Subwavelength Holes Research Articles

Refractive index biosensing using near-UV metal–dielectric–metal nanostructured arrays: fabrication and simulation

This work reports the design, simulation, and fabrication of a metal–dielectric–metal (MDM) nanohole array structure toward a realization of a refractive index biosensor. The MDM nanohole array structure operating near the UV range consists of a subwavelength hole array inscribed through the upper aluminum (Al) layer positioned on a stack of a thin silica ( S i O 2 ) spacer and an Al mirror on a silicon substrate. The quality factor of the resonance at ∼ 390 n m of the present nanostructure is ∼ 1.5 t i m e s higher than the previous result owing to the larger fill factor of Al material on the upper layer. This means at the UV range, the designed nanohole array structure closely resembles the dielectric gratings in the visible range. At resonance, we numerically observed a strong confinement of incident light localized inside the subwavelength apertures and the S i O 2 spacer, and thus high absorption and directive emission of up to 99% and ∼ 8.0 d e g , respectively. The MDM nanohole array structure has been fabricated by using plasma-enhanced chemical vapor deposition combining with the focused ion beam, and its reflection experiments are in good agreement with the finite-difference time-domain simulation results. The proposed MDM nanohole array structure can be applicable for a wide variety of plasmonic sensing, in particular for refractive index biosensors. Because of working in the near-UV range, this refractive index biosensor shows the figure of merit to have a selectivity that is ∼ 4.0 t i m e s higher than that of other established plasmonic MDM biosensors.

Millimeter-Wave Metal Reflectarray Antennas with Sub-Wavelength Holes

Reflectarray antennas composed of rectangular grooves with sub-wavelength holes on a metal plate are designed for millimeter-wave regions. All depths of multiple grooves in the metal reflectarray are elaborately manipulated for a high-gain reflector. A sub-wavelength hole in each groove reduces the mass of the reflectarray antenna, which rarely affects the re-radiated millimeter-wave filed from the groove. In this paper, we have demonstrated light high-gain reflectarray antennas and achieved a 25%-light reflectarray antenna compared with a metal reflectarray without sub-wavelength holes. The designed reflectarray antenna operates within the 15% wide-band bandwidth at 3 dB for millimeter-wave band.

Robust hybrid laser linewidth reduction using Si3N4-based subwavelength hole defect assisted microring reflector

We demonstrate a hybrid laser with a low intrinsic linewidth of 34.2 Hz and a high fiber-coupled output power of 11.7 dBm, by coupling a Si 3 N 4 -based subwavelength hole defect assisted microring reflector (SHDA-MRR) to a commercially available distributed feedback semiconductor laser. The proposed SHDA-MRR structure features an accurately controlled reflection response, with the manipulated modal coupling between two degenerate counterpropagating modes induced by a subwavelength hole defect embedded in the microring waveguide. With further joint optimization of cavity parameters, this Si 3 N 4 SHDA-MRR structure is expected to reduce the laser intrinsic linewidth to a sub-hertz level. This work explores a low-cost and robust linewidth reduction scheme for the applications of high-speed coherent optical communications systems and high-resolution optical metrology.

Smith-Purcell radiation based on the transmission enhancement of a subwavelength hole array with inner tunnels.

The use and control of the extraordinary optical transmission through subwavelength hole arrays has enormous application potential in photonic devices. In this paper, we propose a subwavelength hole array with inner tunnels, for which the Smith-Purcell radiation (SPR) with this enhanced transmission phenomenon in THz is excited when the transmission peak locates in the SPR band. The SPR is monitored using particle-in-cell simulations in order to analyze the mechanisms responsible for improving the radiation coherence. Analysis of the electron energy loss reveals that the proposed subwavelength hole array with inner tunnels outperforms a conventional subwavelength grating array with respect to SPR generation efficiency. As SPR plays a significant role in research on particle diagnosis and terahertz radiation sources, the performance of the proposed structure suggests that it has high application potential.

