Group velocity modulation and wavelength tuning of phonon-polaritons by engineering dielectric-metallic substrates

TL;DR

This paper demonstrates how the group velocity and wavelength of hyperbolic phonon-polaritons in hBN can be modulated by engineering dielectric-metallic substrates, using experimental and theoretical approaches to control their dispersion and acceleration.

Contribution

It introduces a method to modulate phonon-polariton dispersion and acceleration via substrate thickness engineering, combining infrared spectroscopy and semi-classical modeling.

Findings

Group velocity of phonon-polaritons can be tuned by substrate thickness.

Experimental and theoretical acceleration values are in good agreement.

Gradient of dielectric thickness causes pulse acceleration in hBN nanocrystals.

Abstract

In analogy to the observed for single plasmon-polaritons, we show that subdiffractional hyperbolic phonon-polariton (HP2) modes confined in hexagonal boron nitride (hBN) nanocrystals feature wave-particle duality. First, we use Synchrotron Infrared Nanospectroscopy to demonstrate modulation of the HP2 frequency-momentum dispersion relation and group velocity by varying the thickness of the SiO2 layer in the heterostructure hBN/SiO2/Au. These modulations are, then, exploited for the hBN crystal lying on a SiO2 wedge, with a gradient of thickness of such a dielectric medium, built into the Au substrate. Simulations show that a phonon-polariton pulse accelerates as the thickness of the wedge increases. This is explained by a parameter-free semi-classical approach considering the pulse as free quantum particle. Within this picture, an estimated average acceleration value of ~ 1.45 10^18 m.s^(-2) is determined using experimental inputs. This estimation is in good agreement with the value of 2.0 10^18 m.s^(-2) obtained from the theory directly.