Tunable Perfect Absorption Research Articles

Tunable narrowband perfect absorption and active phase transition driven by gain-coupled bound states in the continuum: A basis for sensing

Tunable narrowband perfect absorption and active phase transition driven by gain-coupled bound states in the continuum: A basis for sensing

MEMS-based meta-emitter with actively tunable radiation power characteristic

We propose a meta-emitter based on micro-electro-mechanical system (MEMS) technology. The main structure of the meta-emitter unit cell is composed of four symmetrically split crosses of Au and SiO2 bilayer cantilevers. By changing the size of the cantilevers, this MEMS-based meta-emitter can realize the tunable perfect absorption, and the absorption spectrum is within the longwave infrared (LWIR) wavelength from 8.90 to 11.90 µm. When the surface temperature of the meta-emitter rises, the electrothermal actuation mechanism is performed through the different thermal expansion coefficient (TEC) of the bilayer cantilevers. Therefore, the cantilevers will be bent downward and the bending height of the cantilevers decreases linearly. In such case, the peak value of thermal radiation power can be tuned from the wavelength of 9.52 µm to 10.48 µm when the temperature of meta-emitter is increased from 293 to 1290 K. This proposed MEMS-based meta-emitter is an excellent LWIR light source and has potential application prospects in gas sensing, infrared spectroscopy analysis, medical care and so on.

Dynamically tunable perfect absorption based on quantum plasmonic metal-insulator-metal mirror

Achieving perfect absorption in nanostructures plays a crucial role in applications such as photovoltaics, photoluminescence, and sensing. In recent years, a large number of scholars have devoted themselves to the research of metamaterial absorbers from microwave to optical frequencies. However, almost these absorbers are based on the resonant absorption resulting from strong electromagnetic coupling. In this study, we propose a dynamically tunable perfect absorber based on quantum plasmonic tunneling effect with nonelectromagnetic coupling in a metal–insulator–metal mirror, which has never been demonstrated before as far as we know. With the manipulation of tunneling current and tunneling damping via altering the external bias on the tunneling electrodes, the absorptivity of proposed absorber can be modulated at the Er:YAG laser wavelength (2940nm) with the modulation deep near 92%. Moreover, both the absorptivity and full width at half maximum of this absorber have strong robustness to the incidence angle. This ultra-thin perfect absorber, characterized by its rapid response, exceptional integration capabilities, robust tolerance to the incidence angle, and superior controllability, holds tremendous promise for a broad range of applications within the realm of infrared imaging and treatment.

Optical switch based on tunable perfect photon absorption

We investigate the manipulation of optical field within quantum electrodynamics (QED) scheme. In our proposal, the cold three-level atoms interact with a two-sided cavity driven by two coherent beams with a relative phase. Coherent perfect absorption (CPA) appears when the beams are in phase. However, the perfect outputs occur when the beams are out of phase. On the basis of the phase modulation, an optical switch can be operated at a high efficiency. With a coherent control laser, the excited and metastable levels of the atoms are coupled. Electromagnetically induced transparency (EIT) is produced at two-photon resonance, which leads to four-band CPA. By virtue of the control laser, the switching efficiency can be maximized in a weaker probe field, which provides advantages over two-level atoms on weak-signal switching.

Tunable perfect absorption and versatile applications based on a simple black phosphorus metasurface in terahertz

In this study, we introduce a simple black phosphorus (BP) metasurface consisting of a square BP sheet placed on a silicon substrate. The proposed metasurface achieves perfect absorption through guided modes, with an absorbance spectrum peaking at 99.85 % for an electron doping concentration of 0.8 × 1018 m−2. We demonstrate the tunability of the absorbance spectrum using polarization angles and electron doping concentrations, as evidenced by finite-difference time-domain (FDTD) simulations and coupled-mode theory (CMT) calculations. Furthermore, we assess the sensing capabilities of BP-based metasurface, achieving a maximum sensitivity of 120.1 GHz/RIU. Additionally, we investigate the slow-light effect at various electron doping concentrations, reaching a maximum group index of 149924. This work provides valuable insights into the potential practical applications of BP-based metasurface and contributes to the advancement of sophisticated optical devices.

