Subwavelength Circular Aperture Research Articles

Multianalyte Detection with Metasurface-Based Midinfrared Microspectrometer.

Midinfrared (2.5-25 μm) spectroscopy is an ideal tool for identifying chemicals in a nondestructive manner. The traditional platform is a Fourier transform infrared (FTIR) spectrometer, but this is too bulky, expensive, and power-hungry for many applications. There is therefore a growing demand for small, lightweight, and cost-effective microspectrometers for use in the field. One emerging platform is the filter-array detector-array microspectrometer. It pairs a broadband detector array with a thin and rigid array of spectral filters to offer a robust, compact platform for real-time in situ sensing. However, most demonstrations have only focused on identifying a single chemical against a null sample, even though many applications would involve multianalyte detection. In this work, we show a rare attempt at simultaneously tracking multiple analytes with a metasurface filter-array microspectrometer. The metasurface consists of periodic lattices of subwavelength circular apertures in an aluminum layer to create an array of bandpass filters. The filter array is imaged with an off-the-shelf microbolometer via a reverse-lens imaging setup to simultaneously monitor the concentration of ethanol and methanol in gasoline. This represents an important application of fuel quality monitoring. Chemometric models (PLS and SVR) are trained and tested on gasoline blends with ethanol and methanol contents, both ranging from 0% to 20% v/v. A support vector machine regression (SVR) model with a cubic kernel was found to have the lowest combined prediction errors. The root-mean-square-error of prediction (RMSEP) for ethanol and methanol are 1.23% and 1.84% v/v; the corresponding pseudounivariate limit of detection is found to be 4.22% and 6.86% v/v, respectively. This work takes the emerging field of metasurface-based mid-infrared spectrometers from single- to multianalyte detection, thereby considerably expanding their range of potential applications.

Closed-Form Expressions for the On-Axis Scattered Fields by a Subwavelength Circular Aperture in an Infinite Conducting Plane

Closed-form analytical expressions for the on-axis scattered fields by a subwavelength circular aperture in an infinite perfectly conducting plane were derived using a vector potential formulation and the equivalence principle. The final expressions are valid for the near-field, intermediate-field, and far-field zones. The underlined formulation is based on the equivalent quasi-static magnetic current distributions in the aperture, which were derived by Bethe and Bouwkamp in the mid 40's and early 50's. The resulting scattered-field integrals, which involve the free-space Green's function, were evaluated analytically after introducing Taylor series expansion and using transformations. The final closed-form expressions of the scattered fields are given in terms of a recursion formula. Obtained results based on these closed-form expressions are in excellent agreement with data generated by a numerical integration scheme.

Analysis of a Plane Wave-Excited Subwavelength Circular Aperture in a Planar Conducting Screen Illuminating a Multilayer Uniaxial Sample

A spectral domain analysis is presented of a plane wave-excited subwavelength circular aperture in a planar perfectly conducting screen in close proximity to a multilayer stack, which may comprise uniaxially anisotropic slices. The formulation employs the Bethe-Bouwkamp aperture model for the general case of oblique incidence, and it combines the Hankel transform method with a transmission line network analogue of the medium, which facilitates the inclusion of any number of material layers. The electromagnetic fields are expressed in terms of one-dimensional (1-D) Hankel transform integrals, which are efficiently evaluated by a numerical quadrature along a suitably selected path, with convergence acceleration by extrapolation. Sample numerical results are presented to demonstrate the validity of the method and its utility in the modeling of multilayer near-field lenses and aperture-type probes of near-field optical microscopy.

On the Plane Wave-Excited Subwavelength Circular Aperture in a Thin Perfectly Conducting Flat Screen

The electromagnetic field transmitted by a subwavelength circular aperture in a perfectly conducting screen of infinitesimal thickness and illuminated by an obliquely incident plane wave is analyzed based on the Bethe-Bouwkamp quasi-static model of the field in the aperture plane. The problem is formulated in the spectral domain and the solution is expressed in terms of a minimum number of one-dimensional Hankel-transform integrals involving products of Bessel functions and spherical Bessel functions. An efficient computational scheme for these integrals is implemented, based on numerical quadrature with convergence acceleration by extrapolation. From the rigorous spectral domain formulation, closed form expressions are also derived for the transmitted near-zone and far-zone fields. Sample numerical results are presented to demonstrate the validity of the method and its utility in the modeling of aperture-type near-field optical probes.

