Ultrafast phase transition to hyperbolic metamaterial state

Hyperbolic metamaterials are valuable potential single-photon emitters because of their large density of states at phase boundaries. We grow metamaterial stacks using alternating layers of undoped GaSb (dielectric) and Sidoped InAsSb (metallic). This combination can act as a dielectric in the sample plane, but a metal perpendicular to the plane, forming a hyperbolic metamaterial (IIMM) state depending on the density of free electrons. We demonstrate this behavior by injecting free electrons using an ultrafast 1300-nm pump laser, while probing the differential reflectivity and transrnittivity with a linearly polarized probe in the range of 4-5 Inn. The difference in results for s- and p-polarized probes demonstrates the anisotropic nature of the hyperbolic state, suggesting that single photons at mid infrared (MIR) frequencies may be efficiently emitted in a highly directional manner. The HMM state is also dependent on the metal fraction, which we control via the relative thicknesses of the layers. Additionally, spectroscopic ellipsometry reveals that the metal fraction is consistently lower than the nominal value, a phenomenon we attribute to doped carriers being squeezed to the center of the InAsSb layers. Our analysis of the pump-related shift of the metal/dielectric/HMM phase diagram shows that our sample structure is a highly tailorable avenue to MIR spontaneous photon emission.