  Using broadband (12-45 THz) multi-terahertz spectroscopy, we show that
stimulated Rayleigh scattering dominates the transient optical conductivity of
cadmium arsenide, a Dirac semimetal, under an optical driving field at 30 THz.
The characteristic dispersive lineshape with net optical gain is accounted for
by optical transitions between light-induced Floquet subbands, strikingly
enhanced by the longitudinal plasma mode. Stimulated Rayleigh scattering with
an unprecedentedly large refractive index change may pave the way for slow
light generation in conductive solids at room temperature.
