A multi-dimensional photodetector based on an α-MoO3 grating and graphene.

Integration of multi-dimensional optical information enhances the recognition and anti-interference capabilities of the detection system, allowing for better adaptation to complex environments. Therefore, this technology represents a crucial developmental pathway for the future of infrared optical detectors. In this study, a dual-band polarization photodetector based on a two-dimensional α-MoO3 grating structure is proposed. The structure utilizes the special dispersion property of the α-MoO3 material to excite the localized plasmon resonance, which generates narrowband high absorption peaks with Q-factors as high as 110.24 and 92.65, with peaks close to 1 under TM and TE polarized waves, respectively. The direct measurement of multi-dimensional optical information including intensity, spectrum and polarization states is realized. By adjusting the structural parameters, polarization-dependent dual-band detection can be achieved. Meanwhile, the introduction of graphene material realizes the electronically tunable function of the device. This study provides unexplored strategies for realizing more efficient, flexible and versatile dual-band polarization wave detection.

Coupling polaritons in near-field radiative heat transfer between multilayer graphene/vacuum/α-MoO3/vacuum heterostructures.

Both materials and structures can significantly affect radiative heat transfer, which is more pronounced in the near-field regime of two-dimensional and hyperbolic materials, and has promising prospects in thermophotovoltaics, radiative cooling, and nanoscale metrology. Hence, it is important to investigate the near-field radiative heat transfer (NFRHT) in complicated heterostructures consisting of two-dimensional and hyperbolic materials. Recent studies have reported that adding vacuum layers to multilayer structures can effectively enhance the NFRHT. Take the case of multilayer graphene/α-MoO3 heterostructures: the effect of vacuum layers on these heterostructures has not been studied, and hence investigations on adding vacuum layers between graphene and α-MoO3 layers should be emphasized. In this work, we conduct an investigation of the NFRHT between multilayer graphene/vacuum/α-MoO3/vacuum heterostructures. Compared to unit graphene/α-MoO3 heterostructures without vacuum layers, it is found that NFRHT between the heterostructures with vacuum layers can be suppressed to 49.1% when the gap distance is 10 nm, and can be enhanced to 16.3% when the gap distance is 100 nm. These phenomena are thoroughly explained by the coupling of surface plasmon polaritons and hyperbolic phonon polaritons. Energy transmission coefficients and spectral heat flux are analysed during the calculations changing chemical potentials of graphene, thicknesses of vacuum layers, and α-MoO3 layers. This study is expected to provide guidance in implementing the thermal management of reasonable NFRHT devices based on graphene/α-MoO3 heterostructures.

A tungsten-based metamaterial emitter for solar thermophotovoltaic systems.

Solar thermophotovoltaic systems are capable of showing efficient photoelectric conversion and are expected to surpass the Shockley-Queisser limit, owing to the spectrum-selective functionality of metamaterial selective emitters. Generally, metamaterial emitters are manufactured from multifarious materials, which also makes their manufacturing process complicated. Here, we propose a tungsten-only emitter composed of two rectangular bars with different widths and heights arranged in a cruciform structure, featuring a rectangular cavity at the top. Results from the simulations reveal that the emissivity of the metamaterial emitter exceeds 90% at the wavelength of 950-1590 nm and drops below 20% for wavelengths exceeding 2025 nm, which can effectively match GaSb photovoltaic cells. The outstanding emission performance is attributed to the coupling effect of surface plasmon resonance, cavity resonance and guided mode resonance, as evidenced by the analysis of electric and magnetic fields. We also explored the radiation spectrum in the 500-2500 K temperature range and found that it performed best at 1400 K. It is concluded that the emission performance is slightly affected by structural parameters and angles. This study presents a meaningful exploration of efficient solar utilization.

Comparative analysis of two models for the optical response of a thin slab: a film of finite thickness versus a surface current sheet.

