17 research outputs found
Interplay between evanescence and disorder in deep subwavelength photonic structures
Deep subwavelength features are expected to have minimal impact on wave transport. Here we show that in contrast to this common understanding, disorder can have a dramatic effect in a one-dimensional disordered optical system with spatial features a thousand times smaller than the wavelength. We examine a unique regime of Anderson localization where the localization length is shown to scale linearly with the wavelength instead of diverging, because of the role of evanescent waves. In addition, we demonstrate an unusual order of magnitude enhancement of transmission induced due to localization. These results are described for electromagnetic waves, but are directly relevant to other wave systems such as electrons in multi-quantum-well structures
Quantitative scattering theory of near-field response for 1D polaritonic structures
Scattering-type scanning near-field optical microscopy is a powerful imaging
technique for studying materials beyond the diffraction limit. However,
interpreting near-field measurements poses challenges in mapping the response
of polaritonic structures to meaningful physical properties. To address this,
we propose a theory based on the transfer matrix method to simulate the
near-field response of 1D polaritonic structures. Our approach provides a
computationally efficient and accurate analytical theory, relating the
near-field response to well-defined physical properties. This work enhances the
understanding of near-field images and complex polaritonic phenomena. Finally,
this scattering theory can extend to other systems like atoms or nanoparticles
near a waveguide
Quantum \v{C}erenkov Radiation: Spectral Cutoffs and the Role of Spin and Orbital Angular Momentum
We show that the well-known \v{C}erenkov Effect contains new phenomena
arising from the quantum nature of charged particles. The \v{C}erenkov
transition amplitudes allow coupling between the charged particle and the
emitted photon through their orbital angular momentum (OAM) and spin, by
scattering into preferred angles and polarizations. Importantly, the spectral
response reveals a discontinuity immediately below a frequency cutoff that can
occur in the optical region. Specifically, with proper shaping of electron
beams (ebeams), we predict that the traditional \v{C}erenkov radiation angle
splits into two distinctive cones of photonic shockwaves. One of the shockwaves
can move along a backward cone, otherwise considered impossible for
\v{C}erenkov radiation in ordinary matter. Our findings are observable for
ebeams with realistic parameters, offering new applications including novel
quantum optics sources, and open a new realm for \v{C}erenkov detectors
involving the spin and orbital angular momentum of charged particles.Comment: 27 pages, 3 figure
Deep-subwavelength Phase Retarders at Mid-Infrared Frequencies with van der Waals Flakes
Phase retardation is a cornerstone of modern optics, yet, at mid-infrared
(mid-IR) frequencies, it remains a major challenge due to the scarcity of
simultaneously transparent and birefringent crystals. Most materials resonantly
absorb due to lattice vibrations occurring at mid-IR frequencies, and natural
birefringence is weak, calling for hundreds of microns to millimeters-thick
phase retarders for sufficient polarization rotation. We demonstrate mid-IR
phase retardation with flakes of -molybdenum trioxide
(-MoO) that are more than ten times thinner than the operational
wavelength, achieving 90 degrees polarization rotation within one micrometer of
material. We report conversion ratios above 50% in reflection and transmission
mode, and wavelength tunability by several micrometers. Our results showcase
that exfoliated flakes of low-dimensional crystals can serve as a platform for
mid-IR miniaturized integrated polarization control.Comment: 8 pages, 5 figure
Retrieving optical parameters of emerging van der Waals flakes
High-quality low-dimensional layered and van der Waals materials are
typically exfoliated, with sample cross sectional areas on the order of tens to
hundreds of microns. The small size of flakes makes the experimental
characterization of their dielectric properties unsuitable with conventional
spectroscopic ellipsometry, due to beam-sample size mismatch and
non-uniformities of the crystal axes. Previously, the experimental measurement
of the dielectrirc permittivity of such microcrystals was carried out with
near-field tip-based scanning probes. These measurements are sensitive to
external conditions like vibrations and temperature, and require
non-deterministic numerical fitting to some a priori known model. We present an
alternative method to extract the in-plane dielectric permittivity of van der
Waals microcrystals, based on identifying reflectance minima in spectroscopic
measurements. Our method does not require complex fitting algorithms nor near
field tip-based measurements and accommodates for small-area samples. We
demonstrate the robustness of our method using hexagonal boron nitride and
{\alpha}-MoO3, and recover their dielectric permittivities that are close to
literature values.Comment: 10 pages, 4 figure and 3 table
Engineering 2D material exciton lineshape with graphene/h-BN encapsulation
Control over the optical properties of atomically thin two-dimensional (2D)
layers, including those of transition metal dichalcogenides (TMDs), is needed
for future optoelectronic applications. Remarkable advances have been achieved
through alloying, chemical and electrical doping, and applied strain. However,
the integration of TMDs with other 2D materials in van der Waals
heterostructures (vdWHs) to tailor novel functionalities remains largely
unexplored. Here, the near-field coupling between TMDs and graphene/graphite is
used to engineer the exciton lineshape and charge state. Fano-like asymmetric
spectral features are produced in WS, MoSe and WSe vdWHs
combined with graphene, graphite, or jointly with hexagonal boron nitride
(h-BN) as supporting or encapsulating layers. Furthermore, trion emission is
suppressed in h-BN encapsulated WSe/graphene with a neutral exciton
redshift (44 meV) and binding energy reduction (30 meV). The response of these
systems to electron-beam and light probes is well-described in terms of 2D
optical conductivities of the involved materials. Beyond fundamental insights
into the interaction of TMD excitons with structured environments, this study
opens an unexplored avenue toward shaping the spectral profile of narrow
optical modes for application in nanophotonic devices