Research Project: Ultrafast Spectroscopy for Stand-off Detection of Explosive Devices
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- Zheltikov, Alexey
- Kulatilaka, Waruna
- Yakovlev, Vladislav
- Scully, Marlan
- Svidzinsky, Anatoly
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Abstract or Project Summary
The major goal of this project is to develop new approaches to real-time detection of explosives, based on remote atmospheric superradiance and lasing, and stand-off coherent Raman spectroscopy. Our enhanced technique will allow rapid remote spectroscopic detection of chemicals, with the possibility of fast scanning of a wide area for Improvised Explosive Devices (IED's) and related trace chemicals. The ultimate target species are specific chemical constituents of common "home-made" devices originating from materials such as fertilizer, gunpowder and hydrogen peroxide. For this purpose, we will develop methods and techniques based on approaches we have previously used for remote identification and sensing of chemicals, on the ground and in the atmosphere. The techniques developed are expected to have sufficient sensitivity for detecting explosive chemical signature of substances such as ammonium nitrate, triacetone triperoxide, ethylene glycol, and urea nitrate, for example, at a safe remote distance. As an ultimate goal, we envision the possibility of producing a short-pulse fiber laser -based device that can be reliably deployed in the field. Implementation in the field may utilize spectrum databases and algorithms to quickly determine the constituent molecules and atoms in a sample under investigation. Moreover, a high-energy laser-based device may provide means initiating ignition and detonation of detected explosives from a safe distance. Extension of nonlinear spectroscopic techniques to the stand-off detection mode would enhance the optical return and permit the extraction a highly directional species-selective signal without the trade-off between the detection distance and input laser power that occurs with incoherent techniques. Stand-off optical sensing requires that the wave-vector of one or more impinging or generated laser fields is inverted, so that the beam propagates back towards the observer, providing an optical signature of the target volume. In dense optical media strong nonlinearities enable artificial nonlinear mirrors, based for example on phase-conjugation schemes or on plasma reflection. In low-density gaseous media, on the other hand, in the absence of back-scattering/-reflection at hard surfaces, it is extremely challenging to achieve wave-vector inversion, producing a coherent beam that retraces its way back to the laser. This difficulty is due to momentum conservation which strongly prohibits wave-vector reversal, while the optical nonlinearities in the gas phase are too weak to enable it. To acquire a coherent stand-off signal from a gas volume without the use of back-scattering/reflecting surfaces, one needs to provide a spatially-coherent back-propagating beam originating from the gas itself. Hence the objective of the proposed work is to develop a laser-like source of directional radiation from a remotely-pumped region of air, that will provide a coherent probe beam for nonlinear spectroscopy of target species. We aim to consider and evaluate backward air-lasing, superradiance, and quantum amplification by superradiant emission of radiation (QASER). The utilization of the bright coherent backward-propagating optical beam in remote spectroscopy applications will result in a dramatic improvement of the detection sensitivity compared to the standard light detection and ranging techniques. We will use nonlinear spectroscopy and supplement the backward-generated light by additional laser pulses sent in the forward direction. The end goal is to induce a coherent, directional, signal beam that carries the molecular fingerprint straight back toward the detector. Molecular excitations produced by pairs of photons - one from the backward air-laser beam and the other from the forward interrogation laser - provide a sensitive molecular detection mechanism. Examples include two-photon absorption (TPA ) and stimulated Raman scattering (SRS). For gas molecules, both TPA and SRS will show species-specific (fingerprint-like) structure of vibrational and rotational energy levels.
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