLE OF CONTENTS

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. PROPERTIES OF EXPLOSIVES . . . . . . . . . . . . . . . . A. Physical and chemical properties . . . . . . . . . . B. Spectral properties . . . . . . . . . . . . . . . . . . . . . . . III. PAS AND QCL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Principles of PAS . . . . . . . . . . . . . . . . . . . . . . . . B. Overview of QCL . . . . . . . . . . . . . . . . . . . . . . . . IV. EXPLOSIVES DETECTION BY QCL BASED PAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Extractive sampling . . . . . . . . . . . . . . . . . . . . . . B. Standoff detection . . . . . . . . . . . . . . . . . . . . . . . . C. Effects of field environment . . . . . . . . . . . . . . . V. CONCLUSIONS, CHALLENGES, AND PERSPECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . .

1 2 2 2 3 3 3 4 4 5 6 6 7

I. INTRODUCTION

Fast, effective detection and identification of explosive materials (or illicit substances and chemical warfare) in solid, liquid, or vapor phase has become a key security issue in modern society, especially at public places like airports, railway, or coach stations. Detection of explosives is a very high challenging task because of a number of factors, such as the low vapor pressures of most explosives,1 frequent

a)Author to whom correspondence should be addressed. Electronic

mail: [email protected]. Tel.: +86-551-63861490. Fax: +86-55163861490.

introduction of novel explosive compositions, and concealment and weapon delivery schemes, as well as requirement of non-destructive inspection process and operator safety. Therefore, instrumental techniques for security applications should be rapid (measurements in a few seconds), selective (a low number of false positives and false negatives), sensitive (parts-per-billion, ppb, even sub-ppb), and subject to minimal interference in the presence of numerous atmospheric species. Many well-known explosive detection techniques such as mass spectrometry, chromatography, and ion mobility spectrometry rely on close-contact sampling of surface residues or explosive vapors.2–4 Effective detection of explosive materials using laser-based methods has been demonstrated in closecontact and standoff (tens of meters) configurations. The optical techniques appear to have the greatest potential for remote-detection or standoff capability. An effective standoff explosives detection capability would save lives and prevent losses of mission-critical resources by increasing the distance between the explosives and the intended targets and/or security forces. To date, a variety of laser-based spectroscopy techniques have been investigated as means of identifying and characterizing explosive materials. The literature on this topic to 1998 was summarized by Steinfeld and Wormhoudt5 and by Katon.6 Instrumentation for explosives detection to 2004 was summarized by Moore.7 Detection of peroxide-based explosives to 2009 was reviewed by Burks and Hage.8 Photonic sensor devices for detection of explosives in gas phase and condensed phase as well as the underlying spectroscopic techniques were briefly presented by Willer and Schade.9 Moreover, an excellent overview of established and new methods (such as laser induced break down spectroscopy, Raman spectroscopy,

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laser induced fluorescence spectroscopy, and IR spectroscopy) for standoff detection explosives to 2009 was made by Wallin et al.,10 and advances of spectroscopic approaches between 2006 and 2011 were done by Caygill et al.11 Most recently, the recent trends and developments of Raman spectroscopy and Fourier transform infrared (FTIR) applied to the identification of explosives were very nicely summarized by María and Carmen.12 This review focuses on recent developments in photoacoustic spectroscopy (PAS) for detection of trace amounts of explosives utilizing novel quantum cascade lasers (QCL)laser sources, with the main emphasis on the literature from 2007 to the present, based on results published in journals and proceedings. Recent novel developments are highlighted and future trends are discussed. No current reviews specifically cover this topic, but some may be of interest for readers, such as reviews on QCLs in chemical physics,13 QCLs in atmospheric chemistry,14 QCLs for breath gas analysis,15–18 and plasma chemistry.19 Additionally, certain reviews on the use of QCLs in infrared spectroscopy and analytical devices are relevant,20,21 as is the book chapter dealing with mid-infrared (MIR) coherent sources and applications.22,23

