Ultrasensitive Doppler Raman spectroscopy using radio frequency phase shift detection

We introduce the first method to enable an optical amplification of a coherent Raman spectroscopy signal called radio frequency Doppler Raman spectroscopy. Doppler Raman measurements amplify the optical signals in coherent Raman spectroscopy by converting a spectral frequency shift imparted by an impulsive coherent Raman excitation to a change in a probe pulse transit time. This transit time perturbation is detected through the phase of a radio frequency electronic signal measured at a harmonic of the probe pulse train. By exploiting this new capability to scale the signal of a coherent Raman spectroscopic signal, we open the potential to detect very weak Raman spectroscopy signals that are currently not observable due to limits of illumination intensity imposed by laser damage to the specimen.