Frequency tunable coherent perfect absorption and lasing in radio-frequency system for ultrahigh-sensitive sensing

A coherent perfect absorption-lasing (CPAL) point in non-Hermitian physics systems has infinite output amplitudes (or quality factor) theoretically and has been utilized for applications. However, practical implementation of CPAL systems faces extreme challenges because of the deviations of electronic components from ideal values, which makes it difficult to attain the CPAL state. Moreover, the amplitude-based output is easily interfered by noises or environments and is inferior as compared to frequency shift-based sensing. Through the investigation of the effects of component deviations on general CPAL systems, we propose an eigenstate tuning method specifically tailored for radio-frequency (RF) CPAL systems. The method enables the realization of CPAL states at any desired frequency with dynamic ranges of signal amplitudes exceeding 60 dB experimentally or to work at a frequency shift-based sensing. Then, a CPAL-based RF sensor system is developed for verification, showing an ultrahigh sensitivity of 1.9 dB/10 fF by measuring output coefficient variation or 9.4 MHz/pF by measuring frequency shift, significantly broadening the application scope of CPAL systems.

Mirror-coupled toroidal dipole bound states in the continuum for tunable narrowband perfect absorption

Metasurfaces have received a significant amount of attention in recent years as they offer novel ways for wave front control, subwavelength light focusing, and flat optics. Here, we employ sharp resonances in metasurfaces originated from the physics of bound states in the continuum (BICs) for both engineering and enhancing the absorption of light. Specifically, using temporal coupled-mode theory and numerical simulations, we demonstrate that an all-dielectric metasurface, of which the meta-atom consists of narrow-gap Si dimers with a monolayer graphene, can perfectly absorb the incident near-infrared light through critical coupling to the mirror-coupled toroidal dipole BIC. Due to unique loss engineering capabilities of the mode, multiple tuning knobs, including the narrow gap, meta-atom–mirror distance, and geometrical scaling factor, can be utilized for control over the coupling condition and the electric-field enhancement at the critical coupling condition. These results suggest that our platform could promise in on-chip optoelectronic applications.

Dynamic tunable narrow-band perfect absorber for fiber -optic communication band based on liquid crystal

At present, most of the reported metasurface structure absorbers show that its working band cannot be regulated actively. In this study, a dynamic tunable narrow-band perfect absorption structure for fiber-optic communication band based on liquid crystal (LC) is proposed and studied. The structure is mainly composed of two effective tiers. The top tier gold array and the bottom tier reflective gold film, which are separated by a SiO2-LC dielectric medium interlayer to form a metal–dielectric–metal structure. Due to the unique optical properties of LCs, its index of refraction can be changed by adjusting the bias voltage and temperature, so as to adjust the resonance wavelength actively. The designed structure is analyzed by finite element method and the coupled mode theory is used to verify the analysis results. The designed structure has a 99.92% absorption effect in the most commonly used band of fiber-optic communication. Due to the symmetry of the absorber structure, the device is not sensitive to the polarization state of the excitation source. Moreover, the absorber exhibits an unusual dependence on the incident angle, which can be attributed to the anisotropy of the LC. Based on the dependence of incident angle, a plasma optical switch with large ON/OFF ratio (η) of 27.395 dB and nearly flawless modulation depth of 99.818% can be realized. It is believed that this structure can provide a method for the dynamic control of near infrared electromagnetic waves, and to be applied in electromagnetic energy absorption, filtering and plasma optical switch system.

