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...

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.

We present a computational study of terahertz optical properties of a grating-coupled plasmonic structure based on micrometer-thin InSb layers. We find two strong absorption resonances that we interpret as standing surface plasmon modes and investigate their dispersion relations, dependence on InSb thickness, and the spatial distribution of the electric field. The observed surface plasmon modes are well described by a simple theory of the air/InSb/air trilayer. The plasmonic response of the grating/InSb structure is highly sensitive to the dielectric environment and the presence of an analyte (e.g., lactose) at the InSb interface, which is promising for terahertz plasmonic sensor applications. We determine the sensor sensitivity to be 7200 nm per refractive index unit (or 0.06 THz per refractive index unit). The lower surface plasmon mode also exhibits a splitting when tuned in resonance with the vibrational mode of lactose at 1.37 THz. We propose that such interaction between surface plasmon and vibrational modes can be used as the basis for a new sensing modality that allows the detection of terahertz vibrational fingerprints of an analyte.

Over the years, there has been increasing interest in the integration of metal hole array (MHA) with optoelectronic devices, as a result of enhanced coupling of incident light into the active layer of devices via surface plasmon polariton (SPP) resonances. However, not all incident light contributes to the SPP resonances due to significant reflection loss at the interface between incident medium and MHA. Conventional thin-film antireflection (AR) coating typically does not work well due to non-existing material satisfying the AR condition with strong dispersion of MHA’s effective impedances. We demonstrate a single-layer metasurface AR coating that completely eliminates the refection and significantly increases the transmission at the SPP resonances. Operating at off-resonance wavelengths, the metasurface exhibits extremely low loss and does not show resonant coupling with the MHA layer. The SPP resonance wavelengths of MHA layer are unaffected whereas the surface wave is significantly increased, thereby paving the way for improved performance of optoelectronic devices. With an improved retrieval method, the metasurface is proved to exhibit a high effective permittivity () and extremely low loss (tan δ ~ 0.005). A classical thin-film AR coating mechanism is identified through analytical derivations and numerical simulations.

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.