Arrays Of Subwavelength Apertures Research Articles

Thermally reconfigurable extraordinary terahertz transmission using vanadium dioxide

We numerically demonstrate a reconfigurable extraordinary terahertz transmission based on a phase-change material of vanadium dioxide ( V O 2 ). The proposed hybrid metasurface is composed of an array of subwavelength apertures perforated on a gold film. The holes are partially filled with annular V O 2 and gold disks to control the effective aperture area and the modes inside the aperture. Switching between the insulator and the metallic phase of V O 2 provides a convenient way to shift the transmission window. We present two designs offering redshift or blueshift of the extraordinary terahertz transmission. Upon phase transition from the insulator to the metallic phase, in the first design, the transmission peak redshifts from 1.02 to 0.82 THz while in the second design the transmission peak blueshifts from 0.71 to 0.77 THz. Furthermore, the transmission level and resonance frequency can be modulated by controlling the partial phase transition of the V O 2 . The potential applications for the proposed structures are terahertz modulators and reconfigurable filters.

Symmetry‐Assisted Spectral Line Shapes Manipulation in Dielectric Double‐Fano Metasurfaces

AbstractSpectral line shapes feature many essential resonant phenomena when diagnosing light–matter interactions. Particularly, tailoring spectral line shapes in photonics unveils a robust scheme to control electromagnetic waves and design functional optical devices. Here, the symmetry‐assisted manipulation of spectral line shapes in dielectric double‐Fano metasurfaces is theoretically proposed and experimentally demonstrated. Composed by a free‐standing silicon membrane with subwavelength aperture arrays, the metasurfaces simultaneously support two orthogonal localized modes (symmetric and antisymmetric) with high‐quality (high‐Q) factors and a propagating mode with a low‐Q factor. The symmetric and antisymmetric modes interfere with the propagating mode, resulting in the double‐Fano resonances with opposite signs of Fano parameters. The superposition of such double‐Fano resonances can produce multiple spectral line shapes, including double‐Fano resonances, a single Fano resonance, an electromagnetic‐induced‐transparency‐like (EIT‐like) window, and a complementary EIT‐like window, which have been analytically, numerically, and experimentally verified. Moreover, the EIT‐like window exhibits a steep dispersion, which is beneficial in slow light devices. Due to the anisotropy of the design, this EIT‐like window can transform into a complementary EIT‐like window when the incident polarization is rotated by 90°. Thus, such asymmetry‐assisted double‐Fano metasurfaces provide a new route to reveal photonic phenomena and construct multifunctional metadevices.

Structural Stability of Optofluidic Nanostructures in Flow-Through Operation.

Optofluidic sensors based on periodic arrays of subwavelength apertures that support surface plasmon resonance can be employed as both optical sensors and nanofluidic structures. In flow-through operation, the nanoapertures experience pressure differences across the substrate in which they are fabricated, which imposes the risk for structural failure. This work presents an investigation of the deflection and structural stability of nanohole array-based optofluidic sensors operating in flow-through mode. The analysis was approached using experiments, simulations via finite element method, and established theoretical models. The results depict that certain areas of the sensor deflect under pressure, with some regions suffering high mechanical stress. The offset in the deflection values between theoretical models and actual experimental values is overturned when only the effective area of the substrate, of 450 µm, is considered. Experimental, theoretical, and simulation results suggest that the periodic nanostructures can safely operate under trans-membrane pressures of up to 20 psi, which induce deflections of up to ~20 μm.

Huge field enhancement and high transmittance enabled by terahertz bow-tie aperture arrays: a simulation study.

Sub-wavelength aperture arrays featuring small gaps have an extraordinary significance in enhancing the interactions of terahertz (THz) waves with matters. But it is difficult to obtain large light-substance interaction enhancement and high optical response signal detection capabilities at the same time. Here, we propose a simple terahertz bow-tie aperture arrays structure with a large electric field enhancement factor and high transmittance at the same time. The field enhancement factor can reach a high value of 1.9×104 and the transmission coefficient of around 0.8 (the corresponding normalized-to-area transmittance is about 14.3) at 0.04 µm feature gap simultaneously. The systematic simulation results show that the designed structure can enhance the intensity of electromagnetic hotspot by continuously reducing the feature gap size without affecting the intensity of the transmittance. We also visually displayed the significant advantages of extremely strong electromagnetic hot spots in local terahertz refractive index detection, which provides a potential platform and simple strategy for enhanced THz spectral detection.

