The resonant scattering which is governed by transverse relaxation is physically different from absorption followed by emission which is affected by longitudinal relaxation. A number of physical examples are given.
This work focused on understanding the effects of arbitrary transverse and longitudinal relaxation rates on the susceptibilities of coherently driven three-level systems. The approximation of a single relaxation rate often made in previous work is strongly invalidated by the variation in the spontaneous emission lifetime between various atomic level pairs in systems such as cesium. It is of great importance to the problem of nonlinear infrared generation to determine the dependence of both real and imaginary susceptibility on relaxation rates. The imaginary susceptibility on the pump transition determines the absorption of pump photons and the imaginary susceptibility on the laser transition determines the spectral dependence of the gain. This is of particular importance for pure Raman emission (i.e., absorption at linecenter of the gain transition) as it determines the tunability characteristics we are aiming to predict. The real susceptibility is important when cavities are used at the signal field as this will determine the loaded resonance of the Raman oscillator. Researchers show that in some cases which result from having different relaxation rates mode splitting may result, allowing more than one frequency to have the same Raman wavelength, possibly resulting in a temporal instability.
Two complementary pictures for the resonance Raman scattering are presented, corresponding to two different ways of decomposing the four-time correlation function of dipole moment for this second-order process into a pair of two-time correlation functions. One is to take the auto-correlation of polarizability following the semi-classical picture for Raman scattering and expand it in powers of intra-system coupling; how the Raman spectra of higher orders approach the so-called luminescence is shown in the weak and strong coupling limits. Another is to study the correlation between absorption and emission processes; one can see how the relaxation of the strong coupled system after photoexcitation is reflected in the time-resolved emission spectra.
Selected topics related to the fundamentals of nonlinear optics are discussed. First, a brief description on the theory of nonlinear optical susceptibilities is given. Emphasis is on the basic properties, on the recently developed bond theory, and on the dispersion of nonlinear susceptibilities. Then the coupled wave approach is discussed. It is shown that the transverse resonant excitation in material can be treated as a material excitational wave, and most nonlinear optics problems can be understood as results of coupling between em waves or em and material excitational waves. A number of examples of current interest are presented. These include sum- and difference-frequency generation, Raman scattering by coherent excitation, stimulated Raman and stimulated polariton scattering, four-wave mixing, and vibrational relaxation measurements. Finally, the physical distinction between laser emission and resonant stimulated Raman scattering in an optically pumped three-level system are considered. A similar problem on the distinction of hot luminescence and resonant Raman scattering has recently attracted much attention. It is shown that the hot laser action is connected directly with the population inversion, while the resonant stimulated Raman scattering is connected with transverse excitation. The problem is, however, more complicated for a multi-level system. (auth)