An accurate description of the electromagnetic properties of materials is fundamental to optical and electric devices. As a current research hotspot, thin slabs generally are modeled as a film of finite thickness with a dielectric function. However, inspired by two-dimensional materials, thin slabs can be regarded as surface current sheets with conductivity. Due to the convenience of the latter in simplifying the calculations, it becomes increasingly significant to determine the equivalent conditions of the two models. In this work, we compare the differences between the thin film and surface current models in calculating the transmissivity, reflectivity, and absorptivity of a SiC film. For normal incidence, the difference between the calculations of the two models is only non-negligible when the thickness is large (500 nm), because of the invalidation of surface current models and the excitation of Fabry-Perot resonance. In particular, we derive analytical formulas for the relative error in transmittance versus phase difference, which can be used to predict the difference between the two models as a function of film thickness. For oblique incidence, the two models have significant differences in the vicinity of epsilon-near-zero (ENZ) frequency. The excitation of the Berreman leaky mode in a thin film model causes a narrow blank absorption peak close to the ENZ frequency. However, we found that the surface current model is unable to form this resonance mode and further demonstrate it theoretically. In addition, it is found that the two models are equivalent in the case of a transverse electric wave even though the incidence is oblique. This work can enhance the awareness of the light-matter interaction and open unprecedented avenues for designing ultrathin optical devices.

Epsilon-near-zero characteristics of near-field radiative heat transfer between α-MoO3 slabs.

Epsilon-near-zero (ENZ) materials, which manifest a wealth of exotic optical characteristics, have attracted significant research interest in recent years. However, these characteristics have rarely been considered in the study of near-field radiative heat transfer (NFRHT). In this work, we investigated the ENZ characteristics of the NFRHT between two symmetric biaxial α-MoO3 slabs. The numerical results show that the NFRHT is greatly enhanced around the ENZ frequency of 1.604 × 1014 rad s-1. Notably, near the other two ENZ frequencies (1.832 × 1014 rad s-1 and 1.891 × 1014 rad s-1), only the NFRHT between α-MoO3 slabs of certain thicknesses is enhanced. The reasons can be explained by the fact that the VHPs can be excited in almost all azimuthal angles at angular frequencies of 1.604 × 1014 rad s-1 and 1.891 × 1014 rad s-1. For the ENZ frequency of 1.832 × 1014 rad s-1, the VHPs can be excited at the top and bottom, while the SHPs excited at the left and right sides. It is worth noting that both the hyperbolic and ENZ characteristics affect the NFRHT between α-MoO3 slabs. Moreover, the excitation is strongly dependent on the thickness of the slab. Our findings contribute to understanding the physical mechanisms underlying the characteristics of the NFRHT at ENZ frequencies.

Pattern-free solar absorber driven by superposed Fabry-Perot resonances.

Solar absorbers, which harvest solar irradiation in the form of heat, have promising prospects in electricity, heating, desalination, and energy storage. However, in previous work, absorbers are usually designed as nanostructures involving photolithography to obtain a superior spectral absorption performance, which inevitably increases the cost and complexity of fabrication. Here, we propose a pattern-free absorber consisting of the TiN-SiO2-based multilayer structure for effective solar energy utilization. Numerical results show that the maximum average spectral absorption of a 3-cell multilayer structure is up to 93.5% at wavelengths of 0.3-2.5 µm. The underlying physical mechanism can be explained by the coupling of superposed Fabry-Perot (FP) resonances and the intrinsic absorption of TiN, which is further confirmed via the electric field and power dissipation density distributions. The effect of the geometric parameters and materials of multilayer structures on the spectral absorption performance is investigated. Moreover, we discuss the influence of the incidence angle on solar absorbers and demonstrate that an average spectral absorption of more than 80% can be obtained even at a large incidence angle of 60°. Finally, when the number of cells in the multilayer structure is increased to 6, the average spectral absorption can reach 96%. The findings in this work will deepen the understanding of FP resonance and pave a novel path for efficient solar thermal energy utilization.

