Time and frequency resolved transient-absorption and stimulated-Raman signals of stochastic light.

Covariance spectroscopy signals based on the transmission of broadband stochastic probe light undergoing a nonlinear optical process with matter are studied. The resulting signal depends on intensity correlation functions of the probe pulse. Application is made to transient absorption and stimulated Raman signals, where an ultraviolet pump and a delayed stochastically modulated infrared or optical Raman probe are used for monitoring vibrational excitations. We show that the stochastic properties of light can be exploited to circumvent the limitations on the joint temporal and spectral resolution of the signal. We study a model system with a time-dependent frequency and show that the dynamical information can be fully extracted from the covariance signal. This information is heavily suppressed upon averaging of noisy signals and practically inaccessible in conventional stimulated Raman measurements.