Corner-Promoted Focus Enhancement of Light in Conical Holes for Extraordinary Optical Transmission

Extraordinary optical transmission (EOT) is generated when light transmits through an array of subwavelength holes on a metallic sheet. Most studies have focused on the EOT effect with ideal cylindrical holes. Subsequently, it has been recognized that imperfection in hole fabrication could alter light-matter interaction. Later, adiabatic taper is reported to promote light nano-focusing effect in the optical and near infrared wavelength regime. Due to geometrical and fabrication constraints, it is difficult to fabricate adiabatic taper for mid-infrared responsive nanohole array. In this work, we report the construction of nanohole arrays with varying near-adiabatic taper holes. We demonstrate that when the positive (inverted conic) angle increases from 10° to 20°, the peak full width half maximum (FWHM) improves by 14% from 800 to 700nm, respectively. Furthermore, inverted conical hole is shown to be superior to negative conic angle. Unlike cylindrical hole array, conical hole array maintains high transmission throughput for any given hole dimension while achieving better FWHM. The betterment in performance for conical over cylindrical holes is attributed to a localized plasmonic effect at the terminal exit. Subsequently, the device is demonstrated as a CO2 low flow sensor. The highest sensor sensitivity is determined to be between 1-4sccm.

A Thermopile Device with Sub-Wavelength Hole Arrays by CMOS-MEMS Technology.

A thermopile device with sub-wavelength hole array (SHA) is numerically and experimentally investigated. The infrared absorbance (IRA) effect of SHAs in active area of the thermopile device is clearly analyzed by the finite-difference time-domain (FDTD) method. The prototypes are manufactured by the 0.35 μm 2P4M complementary metal-oxide-semiconductor micro-electro-mechanical-systems (CMOS-MEMS) process in Taiwan semiconductor manufacturing company (TSMC). The measurement results of those prototypes are similar to their simulation results. Based on the simulation technology, more sub-wavelength hole structural effects for IRA of such thermopile device are discussed. It is found from simulation results that the results of SHAs arranged in a hexagonal shape are significantly better than the results of SHAs arranged in a square and the infrared absorption efficiencies (IAEs) of specific asymmetric rectangle and elliptical hole structure arrays are higher than the relatively symmetric square and circular hole structure arrays. The overall best results are respectively up to 3.532 and 3.573 times higher than that without sub-wavelength structure at the target temperature of 60 °C when the minimum structure line width limit of the process is ignored. Obviously, the IRA can be enhanced when the SHAs are considered in active area of the thermopile device and the structural optimization of the SHAs is absolutely necessary.

Coherent terahertz emission from free-electron-driven Fabry–Pérot resonators with coupled grooves

Fabry–Pérot (FP) resonators with metallic subwavelength grooves or holes, which have been extensively applied in optical region, have recently attracted increasing interest for terahertz wave generation and manipulation. However, the practices of such resonators in the terahertz region are greatly restricted by the low energy-capacity and by the difficulty of manufacturing high-aspect-ratio grooves/holes. Here we illustrate a kind of terahertz FP resonator formed by a cluster of coupled grooves with low-aspect-ratio, which has much higher energy-capacity, higher quality-factor and is much easier to fabricate than one formed by a single uncoupled-groove. Using free-electron beams to excite an array of such FP resonators, coherent terahertz emission can be achieved, the intensity of which is more than an order of magnitude higher than that from an array of uncoupled-grooves, affording a promising way for high-power and coherent terahertz wave generation.

Superresolution Imaging via Subwavelength Hole Resonances

This work presents a new super-resolution imaging approach by using subwavelength hole resonances. We employ a subwavelength structure in which an array of tiny holes are etched in a metallic slab with the neighboring distance $\ell$ that is smaller than half of the wavelength. By tuning the incident wave at resonant frequencies, the subwavelength structure generates strong illumination patterns that are able to probe both low and high spatial frequency components of the imaging sample sitting above the structure. The image of the sample is obtained by performing stable numerical reconstruction from the far-field measurement of the diffracted wave. It is demonstrated that a resolution of $\ell/2$ can be obtained for reconstructed images, thus one can achieve super-resolution by arranging multiple holes within one wavelength. The proposed approach may find applications in wave-based imaging such as electromagnetic and ultrasound imaging. It attains two advantages that are important for practical realization. It avoids the difficulty to control the distance the between the probe and the sample surface with high precision. In addition, the numerical reconstructed images are very stable against noise by only using the low frequency band of the far-field data in the numerical reconstruction.