Gradient index effect assisted anisotropic broadband absorption in α-MoO3 metamaterial.

As an excellent natural hyperbolic material (HM), α-M o O 3 has a larger hyperbolic bandwidth and longer polariton lifetime than other HMs, which makes it an ideal candidate for broadband absorbers. In this work, we theoretically and numerically investigated the spectral absorption of an α-M o O 3 metamaterial using the gradient index effect. The results show that the absorber has an average spectral absorbance of 99.99% at 12.5-18 µm at transverse electric polarization. When the incident light is transverse magnetic polarization, the broadband absorption region of the absorber is blueshifted, and a similar strong absorption is achieved at 10.6-12.2 µm. By simplifying the geometric model of the absorber using equivalent medium theory, we find that the broadband absorption is caused by the refractive index matching of the metamaterial to the surrounding medium. The electric field and power dissipation density distributions of the metamaterial were calculated to clarify the location of the absorption. Moreover, the influence of geometric parameters of pyramid structure on broadband absorption performance was discussed. Finally, we investigated the effect of polarization angle on the spectral absorption of the α-M o O 3 metamaterial. This research contributes to developing broadband absorbers and related devices based on anisotropic materials, especially in solar thermal utilization and radiation cooling.

Optical axis-driven tunable Brewster effect in anisotropic materials.

The Brewster effect, which is known as a notable physical law, has promising prospects in perfect absorption and angular selectivity transmission. The Brewster effect in isotropic materials has been investigated extensively in previous works. However, the research on anisotropic materials has been rarely carried out. In this work, we theoretically investigate the Brewster effect in quartz crystals with tilted optical axes. The conditions for the occurrence of the Brewster effect in anisotropic materials are derived. The numerical results show that by changing the orientation of the optical axis, we have effectively regulated the Brewster angle of crystal quartz. The reflection of crystal quartz versus the wavenumber and incidence angle at different tilted angles is studied. In addition, we discuss the effect of the hyperbolic region on the Brewster effect of crystal quartz. The Brewster angle negatively correlates with the tilted angle when the wavenumber is 460c m -1 (Type-II). In contrast, when the wavenumber is 540c m -1 (Type-I), the Brewster angle positively correlates with the tilted angle. Finally, the relationship between the Brewster angle and wavenumber at different tilted angles is investigated. The findings in this work will broaden the research field of crystal quartz and open the door for tunable Brewster devices based on anisotropic materials.

Spinning thermal radiation from twisted two different anisotropic materials.

Thermal radiation has applications in numerous fields, such as radiation cooling, thermal imaging, and thermal camouflage. Micro/nanostructures such as chiral metamaterials with polarization-dependent or symmetry-breaking properties can selectively emit circularly (spin) polarized polarization waves. In this paper, we propose and demonstrate the spinning thermal radiation from two twisted different anisotropic materials. Taking industrial polymer and biaxial hyperbolic material α-MoO3 as an example, it is found that broadband spinning thermal radiation can be obtained from 13 µm to 18 µm. The spin thermal radiation of the proposed twisted structure originates from the combined effect of polarization conversion of circularly polarized wave and selective absorption of linearly polarized wave by the top and bottom layers of anisotropic materials, respectively. Besides, the narrowband spinning thermal radiation with 0.9 circular dichroism is achieved at wavelength of 12.39 µm and 18.93 µm for finite thickness α-MoO3 due to the epsilon-near-zero mode, and the magnetic field distribution can confirm the phenomenon. This work achieves broadband and narrowband spin thermal radiation and significantly enhances circular dichroism, which may have applications in biological sensing and thermal detection.

An ultra-broadband and wide-angle absorber based on a TiN metamaterial for solar harvesting.