Tunable IR perfect absorbers enabled by tungsten doped VO2 thin films

The temperature tunability of complex dielectric constants of vanadium dioxide (VO2) makes it a promising phase-change material for use in active, dynamic, tunable photonics applications. Specifically, the semiconductor-to-metal phase transition in VO2 enables reversible, broadband, and large complex refractive index variation and paves the way for a plethora of applications. Although the critical temperature for phase-transition is 68 °C for VO2 films, its transition temperature can be reduced to room temperature by tungsten-doping of vanadium dioxide. Such a degree of freedom in controlling the critical temperature through tungsten doping provides further tunability of the thermochromic behavior. In this work, we investigate a variety of W-doped VO2 thin films deposited by laser ablation of targets with increasing W doping content and report detailed infrared characterization together with numerical simulations. Our experimental results indicate that the perfect absorption can be achieved at different temperatures, within the VO2 insulator-to-metal phase transition process, as a function of W doping content. Tunable subwavelength layers allow perfect absorption under different temperature conditions around λ = 12 µm. We show that a high dynamic range of reflectivity can be achieved when the temperature is increased above the phase transition temperature. Furthermore, we observe perfect absorption at 11.8 µm at room temperature for a W content of 0.75%. We believe that W-doped VO2 thin films with tunable and controllable perfect absorption will open the way for a class of promising thermo-optical devices including thermos-photovoltaics, infrared filters, radiative cooling devices, and thermal emitters.

Spin-dependent and tunable perfect absorption in a Fabry-Perot cavity containing a multi-Weyl semimetal.

Spin-dependent absorption has been widely studied in metamaterials and metasurfaces with chirality since it develops significant applications in multiplexed holograms, photodection, and filtering. Here, the one-dimensional photonic crystal Fabry-Perot (FP) cavity containing a multi-Weyl semimetal (mWSM) defect is proposed to investigate the spin-dependent perfect absorption. Results denote that the distinct refractive indices of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves are present due to the nonzero off-diagonal term of mWSM, thus supporting the perfect absorption of RCP and LCP waves at distinct resonant wavelengths. The different perfect absorption wavelengths of RCP and LCP waves reveal the spin-dependent perfect absorption. By altering the Fermi energy, tilt degree of Weyl cones, Weyl nodes separation, topological charge, and thickness of the mWSM layer, the perfect absorption wavelength of RCP and LCP waves can be regulated conveniently. Particularly, the linear tunable perfect absorption wavelength with thickness of the mWSM layer supports the accurate determination of perfect absorption wavelength at distinct mWSM thicknesses. Our studies develop simple and effective approaches to acquire the spin-dependent and adjustable perfect absorption without the external magnetic field, and can find practical applications in spin-dependent photonic devices.

Mirror‐Coupled Plasmonic Bound States in the Continuum for Tunable Perfect Absorption

AbstractTailoring critical light‐matter coupling is a fundamental challenge of nanophotonics, impacting fields from higher harmonic generation and energy conversion to surface‐enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, excel at obtaining different coupling regimes by tuning the antenna‐film gap size. However, practical PA applications require constant gap size, making it impossible to maintain critical coupling beyond singular wavelengths. Here, a new approach for plasmonic PAs is introduced by combining mirror‐coupled resonances with the unique loss engineering capabilities of plasmonic quasi‐bound states in the continuum. This novel combination allows to tailor the light–matter interaction within the under‐coupling, over‐coupling, and critical coupling regimes using flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor. The study demonstrates a pixelated PA metasurface with optimal absorption over a broad range of mid‐infrared wavenumbers (950–2000 cm−1) using only a single gap size and applies it for multispectral surface‐enhanced molecular spectroscopy. Moreover, the asymmetry parameter enables convenient adjustment of the quality factor and resonance amplitude. This concept expands the capabilities and flexibility of traditional gap‐tuned PAs, opening new perspectives for miniaturized sensing platforms towards on‐chip and in situ detection.