Fundamental Bounds on Transmission Through Periodically Perforated Metal Screens With Experimental Validation

This paper presents a study of transmission through arrays of periodic sub-wavelength apertures. Fundamental limitations for this phenomenon are formulated as a sum rule, relating the transmission coefficient over a bandwidth to the static polarizability. The sum rule is rigorously derived for arbitrary periodic apertures in thin screens. By this sum rule we establish a physical bound on the transmission bandwidth which is verified numerically for a number of aperture array designs. We utilize the sum rule to design and optimize sub-wavelength frequency selective surfaces with a bandwidth close to the physically attainable. Finally, we verify the sum rule and simulations by measurements of an array of horseshoe-shaped slots milled in aluminum foil.

Embedded arrays of annular apertures with multiband near-zero-index behavior and demultiplexing capability at near-infrared

In this paper, we study transmission through the embedded arrays of subwavelength annular apertures at near-infrared. Single, i.e. non-embedded arrays of annular holes are known for their capability for high-efficiency transmission even through rather thick apertures in a wide frequency range, extending from microwaves to the visible. In the suggested structures, which contain up to four embedded arrays, multiband operation can be obtained, so that each array is mainly responsible for one of four transmission bands. In such a way, a demultiplexing-like functionality can be realized, i.e. the desired parts of the incident-wave spectrum are distributed between several transmission channels. In the studied structures, we obtain (nearly) zero phase advancement and that indicates near-zero-index behavior at the expected propagation thresholds of plasmonic modes in the frequency domain. Therefore, the earlier developed concept of supercoupling and squeezing into very narrow waveguide channels is applicable to the studied structures. The number of near-zero-index bands is determined by the number of the embedded arrays. The effects of thickness, width of the slits, and permittivity of the filling material are numerically studied and discussed in detail. It is shown that multiband transmission may exist in the near-zero-index regime in a very wide range of parameter variations.

Light-Blocking Optical Alignment Rulers for Guiding Layer-by-Layer Integration

Vertical integration of device layers is a prevailing strategy to boost the performance of microchips and optical devices while maintaining a small form factor. In this paper, we introduce light blocking optical alignment rulers (LBOARs) for layer-by-layer alignment of thin films, which can be potentially applied in manufacturing three-dimensional integrated circuits. LBOAR measures the blocking of the light transmitted through two vertically stacked subwavelength aperture arrays (gratings), which are fabricated onto the separate device layers to be aligned. Here, detailed numerical studies and proof-of-concept experiments on LBOAR are presented. The simulation results reveal two different mechanisms governing the light transmission through a pair of rulers: light blocking and induced extraordinary optical transmission (EOT). In the light-blocking regime, the intensity change is very sensitive to the horizontal shift between two rulers. In contrast, EOT-based alignment is significantly disturbed by the vertical separation of the nearly contacting rulers. Therefore, LBOAR can be more advantageous than EOT-based alignment for certain applications that require only horizontal alignment between two layers of devices. To validate the light-blocking concepts, in our experiments, two-layer stacked LBOARs were fabricated using laser lithography, physical vapor deposition, and dry etching. Optical transmission measurements show that in the nearly perfect alignment condition the transmitted intensity in the light-blocking regime drops significantly providing an alignment accuracy better than 200 nm.

Plasmon Enhancement of the Electric Field in Mid-Infrared Ge/Si Quantum-Dot Photodetectors with Different Thicknesses of the Active Region

The spatial distribution of the electric field in Ge/Si photodetector heterostructures coated with a gold film containing a regular two-dimensional array of subwavelength apertures is calculated by the finite-element method. The array period and aperture diameter are 1.2 and 0.7 μm, respectively. The efficiency of field enhancement is determined for different thicknesses of the active region occupied by quantum dots. It is demonstrated that the field-enhancement factor for an electromagnetic wave incident on the structure from the side of the substrate is ~3.5 times larger than that for a wave incident from the opposite side. In the first case, the field-enhancement factor varies nonmonotonically with the thickness of the active region.

Extraordinary Transmission of Light Through Freestanding Au Films with Subwavelength Apertures and Double Slits

We demonstrated the extraordinary transmission of light through a freestanding Au thin film with a 7×7 subwavelength aperture array and double slits. Firstly, we observed the dependence of the change in the light transmission through a plasmonic structure on the pitch size of the aperture array. Also, we fabricated a freestanding Au thin film to avoid the effects of absorption and reflection of the substrate. We perforated the 7×7 subwavelength hole array on the Au film for extraordinary optical transmission caused by the electric field enhancement from the structures. Additionally, we designed a hybrid structure composed of 7×7 subwavelength apertures and double slits to enhance the transmission of the light. The normalized-to-area transmission of the hybrid structures was approximately five times greater than that of the structures without double slits.