The efficient absorption of solar spectrum radiation is the most critical step in solar thermal utilization. In this work, a near-perfect metamaterial solar absorber with broadband, wide angle, polarization insensitivity, and high-temperature resistance is proposed and investigated. The absorber takes advantage of the high melting point material, which consists of a TiN reflector, a SiO2 insulating layer, and a TiN ring array. In the spectral range of 300-2500 nm, an average absorption of 97.6% is achieved. The percentage of absorbed energy in the AM1.5 spectral radiation can reach 95.8%. The electric and magnetic field distributions show that the high absorption is attributed to the coupling effect of surface plasmon resonance, guided mode resonance and cavity resonance. Furthermore, the absorber is found to maintain high absorption performance at large angles of solar radiation and to be insensitive to polarization. The designed absorber maintains its broadband absorption performance well within certain geometric tolerances. This reduces the complexity and cost of manufacturing and facilitates practical applications. The research indicates that this work will benefit the design and application of solar thermal conversion and thermophotovoltaic systems.

The optical properties of dumbbell-type nanorods for solar photothermal conversion.

Due to localized surface plasmon resonance (LSPR), plasmonic nanoparticles have exciting potential for improving solar photothermal conversion performance and have been extensively studied. However, in addition to enhanced solar absorption, scattering is also enhanced with the occurrence of LSPR, which is detrimental to the direct absorption of solar energy. The nanoparticles that can excite magnetic resonance can alleviate the above problem but have rarely been studied. In this work, we propose a dumbbell-type nanorod that can excite both electrical resonance and magnetic resonance. The analysis of its optical properties reveals that the dumbbell-type nanorod can excite multiple absorption peaks to enhance absorption while reducing scattering. The mechanism is the coupling between electrical resonance and magnetic resonance. Furthermore, the analysis with the slotted position of the dumbbell-type nanorod shows that the slotting should not occur at the two ends of the nanorod. It is also shown that the optical properties of the dumbbell-type nanorod can be effectively tuned by adjusting the geometric parameters of the slot. The dumbbell-type nanorods are promising for solar photothermal conversion and can be a candidate material for direct absorption solar collectors (DASCs).

Optical axis-driven modulation of near-field radiative heat transfer between two calcite parallel structures.

Recently, the increasing research on the anisotropic optical axis (OA) has provided a novel way to control light. However, this method is rarely applied to modulate the near-field radiative heat transfer (NFRHT). In this work, we investigate the influences of the OA orientation of calcite on the NFRHT between two calcite parallel structures. The numerical results demonstrate that the near-field radiative heat flux is larger when the OA is along the z-axis than that when the OA is along the x-axis. This is because when the OA is along the z-axis, the excited hyperbolic polaritons exhibit a full range of angles in the type I hyperbolic band. In contrast, the excited hyperbolic polaritons exhibit a finite angle when the OA is along the x-axis. Moreover, it is further investigated that the thickness has a significant impact on the NFRHT between calcite slabs. Our findings may highlight the promising role of calcite in manipulating NFRHT.

Metasurfaces Assisted Twisted α-MoO3 for Spinning Thermal Radiation.

Spinning thermal radiation has demonstrated applications in engineering, such as radiation detection and biosensing. In this paper, we propose a new spin thermal radiation emitter composed of the twisted bilayer α-MoO3 metasurface; in our study, it provided more degrees of freedom to control circular dichroism by artificially modifying the filling factor of the metasurface. In addition, circular dichroism was significantly enhanced by introducing a new degree of freedom (filling factor), with a value that could reach 0.9. Strong-spin thermal radiation resulted from the polarization conversion of circularly polarized waves using the α-MoO3 metasurface and selective transmission of linearly polarized waves by the substrate. This allowed for extra flexible control of spinning thermal radiation and significantly enhanced circular dichroism, which promises applications in biosensing and radiation detection. As a result of their unique properties, hyperbolic materials have applications not only in spin thermal radiation, but also in areas such as near-field thermal radiation. In this study, hyperbolic materials were combined with metasurfaces to offer a new idea regarding modulating near-field radiative heat transfer.