Coherently-enhanced lock-in-free chirped-CARS microscopy by notch filtering

Abstract

Coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy have many potential applications in biology and medicine. Among many variants of the technique itself, the method of single-pulse CARS spectroscopy and microscopy is attractive for its simplicity and quick implementation. Single-pulse CARS microscopy can be performed by shaping the excitation spectrum using a notch filter, yet the resonant signal rides on a large background caused by a non-resonant signal, a background which is usually removed by lock-in detection. Here, we show that the background can be reduced significantly by adding a small chirp to the pulse and can even be made smaller than the resonant signal. In order to enhance the CARS signal and thus the contrast further, double-notch shaping is introduced. The double-notch induces two sets of CARS features shifted by the frequency difference between the two notches, thereby coherently enhancing a particular CARS feature. The experimental results agree well with theoretical simulations. We applied this scheme to perform lock-in free CARS microscopy of bone tissue with enhanced contrast.