Far-infrared absorber based on standing-wave resonances in metal-dielectric-metal cavity

Selective terahertz absorber for angle and polarization-independent spectral sensing

Polarization- and incident-angle-independent narrow-band terahertz (THz) absorbers were developed to enable THz imaging, radar, and spectroscopy applications. The design comprises a transparent fused silica (SiOx) substrate backed by an optically thick metal layer and topped by a periodic array of metal cross patterns. Finite element analysis (FEA) simulations optimized the geometry of devices fabricated by contact photolithography. Resonances were characterized by Fourier-transform reflectance spectroscopy. The design tunable absorption bands appeared in the range 50-200 cm^-1 (1.5-6 THz) with full widths at half maximum of 20-56 cm^-1 (0.6-1.68 THz). Maximum absorption was -8.5 to -16.8 dB. The absorption bands are independent of incidence angle and polarization in agreement with simulation.

Broadband switching of mid-infrared atmospheric windows by VO2-based thermal emitter

Atmospheric windows play an important role in the field of infrared detection and radiative cooling. In this paper, the development of VO₂-based metamaterial emitter brings broadband thermal-switching light to mid-infrared atmospheric windows. At room temperature, the emitter radiates light in both 3-5μm and 8-14μm atmospheric windows. At high temperature, the radiation peaks move out of the atmospheric windows and result a strong radiation at 5-8μm. The underlying mechanism relies on the relationship between VO₂ metal-insulator transition (MIT) and resonant absorption modes coupling. Corresponding thermal imaging experiment exhibits two distinct phenomena. One is the observation of unchanged thermal radiation around MIT temperature. The other phenomenon regards the concealment of the emitter from Al background at specific temperatures. These two phenomena show potential application in infrared anti-detection.

Ultra-sensitive and plasmon-tunable graphene photodetectors for micro-spectrometry

We demonstrate an ultra-sensitive photodetector based on a graphene/monolayer MoS2 vertical heterostructure working at room temperature. Highly confined plasmon waves are efficiently excited through a periodic array of monolayer graphene ribbons in which plasmon resonance has remarkably large oscillator strength, resulting in a sharp optical absorption peak in the normal-incidence transmission spectrum. A significant amount of electron-hole pairs are produced in graphene ribbons by optical absorption, separated by the built-in electric field across the graphene/MoS2 heterojunction. The responsivity reaches up to 1 × 107 A W-1 at room temperature due to very strong resonance in the heterostructure, yielding a highly sensitive graphene-based photodetector. Additionally, the absorption can be tuned over a wide spectral range (6-16 μm) by varying gate biasing. The ultra-sensitive, spectrally tunable photodetector could be potentially used as a promising candidate for mid-infrared micro-spectrometers.

Ultrathin Six-Band Polarization-Insensitive Perfect Metamaterial Absorber Based on a Cross-Cave Patch Resonator for Terahertz Waves

A simple design of an ultrathin six-band polarization-insensitive terahertz perfect metamaterial absorber (PMMA), composed of a metal cross-cave patch resonator (CCPR) placed over a ground plane, was proposed and investigated numerically. The numerical simulation results demonstrate that the average absorption peaks are up to 95% at six resonance frequencies. Owing to the ultra-narrow band resonance absorption of the structure, the designed PMMA also exhibits a higher Q factor (>65). In addition, the absorption properties can be kept stable for both normal incident transverse magnetic (TM) and transverse electric (TE) waves. The physical mechanism behind the observed high-level absorption is illustrated by the electric and power loss density distributions. The perfect absorption originates mainly from the higher-order multipolar plasmon resonance of the structure, which differs sharply from most previous studies of PMMAs. Furthermore, the resonance absorption properties of the PMMA can be modified and adjusted easily by varying the geometric parameters of the unit cell.

Modes Coupling Analysis of Surface Plasmon Polaritons Based Resonance Manipulation in Infrared Metamaterial Absorber

Surface plasmon polaritons (SPPs) and standing wave modes provide interesting and exotic properties for infrared metamaterial absorbers. Coupling of these modes promises further development in this field but restricted by the complexity of modes analysis. In this work, we investigate the general phenomenon of modes coupling supported by a metal (with grating)-dielectric-metal sandwich structure based on rigorous coupled-wave analysis (RCWA) method and experiment results. Through the analysis of fundamental modes, a new approach based on the boundary conditions is introduced to reveal the coupling mechanism and the corresponding resonance shifting phenomenon with simple but rigorous derivations. The strong coupling between SPPs excited on the dielectric-metal interfaces and rigorous modes of standing waves in the dielectric layer can be manipulated to improve the detection sensitivity of sensors and emissivity efficiency of infrared emitters.

Densely-tiled metal-insulator-metal metamaterial resonators with quasi- monochromatic thermal emission

Metal-insulator-metal metamaterial thermal emitters strongly radiate at multiple resonant wavelengths. The fundamental mode, whose wavelength is the longest among resonances, is generally utilized for selective emission. In this paper, we show that parasitic modes at shorter wavelengths are suppressed by newly employed densely-tiled resonators, and that the suppression enables quasi-monochromatic thermal emission. The second-order harmonics, which is excited at half the fundamental wavelength in conventional emitters, shifts toward shorter wavelength. The blue-shift reduces the amplitude of the second-order emission by taking a distance from the Wien wavelength. Other parasitic modes are eliminated by the small spacing between resonators. The densely-tiled resonators are fabricated, and the measured emission spectra agree well with numerical simulations. The methodology presented here for the suppression of parasitic modes adds flexibility to metamaterial thermal emitters.

Unidirectional reflectionless propagation in plasmonic waveguide-cavity systems at exceptional points

We design a non-parity-time-symmetric plasmonic waveguide-cavity system, consisting of two metal-dielectric-metal stub resonators side coupled to a metal-dielectric-metal waveguide, to form an exceptional point, and realize unidirectional reflectionless propagation at the optical communication wavelength. The contrast ratio between the forward and backward reflection almost reaches unity. We show that the presence of material loss in the metal is critical for the realization of the unidirectional reflectionlessness in this plasmonic system. We investigate the realized exceptional point, as well as the associated physical effects of level repulsion, crossing and phase transition. We also show that, by periodically cascading the unidirectional reflectionless plasmonic waveguide-cavity system, we can design a wavelength-scale unidirectional plasmonic waveguide perfect absorber. Our results could be potentially important for developing a new generation of highly compact unidirectional integrated nanoplasmonic devices.