causes the spectral shift of the resonance wavelength. In particular, most organic molecules have a higher refractive index than buffer solution, thus as the concentration of these molecules rises, the local refractive index increases, thereby redshifting the resonance wavelength. The spectral shift of resonance results in the change of the transmission, refl ection, or absorption spectrum, which can be monitored by inexpensive spectrometry. [1][2][3][4][9][10][11][12] However, strong induced electric current in these plasmonic resonators also leads to signifi cant ohmic loss, [6][7][8] which gives rise to the broadening of the resonance spectrum with low-quality factor ( Q -factor) and thus degrade the performance of the sensor characterized by a fi gure of merit (FOM*). Over the years, numerous efforts have been made to improve the performance of plasmon-based sensor, for example, using high Q-factor Fano resonance induced by coupling of surface plasmon polariton and LSPR [ 11 ] and reducing the substrate effect by lifting the LSPR resonators. [ 13 ] Among the efforts, it has been demonstrated that the metamaterial perfect absorbers (MPAs) can be used to signifi cantly increase the Q -factor and thus improve the FOM*. [ 12,14 ] The MPA is a recently developed branch of metamaterial which exhibits nearly unity absorption within certain frequency range. [15][16][17][18][19][20] The optically thin MPA possesses characteristic features of angular-independence, high Q -factor and strong fi eld localization that have inspired a wide range of applications including electromagnetic (EM) wave absorption, [ 17,21,22 ] spatial [ 20 ] and spectral [ 19 ] modulation of light, [ 23 ] selective thermal emission, [ 23 ] thermal detecting, [ 24 ] and refractive index sensing for gas [ 25 ] and liquid [ 12,14 ] targets. The MPA is typically comprised of three layers: a metallic resonator's layer, e.g., cross-type resonators, [ 23,26 ] split-ring resonators, [ 17 ] or metallic nanoparticles, [ 27 ] and a highly refl ective layer, e.g., metallic fi lm [ 17,23,26,27 ] or metallic mesh grid, [ 17 ] separated by a subwavelength-thick dielectric fi lm (spacer). The impedance matching between MPA and free space, and high attenuation of light inside the MPA result in the perfect absorption. [ 17 ] In the sensing application, the spectral shift of perfect absorption peak is attributed to the refractive index change of gas [ 25 ] or liquid. [ 12,14 ] However, the gas or liquid used as the sensing target has been so far only on the surface of the MPA-basedIn most plasmon resonance based sensor to date, only the surface of the sensor is accessible to the gas or liquid as the sensing target. In this work, an interferometric, lithographically fabricated, large-area, mushroom-capped plasmonic perfect absorber whose dielectric spacer is partially removed by a reactive-ion-etch process, thereby enabling the liquid to permeate into the sensitive region to a refractive index change, is demonstrated. Findings of this paper demonstrate experimentall...

