Controlling the spontaneous emission rate of quantum wells in rolled-up hyperbolic metamaterials

We experimentally demonstrate the enhancement of the spontaneous emission rate of GaAs quantum wells (QWs) embedded in rolled-up metamaterials. We investigate the spontaneous emission lifetime of semiconductor QW heterostructures which are directly integrated inside rolled-up microtubes (RMTs) with hyperbolic dispersion. RMTs are prepared by self-rolling of strained metal-semiconductor layer systems [1]. In the rolling-up process, multiple functional layers are closely stacked on top of each other, forming a three dimensional radial metamaterial which operates at optical frequencies. Rolling up metal-semiconductor systems is a unique and elegant route to incorporate optically active QW structures directly inside a multilayer system and results in a high quality active metamaterial composed of identical functional layers. Within this approach, the variable metal and semiconductor layer thicknesses allow to control the effective permittivity tensor of the metamaterial according to an effective medium approach. Thereby, the topology of the dispersion's iso-frequency surface can be changed from a closed ellipsoidal iso-frequency surface to an open hyperboloidal surface corresponding to a so called hyperbolic metamaterial. We show by means of time resolved low temperature photoluminescence (PL) spectroscopy, that the spontaneous emission rate of the embedded QWs is enhanced by a factor of 2 as a signature of the topological transition to a hyperbolic medium. Our work is the first to prove spontaneous emission enhancement of robust QW structures in a hyperbolic metamaterial and at the same time opens-up the pathway to use this ansatz for novel integrated quantum light sources with tailored emission properties.