Shaping an enhanced and highly directional thermal emission with localized surface phonon polariton resonances (LSPhPs)

Semiconductors can have an infrared permittivity response that is reminiscent of noble-metal plasmonic responses to visible light. This occurs for a specific spectral window within a regime known as the reststrahlen band, where EM waves activate out-of-phase phonon vibrations in the material. Microscale objects made of such reststrahlen-band material support then highly localized resonances similar to localized surface plasmons (LSPs) around plasmonic nanoparticles. By analogy, these resonances are called localized surface phonon polariton resonances (LSPhPs). We show here how to utilize individual LSPhP resonances in a judiciously designed structure to achieve a highly forward thermal emission enhanced by an order of magnitude with respect to the bulk homogenous material. This study can be important for thermal photovoltaics, efficient THz sources, and passive radiative cooling devices.