Recent trends in laser-based standoff detection of hazardous molecules

We report here the recent developmental studies in the field of laser-based standoff detection of hazardous molecules, including explosives. Continuous wave lasers to high intensity pulsed lasers and robust quantum cascade lasers that can be tuned widely have been used extensively in the laser-based techniques. We summarize the most important recent advances in the fields of (i) Raman spectroscopy (ii) laser induced breakdown spectroscopy (LIBS) (iii) laser induced fluorescence (LIF) (iv) quantum cascade laser (QCL)/external cavity quantum cascade laser (EC-QCL)-based infrared spectroscopy (v) photoacoustic spectroscopy and (vi) terahertz spectroscopic/imaging studies especially towards the detection of hazardous materials and particularly in standoff configurations. With the improvements in the technology of laser sources development, sensors/detectors, and the availability of the spectroscopic database of various hazardous compounds/materials, the standoff techniques have evolved rapidly in the last few years. From small range and limited detectable compounds with low sensitivity, limited only to laboratory conditions, to wide range of compounds and high sensitivity along with open atmosphere long range and range adaptable scenarios are thoroughly discussed in this review. We account here the different lasers used for the detection and distances at which the signatures of various molecules were identified along with the sensitivity of each experiment. The advantages and limitations of the various techniques are also narrated. Finally, a roadmap for the future is chalked out taking into account the strengths of each of these techniques.