Germanium Monosulfide as a Natural Platform for Highly Anisotropic THz Polaritons

Abstract

Terahertz (THz) electromagnetic radiation is key to access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging topics 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 circumventing the large footprint and inherently low spectral power density of far-field THz radiation. For that purpose, phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials have recently emerged as a promising platform for THz nanooptics. Hence, there is a demand for the exploration of materials that feature not only THz PhPs at different spectral regimes but also host anisotropic (directional) electrical, thermoelectric, and vibronic properties. To that end, we introduce here the semiconducting vdW-material alpha-germanium(II) sulfide (GeS) as an intriguing candidate. By employing THz nanospectroscopy 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 lifetimes (τ > 2 ps) and excellent THz light confinement (λ0/λ 45) but also an intrinsic, phonon-induced anomalous dispersion as well as signatures of naturally occurring, substrate-mediated PhP canalization within a single GeS slab.

Optical properties and polaritons of low symmetry 2D materials

“…where Z 0 is the vacuum impedance, and n s is the refractive index of the substrate. They measured the anisotropic optical [243] and α-GeS [244] respectively. (c) Real part (continuous lines) and imaginary part (dashed lines) of the permittivity along the principal [100], [001], and [010] axes in α-V 2 O 5 .…”

“…Hyperbolic characteristics in other natural materials. (a), (b) Lattice structures of α-V2O5 [243] and α-GeS [244] respectively. (c) Real part (continuous lines) and imaginary part (dashed lines) of the permittivity along the principal [100], [001], and [010] axes in α-V2O5.…”

“…(c) Real part (continuous lines) and imaginary part (dashed lines) of the permittivity along the principal [100], [001], and [010] axes in α-V2O5. [(d)-a] Dispersion of PhPs in α-GeS [244] . Dots are the extracted experimental data from the near-field interference pattern.…”

“…More similar materials have been discovered. Nörenberg et al reported that α-GeS sustains low loss hyperbolic PhPs in the THz regime [244] . As mentioned above, α-GeS is of a layered orthorhombic crystal lattice [Fig.…”

“…[(d)-a] Dispersion of PhPs in α-GeS [244] . Dots are the extracted experimental data from the near-field interference pattern.…”