Chirped time-resolved CARS microscopy with square-pulse excitation.

Time-resolved coherent anti-Stokes Raman scattering (T-CARS) microscopy is a technique known for suppressing non-resonant background by utilizing the different temporal responses of virtual electronic transitions and Raman transitions. However, the previous use of femtosecond excitations in T-CARS microscopy has led to low spectral resolution and difficulty in selectively exciting a single Raman band. Here, we report an improved T-CARS imaging technique with chirped pump and Stokes excitations, and the Stokes pulses were shaped into square pulses. Using a femtosecond probe, we acquired T-CARS images with a high spectral resolution, suppressed non-resonant background, and high resonant signal level. We experimentally demonstrated the selective excitation of two close Raman bands of polystyrene at 1005 cm(-1) and 1035 cm(-1) using our technique; conventional T-CARS would inevitably excite them both with little selectivity. Our novel technique could become an ideal method for high-sensitivity, background-free imaging of single Raman bands for a wide variety of samples.