3 research outputs found
Fundamentals of Polaritons in Strongly Anisotropic Thin Crystal Layers
Polaritons in strongly anisotropic thin layers have recently
captured
considerable attention in nanophotonics because of their directional
propagation at the nanoscale, which offers unique possibilities for
nano-optical applications. However, exploiting the full potential
of anisotropic polaritons requires a thorough understanding of their
properties, including field confinement, energy, and phase propagation
direction and losses. Here, we provide novel insights into some fundamental
aspects of the propagation of anisotropic polaritons in thin biaxial
layers. In particular, we introduce a novel methodology that allows
us to represent isofrequency curves of polaritons in strongly anisotropic
materials, considering that the real and imaginary parts of the wavevector
are not parallel. In fact, we analytically show that the direction
of the imaginary part of the wavevector is parallel to the group velocity,
which can have different, even perpendicular or opposite, directions
with respect to the phase velocity. This finding is crucial for understanding
polaritonic phenomena in anisotropic media; yet, it has so far been
widely overlooked in the literature. Additionally, we introduce a
criterion for classifying the polaritonic modes in biaxial layers
into volume and surface categories and analyze their dispersion, field
structure, and losses. Finally, we discover the existence of previously
unexplored anisotropic transverse-electric-like modes, which can exhibit
natural canalization. Taken together, our results shed light on hitherto
unexplored areas of the theory of electromagnetic modes in thin biaxial
layers. Although exemplified for van der Waals α-MoO3 layers, our findings are general for polaritons in other strongly
anisotropic biaxial hyperbolic crystals
Germanium Monosulfide as a Natural Platform for Highly Anisotropic THz Polaritons
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
Terahertz Twistoptics–Engineering Canalized Phonon Polaritons
The terahertz (THz) frequency range is key to studying
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-molecule 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 (HPhPs) 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 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
α-molybdenum trioxide (α-MoO3), hence extending
twistoptics to this intriguing frequency range. Our images, recorded
by near-field optical microscopy, show the frequency- and twist-angle-dependent
changes 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
α-MoO3 at 8.67 THz for a twist angle of 50°.
Our results demonstrate the precise control and manipulation of confined
collective excitations at THz frequencies, particularly offering possibilities
for nanophotonic applications