Tunable hyperbolic metamaterials for brightening single-photon emission
Hyperbolic metamaterials (HMMs) are widely used to enhance spontaneous emission through their exceptionally large photonic density of states, yet large Purcell factors (>100) rarely translate into brighter emission, exposing both a practical limitation of HMMs as brightening platforms and a flaw in the Purcell factor itself as a figure of merit. Here we systematically evaluate spontaneous emission enhancement and photon outcoupling in metal-dielectric HMMs (TiO$_2$/Ag and phase-change Sb$_2$S$_3$/Ag) using effective medium theory, transfer matrix calculations, and full-wave FDTD simulations. We show that planar HMMs with Purcell factors exceeding 100 have external quantum efficiencies below 0.01, yielding an actual radiative Purcell factor (PF$_{rad}$) of only 1-2. Nanopatterning the HMM into a photonic crystal efficiently couples trapped high-k modes into free space, raising PF$_{rad}$ by an order of magnitude. Incorporating the phase-change material Sb$_2$S$_3$ further enables dynamic tuning of the hyperbolic spectral window and active control over emission brightness: an optimized nanostructured design reaches PF$_{rad}$ ~ 10-15 in the near-infrared, with switching contrast exceeding a factor of 15 upon phase transition. These results establish the radiative Purcell factor as a more reliable figure of merit than the conventional Purcell factor for engineering quantum emitters, and provide a practical route toward actively tunable, bright single-photon sources based on hyperbolic metamaterials.