Spectral focusing: High spectral resolution spectroscopy with broad-bandwidth laser pulses

In this work, we show how broad-bandwidth femtosecond pulses can be used to achieve high spectral resolution in nonlinear spectroscopy and microscopy. Our approach is based on chirping the excitation pulses in order to focus their entire bandwidth into a narrow spectral region. We show that spectral features which are 100 times narrower than the excitation light can be resolved with this simple spectral focusing. The gain in spectral selectivity and sensitivity makes its application to nonlinear microscopy very convenient. This is demonstrated with diffraction-limited coherent anti-Stokes Raman scattering microscopy.