Tunable nonlinear coherent perfect absorption in cavity QED

We propose and analyze a scheme for tunable coherent perfect absorption (CPA) and near-perfect reflection (near CPR) in a three-level $\mathrm{\ensuremath{\Lambda}}$-type atom-cavity system. With electromagnetically-induced-transparency-type interference induced by a coherent coupling laser, it provides a method of tunable near CPR at two-photon resonance. By virtue of different coupling strengths, CPA can be tuned from the linear regime to the nonlinear or bistable regime at a fixed input-field frequency. In addition, the CPA regime can be switched to the near-CPR regime with variable linear absorption or gain by an incoherent pump field. This work suggests a mechanism for manipulation on perfect absorption and reflection theoretically which can potentially be applied to logical gates and optical switches for coherent optical computing and communication.

Terahertz metamaterial using reconfigurable H-shaped resonator with tunable perfect absorption characteristic

A design of H-shaped tunable metamaterial absorber (HTMA) is proposed to show multi-resonance and tunable perfect absorption characteristics. It consists of a reconfigurable H-shaped Al resonator suspended on the bottom Al layer. HTMA shows multi-resonance characteristic that can be actively tuned by reconfiguring the H-shaped resonator. The transverse magnetic (TM) responses can be switched from single perfect absorption resonance at 3.04 THz to dual perfect absorption resonances at 3.22 THz and 4.82 THz. While the transverse electric (TE) responses exhibit triple-resonance with great tunability. The corresponding quality factor (Q-factor) and sensitivity are calculated. The sensitivities are 1.16 THz/RIU, 1.76 THz/RIU, 1.27 THz/RIU, 2.24 THz/RIU, and 2.94 THz/RIU for dual-resonance in TM mode and triple-resonance in TE mode, respectively. In consideration of the above optical characteristics, it can be used for the sensing application when the device is exposed to different surrounding refractive index. These results indicate the potential of the proposed HTMA to be applied in optical gas sensors, biosensors, and polarization switches.

Numerical investigation of graphene-silica-silver based VLSI interconnects with tunable multiband infrared absorber for 3D integrated circuits

We propose the multilayer silver-silica-graphene-based very large-scale integration interconnects assisted by a tunable perfect absorption structure over the infrared frequency spectrum of the third window of the optical communication for three-dimensional (3D) integrated circuits (ICs). This absorber is numerically investigated for the different cylindrical silver resonator-based squared geometries. The overall structure is investigated for the wavelength range of 1.4 to 1.6 μm over the infrared spectrum. The adjustable behavior of the absorption spectrum is observed when this structure is studied for various chemical potentials of the graphene sheet. We also present the electric and magnetic field intensity for the nearly perfect absorption conditions to identify the effect of energy concentration over different pixel structures and wavelengths. We also showcased the possible fabrication process for the proposed numerical investigation analysis. Resonator height and width have also been simulated numerically to find the resonance shift in the absorber. The adaptable behavior of the suggested structure has potential applications in a wide range of scientific fields, including biosensors, solar absorbers, optical communication, and the fabrication of 3D ICs. Simulations are performed using the COMSOL Multiphysics software module

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.

Tunable underwater low-frequency sound absorption via locally resonant piezoelectric metamaterials

Acoustic metamaterials with passive dissipative elements have demonstrated excellent underwater low-frequency sound absorption abilities, but still suffer from the narrow bandwidth, fixed absorption frequencies and bulky size. In this research, tunable underwater low-frequency sound absorption of the locally resonant piezoelectric metamaterials (LRPM) is systematically studied. A theoretical model of underwater sound absorption based on the LRPM is established. From the perspective of effective materials, the tunable sound absorption characteristics and perfect absorption mechanism are analyzed. The theoretical results are in good agreement with the numerical ones. It is demonstrated that a thin LRPM layer can achieve perfect sound absorption at targeted low-frequency which can be actively tuned by manipulating the resonant shunt circuit. Furthermore, the negative capacitance (NC) shunt can be introduced to the resonant circuit, which significantly improves the sound absorption bandwidth. By applying causality principle to the proposed LRPM, the causal constraints are discussed, which results in an improvement of causal optimality by NC shunt. This research can provide useful guidance for the realization of efficient and widely adjustable ultra-thin underwater sound absorbers.