II. PROPERTIES OF EXPLOSIVES A. Physical and chemical properties

Before discussing about explosives detection, it is necessary to understand the physical and chemical properties of explosives. Explosives are chemical compounds that can be initiated to undergo self-propagating decomposition resulting in the sudden release of heat and pressure. Generally, explosives are classified into primary and secondary based on their susceptibility to initiation. Primary explosives, which include lead azide and lead styphnate, are highly susceptible to initiation. Secondary explosives, which include trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX or cyclonite), high melting explosives (HMX), and tetryl, are much stable and require primary explosives to initiate explosion. According to their chemistry, explosives are organic compounds and can be classified into seven broad classes:24

(1) Aliphatic nitro compounds, such as nitromethane and hydrazine nitrate. (2) Nitroaromatic compounds, such as TNT, dinitrobenzene (DNB), hexanitrostilbene, and picric acid. (3) Nitramines or nitrosamines, such as octogen (HMX) or RDX. (4) Nitrate esters, such as pentaerythritol tetranitrate (PETN), ethylene glycol dinitrate (EGDN), nitroglycerine, and nitroguanidine (NQ). (5) Acid salts, such as ammonium nitrate. (6) Organic peroxides, such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD). (7) Precursors of explosives, such as acetone a precursor to TATP and 2,4-Dinitrotoluene (DNT) or dinitro, a precursor to TNT, and so on. These compounds are often used in combination, for example, the commercial explosive SEMTEX is composed of the active ingredients RDX and PETN along with plasticizers. Some explosives like TATP and EGDN have high vapor pressures even at room temperature which could facilitate vapor phase detection. However, most common explosives have extremely low vapor pressures at ambient temperature, as shown in Figure 1.25 It should be noted that the vapor pressures of explosives increase rapidly with temperature.26 Unlike most other high explosives, TATP contains no nitrogen or nitrates. B. Spectral properties

Almost all explosive chemicals exhibit strong, characteristic absorbance patterns in the MIR spectral range.27 However, water vapor, which occurs in high concentrations in air and absorbs through much of the IR, was found to be the main interferent. Transparency of the atmosphere is another crucial prerequisite for detection of trace amounts of explosives utilizing optical spectroscopic techniques, especially for open path detection. Figure 2 shows the mid-IR transmission spectrum for humid air, TNT, and RDX.28 Since they share a common functional -NO2 group, these explosives exhibit similar absorption spectral features including the band at 6.25 µm associated with the anti-symmetric N–O stretch, and where

FIG. 1. Vapor pressure and structural formulas of typical explosive compounds. Reprinted with permission from Hildenbrand et al. Proc. SPIE 7222, 72220B (2009). Copyright 2009 The Society of Photo-Optical Instrumentation Engineers. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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FIG. 2. MIR transmission spectra of air, TNT, and RDX. Shared absorption bands are highlighted. Reprinted with permission from Furstenberg et al. Appl. Phys. Lett. 93, 224103 (2008). Copyright 2008 AIP Publishing LLC.

humid air is transparent, which would allow propagation over long distances. In the TNT molecule, -NO2 groups are bonded to the aromatic ring; in the RDX molecule, -NO2 groups are bonded to nitrogen. The spectrum of TNT is characterized by a strong absorption at 7.41 µm, corresponding to excitation of the symmetric stretch of the -NO2 groups. Several other absorption features in common can also be found from this figure. In 1997, infrared absorption band strengths for TNT, RDX, and PETN were experimentally reported by Janni et al.29 using Fourier transform spectrometer, and compared with theoretical calculations. Most recently, infrared spectra of tetryl, RDX, and PETN with high signal-to-noise ratio (SNR) were reported using external cavity quantum cascade lasers (EC-QCLs) based scattering-type scanning near-field optical microscopy (s-SNOM) between 7.1 and 7.9 µm wavelength range.30