Plasmonic-Layered InAs/InGaAs Quantum-Dots-in-a-Well Pixel Detector for Spectral-Shaping and Photocurrent Enhancement.

The algorithmic spectrometry as an alternative to traditional approaches has the potential to become the next generation of infrared (IR) spectral sensing technology, which is free of physical optical filters, and only a very small number of data are required from the IR detector. A key requirement is that the detector spectral responses must be engineered to create an optimal basis that efficiently synthesizes spectral information. Light manipulation through metal perforated with a two-dimensional square array of subwavelength holes provides remarkable opportunities to harness the detector response in a way that is incorporated into the detector. Instead of previous experimental efforts mainly focusing on the change over the resonance wavelength by tuning the geometrical parameters of the plasmonic layer, we experimentally and numerically demonstrate the capability for the control over the shape of bias-tunable response spectra using a fixed plasmonic structure as well as the detector sensitivity improvement, which is enabled by the anisotropic dielectric constants of the quantum dots-in-a-well (DWELL) absorber and the presence of electric field along the growth direction. Our work will pave the way for the development of an intelligent IR detector, which is capable of direct viewing of spectral information without utilizing any intervening the spectral filters.

Parametric study of optical transmission through plasmonic hole arrays modulated by the phase transition of vanadium dioxide

We have performed comprehensive electromagnetic simulations and preliminary experiments to explore the effects of geometrical and material parameters on the extraordinary optical transmission (EOT) through periodic arrays of subwavelength holes in a bilayer stack consisting of a gold or silver film atop a vanadium dioxide film (Au/Ag + VO2), where the latter undergoes a semiconductor-to-metal phase transition. Using the finite-difference time-domain (FDTD) and finite-element methods (FEM), we vary iteratively the array periodicity, VO2 film thickness and hole diameters, as well as the refractive index inside the VO2-layer holes and the VO2 optical constants. For each variation, we compare the metallic-to-semiconducting ratios of the zero-order transmission (T00) peaks and find sharp maxima in these ratios within narrow parameter ranges. The maxima arise from Fabry-Perot and Fano-type resonances that minimize T00 in the semiconducting phase of the perforated bilayers. At a fixed array period, the primary factors controlling the VO2-enabled EOT modulation are the VO2 thickness, diameter of the VO2-layer holes, and absorption in the two VO2 phases. Besides uncovering the origins of the higher metallic-phase T00, this study provides a protocol for optimizing the performance of the bilayer hole arrays for potential uses as dynamically tunable nano-optical devices.

Quasiordered, subwavelength TiO2 hole arrays with tunable, omnidirectional color response

Fabrication, optical characterization, and simulation of subwavelength TiO2 hole arrays exhibiting geometry-tunable, omnidirectional color response across the visible spectrum is described. Partially suspended TiO2 grating “membranes” (hole arrays supported by a high void-fraction, low-index underlayer) with quasiperiodic hexagonal order were created on an Si substrate using colloidal lithography, metal mask, plasma-based pattern transfer, and XeF2 etching. Optical measurements under specular and diffuse reflection conditions, along with finite-difference time-domain simulations, indicated that the omnidirectional color response of the hole arrays emerges from a broad distribution of Fano resonance states formed by coupling between guided and internal Fabry–Pérot (FP) cavity modes of the TiO2 layer. Higher-order FP resonances from the external cavity formed between the TiO2 layer and substrate control the apparent color when devices are viewed in direct light. The simulated modal behavior of arrays was found to be very sensitive to the degree of Si removal from the underlayer, in agreement with experimental observations. The fabrication methodology presented herein is substrate-agnostic and can be employed to fabricate suspended, subwavelength hole arrays in many material systems, with potential application to optical filters and reflectors, photocatalytic electrodes, photovoltaics, and sensors.

