Local control and lateral nanofocusing of hyperbolic phonon polaritons

TL;DR

This paper demonstrates a novel substrate engineering method using sinusoidal gold surface corrugation to achieve local control and nanofocusing of hyperbolic phonon polaritons in hexagonal boron nitride.

Contribution

It introduces a continuous, non-binary substrate patterning approach for precise local control of polariton wavelengths and lateral nanofocusing.

Findings

Achieved nearly threefold local variation of polariton wavelength.

Demonstrated lateral nanofocusing with a compression factor of around 2.5.

Verified control through near-field optical microscopy and theoretical calculations.

Abstract

Phonon polaritons in van der Waals crystals enable exceptional light confinement and control over low-loss nanolight propagation. The polariton wavelength can be controlled by the crystal geometry, isotopic composition, or surrounding environment -- for which substrate engineering is particularly effective. However, existing approaches of substrate nanopatterning are binary and offer limited leverage. Here, we demonstrate local control over the wavelength of phonon polaritons in hexagonal boron nitride by employing a sinusoidally corrugated gold surface to smoothly vary the gap between the van der Waals crystal and metallic substrate. The nonuniform gap provides a continuous and nearly threefold local variation of the polariton wavelength across the structure, verified by near-field optical microscopy. Our platform further enables lateral nanofocusing by gradually compressing and decompressing the wavelength of propagating polaritons by a factor of around 2.5 achieved solely through substrate geometry, consistent with our local control experiments and theoretical calculations. Our results push the boundaries of substrate engineering and showcase a powerful method for precise and local tailoring of polaritonic modes.