Nanofocusing of Hyperbolic Phonon Polaritons in a Tapered Boron Nitride Slab

A. Y. Nikitin, E. Yoxall, M. Schnell, S. Vélez, I. Dolado, P. Alonso-Gonzalez, F. Casanova, L. E. Hueso, and R. Hillenbrand

ACS Photonics, Article ASAP (2016)
Nanofocusing of light offers new technological opportunities for the delivery and manipulation of electromagnetic fields at subdiffraction-limited length scales. Here, we show that hyperbolic phonon polariton (HPP) modes in the mid-infrared as supported by a hexagonal boron nitride (h-BN) slab can be nanofocused (i.e., both field enhanced and wavelength compressed) by propagation along a vertical taper. Via numerical simulations, we demonstrate that field enhancement factors of 90—for steep tapers—and wavelength compression of more than an order of magnitude—for adiabatic tapers—can be expected. Employing scattering-type scanning near-field optical microscopy, we provide, for the first time, proof-of-principle experimental evidence of a significant HPP wavelength compression. We expect these functionalities to provide diverse applications, from biosensing and nonlinear optics to optical circuitry.