A Phase-Sensitive Detection Method Using Diffractive Optics for Polarization-Selective Femtosecond Raman Spectroscopy

A phase-sensitive detection method that uses two diffractive optics for femtosecond nonresonant Raman spectroscopy is demonstrated. One diffractive optic is used for generating the three input pulses for the third-order nonlinear experiment, and the second is used for recombining the signal with a passively phase-locked local oscillator derived from the probe pulse. This approach allows for phase-sensitive detection, direct phase calibration, control over all field polarizations, and removal of unwanted two-pulse signal contributions. Experiments on the intermolecular dynamics of CS₂ and CH₃CN demonstrate that the birefringent (in-quadrature) signal amplitude is significantly greater than the dichroic (in-phase) contribution. Polarization-selective measurements are used to project the isotropic birefringent response for CS₂, which suppresses reorientational dynamics and allows interaction-induced effects to be observed.