Abstract:In this work, we propose an optimum unit cell arrangement to obtain near absolute polarization insensitivity in a metal-insulator-metal (MIM) based ultra-broadband perfect absorber. Our findings prove that upon utilizing this optimum arrangement, the response of the absorber is retained and unchanged over all arbitrary incidence light polarizations, regardless of the shape of the top metal patch. First, the impact of the geometry of the top nanopatch resonators on the absorption bandwidth of the overall structure is explored. Then, the response of the MIM design for different incidence polarizations and angles is scrutinized. Finally, the proposed design is fabricated and characterized.
We present a theoretical study on the plasmonic response of borophene, a monolayer 2D material that is predicted to exhibit metallic response and anisotropic plasmonic behavior in visible wavelengths. We investigate plasmonic properties of borophene thin films as well as borophene nanoribbons and nanopatches where polarization-sensitive absorption values in the order of 50% is obtained with monolayer borophene. It is demonstrated that by adding a metal layer, this absorption can be enhanced to 100%. We also examine giant dichroism in monolayer borophene which can be tuned passively (patterning) and actively (electrostatic gating) and our simulations yield 20% reflected light with significant polarization rotation. These findings reveal the potential of borophene in the manipulation of phase, amplitude and polarization of light at the extreme subwavelength scales.
We theoretically investigate mid-infrared electromagnetic wave propagation in multilayered graphene-hexagonal boron nitride (hBN) metamaterials. Hexagonal boron nitride is a natural hyperbolic material that supports highly dispersive phonon polariton modes in two Reststrahlen bands with different types of hyperbolicity. Due to the hybridization of surface plasmon polaritons of graphene and hyperbolic phonon polaritons of hBN, each isolated unit cell of the graphene-hBN metamaterial supports hybrid plasmon-phonon polaritons (HPPs). Through the investigation of band structure of the metamaterial we find that, due to the coupling between the HPPs supported by each unit cell, the graphene-hBN metamaterial can support HPP bands. The dispersion of these bands can be noticeably modified for different thicknesses of hBN layers, leading to the appearance of bands with considerably flat dispersions. Moreover, analysis of light transmission through the metamaterial reveals that this system is capable of supporting high-k propagating HPPs. This characteristic makes graphene-hBN metamaterials very promising candidates for the modification of the spontaneous emission of a quantum emitter, hyperlensing, negative refraction, and waveguiding.
Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study, we investigate the polarization-dependent optical characteristics of cavities formed using α-MoO3 to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry-Perot structure with α-MoO3 is reported without the need for nanoscale fabrication on the α-MoO3. We observe coupling between the α-MoO3 optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. These findings can open new avenues in the quest for polarization filters and low-loss, integrated planar IR photonics and in dictating polarization control.
2D van der Waals materials have attracted increasing attention in recent years due to their exciting physical properties and offer new opportunities for creating devices with enhanced or novel functionalities. In particular, α‐MoO3 is an emerging member of the fast‐growing 2D family with strong natural anisotropic optical properties. However, anisotropic optical properties of ‐MoO3 in the visible frequency range remain elusive. Here, α‐MoO3 is investigated as an optical material at the visible frequency (450–750 nm), which exhibits a polarization‐dependent complex refractive index due to the anisotropic crystal structure. As a proof of concept, polarization‐sensitive photonic devices including polarization reflectors and polarization color filters are designed and realized by constructing metal–insulator–metal Fabry–Perot cavities. It is observed that resonance frequencies for designed transmission and reflection filters change up to 25 nm with incident polarization which stems from the polarization‐dependent complex refractive indices of α‐MoO3. The largest contrasts are observed for two orthogonal polarization states parallel to the two orthogonal in‐plane crystal directions. The approach in this study offers new directions for potential applications in the development of polarization‐dependent devices based on 2D van der Waals materials for visible frequencies.
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