Плазмонное усиление поля в фотоприемниках среднего ИК-диапазона на базе квантовых точек Ge/Si с различной толщиной активной зоны

AbstractThe spatial distribution of the electric field in Ge/Si photodetector heterostructures coated with a gold film containing a regular two-dimensional array of subwavelength apertures is calculated by the finite-element method. The array period and aperture diameter are 1.2 and 0.7 μm, respectively. The efficiency of field enhancement is determined for different thicknesses of the active region occupied by quantum dots. It is demonstrated that the field-enhancement factor for an electromagnetic wave incident on the structure from the side of the substrate is ~3.5 times larger than that for a wave incident from the opposite side. In the first case, the field-enhancement factor varies nonmonotonically with the thickness of the active region.

Multi-spectral frequency selective mid-infrared microbolometers.

Frequency selective detection of low energy photons is a scientific challenge using natural materials. A hypothetical surface which functions like a light funnel with very low thermal mass in order to enhance photon collection and suppress background thermal noise is the ideal solution to address both low temperature and frequency selective detection limitations of present detection systems. Here, we present a cavity-coupled quasi-three dimensional plasmonic crystal which induces impedance matching to the free space giving rise to extraordinary transmission through the sub-wavelength aperture array like a "light funnel" in coupling low energy incident photons resulting in frequency selective perfect (~100%) absorption of the incident radiation and zero back reflection. The peak wavelength of absorption of the incident light is almost independent of the angle of incidence and remains within 20% of its maximum (100%) up to θi≤45˚. This perfect absorption results from the incident light-driven localized edge "micro-plasma" currents on the lossy metallic surfaces. The wide-angle light funneling is validated with experimental measurements. Further, a super-lattice based electronic biasing circuit converts the absorbed narrow linewidth (Δλ/λ0< 0.075) photon energy inside the sub-wavelength thick film (< λ/100) to voltage output with high signal to noise ratio close to the theoretical limit. Such artificial plasmonic surfaces enable flexible scaling of light funneling response to any wavelength range by simple dimensional changes paving the path towards room temperature frequency selective low energy photon detection.

Optical Response of Nanohole Arrays Filled with Chalcogenide Low‐Epsilon Media

AbstractWhen an array of subwavelength apertures in a metal film is filled with a chalcogenide semiconductor acting as a low‐epsilon medium at optical frequencies, an increase in transmission is observed over a broad range of plasmonic frequencies, and found to be enabled by “laminar” flow of energy through the chalcogenide inclusions. As losses decrease in such a low‐epsilon medium, with causality‐bound dispersion of complex relative permittivity, a peak in transmission emerges at a wavelength tending toward that of unitary refractive index, while counterintuitively the absorption of the composite structure increases.

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.

Electromagnetic behavior of dielectric objects on metallic periodically nanostructured substrates.

In this research, we investigate the electromagnetic behavior of a metallic thin-film with a periodic array of subwavelength apertures when dielectric objects are located on it. The influence of size, geometry and optical properties of the objects on the transmission spectra is numerically analyzed. We study the sensitivity of this system to changes in the refractive index of the illuminated volume induced by the presence of objects with sizes from hundreds of nanometers (submicron-sized objects) to a few microns (micron-sized objects). Parameters such as the object volume within the penetration depth of the surface plasmon in the buffer medium or the contact surface between the object and the nanostructured substrate strongly affect the sensitivity. The proposed system models the presence of objects and their detection through the spectral shifts undergone by the transmission spectra. Also, we demonstrate that these can be used for obtaining information about the refractive index of a micron-sized object immersed in a buffer and located on the nanostructured sensitive surface. We believe that results found in this study can help biomedical researchers and experimentalists in the process of detecting and monitoring biological organisms of large sizes (notably, cells).

An Electrically Tunable Terahertz Plasmonic Device Based on Shape Memory Alloys and Liquid Metals

AbstractAn electrically tunable terahertz (THz) plasmonic device is designed and fabricated using liquid metals (eutectic gallium indium) and shape memory alloy wires (Flexinol). The liquid metal is injected into the voids of a poly(dimethyl) siloxane microfluidic mold forming a periodic array of subwavelength apertures, while the wires are inserted into the elastomer below the metal plane. When a DC voltage is applied to the wires, they contract via Joule heating, reducing the aperture periodicity and blueshifting the transmission resonances of the device. When the voltage is removed, the wires cool and elongate back to their original length, allowing the transmission spectrum to return to its original state. The magnitude of this change depends upon the applied voltage. The device is shown to thermally cycle between the relaxed state and the fully contracted state reproducibly over at least 500 thermal cycles. The asymmetric geometry of the device and the contraction process yield transmission properties that are unexpected: two closely spaced resonances, where both resonances correspond to the same scattering indices, and an increase in the transmission amplitude of the lowest order resonance upon contraction. Numerical simulations are used to understand these features.

