Tunable Perfect Narrow-Band Absorber Based on a Metal-Dielectric-Metal Structure

Spatially selective narrow band and broadband absorption in Ag/SiO2/Ag based trilayer thin films by oblique angle deposition of SiO2layer

We have experimentally demonstrated spatially selective absorption in Ag-SiO2-Ag based trilayer thin films by tuning the deposition angle of SiO2layer. These structures generate cavity resonance which can be tuned across the substrate locations due to spatially selective thickness and refractive index of silicon oxide (SiO2) film sandwiched between metallic silver (Ag) mirrors. Spatially selective property of SiO2film is obtained by oblique angle deposition technique using an electron beam evaporation system. The resonance wavelength of absorption in this trilayer structure shifts across the substrate locations along the direction of oblique deposition. The extent of shift in resonance increases with increase in angle of deposition of SiO2layer. 4.14 nm mm-1average shift of resonance wavelength is observed when SiO2is deposited at 40° whereas 4.76 nm mm-1average shift is observed when SiO2is deposited at 60°. We observed that the width of resonance increases with angle of deposition of the cavity layer and ultimately the resonant absorption disappears and becomes broadband when SiO2is deposited at glancing angle deposition (GLAD) configuration. Our study reveals that there is a suitable range of oblique angle of deposition from 40° to 60° for higher spatial tunability and resonant absorption whereas the absorption becomes broadband for glancing angle deposition.

A tunable wide-angle narrowband perfect absorber based on an optical cavity containing hyperbolic metamaterials

We demonstrate that wide-angle narrowband absorption can be achieved from a microcavity where a hyperbolic metamaterial and a dielectric layer are sandwiched between two metal reflectors. As the incident angle changes, the phase-shift variation in the hyperbolic metamaterial can compensate that in the dielectric layer and, consequently, result in the angle-insensitive Fabry-Perot resonance in the proposed cavity. Silicon, indium tin oxide (ITO), and gold layers are used to construct the microcavity to produce a narrow absorption band in the near-infrared region. Our device exhibits good absorption stability over a wide angle range of incidence from 0° to 70°. Moreover, the absorption wavelength can be tuned by changing the thickness of the resonator. The presented absorber may find potential applications in the design of narrowband thermophotovoltaic emitters, sensitive detectors, filters, etc.

Tricolor narrowband planar perovskite photodetectors based on FP microcavity structure

This paper presents a novel tunable narrowband photodetector based on Ag-MgF2-Ag (metal-dielectric-metal: MDM) Fabry-Perot (FP) microcavity structure. The tunability is achieved through precise adjustment of the thickness of the metal and intermediate dielectric layers of the FP microcavity, taking into account the response spectral range of planar perovskite. After optimizing the parameters mentioned above, the prototype devices were prepared by combining the perovskite layer and MDM layer. The center wavelength of the planar detector can be tuned from 430 nm to 680 nm within the detection band of 400-800 nm, with a narrow FWHM about 30 nm and a relatively high response of 0.05 A/W @ 5 V bias voltage for 500 nm. Meanwhile the rise and fall times of the detector are 375 ms and 550 ms, respectively. The experimental results are corroborated by the theory. Our design is highly beneficial to such applications as hyperspectral photography and color-related active optical devices, which paves the way to design this kind of triple structure.

Fano resonance in a dolomite phase-change multilayer design for dynamically tunable omnidirectional monochromatic thermal emission

In this Letter, we unveil the unprecedented optical phonon response of CaMg(CO₃)₂ (dolomite) thin film in the design of a planar ultra-narrowband mid-infrared (MIR) thermal emitter. Dolomite (DLM) is a carbonate mineral composed of calcium magnesium carbonate, which can inherently accommodate highly dispersive optical phonon modes. Utilizing strong interference in the Al-DLM bilayer, a lithography-free planar thermal emitter is realized with near-unity omnidirectional emission at a specific resonance wavelength of 7.12 µm. Further incorporation of embedded vanadium dioxide (VO₂) phase change material (PCM) enables the excitation of hybrid Fano resonances with dynamic spectral tunability. The findings of this study can have multiple applications, ranging from biosensing and gas sensing to thermal emission.

