http://arxiv.org/pdf/1305.0331
Spontaneous and Stimulated Raman Scattering near Metal Nanostructures in the Ultrafast, High-Intensity regime This paper explains how nanoantennas amplify electric fields through Stokes and anti-Stokes field conversion. Stokes and anti-Stokes scattering is the way materials convert photons to less and more energy states. * the material absorbs energy and the emitted photon has a lower energy than the absorbed photon. This outcome is labeled Stokes Raman scattering. * the material loses energy and the emitted photon has a higher energy than the absorbed photon. This outcome is labeled anti-Stokes Raman scattering. The energy difference between the absorbed and emitted photon corresponds to the energy difference between two resonant states of the material and is independent of the absolute energy of the photon. The spectrum of the scattered photons is termed the Raman spectrum. It shows the intensity of the scattered light as a function of its frequency difference Δν to the incident photons. The locations of corresponding Stokes and anti-Stokes peaks form a symmetric pattern around Δν=0. The frequency shifts are symmetric because they correspond to the energy difference between the same upper and lower resonant states. The intensities of the pairs of features will typically differ, though. It depends on the population of the initial state of the material, which in turn depends on the temperature. In thermodynamic equilibrium, the upper state will be less populated than the lower state. Therefore, the rate of transitions from the lower to the upper state (Stokes transitions) will be higher than in the opposite direction (anti-Stokes transitions). Correspondingly, Stokes scattering peaks are stronger than anti-Stokes scattering peaks. Their ratio depends on the temperature (which can practically be exploited for the measurement of temperature). This paper explains how electric fields are amplified by "Stokes and anti-Stokes gains saturate. In this paper we show that by using sub-picosecond pulses and by allowing the system to depart from the ground state, a number of effects never discussed before in this context are triggered: (i) population inversion may occur within the hot spots; (ii) Stokes and anti-Stokes gains saturate; (iii) the transition to what we refer to as the fully nonlinear or stimulated regime occurs in a switch-like fashion reminiscent of a bistable state, as predicted conversion efficiencies jump by an additional twelve orders of magnitude beyond the enhancement achievable in the spontaneous emission regime. Then, within each hot spot the local field intensity can reach values of several tens of TW/cm2, which may suffice to trigger ionization effects and plasma formation that we have not considered here, but that we realize could introduce additional, unforeseen effects" Take note: When the radiation of photons is suppressed in a dark mode, the energy is stored in a photon population inversion to a higher energy state and conversion efficiencies jump by an additional twelve orders of magnitude Then, within each hot spot the local field intensity can reach values of several tens of TW/cm2 It seems to me that a Terawatt is a large power factor to start off with for a non-optimized system.
