Nonlinear behavior of vibrating molecules on suspended graphene waveguides

Suspended graphene waveguides were deposited on micron-scale periodic metal (plasmonic) structures. Raman scattering of test molecules (B. Megaterium), deposited on the waveguides' surface, exhibited azimuthal cycles upon rotation: at these micron scales, spontaneous Raman ought to be independent of phase matching conditions. In addition, we observed angular-selective quadratic intensity dependence contrary to the typical linear behavior of spontaneous Raman. The effects were observed at very modest pump laser intensities (<10 MW/cm2 at the sample surface, oftenly used in Raman experiments). We attributed these observations to nonlinear coupling between the vibrating molecules and surface plasmon polariton (SPP) modes at the molecular vibration frequency. It was assessed that the polariton mode propagates through fairly long distances (over 100 microns).