Germanium monosulfide as a natural platform for highly anisotropic THz polaritons

TL;DR

This paper demonstrates that germanium monosulfide (GeS) is a promising anisotropic material for THz polaritons, showing long-lived phonon polaritons with strong confinement and unique dispersion properties, advancing THz nanooptics.

Contribution

It introduces GeS as a novel natural platform for highly anisotropic THz polaritons with detailed experimental and theoretical characterization.

Findings

Long-lived PhPs with τ > 2 ps

Excellent THz light confinement (λ₀/λ > 45)

Intrinsic anomalous dispersion and PhP canalization in GeS

Abstract

Terahertz (THz) electromagnetic radiation is key to optically access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging between optics and solid-state physics. Confinement of THz light to the nanometer length scale is desirable for local probing of such excitations in low dimensional systems, thereby inherently circumventing the large footprint and low spectral density of far-field THz optics. For that purpose, phonon polaritons (PhPs, i.e., light coupled to lattice vibrations in polar crystals) in anisotropic van der Waals (vdW) materials have recently emerged as a promising platform for THz nanooptics; yet the amount of explored, viable materials is still exiguous. Hence, there is a demand for the exploration of novel materials that feature not only THz PhPs at different spectral regimes, but also exhibit unique anisotropic (directional) electrical, thermoelectric, and vibronic properties. To that end, we introduce here the semiconducting alpha-germanium(II) sulfide (GeS) as an intriguing candidate. By employing THz nano-spectroscopy supported by theoretical analysis, we provide a thorough characterization of the different in-plane hyperbolic and elliptical PhP modes in GeS. We find not only PhPs with long life times ($\tau$ > 2 ps) and excellent THz light confinement ($\lambda_0/\lambda$ > 45), but also an intrinsic, phonon-induced anomalous dispersion as well as signatures of naturally occurring PhP canalization within one single GeS slab.