  We demonstrate a single-beam, standoff (>10m) coherent anti-Stokes Raman
scattering spectroscopy (CARS) of various materials, including trace amounts of
explosives and nitrate samples, under ambient light conditions. The multiplex
measurement of characteristic molecular vibrations with <20cm-1 spectral
resolution is carried out using a single broadband (>550cm-1) phase-shaped
femtosecond laser pulse. We exploit the strong nonresonant background signal
for amplification of the weak backscattered resonant CARS signal by using a
homodyne detection scheme. This facilitates a simple, highly sensitive
single-beam spectroscopic technique, with a potential for hazardous materials
standoff detection applications.
