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

中文翻译：

---

时域布里渊散射探测的多个量子阱中的极化激元诱导透明度


从介质表面反射的入射光和通过介质传播的皮秒应变脉冲的干涉导致反射强度的时间振荡。这种现象称为时域布里渊散射，使我们能够获得有关介质内部光场的信息。如果入射光被强烈吸收，则振荡幅度随着应变脉冲到表面距离的增加而减小，而如果介质是透明的，则振荡幅度保持不变。在这里，我们利用时域布里渊散射来探测多量子阱层内的光场，以获得与激子强烈耦合并形成极化激元的光。在低激发密度下，我们观察到传统的布里渊振荡，由于激子共振附近极化激元的强烈吸收，当应变脉冲远离表面时，其振幅很小。在光密度升高时，吸收消失，介质变得透明，振荡的幅度不取决于应变脉冲与表面的距离。我们通过极化激元散射产生的非相干激子密度的增加来解释极化激元诱导的透明度的这种效应。最后，激子密度的增加导致激子气体转变为集体状态，导致极化子状态崩溃，入射光在介质中传播而不被吸收。