Experimental study of a tunable perfect flexural wave absorber with a piezoelectric shunted resonator

Metamaterials and metasurfaces have been widely developed recently for extraordinary acoustic and elastic wave control at a deep subwavelength scale. Perfect wave absorption as an extreme case to totally absorb the impinged waves has gained great attention, whereas most existing designs based on local resonance lack tunabilities, making perfect absorption be observed at a single frequency. To overcome this drawback, in this work, we design and fabricate a tunable inductance-resistance (LR) shunted mechanical resonator via a bonded piezoelectric patch for perfect flexural wave absorption at low frequency. The LR shunted absorber could be reconfigured to a broad frequency range for perfect flexural wave absorption. The tunable perfect absorption performances are validated through experiments and unit absorption is achieved in experiments. In the end, to further highlight the advantages of shunted damping we numerically demonstrate that the absorption spectrum could be enhanced to broadband absorption with a negative capacitance and an inductance-resistance circuit (NC-LR) connected in parallel. The approach proposed provides an alternative solution to achieve perfect wave absorption in the low-frequency range and enables practical application in complex engineering structures.

Active metasurface in the near-infrared region by gating ultrathin TiN films.

Ultrathin titanium nitride (TiN) films have become a novel material flatform for constructing active metasurfaces in the near-infrared region. In this Letter, we numerically achieved the dual functions of switchable linear dichroism (LD) and tunable perfect absorption in a G-shape gold resonators/TiN film hybrid metasurface by gating ultrathin TiN films. As the carrier density of TiN decreases, the modulation depth for LD strength is about 70% at 1211 nm. Meanwhile, the response wavelength of perfect absorption (∼1) shifts to the blue by around 130 nm with a change of carrier density of 12%. Our proposed active metasurface with the capability of strength-switchable LD and wavelength-tunable perfect absorption has considerable potential in dynamic electro-optic modulation and flat photonic devices with reconfigurable functionalities.

Tunable perfect optical absorption in truncated photonic crystals with lossy defects

We theoretically investigate tunable optical absorption properties of photonic crystals containing lossy materials as defects. It is found that a lossy defect can induce one or multiple perfect absorption peaks in the bandgap of photonic crystals and the number of the peaks mainly depends on the thickness of the defect layer. On the one hand, multiple complete absorption peaks can also emerge in the photonic bandgap when multiple lossy defects are inserted within the photonic crystals, and the resonant wavelengths of absorption peaks can be modulated by changing the distances among the defects. On the other hand, the optical absorption away from resonant wavelengths is nearly zero in the whole visible range. Such nanostructures can be used to engineer novel optical devices such as tunable single-channel and multi-channel perfect optical absorbers.

Perfect absorption and strong phonon polariton with a graphene-based silicon carbide grating and Tamm plasmonic structures

This study proposes a tunable dual-band mid-infrared graphene-based one-dimensional photonic crystal absorber with strong surface phonon polaritons and Tamm phonon polariton coupling. We use an LC circuit, transfer matrices, and coupled harmonic oscillator models to theoretically analyze the different modes, and the theoretical results are consistent with the simulation results. The resonance wavelengths and absorption intensities of the coupled mode can be adjusted by the Fermi level and structure parameters. Moreover, because the perfect dual-band absorption peaks of the designed structure are sensitive to the air layer’s refractive index, we demonstrate the possibility of its application in the field of refractive index sensors and analyze its potential in the field of biomolecular layer sensors. The designed structure also has broad applications in absorbers, photodetectors, and energy harvesting devices due to the excellent performance of the tunable perfect absorption peaks.