Heterodyne single‐beam CARS microscopy

In single‐beam coherent anti‐Stokes Raman spectroscopy (CARS), a complete four‐wave‐mixing scheme is accomplished within a single pulse from an ultrashort femtosecond oscillator. Spectral information is achieved by coherent control of the non‐linear signal generation using phase modulation of the excitation with a pulse shaper. This single‐beam approach can be even extended to heterodyne detections schemes wherein a strong local oscillator (LO) field, also generated by pulse shaping from the same excitation laser, interferes with the CARS signal. Here, we explore the potential of this promising technique. A careful optimisation of the experimental scheme is presented together with an analysis of the different relations between LO field and CARS signal intensity, discovering an optimal LO field at a fraction of about 10 −4 compared to the overall CARS excitation energy. With the optimised set‐up, an absolute number of molecules as low as 5 × 10 6 corresponding to concentrations in the attomole regime in the focal volume can be detected. Using such an optimal parameterisation, heterodyne single‐beam CARS based on a broadband laser source is introduced to microscopic applications, combining chemical selectivity and a high spatial resolution of the non‐linear CARS process with the sensitivity of the heterodyne approach. Copyright © 2009 John Wiley & Sons, Ltd.