Ultrathin High Quality‐Factor Planar Absorbers/Emitters Based on Uniaxial/Biaxial Anisotropic van der Waals Polar Crystals

Abstract

Tailoring the absorption/emission through nanostructures from broadband to narrowband has attracted increasing attention due to the merits of high efficiency and compactness. However, most of the reported narrowband emitters require either complex fabrication process or thick coating of the suitable materials (~mm). In this paper, by exciting Berreman mode through a combination of Au mirrors and ultra‐thin layers of low‐loss uniaxial/biaxial anisotropic 2D van der Waals polar crystal hBN/α‐MoO3, a narrowband absorption/emission peak is realized in the mid‐infrared region. The measured quality‐factor of Berreman mode for hBN/Au and α‐MoO3/Au structure can reach up to 134 and 164, respectively. The deep subwavelength thickness (~nm) and bilayer architecture simplify the fabrication process. Furthermore, taking advantage of the in‐plane birefringence originating from α‐MoO3, the polarization of the absorbed/emitted radiation can be controlled through this planar configuration. Such ultrathin narrowband absorbers/emitters provide new opportunities for the advancements in thermal sources, infrared sensors, and thermophotovoltaic power generation systems.

A redshifted photonic bandgap and wide-angle polarization selection in an all-hyperbolic-metamaterial one-dimensional photonic crystal

“…[41][42][43][44] The iso-frequency curves (IFCs) of HMMs under TM polarization are hyperbolas while those under TE polarization are circles. Owing to their hyperbolic IFCs, HMMs have been utilized to achieve absorbers, [45][46][47][48][49][50] fibers, 51,52 lasers, 53,54 enhancement of the photonic spin Hall effect, 55,56 and surface/volume plasmon polariton engineering. 57,58 In particular, researchers proposed a class of 1-D PhCs composed of alternating HMMs and isotropic dielectrics to achieve PBG engineering.…”

Coupled Tamm Phonon and Plasmon Polaritons for Designer Planar Multiresonance Absorbers

“…[55,56] Notably, the Berreman modes near the longitudinal optical phonon of hBN (1650 cm −1 ) are excited in this artificially isotropic hBN, but they are absent in the real material because they can only be excited along the out-of-plane axis. [57] As such, using hyperbolic media allows for selectively turning off some modes, which in many applications may be desirable (Sections S5 and S8, Supporting Information). Additionally, the responses of the isotropic and anisotropic hBNs differ at high incident angles, e.g., 60°, while they are nearly identical at low angles.…”

“…Note that the resonance at ≈1400 cm −1 for the THP-absorbers cannot be avoided, as they originate from the TO phonon absorption and the high positive permittivity of hBN near the TO phonon. [11,57] However, considering the extremely narrow linewidth (≈5 cm −1 ), the influence of that mode can be safely neglected in most cases. For instance, for thermal emitter applications, assuming 600 °C working temperature, the emitted power at ≈1400 cm −1 only counts for 4% of the total emitted power in the 1000-2000 cm −1 range (Section S6, Supporting Information) for these designs.…”

Thermal Vertical Emitter of Ultra‐High Directionality Achieved Through Nonreciprocal Magneto‐Optical Lattice Resonances

“…[1][2][3][4][5] Thermal emitters with a well-defined emission direction and narrow bandwidth have important applications in thermal imaging, [6] infrared (IR) camouflage and countermeasures, [7,8] gas/ optical sensing with IR absorption, [9,10] etc. Metamaterials or metasurfaces like multilayer structures, [11,12] diffraction gratings, [13][14][15][16] van der Waals crystals, [17] or photonic crystals [18] allow tailoring of the emission/absorption spectrum at certain directions. Lamber's cosine law shows that a high-efficiency thermal emitter (i.e., low power consumption) requires near-unity emissivity at zero angle without any emission at other polar angles.…”