**Four-wave mixing and CARS **

We investigate the unique two-photon correlation properties of broadband four wave mixing and their applications for coherent Raman spectroscopy and microscopy. In the nonlinear process of four wave mixing a pair of optical frequencies , (pump) are converted to another pair of frequencies , (signal and idler) such that . Quantum mechanically, this process converts a pair of pump photons to a pair of signal-idler photons, which are correlated in energy-time, similar (with very important differences) to entangled photon pairs generated by parametric down conversion.

This correlation is too short in time to be measured directly. In our lab we use sum-frequency generation as an ultrafast two-photon detector, capable of resolving both the tight time difference (down to 10s of fs) and energy-sum correlation (down to the pump linewidth ~0.3nm) between the beams, well below the actual time duration )several ps) and bandwidth (~100nm) of the signal-idler fields. We recently demonstrated time-energy coupling and splitting in the two-photon correlation that are unique to four waves mixing due to the special nonlinear matching conditions due to self and cross phase modulation effects. While these effects are minute compared to the signal idler spectrum / temporal intensity and cannot be observed directly, they are easily observed with an SFG correlation detector.

The narrowband pump laser generates an ultra broad four waves mixing spectrum (100nm wide signal and matching idler). In order to investigate the correlation properties of the signal-idler pairs we use non collinear sum frequency generation in a non-linear crystal (BBO) as a cross correlation detector. We measured the temporal correlation by recording the sum frequency generation signal in a spectrometer coupled to a cooled intensified CCD camera while scanning the relative signal-idler delay. The measurement results demonstrate the unique behavior of FWM, showing splitting of the SFG spectrum into two peaks (Paper , Poster).