III. PAS AND QCL A. Principles of PAS

The origins of PAS date back to the discovery of the photoacoustic (PA) or optoacoustic effect by Alexander Graham Bell in 1880.31 He found that when a beam of sunlight rapidly interrupted with a rotating slotted disk was focused onto thin diaphragms, sound was emitted. The absorbed energy from the light is transformed into kinetic energy of the sample by energy exchange processes. This results in local heating and thus a pressure wave or sound. A PA spectrum of a sample can be recorded by measuring the sound at different wavelengths. PAS can be applied to solids, liquids, and gases and is only sensitive to sample absorption, not scattering losses. PAS is a very promising technique for trace gaseous species detection.32 In the traditional PAS experiment, sample materials in a closed cell are irradiated using either a pulsed or

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FIG. 3. Principle of photoacoustic effect.

a modulated light source. The sample species absorb the light and are thermally excited, causing expansion and contraction of the gas within the cell in synchronization with the modulation frequency or laser pulse repetition frequency. The resulting dynamic motion of the gas creates an acoustic pressure front that can be monitored by microphone or piezoelectric crystal. In generally, it can be summarized as Figure 3. After more than 100 years, PAS has gradually matured in utility with significant improvements in light sources and modulators, in wavelength tunability, and in PA signal transducers, e.g., cantilever enhanced PAS and quartz-enhanced (QE) PAS,33,34 which have the potential for compact, robust, highly selective, highly sensitive, large dynamic range, and relatively low-cost trace gas analysis. Furthermore, because the acoustic properties of the PA system (e.g., acoustic detector) do not depend on the spectral distribution of the absorbed laser radiation, PAS instruments are optically broadband devices, that is, wavelength independent. B. Overview of QCL

MIR spectroscopy has proven to be a powerful platform for designing applications like breath analysis, environmental monitoring, airborne measurements, security applications, isotope ratio measurements, and many other disciplines. Especially for security applications, optical methods are advantageous because of their capability for standoff detection increasing the distance between the explosives and the possible victims, analyzing equipment and operators. Spectroscopy analysis of larger molecules requires a broader tuning range and is thus traditionally the domain of FTIR spectroscopy. However, the development of tunable external cavity (EC) QCLs is starting to change this situation.35 QCLs known as ideal spectroscopic sources are attributed to their wavelength range, high laser power, narrow linewidth, wide tunability, and low beam divergence. The original idea of a solid-state semiconductor laser with a superlattice was

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proposed by Kazarinov and Suris in 1971,36 with the first experimental demonstration of a pulsed QCL at cryogenic temperature in 1994 at Bell Laboratories.37 Since the first continuous wave (CW) demonstration in 2002 (about 10 mW of power), QCLs have experienced rapid and dramatic improvements in power, efficiency, and wavelength range. The first commercialization of a QCL product was in 2004,38 and broadly tunable EC-QCL was first commercialized in 2006.39 By carefully designing the quantum wells, lasing has been achieved at wavelengths as short as 2.75 µm and as long as 3000 µm (i.e., 0.1 Terahertz (THz)).40 The longer wavelength devices require cryogenic cooling still, but room temperature operation has been observed to at least 16 µm. Interest has concentrated in the mid-infrared (3.5-13 µm) and the THz spectra (2-5 THz). THz QCLs are particularly attractive for a variety of security and public safety applications because many materials, such as clothing, plastics, biological materials, and packaging materials, are transparent (or semitransparent) at terahertz frequencies. QCLs are novelty semiconductor lasers that emit from the mid- to THz bands and are finding numerous new applications in spectroscopic fields.14 The wide tuning range and fast response time of QCLs allow for faster and more precise compact trace element detectors and gas analyzers that are replacing slower and larger FTIR, mass spectroscopy, and chromatographic systems. Indeed, QCL technology is still an active area of research, and the number of annual publications on QCLs and their applications are still increasing. For an indepth review of recent advances in QCL technology, the reader is referred to other papers of particular interest.41–46

