38 research outputs found
Near-field heat transfer between graphene/hBN multilayers
We study the radiative heat transfer between multilayer structures made by a
periodic repetition of a graphene sheet and a hexagonal boron nitride (hBN)
slab. Surface plasmons in a monolayer graphene can couple with a hyperbolic
phonon polaritons in a single hBN film to form hybrid polaritons that can
assist photon tunneling. For periodic multilayer graphene/hBN structures, the
stacked metallic/dielectric array can give rise to a further effective
hyperbolic behavior, in addition to the intrinsic natural hyperbolic behavior
of hBN. The effective hyperbolicity can enable more hyperbolic polaritons that
enhance the photon tunneling and hence the near-field heat transfer. However,
the hybrid polaritons on the surface, i.e. surface plasmon-phonon polaritons,
dominate the near-field heat transfer between multilayer structures when the
topmost layer is graphene. The effective hyperbolic regions can be well
predicted by the effective medium theory (EMT), thought EMT fails to capture
the hybrid surface polaritons and results in a heat transfer rate much lower
compared to the exact calculation. The chemical potential of the graphene
sheets can be tuned through electrical gating and results in an additional
modulation of the heat transfer. We found that the near-field heat transfer
between multilayer structure does not increase monotonously with the number of
layer in the stack, which provides a way to control the heat transfer rate by
the number of graphene layers in the multilayer structure. The results may
benefit the applications of near-field energy harvesting and radiative cooling
based on hybrid polaritons in two-dimensional materials.Comment: 10 pages, 11 figure
Optimization of highly circularly polarized thermal radiation in -MoO/-GaO twisted layers
We investigate a bi-layer scheme for circularly polarized infrared thermal
radiation. Our approach takes advantage of the strong anisotropy of
low-symmetry materials such as -GaO and -MoO. We
numerically report narrow-band, high degree of circular polarization (over
0.85), thermal radiation at two typical emission frequencies related to the
excitation of -GaO optical phonons. Optimization of the degree
of circular polarization is achieved by a proper relative tilt of the crystal
axes between the two layers. Our simple but effective scheme could set the
basis for a new class of lithography-free thermal sources for IR bio-sensing.Comment: 11 pages, 6 figure
Large-area polycrystalline -MoO3 thin films for IR photonics
In recent years, excitation of surface phonon polaritons (SPhPs) in van der
Waals materials received wide attention from the nanophotonics community.
Alpha-phase Molybdenum trioxide (-MoO3), a naturally occurring biaxial
hyperbolic crystal, emerged as a promising polaritonic material due to its
ability to support SPhPs for three orthogonal directions at different
wavelength bands (range 10-20 m). Here, we report on the fabrication and
IR characterization of large-area (over 1 cm size) -MoO3
polycrystalline films deposited on fused silica substrates by pulsed laser
deposition. Single alpha-phase MoO3 films exhibiting a polarization-dependent
reflection peak at 1006 cm with a resonance Q-factor as high as 53 were
achieved. Reflection can be tuned via changing incident polarization with a
dynamic range of R=0.3 at 45 deg. incidence angle. We also report a
polarization-independent almost perfect absorption condition (R<0.01) at 972
cm which is preserved for a broad angle of incidence. The development of
a low-cost polaritonic platform with high-Q resonances in the mid-infrared
(mid-IR) range is crucial for a wide number of functionalities including
sensors, filters, thermal emitters, and label-free biochemical sensing devices.
In this framework our findings appear extremely promising for the further
development of lithography-free, scalable films, for efficient and large-scale
devices operating in the free space, using far-field detection setups.Comment: 17 pages, 12 figure
Large-area polycrystalline α-MoO3 thin films for IR photonics
In recent years, the excitation of surface phonon polaritons (SPhPs) in van der Waals materials
received wide attention from the nanophotonics community. Alpha-phase Molybdenum trioxide
(α-MoO3), a naturally occurring biaxial hyperbolic crystal, emerged as a promising polaritonic
material due to its ability to support SPhPs for three orthogonal directions at different
wavelength bands (range 10–20 µm). Here, we report on the fabrication, structural,
morphological, and optical IR characterization of large-area (over 1 cm2
size) α-MoO3
polycrystalline film deposited on fused silica substrates by pulsed laser deposition. Due to the
random grain distribution, the thin film does not display any optical anisotropy at normal
incidence. However, the proposed fabrication method allows us to achieve a single α-phase,
preserving the typical strong dispersion related to the phononic response of α-MoO3 flakes.
Remarkable spectral properties of interest for IR photonics applications are reported. For
instance, a polarization-tunable reflection peak at 1006 cm−1 with a dynamic range of ∆R = 0.3
and a resonance Q-factor as high as 53 is observed at 45â—¦
angle of incidence. Additionally, we
report the fulfillment of an impedance matching condition with the SiO2 substrate leading to a
polarization-independent almost perfect absorption condition (R < 0.01) at 972 cm−1 which is
maintained for a broad angle of incidence. In this framework our findings appear extremely
promising for the further development of mid-IR lithography-free, scalable films, for efficient
and large-scale sensors, filters, thermal emitters, and label-free biochemical sensing devices operating in the free space, using far-field detection setups
Tailoring full-Stokes thermal emission from twisted-gratings structures
Polarized thermal emission finds extensive applications in remote sensing, landmine detection, and target detection. In applications such as ellipsometry and biomedical analysis, the generation of emission with controllable polarization is preferred. It is desired to manipulate the polarization state over the full Stokes parameters. While numerous studies have demonstrated either linear or circular polarization control using metamaterials, full-Stokes thermal emission has not been explored. Here, a microstructure based on two layers of silicon carbide gratings is proposed to tailor the polarization state of thermal emission, covering the full-Stokes parameter range. The bilayer twisted-gratings structure breaks mirror symmetry. Wave interference at the interfaces and diffraction by the gratings enhance the emission dichroism, resulting in almost completely polarized emission. By adjusting the twist angle between the gratings, the polarization state can be continuously tuned from linear to circular, nearly covering the entire surface of Poincaré sphere. This study provides a design for tailoring full-Stokes emission with notable advantages over other plasmonic metasurfaces