Tunable Hyperbolic Phonon Polaritons in a Gradiently-Suspended Van Der Waals {\alpha}-MoO3

TL;DR

This paper demonstrates a method to tune hyperbolic phonon polaritons in a gradiently suspended { extalpha}-MoO3 crystal by varying air gaps, achieving significant wavelength elongation and damping reduction, advancing tunable nanophotonics.

Contribution

It introduces a novel approach to tune HPhPs in { extalpha}-MoO3 using gradient air gaps, enabling control over polariton properties for nanophotonic applications.

Findings

Wavelength tuning range up to 160%

Damping rate reduction up to 35%

Opposite dependence of wavelength on gap in different bands

Abstract

Highly confined and low-loss hyperbolic phonon polaritons (HPhPs) sustained in van der Waals crystals exhibit outstanding capabilities of concentrating long-wave electromagnetic fields deep to the subwavelength region. Precise tuning on the HPhP propagation characteristics remains a great challenge for practical applications such as nanophotonic devices and circuits. Here, we show that by taking advantage of the varying air gaps in a van der Waals {\alpha}-MoO3 crystal suspended gradiently, it is able to tune the wavelengths and dampings of the HPhPs propagating inside the {\alpha}-MoO3. The results indicate that the dependences of polariton wavelength on gap distance for HPhPs in lower and upper Reststrahlen bands are opposite to each other. Most interestingly, the tuning range of the polariton wavelengths for HPhPs in the lower band, which exhibit in-plane hyperbolicities, is wider than that for the HPhPs in the upper band of out-of-plane hyperbolicities. A polariton wavelength elongation up to 160% and a reduction of damping rate up to 35% are obtained. These findings can not only provide fundamental insights into manipulation of light by polaritonic crystals at nanoscale, but also open up new opportunities for tunable nanophotonic applications.