Physical Nature of “Slow Light” in Stimulated Brillouin Scattering

Abstract

It is well known that the velocity of a light pulse in a medium, referred to as the group velocity, is smaller than the phase velocity of light, c/n, where c is the speed of light in vacuum, and n is the refractive index of the medium. The difference between phase and group velocity of light is a result of two circumstances: a pulse is generically composed of a range of frequencies, and the refractive index, n, of a material is not constant but depends on the frequency, ω, of the radiation, n = n(ω). A group index ng(ω) = n + ω(dn/dω) is used to quantify the delay (or advancement), Δtg, of an optical pulse, Δtg = ngL/c, which propagates in a medium of length L, where c/ng is called the group velocity (Brillouin, 1960). For about a century studies of this phenomenon, now topically referred to as slow light (SL), were mostly of a scholastic nature. In general the effect is very small for propagation of light pulses through transparent media. However when the light resonantly interacts with transitions in atoms or molecules, as for gain and absorption, the effect is greatly enhanced. Fig. 1 shows the gain (inverted absorption) spectral profile around a resonance together with its refractive index dispersion profile, the gradient of which results in ng(ω).