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.
Most of hyperbolic metamaterials (HMMs) investigated to date are based on isotropic materials resulting in uniaxial HMMs in which dielectric permittivities perpendicular to the propagation direction are the same. Using an anisotropic material constituent to form a HMM is a promising research direction providing opportunities to control the dielectric permittivity in all three directions independently. Herein, we propose and theoretically demonstrate novel biaxial HMMs composed of multilayer stacks of few-layer black phosphorus (BP) and Au thin films. Black phosphorus is an anisotropic material exhibiting crystal axis-dependent dielectric permittivity due to its puckered crystal structure. The proposed HMM provides previously unattained hyperbolic dispersion relations in which the dielectric permittivity in Z-direction of the structure shows opposite sign from that in X- and Y-directions in the most wavelengths from 400~900nm. Furthermore, we calculated the Purcell factor of the proposed biaxial HMMs using full-field electromagnetic simulations.
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.
Here, a metasurface absorber with switchable spectral response is proposed and discussed. The device is constructed by combining a phase change material (vanadium dioxide, VO 2 ) and a naturally hyperbolic materials (hexagonal boron nitride, hBN), utilizing surface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs), respectively, at different thermal conditions in the same system. Specifically, we take advantage of the excitation of SPhP in hBN below the VO 2 critical temperature (T < T c ) and SPP from VO 2 layers at higher temperatures (T > T c ), to switch between narrowband nearperfect absorption at ~7.2 μm to broadband absorption at 8-12 μm. It is shown that SPPs and SPhPs can be excited, respectively, by controlling the temperature, offering new opportunities in designing tunable artificial materials and devices for promising nanophotonics applications.
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