Near- and mid-infrared plasmonic Fano resonances induced by different geometric configurations in subwavelength nanostructures

Abstract Plasmonic Fano resonances, which result from the interaction between two plasmon modes in subwavelength nanostructures, provide a unique way to realize promising applications in light manipulation technology. Here, we investigate the infrared plasmonic Fano resonances of subwavelength periodic hole arrays with different geometric configurations in a metallic film containing Z-shaped hole arrays, cross-shaped aperture arrays, and gammadion-shaped hole arrays (GSHAs). Specifically, the infrared plasmonic Fano resonance in the GSHA structure, without considering rotational symmetry breaking, can be interpreted as stemming from the coupling between the bonding and antibonding localized surface plasmon resonance modes. By considering the rotational symmetry breaking in the GSHA structure, it is possible to actualize plasmonic double- and triple-Fano resonances in the near-infrared or mid-infrared region. Moreover, the plasmonic double- and triple-Fano resonances between the first- and second-NIR optical windows, as well as in the mid-infrared region can be controlled by appropriately regulating the related length of the arms in the GSHA structure. Our results show that infrared plasmonic Fano resonances might pave the way for the exploration of applications in optical communications and information processing.

Quantum teleportation mediated by surface plasmon polariton

Surface plasmon polaritons (SPPs) are collective excitations of free electrons propagating along a metal-dielectric interface. Although some basic quantum properties of SPPs, such as the preservation of entanglement, the wave-particle duality of a single plasmon, the quantum interference of two plasmons, and the verification of entanglement generation, have been shown, more advanced quantum information protocols have yet to be demonstrated with SPPs. Here, we experimentally realize quantum state teleportation between single photons and SPPs. To achieve this, we use polarization-entangled photon pairs, coherent photon–plasmon–photon conversion on a metallic subwavelength hole array, complete Bell-state measurements and an active feed-forward technique. The results of both quantum state and quantum process tomography confirm the quantum nature of the SPP mediated teleportation. An average state fidelity of 0.889pm 0.004 and a process fidelity of 0.820pm 0.005, which are well above the classical limit, are achieved. Our work shows that SPPs may be useful for realizing complex quantum protocols in a photonic-plasmonic hybrid quantum network.

Fano resonance in metallic grating via strongly coupled subwavelength resonators

We investigate the Fano resonance in grating structures using coupled resonators. The grating consists of a perfectly conducting slab with periodically arranged subwavelength slit holes, where inside each period, a pair of slits sit very close to each other. The slit holes act as resonators and are strongly coupled. It is shown rigorously that there exist two groups of resonances corresponding to poles of the scattering problem. One sequence of resonances has imaginary part in the order of ε, where ε is the size of the slit aperture, while the other sequence has imaginary part in the order of ε2. When coupled with the incident wave at resonant frequencies, the narrow-band resonant scattering induced by the latter will interfere with the broader background resonant radiation induced by the former. The interference of these two resonances generates the Fano-type transmission anomaly, which persists in the whole radiation continuum of the grating structure as long as the slit aperture size is small compared to the incident wavelength.

Quantum dot based mid-infrared photodetector enhanced by a hybrid metal-dielectric optical antenna

Surface plasmon waves and Rayleigh anomaly are characteristic optical phenomena exhibited by periodic subwavelength grating structures. In this paper, a hybrid metal-dielectric metasurface is proposed to improve the photoresponse of Ge/Si quantum dot infrared photodetectors (QDIPs) with limited absorption layer thickness. The composite metasurface consists of a regular array of silicon pillars. The pillars protrude through subwavelength holes in a perforated gold film on the detector top. We demonstrate that by combining effects of dielectric and plasmonic metal components, the QDIP photoresponse can be significantly improved compared to case when all-dielectric and metal gratings work alone. We observe about four times photoresponse enhancement with the hybrid metasurface device relative to a common plasmonic design with a two-dimensional metal hole array. Compared with a bare QDIP, the peak responsivity of the hybrid detector at a wavelength of 4.4 µm is increased by a factor of 15. The enhanced sensitivity is supposed to arise from coupling of the surface plasmon resonance and diffractive effect related to the Rayleigh anomaly.

Third harmonic working based on the Smith–Purcell radiation in a closed structure

Smith–Purcell radiation is well analyzed in an open structure; however, it is seldom studied in a closed structure. In a high order mode resonant system, the harmonics of the spoof surface plasmon in the subwavelength hole array will be transformed into Smith–Purcell radiation and the radiation energy will be embedded in a high order mode of this resonant system. The spoof surface plasmon and the high order mode have the same field distribution in the beam channel by a special design, and then, the electron beam will interact with them simultaneously. Based on the phenomenon and mechanism, an effective method that works on the higher order harmonic is proposed. It is possible to mitigate the frequency limits as well as the start current limits in vacuum electronic devices, and the mode competition issue is also improved because of the frequency-locked characteristic.