IV. EXPLOSIVES DETECTION BY QCL BASED PAS

PAS offers unique capabilities for investigating gaseous and condensed phase samples due to its simplicity and resistance to scattering effects, which is often an issue associated with other common techniques such as Raman and IR reflectance spectroscopy. Previously, CO and CO2 lasers have been widely used for PA spectroscopic detection47–50 of vapors of explosives due to their high output power. However, both of these laser sources are step tunable between 9 and 11 µm, and neither of the lasers is able to access the strong absorption features of TNT that lies in the 6.0-7.5 µm region. Recently, the small size, simplicity, high output power, wide tunable frequency range, and long-term reliability of EC-QCLs could make QCL-PAS a more desirable technique for the detection of explosives. Most work on developing explosives sensors based on PAS and the use of novel QCLs as light sources was carried out by Patel’s group at Pranalytica, Inc. (also at University of California), Thundat’s group at Oak Ridge National Laboratory (ORNL, USA) and Choa’s group at University of Maryland, as well as their collaborators. The former mostly focused on analysis of explosives by sampling the vapor into the PA cell. Two different strategies were employed by the latter two groups using standoff detection of explosives. An overview of explosives detection mainly from these representative groups are presented in Sec. IV.

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A. Extractive sampling

The Patel’s group has been engaged over 10 years on developing high sensitivity laser sensors for the detection of chemical warfare agents, explosives, and industrial and environmental pollutants. In the beginning, by employing PAS with broadly tunable CO2 laser as a radiation source and resonant PA cell, they demonstrated sensitivity detection of chemical warfare agents and toxic industrial chemicals, which commonly need long measurement time. Actually, in this case, the PA cell works as an acoustic amplifier, and the absorbed light power is accumulated in the acoustic mode of the resonator; this effect embodied with the quality factor Q of the acoustic resonator, typically between 10 and 50 for the longitudinal resonator but can reach up to over 100 depending on the geometrical design of the PA cavities.33 In 2006, researchers at Patel’s group51 first demonstrated high-sensitivity detection of TNT by using a resonant PA cavity with a longitudinal resonant frequency of ∼1800 Hz and a grating cavity QCL, which is continuously tunable over 400 nm around 7.3 µm and produces a maximum continuouswave power of 200 mW. Practical applicability of the method might be impaired by interferences with surrounding gases. Therefore, a detailed analysis of laser PAS techniques for the detection of chemical warfare agents and toxic industrial chemicals in real-world conditions was discussed by this group.52 The limited tuning range of QCLs, typically 150200 nm, is a limitation on a single QCL tunable source. To cover as much as possible of the relevant mid-IR spectral region using a single EC-QCL unit, several EC-QCLs emitting at different wavelengths can be combined internally and placed in one housing (see Figure 4). In 2008, Patel et al. have multiplexed as many as five QCLs, each centered at a different wavelength to create a source that provides high power (>300 mW) tunable radiation over broad spectral regions covering ∼4 µm, from 6 µm to 10 µm.53–55 The combination of high power QCL and high sensitivity PA detection scheme has permitted to simultaneously detect multi explosives including military explosives such as TNT, RDX, and PETN, homemade explosives such as TATP and its precursor acetone, HMTD, and commercial explosives such as dynamite.56 Switching from one laser to another can be finished in less than 1 s. Detection of TNT at a level of 0.1 ppb, TATP at approximately 1 ppb, ∼20 ppb of dimethyl methyl phosphonates (DMMP), ∼30 ppb of acetone, and ∼40 ppb of EGDN by use of multiple wavelengths and reduced pressure in PA cell was realized. The list of their research documents relevant to trace gases, chemical warfare agents, and explosives detection by PAS can be found in their homepage: http://www.pranalytica.com. Integration of this technique with walk-through portal devices may be possible in the future. In addition, Holthoff et al.57 at Army Research Laboratory (USA) first reported study detailing the use of PAS, employing continuously tunable QCLs emitting from 9.3 to 10 µm and a micro-electro-mechanical systems (MEMS)-scale PA cell, for the simultaneous detection and molecular discrimination of numerous molecules of interest. Exceptional agreement between the measured PA spectra and the infrared spectra for acetic acid, acetone, 1,4-dioxane, and vinyl acetate was

