Shaped femtosecond pulses for remote chemical detection

A single-beam coherently controlled femtosecond pulse coherent anti-Stokes Raman scattering (CARS) technique for remote detection and identification of minute amounts of solids and liquids at a standoff distance is demonstrated. Multiplex measurement of characteristic molecular vibrations using a single broadband phase-shaped femtosecond laser pulse, which supplies the pump, Stokes, and probe photons, is also carried out. A narrowband probe within the wide pulse spectrum is selected and a spectroscopic resolution or more than 30 times better than the pulse bandwidth is maintained. The coherent nature of nonresonant signal is harnessed to amplify the weak, background-scattered vibrationally resonant radiation by interfering them together coherently. Fetosecond CARS spectroscopy is found to exhibit higher efficiency at low energy powers compared to the longer pulses used in conventional CARS technique.