A deeply subwavelength phonon-polaritonic crystal in the mid-IR based on Boron Nitride

Abstract

Photonic crystals (PCs) are periodically patterned dielectrics providing opportunities to shape and slow down the flow of light for processing of optical signals, lasing and spontaneous emission control. Unit cells of conventional PCs are comparable to the wavelength of light and are thus not suitable for subwavelength-scale applications. Here we introduce a deeply subwavelength PC slab for mid infrared frequencies, with unit cell volume 1000 times smaller than that in an equivalent Si PC slab. We engineer a nanoscale hole array in a Van der Waals material (hexagonal Boron Nitride, h-BN) slab that supports ultra-confined phonon-polaritons (PhPs) – atomic lattice vibrations coupled to electromagnetic fields. This hole array is a polaritonic crystal, that supports ultra-confined polaritonic Bloch modes with high quality factors. We will describe how this deeply subwavelength arrays, generally, can present both angle- and polarizationindependent Bragg resonances. We verify these characteristics by far-field spectroscopy and near-field optical microscopy. Our findings open the door for novel miniaturized angle- and polarization-independent tunable infrared narrow-band couplers, absorbers and thermal emitters based on van der Waals materials.