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FIG. 4. Scheme (left) and photograph (right) for multiplexing five external grating cavity tuned QCLs for covering a broad spectral range for PA detection of chemical warfare agents and explosives. Reprinted with permission from Mukherjee et al. Appl. Opt. 47, 4884-4887 (2008). Copyright 2008 The Optical Society of America.

observed. A minimum detection level of 20 parts-per-billion (ppb) for DMMP was reported.58 B. Standoff detection

The traditional PAS is not suitable for standoff applications because of the requirement for sampling the analyte in the specialized PA cell. Standoff detection defined as where equipment and operator stay away from the sample is mostly based on optical spectroscopic techniques such as IR spectroscopy, laser induced breakdown spectroscopy, laser-induced fluorescence, Lidar, Raman, and so forth because spectroscopic techniques offer very high selectivity. Since standoff techniques do not require collecting sample, they are ideal for detection of low vapor pressure analytes, such as explosives. With laser based standoff spectroscopies, the detection distance can be tens of meters. Because of the inverse square dependence of light intensity, larger distances require high power light sources. For homeland security applications such as detection of suicide bombers or improvised explosive devices, a distance of 50-100 m is generally sufficient. One earlier study done by Brassington, who called it PADAR (photoacoustic detection and ranging), demonstrated standoff gas detection and ranging using PA effect.59 Range resolution of better than 10 mm can be obtained with a probable maximum range of 100 m using an optical parametric oscillator (OPO) for methane detection. Later, Yönak and Dowling60 presented leak detection and localization system based on PA effect which could perform standoff sulfur hexaflouride (SF6) gas detection using carbon dioxide (CO2) laser. However, a recently developed variant of the PAS technique has exhibited strong potential for use as a standoff detector for explosive materials. In 2007, the researchers at ORNL and collaborators reported a variation of PAS which involves illuminating the target sample with a pulsed QCL and using a quartz crystal tuning fork (QCTF) as the acoustic detector,61 QCTF based PAS technique was first proposed by the Tittel’ group at Rice University in 2002 and called QE PAS.62 The pulse frequency of the illuminating light is modulated with the mechanical resonant frequency of the QCTF, generating acoustic waves at the tuning fork’s air/surface interface. This produces oscillating localized pressures which drive the tuning fork into resonance. The amplitude of this vibration is proportional to

the intensity of the scattered or reflected light beam falling on the QCTF. Due to the material nature of quartz, a piezoelectric voltage is produced as the QCTF vibrates. The light stimulating the QCTF is diminished by the amount absorbed by the target, and the contribution of the residue is determined by subtraction of the substrate background. Using this technique, they reported sensitive and selective detection of surface adsorbed compounds such as tributyl phosphate (TBP) and three explosives (RDX, TNT, and PETN) at standoff distances ranging from 0.5 to 20 m, with a detection limit on the order of 100 ng/cm.2 Moreover, they also demonstrated standoff detection of trace quantities of surface adsorbed chemicals using two QCLs operated simultaneously (see Figure 5), with tunable wavelength windows that match with absorption peaks of the analytes,63 and anticipate that a better identification of absorbance may be accomplished by illuminating the sides of both tines of QCTF, yet only exposing one tine to the analyte.64 Recently, the ORNL researchers and collaborators demonstrated this probing technique combining with UV photodecomposition of explosive residues on a surface,65,66 which allows to obtain a reference spectrum with very low baseline fluctuation. In addition, researchers at Laser Science

