The Progress in Study of the Raman Effect After the Laser Invention

This chapter delves into the amplification of the Raman effect through incident light coherence. We illustrate that when Raman-active molecules are illuminated by two coherent electromagnetic waves with slightly different frequencies, the resulting Raman signal attains coherence. This dual-frequency illumination introduces a novel characteristic frequency–the beat frequency. When this beat frequency aligns with the vibron frequency, a resonant enhancement of nuclei vibration significantly augments the Raman response. We explore this enhancement both from a quantum perspective and within the semiclassical framework, employing the third-order nonlinear susceptibility for quantification. Furthermore, we study stimulated Raman scattering (SRS) and Raman lasing, highlighting a crucial distinction between Raman and conventional lasers. Unlike conventional lasers, Raman lasing stems not from an inverted population of the excited (virtual) level but rather from energy transfer between the pump wave and the cavity mode, driven by the nonlinear properties of the active medium. Finally, this chapter also explores coherent anti-Stokes Raman scattering (CARS) within both quantum and semiclassical paradigms. We determine the amplitude of the CARS signal and analyze the conditions conducive to amplification.