Subwavelength hole defect assisted microring resonator for a compact rectangular filter.

We propose and demonstrate a subwavelength hole defect assisted microring resonator (SHDAMR) structure. With the manipulated modal coupling between two degenerate counterpropagating modes induced by a subwavelength hole defect embedded in the microring waveguide, the SHDAMR structure shows a rectangular resonance lineshape instead of the Lorentzian resonance lineshape of a conventional microring. As a proof of concept, the SHDAMR structure is fabricated on the Si3N4 waveguide platform, for achieving a rectangular filter with a 3-dB bandwidth of 2.03 GHz and an improved shape factor. The demonstrated SHDAMR structure shows the advantages of compact footprint, simplified tunability, and large tolerance of fabrication errors, showing great potential for various applications.

An ultrasonic metallic Fabry–Pérot metamaterial for use in water

Fabry-Pérot ultrasonic metamaterials have been additively manufactured using laser powder bed fusion to contain subwavelength holes with a high aspect-ratio of width to depth. Such metamaterials require the acoustic impedance mismatch between the structure and the immersion medium to be large. It is shown for the first time that metallic structures fulfil this criterion for applications in water over the 200–800 kHz frequency range. It is also demonstrated that laser powder bed fusion is a flexible fabrication method for the ceration of structures with different thicknesses, hole geometry and tapered openings, allowing the acoustic properties to be modified. It was confirmed via both finite element simulation and practical measurements that these structures supported Fabry-Pérot resonances, needed for metamaterial operation, at ultrasonic frequencies in water. It was also demonstrated the the additively-manufactured structures detected the presence of a sub-wavelength slit aperture in water.

Tailoring the optical field enhancement in Si-based structures covered by nanohole arrays in gold films for near-infrared photodetection

We performed numerical simulations of plasmonic near-field enhancement in Si-based structures in near infrared region. Gold films perforated with periodic two-dimensional subwavelength hole arrays were used as the plasmonic couplers. The array periodicity was adjusted to excite the surface plasmon modes at the telecom wavelengths (between 1.3 and 1.55 μm). The field intensity enhancement factor and its spectral position, as a function of hole diameter, demonstrate the maximum at which the Bloch plasmon polariton waves propagating along the Au–Si interface change by a localized surface plasmon mode. The maximum peak wavelength and field intensity enhancement are reached at d/a = 0.5, where d is the hole diameter and a is the array periodicity. An over 14 times field intensity enhancement was obtained at λ = 1.54 μm for d = 200 nm and a = 400 nm. We found that the localized surface plasmon mode is confined mainly under the Au regions along the diagonals of the square lattice of holes. The lateral field distribution for propagating modes has either a hexagonal or square shape, reflecting in-pane symmetry of the grating. For structures with largest holes, the anticrossing of localized mode with the propagating one was observed implying coupling between the modes and formation of a mixed near-field state. The information acquired from the study is valuable for feasible device applications.

Mid-infrared optical resonances in quantum dot photodetectors coupled with metallic gratings with different aperture diameters

The paper is devoted to optical testing of mid-infrared Ge/Si photodetectors obtained by stacking of self-assembled Ge quantum dots in multilayer structures, which are near-field coupled to the adjacent nanoplasmonic arrays of subwavelength holes in metallic films. It is shown that photocurrent and near-field spectra consist of several sets of peaks, which are attributted to surface plasmon waves, localized surface plasmon modes or diffractive Rayleigh anomaly depending on the hole diameter and the angle of incidence θ. We find that for small holes the greatest contribution to the photocurrent enhancement is due to the excitation of the surface plasmon-polariton waves for all θ. As the hole diameter is increased and becomes comparable with the array periodicity, the normal-incident photoresponse improvement is provided by the Rayleigh anomaly. With the increase of incident angle, the photocurrent enhancement is supposed to arise from coupling of the localized shape resonance and propagating plasmon modes.