Shot-noise limited detection sensitivity in multiplex CARS microscopy

In multiplex CARS microscopy the generated anti-Stokes signal is generated and detected simultaneously over a significant part of the vibrational spectrum. The signal-to-noise ratio of the thus detected spectra is limited only by shot-noise. This principle is demonstrated using a dilution series of 2-propanol in water. It is derived theoretically and shown experimentally that for low solute concentrations - in contrast to methods that suppress the non-resonant background - the CARS signal strength from a particular vibrational mode depends linearly on its concentration. Furthermore, excellent agreement is shown between the experimental data and fits to the theory. It is shown that this approach permits rapid (20 ms acquisition) detection of a single lipid mono-layer, with sufficient signal-to-noise to determine the order parameter for the acyl chain packing. Also it is demonstrated that this detection scheme provides an absolute measure of the solute concentration.