Photoacoustic remote sensing of suspicious objects for defence and forensic applications.

Quantum cascade laser (QCL) based photoacoustic spectroscopic technique has been developed for detection of hazardous molecules contaminants/adsorbed on surfaces such as plastic and cloth from short standoff distances. The laser source and detection system is integrated together in a single unit. Spectra were recorded for traces of various molecules in mid-infrared spectral band (1130-1430 cm-1) from distance of 0.5 m. Pulsed quantum cascade laser source, modulated at 25 kHz and 42 kHz frequency was used to detect molecular species adsorbed on surfaces of cloth and plastic. Ultrasonic microphones operating at 25 and 42 kHz resonant frequencies were used as detectors. The photoacoustic spectra of hazardous chemicals, explosive materials and bio-chemicals such as acetic acid, PETN (pentaerythritol tetranitrate), DPA (dipicolinic acid) in very low quantities were recorded. Sensitivities of 5 and 10 μg/cm2 of these analytes were achieved with frequencies of 25 and 42 kHz respectively at distance of 0.5 m. In the present technique there was no interference of audio frequency (20 Hz to 20 kHz) and bright sunlight. The technique can be applied for screening of suspicious objects for homeland security and forensic applications. The present spectroscopic technique can be developed in man portable standalone product.