Gallium platinum alloys – a new material system for UV plasmonics

We describe a new material system based on alloys of gallium and platinum that is well-suited for ultraviolet (UV) plasmonics. Although gallium has previously been shown to be useful for such studies, creating a continuous, pinhole-free thin film has been technically challenging. For example, when vacuum deposition techniques are used, gallium forms as isolated spherical nanoparticles on a wide variety of substrates. We demonstrate that when a platinum wetting layer is deposited first on a substrate followed by a thick gallium layer, a Ga-Pt alloy thin film is formed near the interface. The excess surface gallium can then be removed using a focused ion beam (FIB), exposing the alloy film. Ellipsometry measurements show that the alloy largely retains the dielectric properties of solid gallium throughout the UV, although the properties of the two diverge somewhat in the visible. We fabricate periodic subwavelength aperture arrays in the alloy thin film and observe enhanced optical transmission resonances that are sharper in the UV than in the visible. The patterned films appear to be stable over time periods exceeding six months based on optical measurements.

Plasmonic metasurface for simultaneous detection of polarization and spectrum.

We present a new plasmonic metasurface for simultaneous detection of polarization and spectrum of incident light. The demonstrated metasurface is a rationally designed cluster of artificial atoms that are engineered to exhibit polarization and wavelength-selective optical transmission. The fundamental building block of this structure is periodically coupled subwavelength aperture arrays with different orientations and lattice constants. When integrated with pixelated photodetectors, the metasurface can be used to measure the polarization and spectral information of an optical input. In this Letter, simultaneous detection of the polarization and spectrum of polarized light was experimentally demonstrated by analyzing the transmitted intensity distribution through the metasurface. The demonstrated metasurface offers great potential for many applications, such as polarimetric multispectral imaging and polarization-division multiplexing in optical communications.

Terahertz Transparent Electrode Using Tripod Metal Aperture Array

In this paper, a high transmission terahertz (THz), polarization insensitive electrode was designed and fabricated to modulate a liquid crystal electro-optic device in the THz regime. The designed electrode consists of a subwavelength periodic tripod aperture array structure patterned in a gold film deposited on a transparent dielectric substrate. The maximum feature size of the structure is maintained below a fraction of the thickness of the electro-optic device to allow a fringing field to uniformly excite the entire electrode area. The transmission reaches as high as 93% in measurements.

New approach for extraordinary transmission through an array of subwavelength apertures using thin ENNZ metamaterial liners.

Extraordinary transmission (ET) through a periodic array of subwavelength apertures on a perfect metallic screen has been studied extensively in recent years, and has largely been attributed to diffraction effects, for which the periodicity of the apertures, rather than their dimensions, dominates the response. The transmission properties of the apertures at resonance, on the other hand, are not typically considered 'extraordinary' because they may be explained using more conventional aperture-theoretical mechanisms. This work describes a novel approach for achieving ET in which subwavelength apertures are made to resonate by lining them using thin, epsilon-negative and near-zero (ENNZ) metamaterials. The use of ENNZ metamaterials has recently proven successful in miniaturizing circular waveguides by strongly reducing their natural cutoff frequencies, and the theory is adapted here for the design of subwavelength apertures in a metallic screen. We present simulations and proof-of-concept measurements at microwave frequencies that demonstrate ET for apertures measuring one-quarter of a wavelength in diameter and suggest the potential for even more dramatic miniaturization simply by engineering the ENNZ metamaterial dispersion. The results exhibit a fano-like profile whose frequency varies with the properties of the metamaterial liner, but is independent of period. It is suggested that similar behaviour can be obtained at optical frequencies, where ENNZ metamaterials may be realized using appropriately arranged chains of plasmonic nanoparticles.

Hollow waveguides as polarization converting elements: a theoretical study

Subwavelength apertures in a metallic film act as hollow waveguides. By using a non-quadratic cross-section, an anisotropic transmission behaviour results for the two polarization states. Thus, an array of metallic subwavelength apertures may be used as polarization converter, e.g., as a half-wave plate. By varying orientation and shape of the cross-sections locally, one can design polarization shifting elements for complex wave fields. Here, we present a theoretical consideration on the physical properties and compare with dielectric form birefringence.