The vibrationai spectrum of molecules can be used to detect hazardous materials at standoff distances. One of the best optical pathways for revealing a molecule's vibrationai spectrum is through coherent anti-Stokes Raman scattering (CARS) nonlinear spectios-copy. In CARS, a molecule's vibrationai modes are excited into motion by the simultaneous interaction of pump and Stokes photons. This excitation is subsequently probed by a narrow-frequency probe beam, and the vibiational level energies are resolved by measuring the amount of blue-shift of the scattered anti-Stokes photons.
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