Annular Aperture Arrays Research Articles

Plasmonic biosensor with annular aperture array integrated on a resonant cavity LED

In this paper, we propose an intensity-sensitive compact plasmonic sensor by integrating an annular aperture array with a resonant cavity LED (RCLED). The extraordinary optical transmission (EOT) peak originates from the mutual coupling between surface plasmon polariton (SPP) and Localized surface plasmon resonance (LSPR), which utilizes the EOT peak with narrower linewidth for intensity modulation. Simulation results show that by optimizing the resonant wavelength of the annular aperture array to match the emission wavelength of the RCLEDs, the relative intensity change reaches 97%, and the intensity sensitivity is 9744 %/RIU as a protein molecular layer covered on the sensor.

Efficient annular aperture array (AAA) filter for thermophotovoltaic by sidewall lithography

Thermophotovoltaic (TPV) systems consume near infrared (NIR) filters, which are key components for high conversion efficiency. Bandpass filters employing annular aperture array (AAA) on a fused silica substrate are one of candidates for matching GaSb cells in high-temperature TPV. It is often placed independently between the emitter and cell, and thus spectral control and heat insulation are simultaneously realized. For high efficiency, the smaller the characteristic size and the larger the size is beneficial for the filter. Novel sidewall lithography based on conventional interference lithography and ion-beam etching is first proposed and used for cost-effective fabrication. An AAA filter with a 80 nm critical dimension in a 100 mm × 160 mm area has been fabricated successfully. After the annealing process at 500 °C, the peak transmittance is increased to 70% from 65%, and no deformation is observed in the structural parameters of the filter before and after annealing, and calculated results demonstrate that the spectral efficiency is about 61%.

High selectivity plasmonic color filters based on tapered annular aperture arrays

Plasmonic color filters (PCFs) with subwavelength nanometallic structures have irreplaceable advantages over traditional organic dye color filters due to their unique characteristics. However, most PCFs excite weak resonances and show low selectivity, which will limit their accuracy and further applications. In this paper, we introduce a PCF design utilizing tapered annular aperture arrays, which not only shows high resonance intensity, but also achieves the light-splitting effect in the wavelength ranges of visible and near-infrared. The proposed device is comprehensively verified by COMSOL Multiphysics. The results show that ideal filtering effect can be achieved with the tilting angle ranging from 0°to 40°. Geometric parameters (period and aperture range) are also investigated, which can provide an instructive theoretical basis for the preparation of PCFs. The proposed device may find extensive potential applications in micro–nano sensing and optical imaging and inspection.

Plasmonless polarization-selective metasurfaces in the visible range

In this paper, we perform numerical and experimental studies of the optical response of an original configuration that consists of enhanced transmission guided mode-based metamaterials. The proposed structure is inspired by an annular aperture array where the cylindrical symmetry is broken in order to acquire polarization-sensitive metasurfaces. The experimental results, which are in good agreement with numerical simulations, demonstrate that the structure acts as a polarizer exhibiting an extinction ratio of (15:1) with a maximum transmission coefficient up to 85%, which is more efficient than what is expected with a typical plasmonic resonance.

Annular Apertures in Metallic Screens as Extraordinary Transmission and Frequency Selective Surface Structures

A 2-D periodic array of annular apertures (or ring slots) is studied using an accurate circuit model. The model accounts for distributed and dynamic effects associated with the excitation of high-order modes operating above or below cutoff but not far from their cutoff frequencies. This paper allows to ascertain the substantial differences of the underlying physics when this structure operates as a classical frequency selective surface or in the extraordinary-transmission (ET) regime. A discussion of two different designs working at each regime is provided by means of the equivalent circuit approach (ECA), full wave simulation results, and experimental characterization. The agreement between the equivalent circuit calculation applied here and the simulation and experimental results is very good in all the considered cases. This validates the ECA as an efficient minimal-order model and a low computational-cost design tool for frequency selective surfaces and ET-based devices. Additional scenarios such as oblique incidence and parametric studies of the structural geometry are also considered.

Oblique Colloidal Lithography for the Fabrication of Nonconcentric Features.

