All-dielectric perfect absorber based on quadrupole modes

Tunable ultraviolet polarized light switch based on all-dielectric metasurfaces on a stretchable substrate

Most ultraviolet (UV) passive optics are currently non-tunable and lack external modulation methods because of the poor tunability of wide-bandgap semiconductor materials in UV working media. This study investigates the excitation of magnetic dipole resonances in the solar-blind UV region by hafnium oxide metasurfaces using elastic dielectric polydimethylsiloxane (PDMS). The near-field interactions between the resonant dielectric elements can be modulated by the mechanical strain of the PDMS substrate, which can flatten the structure's resonant peak beyond the solar-blind UV wavelength range, thereby turning on or off the optical switch in the solar-blind UV region. The device has a facile design and can be used in various applications, such as UV polarization modulation, optical communications, and spectroscopy.

Plasmonic Perfect Absorber Utilizing Polyhexamethylene Biguanide Polymer for Carbon Dioxide Gas Sensing Application

In this paper a perfect absorber with a photonic crystal cavity (PhC-cavity) is numerically investigated for carbon dioxide (CO₂) gas sensing application. Metallic structures in the form of silver are introduced for harnessing plasmonic effects to achieve perfect absorption. The sensor comprises a PhC-cavity, silver (Ag) stripes, and a host functional material-Polyhexamethylene biguanide polymer-deposited on the surface of the sensor. The PhC-cavity is implemented within the middle of the cell, helping to penetrate the EM waves into the sublayers of the structure. Therefore, corresponding to the concentration of the CO₂ gas, as it increases, the refractive index of the host material decreases, causing a blue shift in the resonant wavelength and vice versa of the device. The sensor is used for the detection of 0-524 parts per million (ppm) concentration of the CO₂ gas, with a maximum sensitivity of 17.32 pm (pico meter)/ppm achieved for a concentration of 366 ppm with a figure of merit (FOM) of 2.9 RIU^-1. The four-layer device presents a straightforward and compact design that can be adopted in various sensing applications by using suitable host functional materials.

Graphene perfect absorber based on degenerate critical coupling of toroidal mode

Graphene is a two-dimensional material with great potential for photodetection and light modulation applications owing to its high charge mobility. However, the light absorption of graphene in the near-infrared range is only 2.3%, limiting the sensitivity of graphene-based devices. In this study, we propose a graphene perfect absorber based on degenerate critical coupling comprising monolayer graphene and a hollow silicon Mie resonator array. In particular, monolayer graphene achieves perfect absorption by controlling the periods and holes of the Mie resonators. The proposed graphene perfect absorber can significantly improve the sensitivity of graphene-based devices.

High-quality factor mid-infrared absorber based on all-dielectric metasurfaces

The absorption spectrum of metasurface absorbers can be manipulated by changing structures. However, narrowband performance absorbers with high quality factors (Q-factor) are hard to achieve, mainly for the ohmic loss of metal resonators. Here, we propose an all-dielectric metasurface absorber with narrow absorption linewidth in the mid-infrared range. Magnetic quadrupole resonance is excited in the stacked Ge-Si₃N₄ nanoarrays with an absorption of 89.6% and a Q-factor of 6120 at 6.612 µm. The separate lossless Ge resonator and lossy Si₃N₄ layer realize high electromagnetic field gain and absorption, respectively. And the proposed method successfully reduced the intrinsic loss of the absorber, which reduced the absorption beyond the resonant wavelength and improved the absorption efficiency of Si₃N₄ in the low loss range. Furthermore, the absorption intensity and wavelength can be modulated by adjusting the geometric parameters of the structure. We believe this research has good application prospects in mid-infrared lasers, thermal emitters, gas feature sensing, and spectral detection.

Flexible and efficient fabrication of a terahertz absorber by single-step laser direct writing

Laser direct writing (LDW) is a promising candidate for the fabrication of all-dielectric THz absorbers for its high flexibility and material compatibility. However, multi-step processing or multi-layer materials are required to compensate for the nonideal features of LDW to realize good absorption performance. To further explore the potential of LDW in flexible and cost-effective THz absorber fabrication, in this work, we demonstrate a design method of THz absorbers fully considering and utilizing the characteristics of laser processing. Specifically, we first numerically analyze that by properly combining basic structures processed by single-step LDW, good and adjustable absorption performance can be achieved on a single-layer substrate. Then we experimentally fabricate THz absorbers by processing periodic composite structures, which are combined by grooves and circular holes, on single-layer doped silicon using LDW. Experimental results show that our method can fabricate THz absorbers at a speed of 3.3 mm²/min with an absorptivity above 90% over a broadband of 1.8-3 THz. Our method provides a promising solution for the flexible and efficient fabrication of all-dielectric broadband THz absorbers.

Multiple dipolar resonant silicon-based metamaterials for high-performance optical switching and sensing

Dielectric nanostructures reinforcing light-matter interactions by manipulating geometric parameters have a sound momentum in optoelectronic applications. Here, we construct and numerically demonstrate a new platform with multiple dipolar resonant behaviors or impressive switching operation and optical sensing with a high sensitivity and figure of merit (FOM) via the graphene-silicon combined metamaterials. Ultra-sharp resonances are excited by introducing broken symmetry in such all-dielectric metamaterials (ADMs) consisting of two silicon trapezoidal bodies on a silica substrate. By analyzing the distributions of the electromagnetic fields and current densities, we find that two types of multipole modes have been excited to support multiple ultra-narrowband resonances in the near-infrared range. The influence of geometers, such as period, thickness, asymmetry parameters, and polarization angle of the incident light, has also been studied. In addition, by adjusting the Fermi levels of graphene, we realize a 95% amplitude modulation efficiency, which manifests perfect capacity for an optical switch. According to the calculated results, the highest sensitivity can reach 447.5 nm/RIU and a large FOM is also up to 1173 RIU^-1. This platform not only introduces new insight onto the achievement of high-quality ultra-sharp resonant responses but also offers a distinct possibility for the further development of high-quality related applications in optical sensors, notch filtering, strong light-matter interactions including the nonlinear optics, and multispectral optoelectronics.