Stimulation of inelastic light scattering in plasmon structures with giant Raman enhancement

The dependences of the intensities of the Stokes and anti-Stokes scattering components on the pump power are studied in plasmon-dielectric structures optimized to obtain giant enhancement of Raman scattering (surface-enhanced Raman scattering, SERS) in the infrared spectral range (for a 1064 nm laser, the SERS enhancement factor is 10$^8$). It is found that the intensity of the anti-Stokes scattering channel increases superlinearly with the pump power, and this increase becomes much faster above a certain threshold power. In this case, the Stokes scattering intensity is a linear and then sublinear (at high pump powers) function of the laser pump power. It is shown that the threshold power depends on the concentration of organic molecules deposited on the amplifying structure and on the SERS enhancement factor. The observed behavior of the intensities of the Raman Stokes and anti-Stokes components indicates the importance of the stimulated light scattering mechanism in SERS structures. According to an analysis of the spectral positions of the scattering lines, molecules are excited to high vibrational energy levels under above-threshold laser excitation. This is manifested in the redshift of the anti-Stokes scattering lines.