Polariton-Induced\nTransparency in Multiple Quantum\nWells Probed by Time Domain Brillouin Scattering

The interference of the incident light reflected from\nthe surface\nof a medium and from a picosecond strain pulse propagating through\nit results in temporal oscillations of the reflected intensity. This\nphenomenon, called time-domain Brillouin scattering, enables us to\ngain information about the optical field inside the medium. The oscillation\namplitude decreases with increase of the distance from the strain\npulse to the surface if the incident light is strongly absorbed, while\nit remains constant if the medium is transparent. Here we exploit\ntime domain Brillouin scattering to probe the optical field inside\na multiple quantum well layer for light strongly coupling to excitons\nand forming polaritons. At low excitation density, we observe conventional\nBrillouin oscillations whose amplitude is small when the strain pulse\nis positioned far from the surface due to the strong absorption of\npolaritons in the vicinity of the exciton resonance. At elevated optical\ndensity, the absorption disappears, the medium becomes transparent,\nand the amplitude of the oscillations does not depend on the distance\nof the strain pulse from the surface. We explain this effect of polariton-induced\ntransparency by the increase of the incoherent exciton density generated\nas result of polariton scattering. Finally, the increase of the exciton\ndensity leads to transition of the exciton gas to a collective state,\nresulting in collapse of the polariton state and propagation of the\nincident light in the medium without absorption.