High spectral resolution multiplex CARS spectroscopy using chirped pulses

摘要

A novel technique for achieving high spectral resolution with a femtosecond laser system is presented. Transform-limited 800 nm, 90 femtosecond (fs) pulses pass off two gratings, stretching the pulse in time to a pulse width of several picoseconds due to an induced linear temporal chirp directly proportional to the grating separation. This chirped pulse is the degenerate pump (ω_p) and probe (ω_p) pulse for the CARS experiment. When overlapped in time with the 1050 nm, 90 fs transform-limited Stokes (ω_s) pulse, only a fraction of the chirped ω_p pulse generates the CARS signal, thereby creating a temporal slit that defines the spectral resolution of the technique. Spectra for liquid methanol and liquid isooctane are presented, with ～6 cm~(-1) spectral resolution achieved for isooctane. Resonance enhancement and the mechanism of achieving high spectral resolution are shown by adjusting the ω_s wavelength and ω_p delay relative to the cos pulse.