Ultrasmall polaritonic cavities with graphene and 2D material heterostructures*

A conventional optical cavity supports one or more modes in the cavity core, which are unable to leak through the cavity cladding. The unconventional alternative, known as bound state in continuum (BIC) cavities, are modes which have an available channel to escape the cavity, but nevertheless remain confined due to destructive interferences. BICs have never been demonstrated in nanophotonics, utilizing high momenta plasmon or phonon polaritons.
Here, we introduce and implement the first deep subwavelength BIC cavity, and realize this by employing a novel multi-mode interference concept of hyperbolic phonon polaritons. We demonstrate mid-IR cavities with volumes more than a billion below the free-space mode volume, while maintaining quality factors above 100.
In general, the alliance of HyM with BICs provides a radically new way to confine light and is expected to have far reaching consequences wherever strong optical confinement is being used, from light-matter interaction experiments, to mid-IR nonlinear optics and a range of sensing applications.