Herein, we describe the development of oblique colloidal lithography (OCL) and establish a systematic patterning strategy for creating libraries of nanosized nonconcentric plasmonic structures. This strategy combines OCL, capillary force lithography, and several wet and ion etching steps. Hexagonal arrays of nonconcentric gold features were created on glass substrates with highly controllable geometric parameters. The size, geometry, and eccentricity of the gold features could be independently tuned by controlling the experimental conditions. Gaps within surface elements could be shrunk to as small as 30 nm, while the total patterned area was about l cm2. The goal was to devise a method that offers a high degree of control over the resolution and morphology of asymmetric structures without the need to resort to electron beam lithography. This technique also enabled the development of numerous surface patterns through the stepwise fabrication of separate elements. Complex features, including dots-surrounded nonconcentric targets, nonconcentric hexagram-disks, and nonconcentric annular aperture arrays, were demonstrated, and their optical properties were characterized. Indeed, spectroscopic studies and FDTD simulations demonstrated that Fano resonances could readily be generated by the nonconcentric gold features. Consequently, our patterning strategy should enable the high-throughput investigation of plasmonic coupling and Fano resonances as a function of the physical parameters of the elements within the nanopattern array.

Maskless fabrication of slanted annular aperture arrays

We report maskless fabrication of high-aspect-ratio slanted annular aperture arrays (SAAAs) in gold films using focused ion beam lithography. By tilting the substrate, SAAAs with the desired tilting angle can be fabricated. Our experimental results demonstrate accurate control over aperture size, obliqueness, and reproducibility. We also show that the resulted plasmonic resonances of SAAAs can be effectively tuned via obliqueness control. This versatile approach may enable fabrication of more complicated plasmonic nanostructures. The demonstrated gold SAAAs could also find many potential applications in plasmon-assisted sensing and surface enhanced spectroscopy.

Independent Tailoring of Super‐Radiant and Sub‐Radiant Modes in High‐Q Plasmonic Fano Resonant Metasurfaces

Fano resonances in plasmonic metasurfaces arise from the interference between a super‐radiant and a sub‐radiant plasmon mode. The interference of the plasmon modes, which gives rise to the Fano resonance phenomenon in a plasmonic metasurface, also restricts the independent control of the individual resonance modes. Independent tailoring of super‐radiant and sub‐radiant plasmon modes at nanoscale is one of the challenges to be addressed for the realization of targeted functionalities and fundamental understanding of plasmon mode coupling. Here, it is experimentally and numerically shown that the spectral position and line‐width of both the super‐radiant and sub‐radiant plasmonic modes of a Fano resonance can be independently controlled through the variation of metal film thickness at the skin depth scale and polarization of the incident light. The metasurface consists of a conductively coupled annular and rectangular aperture array that supports multiple high‐Q Fano resonances at near‐infrared frequencies. Fano resonances are excited via interference between the azimuthal plasmon mode of the annular aperture and the dipolar plasmon mode of the rectangular aperture. The multiple Fano resonances excited in the proposed design show remarkable sensitivity to skin‐depth scale film thicknesses, enabling independent control of spectral position and line‐shape of super‐radiant and sub‐radiant modes in high‐Q plasmonic Fano resonant metasurfaces.

Fabrication and optical measurement of double-overlapped annular apertures

We demonstrate double-overlapped annular aperture (DOAA) arrays fabricated in a gold film via focused ion beam milling. The high order resonance modes of DOAA are investigated both theoretically and experimentally. Polarization dependency is observed for DOAA arrays and lower order resonance modes in the mid-infrared range exhibit extraordinary optical transmission and dependency on geometric parameters.