Ultra-broadband, wide-angle plus-shape slotted metamaterial solar absorber design with absorption forecasting using machine learning

Energy utilization is increasing day by day and there is a need for highly efficient renewable energy sources. Solar absorbers with high efficiency can be used to meet these growing energy demands by transforming solar energy into thermal energy. Solar absorber design with highly efficient and Ultra-broadband response covering visible, ultraviolet, and near-infrared spectrum is proposed in this paper. The absorption response is observed for three metamaterial designs (plus-shape slotted design, plus-shape design, and square-shape design) and one optimized design is used for solar absorber design based on its high efficiency. The design results are compared with AM 1.5 spectral irradiance response. The electric field response of the plus-shape slotted metamaterial design is also presented which matches well with the absorption results of different solar spectrum regions. The results proved that the attained absorption response showing wide angle of incidence. Machine learning is also used to examine the design data in order to forecast absorption for various substrate thickness, metasurface thickness, and incidence angles. Regression and forecasting simulations based on machine learning are used to try to anticipate absorber behaviour at forthcoming and intermediate wavelengths. Simulation results prove that Machine Learning based methods can lessen the obligatory simulation resources, time and can be used as an effective tool while designing the absorber. The proposed highly efficient, wide-angle, ultra-broadband solar absorber design with its behavior prediction capability using machine learning can be utilized for solar thermal energy harvesting applications.

Suppression of gap plasmon resonance for high-responsivity metal-insulator-metal near-infrared hot-electron photodetectors

Binary metal layers composed of a grating and a thin film are designed for high-responsivity metal-insulator-metal (MIM) near-infrared hot-electron photodetectors (HEPDs). The binary metal grating structure HEPDs demonstrate a strong asymmetrical optical absorption and result in a high current responsivity. In our devices, the top and bottom absorption ratio is as high as 76:1, much higher than that in the traditional grating structure HEPDs. The maximum zero-biased responsivity is 0.585 mA/W at 1550 nm by employing a five-step electrical model, which is 3.42 times that of the traditional silver grating structure devices. Simply changing the grating period enables spectrally selective photodetection covering a wide range of 500 nm at the near-infrared band with net absorption higher than 0.95 and linewidths narrower than 0.7 meV.

Narrowband and flexible perfect absorber based on a thin-film nano-resonator incorporating a dielectric overlay

We developed a flexible perfect absorber based on a thin-film nano-resonator, which consists of metal-dielectric-metal integrated with a dielectric overlay. The proposed perfect absorber exhibits a high quality (Q-)factor of ~ 33 with a narrow bandwidth of ~ 20 nm in the visible band. The resonance condition hinging on the adoption of a dielectric overlay was comprehensively explored by referring to the absorption spectra as a function of the wavelength and thicknesses of the overlay and metal. The results verified that utilizing a thicker metal layer improved the Q-factor and surface smoothness, while the presence of the overlay allowed for a relaxed tolerance during practical fabrication, in favor of high fidelity with the design. The origin of the perfect absorption pertaining to zero reflection was elucidated by referring to the optical admittance. We also explored a suite of perfect absorbers with varying thicknesses. An angle insensitive performance, which is integral to such a flexible optical device, was experimentally identified. Consequently, the proposed thin-film absorber featured an enhanced Q-factor in conjunction with a wide angle of acceptance. It is anticipated that our absorber can facilitate seminal applications encompassing advanced sensors and absorption filtering devices geared for smart camouflage and stealth.

Special Issue: “Thin Films for Energy Harvesting, Conversion, and Storage”

Efficient clean energy harvesting, conversion, and storage technologies are of immense importance for the sustainable development of human society. To this end, scientists have made significant advances in recent years regarding new materials and devices for improving the energy conversion efficiency for photovoltaics, thermoelectric generation, photoelectrochemical/electrolytic hydrogen generation, and rechargeable metal ion batteries. The aim of this Special Issue is to provide a platform for research scientists and engineers in these areas to demonstrate and exchange their latest research findings. This thematic topic undoubtedly represents an extremely important technological direction, covering materials processing, characterization, simulation, and performance evaluation of thin films used in energy harvesting, conversion, and storage.