Two-color suppression of stimulated Raman scattering

Sub-diffraction limited imaging schemes have become widely used in fluorescence microscopy [1]. However, the need for labeling with fluorescent dyes remains a major downside of fluorescence microscopy. The size, availability, toxicity as well as photo-bleaching of the used dyes can complicate measurements [2]. In contrast, Raman imaging is inherently label-free. Unfortunately, the super-resolution schemes used in fluorescence microscopy are currently not transferable to Raman microscopy. This paper presents the development of a scheme for the suppression of coherent Raman scattering through the depletion of probe photons, inspired by the work of M. Cho et al. [3, 4], who showed the suppression of coherent Raman scattering using laser pulses at three different wavelengths and two Raman resonances. In our case, probe depletion is achieved through saturated stimulated Raman scattering in a three-beam setup with only two colors involved. Fig. 1 (a) describes the working principle: in the unsuppressed case, the combination of pump and probe pulse induces stimulated Raman scattering at the Raman resonances, leading to stimulated Raman loss (SRL) of the probe and and stimulated Raman gain (SRG) of the pump. The Raman spectrum then results from the difference between two spectra, recorded with pump on and off, respectively. In the suppressed case a second strong pump pulse, working as a depletion pulse, depletes the probe pulse via stimulated Raman scattering (SRL_D) and induces a shortage of the probe photons at the Raman resonances, saturating the stimulated Raman scattering. Thus, only a small amount of additional SRLSup is induced by the original pump. If SRL is detected in addition to the spectral change, induced by the depletion pulse, the SRL will be reduced resulting in SRL_Sup < SRL.