FIG. 5. Diagram of the experimental setup based-on two pulsed QCLs. Reprinted with permission from Van Neste et al. Anal. Chem. 81, 1952-1956 (2009). Copyright 2009 The American Chemical Society. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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FIG. 6. Photograph of the actual experimental setup in the laboratory for standoff photoacoustic chemical detection. Reprinted with permission from Chen et al. Appl. Opt. 52, 2626-2632 (2013). Copyright 2013 The Optical Society of America.

and Technology Center in India67 have developed a portable trolley mounted explosives detection system based on QCTFPAS technique for field applications. By using an ellipsoidal mirror to collect the backscattered laser radiations and utilizing a motorized linear translation stage to allow the detector to remain at the focal point for varying target distance, standoff distance of up to 25 m was realized with a SNR of 10, while theoretical calculation shows that standoff QEPAS signal from aluminum and wood surfaces can be detected from a distance of up to 50 m for incident laser power of 12 mW. Most recently, a researcher group at University of Maryland (USA) led by Professor Choa demonstrated standoff PA detection of isopropanol (IPA) vapor and TNT in solid phase using pulse QCLs operated at 7.9 µm (1000 cm−1, www.blockeng.com), and long-wave IR bands (such as terahertz), which provide critical requirements for developing new types of portable, sensitive, selective, real-time gas, and explosive analyzers to meet the challenges of modern atmospheric and environmental applications.73,74 Widely tunable laser sources will expand the capabilities to encompass the standoff detection of broadband absorbers and complex molecular mixtures. Finally, the rapid development of modern microelectronic and micromechanical fabricating technologies makes highly sensitive acoustic detectors technically and economically more promising;75 thus, the costs of constructing a QCL based PA explosives detectors are expected to be reduced significantly.

12L.

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Ebrahim-Zadeh and I. T. Sorokina (Springer, München, 2008), pp. 535-555. 18M. R. McCurdy, Y. Bakhirkin, G. Wysocki, R. Lewicki, and F. K. Tittel, “Recent advances of laser-spectroscopy-based techniques for applications in breath analysis,” J. Breath Res. 1, 014001 (2007). 19S. Welzel, F. Hempel, M. Hübner, N. Lang, P. B. Davies, and J. Röpcke, “Quantum cascade laser absorption spectroscopy as a plasma diagnostic tool: An overview,” Sensors 10, 6861-6900 (2010). 20J. Hodgkinson and R. P. Tatam, “Optical gas sensing: A review,” Meas. Sci. Technol. 24, 012004 (2013). 21L. Bush, B. Lendl, and M. Brandstetter, “Quantum cascade lasers for infrared spectroscopy: Theory, state of the art and applications,” Spectroscopy 28, 26-33 (2013), available online at http://www.highbeam.com/doc/ 1P3-2981145761.html. 22F. K. Tittel, G. Wysocki, A. A. Kosterev, and Y. Bakhirkin, “Semiconductor laser based trace gas sensor technology: Recent advances and applications,” in Mid-Infrared Coherent Sources and Applications, edited by M. EbrahimZadeh and I. T. Sorokina (Springer, Berlin, 2008), pp. 467-493. 23Ch. Mann, Q. K. Yang, F. Fuchs, W. Bronner, R. Kiefer, K. Köhler, H. Schneider, R. Kormann, H. Fischer, T. Gensty, and W. Elsäßer, “Quantum cascade lasers for the mid-infrared spectral range-devices and applications,” ACKNOWLEDGMENTS in Advances in Solid State Physics, edited by B. Kramer (Springer, Berlin, 2003), pp. 351-368. This work was supported in part by Anhui University 24L. Senesac and T. G. Thundat, “Nanosensors for trace explosive detection,” personnel recruiting project of academic and technical leaders Mater. Today 11, 28-36 (2008). 25J. Hildenbrand, J. Herbst, J. Wöllenstein, and A. Lambrecht, “Explosive (Grant No. 10117700014), the Natural Science Fund of Anhui detection using infrared laser spectroscopy,” Proc. 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