CARS Microscopy using linearly-chirped ultrafast laser pulses

We have developed a home-built CARS microscope which exploits linearly-chirped ultrafast laser pulses to achieve spectral focussing of the vibrational excitation. CARS Stokes pulses are generated by a Ti:Sapphire laser, which pumps an optical parametric oscillator (OPO). The intra-cavity frequency-doubled signal of the OPO provides the CARS Pump. By varying the delay time between pump and Stokes their instantaneous frequency difference (IFD) changes so that CARS spectra can be measured. In fact, since CARS is generated modulating the pump by the induced coherent vibration, a pump pulse arriving after the Stokes pulse optimizes the CARS signal and reduces the non-resonant CARS background. Our microscope also features two-photon fluorescence, second harmonic generation, and differential interference contrast imaging modalities. We investigated polystyrene beads in 2% agar solution by detecting the forward-collected CARS at the C-H aromatic band.