relies on the Faraday effect and a pair of polarizers with relative 45° orientation to block the back-reflected beam from reaching a laser source. This architecture uses the Faraday rotation in nonabsorbing media and breaks down at THz wavelengths, where one of the eigenpolarizations in the Faraday medium (either right-circular or left-circular) typically experiences a high loss. After the THz wave propagates through the medium, it acquires a high degree of ellipticity. Often, one of the eigenpolarizations is fully absorbed, and the remaining light is fully circularly polarized. [7] As a result, there are no widely adopted solutions for THz optical isolators that work with linearly polarized inputs and outputs. Operation with linear polarization is the most desirable because many photonic THz emitters and receivers operate with linearly polarized light.Faraday effect and magneto-optical Kerr effect can still be used in THz isolators if polarization-converting elements, such as waveplates, are included, even though they may add significant complexity and degrade the performance of the device. High-mobility semiconductors, [7][8][9][10][11][12][13][14] graphene and other 2D van der Waals materials, [15][16][17][18][19][20][21][22][23] ferrofluids, [24] and magnetic materials [25] have been explored as a suitable Faraday medium. The best known performance of this type of isolator was demonstrated by Tamagnone et al. [19] in a graphene-based device with the isolation of 18 dB and insertion loss of 7.5 dB in applied magnetic field of 7 T. The quoted performance is for circular, not linear, polarization of input and output THz beams.Another fundamental approach to THz optical isolation is based on nonreciprocal directional dichroism in magnetic materials that are also structurally chiral [26] or polar. [27][28][29][30] While the THz isolation performance of these materials can be very high, [30] they typically operate at impractically low temperatures (≤100 K) and high magnetic fields (≥7 T). Alternatively, the nonreciprocal dispersion of surface magnetoplasmons in parallel magnetic field [31,32] has been proposed as the fundamental basis for one-way THz waveguide-based devices. [33,34] Other theoretical proposals have employed magnetoplasmonic metasurfaces and metamaterials to achieve THz isolation. [35][36][37] These theoretical proposals have not been implemented and may be difficult and expensive for practical fabrication.We use the nonreciprocal reflection of magnetoplasmas to design and implement a simple, tunable, and high-performance THz isolator working with linearly polarized THz light in reflection geometry. Our isolator works at room temperature in very moderate magnetic field (≈0.2 T). Therefore, it can serve as a practical alternative to Faraday-based and other proposedThe magneto-optical properties of conduction electrons in InSb in Voigt geometry at oblique incidence angles are explored. In parallel magnetic field, the oblique incidence reflectance exhibits high nonreciprocity, while the transmittance r...

We demonstrate that the metamaterial perfect absorber behaves as a meta-cavity bounded between a resonant metasurface and a metallic thin-film reflector. The perfect absorption is achieved by the Fabry-Perot cavity resonance via multiple reflections between the “quasi-open” boundary of resonator and the “close” boundary of reflector. The characteristic features including angle independence, ultra-thin thickness and strong field localization can be well explained by this meta-cavity model. With this model, metamaterial perfect absorber can be redefined as a meta-cavity exhibiting high Q-factor, strong field enhancement and extremely high photonic density of states, thereby promising novel applications for high performance sensor, infrared photodetector and cavity quantum electrodynamics devices.

This paper is focused on analyzing the impact of a two-dimensional metal hole array structure integrated to the back-illuminated quantum dots-in-a-well (DWELL) infrared photodetectors. The metal hole array consisting of subwavelength-circular holes penetrating gold layer (2D-Au-CHA) provides the enhanced responsivity of DWELL infrared photodetector at certain wavelengths. The performance of 2D-Au-CHA is investigated by calculating the absorption of active layer in the DWELL structure using a finite integration technique. Simulation results show that the performance of the DWELL focal plane array (FPA) is improved by enhancing the coupling to active layer via local field engineering resulting from a surface plasmon polariton mode and a guided Fabry-Perot mode. Simulation method accomplished in this paper provides a generalized approach to optimize the design of any type of couplers integrated to infrared photodetectors. Experimental results demonstrate the enhanced signal-to-noise ratio by the 2D-Au-CHA integrated FPA as compared to the DWELL FPA. A comparison between the experiment and the simulation shows a good agreement.

Because of the strong inherent resonances, the giant optical activity obtained via chiral metamaterials generally suffers from high dispersion, which has been a big stumbling block to broadband applications. In this paper, we propose a type of planar chiral metamaterial consisting of interconnected metal helix slat structures with four-fold symmetry, which exhibits nonresonant Drude-like response and can therefore avoid the highly dispersive optical activity resulting from resonances. It shows that the well-designed chiral metamaterial can achieve nondispersive and pure optical activity with high transmittance in a broadband frequency range. And the optical activity of multi-layer chiral metamaterials is proportional to the layer numbers of single-layer chiral metamaterial. Most remarkably, the broadband behaviors of nondispersive optical activity and high transmission are insensitive to the incident angles of electromagnetic waves and permittivity of dielectric substrate, thereby enabling more flexibility in polarization manipulation.