Strong Plasmonic Coupling between Graphene Ribbon Array and Metal Gratings

The collective oscillation of the massless electrons in graphene ribbons can interact with photons to create graphene plasmon polaritons. The resonance-induced absorption is critical in signal detection and energy harvesting applications. However, because of their atomic thickness, high absorptance is difficult to achieve with graphene ribbons alone. In this work, a hybrid plasmonic system composed of an array of graphene ribbons over a periodic metal grating is theoretically investigated. It is shown that the localized resonances, that is, magnetic polaritons, in metal gratings can couple with the plasmonic resonance in graphene ribbons, resulting in significantly enhanced absorption in graphene. Moreover, the coupling phenomenon depends on the width of the ribbons and the relative positions of the ribbon and the grating. The coupling between the grating and a continuous graphene monolayer sheet is also investigated and the results are compared to those with graphene ribbons. The findings of this work may facilitate the design of optoelectronic devices and metamaterials structures based on hybrid nanostructures and graphene

Polarimetric analysis of thermal emission from both reciprocal and nonreciprocal materials using fluctuational electrodynamics

Coherent thermal emission for a given polarization has been observed in many metamaterials with micro/nanostructures. A complete description of the thermal emission requires the full characterization of the spectral angular emissivity for all polarization states. Emissivity is typically obtained based on the equivalence between the absorptivity and emissivity according to Kirchhoff's law; however, such relation may be invalid for nonreciprocal media. More general approaches without the constrain of optical reciprocity are necessary when dealing with magneto-optical materials and magnetic Weyl semimetals. Here, a polarimetric analysis of thermal emission is carried out based on fluctuational electrodynamics. The Stokes parameters are obtained using coherency matrix for a multilayered system with anisotropic media, including nonreciprocal materials. The results demonstrate that thermal emission may be circularly or linearly polarized in different directions and frequencies. The findings are consistent with the statements of the modified Kirchhoff's law provided by several groups in recent years, and therefore, justify the appropriateness of both the direct and indirect methods. This study will help the design of desired thermal emitters for energy harvesting and thermal control

A Novel Near-field Photonic Thermal Diode with hBN and InSb

Similar to the diode in electronics, a thermal diode is a two-terminal device that allows heat to transfer easier in one direction (forward bias) than in the opposite direction (reverse bias). Unlike conductive and convective thermal diodes, a photonic thermal diode operates in a contactless mode and may afford a large operating temperature range. In this work, a novel near-field photonic thermal diode with hexagonal boron nitride (hBN) and indium antimonide (InSb) is proposed and theoretically demonstrated. The temperature dependence of the interband absorption of InSb is used to couple (or decouple) with the hyperbolic phonon polaritons in hBN. The numerical analysis predicts a rectification ratio greater than 17 for a 10 nm vacuum gap when operating at an average temperature of 300 K and a temperature difference of 200 K. The calculated rectification ratio exceeds 35 with higher average temperatures and larger temperature differences. The mechanism proposed here for achieving photonic thermal rectification provides a new way of controlling radiative heat transfer

Optimization of highly circularly polarized thermal radiation in α\alpha-MoO3_3/β\beta-Ga2_2O3_3 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 $\beta$-Ga$_2$O$_3$ and $\alpha$-MoO$_3$. 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 $\beta$-Ga$_2$O$_3$ 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

Large-area polycrystalline α\alpha-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 ($\alpha$-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 $\mu$m). Here, we report on the fabrication and IR characterization of large-area (over 1 cm$^2$ size) $\alpha$-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$^{-1}$ with a resonance Q-factor as high as 53 were achieved. Reflection can be tuned via changing incident polarization with a dynamic range of $\Delta$R=0.3 at 45 deg. incidence angle. We also report a polarization-independent almost perfect absorption condition (R<0.01) at 972 cm$^{-1}$ 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