Lithography-free tunable absorber at visible region via one-dimensional photonic crystals consisting of an α-MoO3 layer

Highly Tunable Light Absorber Based on Topological Interface Mode Excitation of Optical Tamm State

Optical absorbers based on Tamm plasmon states are known for their simple structure and high operational efficiency. However, these absorbers often have limited absorption channels, and it is challenging to continuously adjust their light absorption rates. Here, we propose a Tamm plasmon state optical absorber composed of a layered stack structure consisting of one-dimensional topological photonic crystals and graphene nano-composite materials. Using the four-by-four transfer matrix method, we investigate the structural relationship of the absorber. Our results reveal that topological interface states (TISs) effectively excite the optical Tamm state (OTS), leading to multiple absorption peaks. This expands the number of absorption channels, with the coupling number of the TIS determining the transmission quality of these channels-a value further adjustable by the period number of the photonic crystals. Tuning the filling factor, refractive index, and thickness of the graphene nano-composite material allows for a wide range of control over the device's absorption rate, from 0 to 1. Additionally, adjusting the defect layer thickness, incident angle, and Fermi energy enables us to control the absorber's operational bandwidth and the switching of its absorption effect. This work presents a new approach to expanding the tunability of optoelectronic devices.

Active control of resonant asymmetric transmission based on topological edge states in paired photonic crystals with a Ge2Sb2Te5 film

For many high-precision applications such as filtering, sensing, and photodetection, active control of resonant responses of metasurfaces is crucial. Herein, we demonstrate that active control of resonant asymmetric transmission can be realized based on the topological edge state (TES) of an ultra-thin G e 2 S b 2 T e 5 (GST) film in a photonic crystal grating (PCG). The PCG is composed of two pairs of one-dimensional photonic crystals (PCs) separated by a GST film. The phase change of the GST film re-distributes the field distributions of the PCG; thus active control of narrowband asymmetric transmission can be achieved due to the switch of the on-off state of the TES. According to multipole decompositions, the appearance and disappearance of the synergistically reduced dipole modes are responsible for the high-contrast asymmetric transmission of the PCG. In addition, the asymmetric transmission performances are robust to the variation of structural parameters, and good unidirectional transmission performances with a high peak transmission and high contrast ratio can be balanced, as the layer number of the two PCs is set as four. By changing the crystallization fraction of GST, the peak transmission and peak contrast ratio of asymmetric transmission can be flexibly tuned with the resonance locations kept almost the same.

Enhanced efficiency of launching hyperbolic phonon polaritons in stacked α-MoO3 flakes

In this work, we reported a systemic study on the enhanced efficiency of launching hyperbolic phonon polaritons (PhPs) in stacked α-phase molybdenum trioxide (α-MoO₃) flakes. By using the infrared photo-induced force microscopy (PiFM), real-space near-field images (PiFM images) of mechanically exfoliated α-MoO₃ thin flakes were recorded within three different Reststrahlen bands (RBs). As referred with PiFM fringes of the single flake, PiFM fringes of the stacked α-MoO₃ sample within the RB 2 and RB 3 are greatly improved with the enhancement factor (EF) up to 170%. By performing numerical simulations, it reveals that the general improvement in near-field PiFM fringes arises from the existence of a nanoscale thin dielectric spacer in the middle part between two stacked α-MoO₃ flakes. The nanogap acts as a nanoresonator for prompting the near-field coupling of hyperbolic PhPs supported by each flake in the stacked sample, contributing to the increase of polaritonic fields, and verifying the experimental observations Our findings could offer fundamental physical investigations into the effective excitation of PhPs and will be helpful for developing functional nanophotonic devices and circuits.

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.

An active tunable terahertz functional metamaterial based on hybrid-graphene vanadium dioxide

In this paper, we propose a switchable and tunable functional metamaterial device based on hybrid graphene-vanadium dioxide (VO₂). Using the properties of the metal-insulator transition in VO₂, the proposed metamaterials can enable switching between tunable circular dichroism (CD) and dual-band perfect absorption in the terahertz region. When VO₂ is in the insulator state, a polarization-selective single-band perfect absorption can be achieved for circularly polarized waves, thus resulting in a strong CD response with a maximum value of 0.84. When VO₂ acts as a metal, there is a tunable dual-band perfect absorption for the designed metamaterial device under the illumination of x-polarization waves. The operation mechanism behind the phenomena can be explained by utilizing the electric field distribution and the coupled mode theory. Moreover, the influences of the Fermi energy of graphene and geometrical parameters on the CD and absorption spectra are discussed in detail. Our proposed switchable and tunable metamaterial can provide a platform for designing versatile functional devices in the terahertz region.

Selective excitation of hyperbolic phonon polaritons-induced broadband absorption via α-MoO3 square pyramid arrays

Optical anisotropy of α-MoO₃ in its reststrahlen (RS) bands provides exciting opportunities for constructing the polarization-dependent devices. However, achieving broadband anisotropic absorptions through the same α-MoO₃ arrays is still challenging. In this study, we demonstrate that selective broadband absorption can be achieved by using the same α-MoO₃ square pyramid arrays (SPAs). For both the x and y polarizations, the absorption responses of the α-MoO₃ SPAs calculated by using the effective medium theory (EMT) agreed well with those of the FDTD, indicating the excellent selective broadband absorption of the α-MoO₃ SPAs are associated with the resonant hyperbolic phonon polaritons (HPhPs) modes assisted by the anisotropic gradient antireflection (AR) effect of the structure. The near-field distribution of the absorption wavelengths of the α-MoO₃ SPAs shows that the magnetic-field enhancement of the lager absorption wavelength tends to shift to the bottom of the α-MoO₃ SPAs due to the lateral Fabry-Pérot (F-P) resonance, and the electric-field distribution exhibits the ray-like light propagation trails due to the resonance nature of the HPhPs modes. In addition, broadband absorption of the α-MoO₃ SPAs can be maintained if the width of the bottom edge of the α-MoO₃ pyramid is large than 0.8 μm, and excellent anisotropic absorption performances are almost immune to the variations of the thickness of the spacer and the height of the α-MoO₃ pyramid.