High‐Q Whispering‐Gallery‐Mode‐Based Plasmonic Fano Resonances in Coupled Metallic Metasurfaces at Near Infrared Frequencies

Fano resonances in metallic and dielectric structures have received much attention in recent years due to their promising applications in surface enhanced phenomena, sensing, and nonlinear optics. The lossless high refractive index dielectric structures have been shown to have ultranarrow resonances at near infrared and optical frequencies. However, it is rather challenging to realize such narrow Fano resonances using metallic nanostructures due to large optical losses in metals that cause significant broadening of the plasmonic resonances. Herein, whispering gallery mode based sharp Fano resonances are demonstrated experimentally and numerically in plasmonic nanostructures at near infrared frequencies with highest quality factor. The design here consists of conductively coupled annular and rectangular aperture arrays that support multiple Fano resonances. The sharp Fano resonances are observed for both the orthogonal polarizations of the incident electric field due to the coupling between quadrupolar resonance of the annular aperture with the dipolar and quadrupolar resonance of the rectangular aperture. The response of the plasmonic metasurface can be switched from single to multiple Fano resonances by switching the polarization of the incident radiation which is highly desirable for hyperspectral sensing and imaging. The spectral tuning of Fano resonances are further achieved by tailoring the coupling between the annular and the rectangular apertures.

Gap Plasmon Enhanced Metasurface Third-Harmonic Generation in Transmission Geometry

The influence of gap plasmons in the third-harmonic generation of annular and H-shaped aperture arrays has been investigated. Surprisingly, the maximum third harmonic in the annular ring structures is not from the smallest gap, but rather appears for a gap of 14 nm. We interpret this result in terms of resonant plasmonic antenna design to match scattering and absorption cross sections. Unlike the annular aperture, continuous gold apertures, like the H-structure, achieve 0.45% conversion efficiency for 2 mW/μm2 CW pump power of a 100 fs pulse, which is 9 times larger than rectangular apertures we have reported in the past. Nonlinear scattering theory has been used to estimate the transmitted third-harmonic signal and is in generally good agreement with the experimental results.

Tunable plasmonic resonances based on elliptical annular aperture arrays on conducting substrates for advanced biosensing

Introducing a conducting metal layer and the structural asymmetry to elliptical annular aperture arrays, multiple plasmonic coupled-resonant modes are generated under normal incidence in the visible light range. The electromagnetic fields can be strongly enhanced at resonant modes in this device, which increases the interaction volume of the detected analyte and optical fields; therefore, multiple plamonic coupled modes exhibit higher refractive index sensitivity than as large as 610nm/RIU. The distinct Fano-like resonance around a wavelength of 681nm originates from the interference between bonding dipolar and the quadrupolar modes. Due to the excitation of sharp spectral features as narrow as 7nm, high figure of merits of 94 at the Fano-like dip is obtained in a wide refractive index range of 1.33-1.40. Furthermore, to generate strong Fano-like resonance, the geometric shape of ellipse is selected, which is a good geometric shape candidate compared to the circle shape. This device is promising for biosensing applications with high sensitivity and low limit of detection.

Transmission properties of slanted annular aperture arrays. Giant energy deviation over sub-wavelength distance.

This paper is devoted to the study of the transmission properties of Slanted Annular Aperture Arrays made in perfectly conducting metal. More precisely, we consider the transmission based on the excitation of the cutoff-less guided mode, namely the TEM mode. We numerically and analytically demonstrate some intrinsic properties of the structure showing a transmission coefficient of at least 50% of an unpolarized incident beam independently of the illumination configuration (angle and plane of incidence). The central symmetry exhibited by the structure is analytically exploited to demonstrate the existence of a polarization state for which all the incident energy is transmitted through the sub-wavelength apertures when the eigenmode is excited, whatever are the illumination and the geometrical parameters. For this state of polarization, the laminar flow of the energy through the structure can exhibit giant deviation over very small distances. An example of energy flow deviation of 220° per wavelength is presented for illustration. The results presented in this paper could be considered as an important contribution to the understanding of the enhanced transmission phenomenon based on the excitation of guided modes.

Design, fabrication and characterization of resonant metamaterial filters for infrared multispectral imaging

We present the design of infrared filters for multispectral imaging applications, based on square annular aperture arrays in a thin gold film. These structures function as band pass filters with large bandwidth and high transmission at resonance. A modal analysis based on the Finite Element Method (FEM) is performed to obtain quickly the features of this resonance. The center wavelength can be tuned in the 7–12μm range while keeping constant the quality factor and maximum transmission by scaling all transverse dimensions of the apertures, which allows to obtain filters with different centering on the same substrate in a single fabrication step. Large area samples have been fabricated on a silicon wafer by electronic lithography. Spectrophotometric measurements are in rather good agreement with numerical predictions. In addition, angle resolved measurements show that the filters are quite tolerant to the incidence angle up to 30° for both polarizations which is consistent with our FEM simulations. Finally, a complete sensitivity analysis allows us to evaluate acceptable opto-geometric tolerances of fabrication and thus to improve reproducibility on large areas. The impact of fabrication defaults (rounded corners, aperture anisotropy, aperture edge roughness, sloping aperture edges) on the filtering performances is analyzed. The simulations of realistic structures allow to explain and reduce the differences between measured and simulated spectra.

