Knowledge Bank

By choosing $\omega 2$ (4880 {\AA}) preresonant with the $S1-S0$ transition of rhodamine 6G, and $\omega 1$ (4537 {\AA}) resonant with the $S2-S1$ absorption, the resonant CARS spectra for lasing rhodamine 6G in methanol, ethanol and acetonitrile have been observed in the $1500 - 1650;cm^{-1}$0 range. Strong positive bands, attributed to CARS from the $S1$ state, replace the negative ground state preresonant bands as the $S0$ state saturates at high laser beam power densities. A water solution behaves quite differently, possibly as a result of strong association of water with rhodamine in the excited state.