  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.
