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Project CROSS TRAP tackles the ICT objective of photonic components and subsystems for sensing for environment, well-being safety and security. The project aims at developing a versatile method for standoff chemical identification of trace amounts of airborne pollutants, such as biochemicals, bacterial threats and explosive materials that can be fingerprinted using their characteristic vibrational Raman spectral signatures.
The core idea of the proposal is to enable a free-space scheme for coherent anti-Stokes Raman scattering (CARS) in the direction exactly reversed with respect to an outgoing laser excitation, so that the probe beam can be arbitrarily pointed in any unobstructed direction and an enhanced backward propagating signal detected at the laser source using a LIDAR-type apparatus. The radical advantage, as compared to incoherent light probing techniques, lies in coherent enhancement, which implies that light fields are phase-matched, i.e. added in phase, so that the signal propagation is confined to very narrow solid angle and the signal magnitude scales quadratically with interaction length and the concentration of the resonantly vibrationally excited molecules.
Although this sounds like a perfect recipe for increasing the range, speed, and sensitivity of a chemically sensitive standoff detection method and/or decreasing the minimum required laser power, the scheme with a CARS signal returning from a free-space unobstructed laser beam has not been demonstrated yet because of a fundamental challenge: to initiate phase-matched generation in the backward direction, a fraction of the excitation light has to be reflected or re-scattered backwards. However, in natural atmospheric conditions, backward light scattering in air is extremely weak. The grand technological and scientific challenge in this proposal is to apply free-space CARS that would rely on the air itself as a diffuse back-reflector controllable through a reversible interaction with a strong field of an ultrashort intense laser pulse, the so-called light filamentation.