Optical transmission through silver film with compound periodic array of annular apertures

Recently, some kinds of structures have been found to show the property of extraordinary optical transmission (EOT). In this paper, we present a novel composite structure based on array of annular apertures (AAA) with compound lattice. The lattice includes two kinds of annular apertures with the same outer radius and different inner radii. The transmission spectrum of this compound periodic AAA can be achieved by adding up the spectra of two corresponding simple periodic AAAs, and the transmission shows EOT property. The transmission peaks of this kind of structure can be adjusted to desire wavelengths by changing the inner radius of aperture or the index of the dielectric material in the aperture. This structure can be used as a filter with dual pass bands when the difference between inner radii or indices of dielectric inside is large enough for two kinds of apertures.

基于磁谐振的亚波长空气环金属阵列透射特性研究

基于磁谐振的亚波长空气环金属阵列透射特性研究

Resonant optical transmission through sub-wavelength annular apertures caused by a plasmonic transverse electromagnetic (TEM) mode

We demonstrate enhanced transmission through annular aperture arrays (AAA) by the excitation of the transverse electromagnetic () guided mode. A complete numerical study is performed to correctly design the structure before it is experimentally characterized. Actually, the challenge was to get efficient -based transmission in the visible range. It turned out to be a hard task because of the strong absorption associated with this guided mode. Nevertheless, we have succeeded to experimentally prove its excitation thanks to the enhanced transmission measured in the far-field. This is the first time we demonstrate experimental evidence of this phenomenon with such AAA structure illuminated at oblique incidence in the visible range. This increases the potential applications of such structures as well, single molecule spectroscopy, photovoltaic, spectral filtering, optical trapping, etc...

SFM-FDTD analysis of triangular-lattice AAA structure: Parametric study of the TEM mode

This theoretical work reports a parametric study of enhanced transmission through annular aperture array (AAA) structure arranged in a triangular lattice. The effect of the incidence angle in addition to the inner and outer radii values on the evolution of the transmission spectra is carried out. To this end, a 3D Finite-Difference Time-Domain code based on the Split Field Method (SFM) is used to calculate the spectral response of the structure for any angle of incidence. In order to work through an orthogonal unit cell which presents the advantage to reduce time and space of computation, special periodic boundary conditions are implemented. This study provides a new modeling of AAA structures useful for producing tunable ultra-compact devices.

Slanted annular aperture arrays as enhanced-transmission metamaterials: Excitation of the plasmonic transverse electromagnetic guided mode

We present here the fabrication and the optical characterization of slanted annular aperture arrays engraved into silver film. An experimental enhanced transmission based on the excitation of the cutoff-less plasmonic guided mode of the nano-waveguides (the transmission electron microscopy mode) is demonstrated and agrees well with the theoretical predicted results. By the way, even if it is less efficient (70% → 20%), an enhanced transmission can occur at larger wavelength value (720 nm–930 nm) compared to conventional annular aperture arrays structure by correctly setting the metal thickness.

Active control of terahertz multimode resonance transmission through subwavelength metal annular aperture arrays

Ultrafast optical properties of U-shaped annular aperture arrays (UAAAs) fabricated on a silicon substrate were investigated for varying photodoping levels of the silicon from the back to front interfaces of the sample. Experimental data demonstrate that the transmission modulation depth of the multimode resonance can be realized about 90% under optical power 60 mW, and the off switch of these resonance peaks transmission can be also simultaneously achieved within several picoseconds. Such plasmonic structure is able to be used for ultrafast optical modulators, active terahertz plasmonics, and ultrafast optical off switch.