Terahertz Twistoptics -- engineering canalized phonon polaritons

TL;DR

This paper demonstrates the first use of twist-angle manipulation of hyperbolic phonon polaritons in alpha-molybdenum trioxide to control THz radiation propagation, enabling canalization and advanced nanophotonic control.

Contribution

It introduces twistoptics at THz frequencies in vdW materials, showing how twist angle and frequency can tune polariton propagation and enable canalization of confined THz radiation.

Findings

Frequency- and twist-dependent change between hyperbolic and elliptic polariton propagation.

Polariton canalization achieved at 8.67 THz with a 50° twist angle.

Unprecedented control of confined excitations at THz frequencies.

Abstract

The terahertz (THz) frequency range is key to study collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecular arrangements, requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhP) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options such as control over the wavefront shape and propagation direction. Here, we demonstrate the first THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material alpha-molybdenum trioxide ($\alpha-MoO_{3}$), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent change between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in $\alpha-MoO_{3}$ at 8.67 THz for a twist angle of 50{\deg}. Our results demonstrate an unprecedented control and the manipulation of highly-confined collective excitations at THz frequencies, offering novel possibilities for nanophotonic applications.