Bound in the continuum modes in indirectly-patterned hyperbolic media

The general premise of nanophotonics involves shrinking light to the subwavelength nanometric scale, which can be thought of as compressing light and thereby enhancing its interaction with material systems. For example, innovations in nanocavity design have allowed even single emitters to be strongly bound to cavity polaritons [1] . The state of the art in nanocavity research is summarized in Fig. 1a , showing that despite the immense progress made in this field virtually all cavities beneath the 100 nm scale (i.e. for V < 10 6 nm 3 ) show low Q -factors, on the order of ten or less. An alternative route to plasmonic cavities lies in hyperbolic phonon polaritons (PhPs), which can achieve high confinement with small losses. Indeed, PhP cavities with Q > 300 have been demonstrated on the 300nm size scale [2] , [3] , but further size reduction seemingly requires a drastically different cavity design.