Resonant Power Transmission Through Coupled Subwavelength Ridged Circular Apertures

The enhanced electromagnetic power transmission through single and coupled subwavelength ridged circular apertures in a thin metal screen is analyzed. The optimum distance between the apertures for the maximum transmission cross section (TCS), corresponding to the resonance condition, is identified with different relative arrangements. Based on an analysis of the TCS at resonance and a duality analogy with small dipole antennas, equivalent circuits for the resonant transmission are drawn that allow the self and mutual conductances to be quantitatively extracted, thereby providing a deeper comprehension of the enhanced power transmission mechanism.

리지가 장하된 소형 원형 개구의 공진 투과에 관한 연구

본 논문에서는 파장에 비해 작은 원형 개구를 통한 전자파 투과를 고려하였다. 무한 도체판의 개구에 대해 수직으로 평면파가 입사될 때 투과 문제를 모멘트 방법으로 풀었다. 투과된 전력과 개구로부터의 복사 패턴에 대한 결과들로부터 파장에 비해 작은 원형 개구, 리지가 장하된 원형 개구 및 반 파장 슬롯의 투과 특성을 비교하였다. 그리고 이론 연구의 타당성을 검증하기 위해 FPCB상에 제작된 개구의 투과 특성을 실험하였으며, 그 결과 시뮬레이션 결과들과 잘 일치함을 관찰하였다. In this paper, the electromagnetic wave transmission through a ridge-loaded small circular aperture is considered. The transmission problem when a plane wave is normally incident on the aperture in an infinite conducting plane is solved by a method of moments(MoM). From the results for the transmitted power and the patterns of radiation from the aperture, the transmission characteristics of a small sub-wavelength circular aperture, a ridge-loaded circular aperture, and a half wavelength slot are compared. In addition, the theoretical study is verified through the experiments for the apertures fabricated on an Flexible Printed Circuit Board(FPCB), which shows fairly good agreements with the simulated results.

Terahertz near-field imaging using subwavelength plasmonic apertures and a quantum cascade laser source

The first demonstration, to our knowledge, of near-field imaging using subwavelength plasmonic apertures with a terahertz quantum cascade laser source is presented. "Bull's-eye" apertures, featuring subwavelength circular apertures flanked by periodic annular corrugations were created using a novel fabrication method. A fivefold increase in intensity was observed for plasmonic apertures over plain apertures of the same diameter. Detailed studies of the transmitted beam profiles were undertaken for apertures with both planarized and corrugated exit facets, with the former producing spatially uniform intensity profiles and subwavelength spatial resolution. Finally, a proof-of-concept imaging experiment is presented, where an inhomogeneous pharmaceutical drug coating is investigated.

SPECTRAL DOMAIN ANALYSIS OF A CIRCULAR NANO-APERTURE ILLUMINATING A PLANAR LAYERED SAMPLE

A rigorous and e-cient spectral domain formalism is presented of a plane wave-excited subwavelength circular aperture in a planar perfectly conducting metallic screen of inflnitesimal thickness, based on the Bethe-Bouwkamp quasi-static model. The formulation utilizes a transmission line analogue of the medium, which facilitates the inclusion of planar multilayered material samples, where the latter may exhibit uniaxial anisotropy. The transmitted fleld components are expressed in terms of one-dimensional Hankel transform integrals, which can be evaluated by e-cient numerical procedures. Sample results are presented showing the intensity proflles and polarization states of transmitted light penetrating into a semiconductor layer.

COMPLEX IMAGE METHOD ANALYSIS OF A PLANE WAVE-EXCITED SUBWAVELENGTH CIRCULAR APERTURE IN A PLANAR SCREEN

A complex image method is presented for the analysis of a subwavelength circular aperture in a perfectly conducting screen of inflnitesimal thickness illuminated by a plane wave. The method is based on the Bethe-Bouwkamp quasi static model of the aperture fleld and uses the spectral domain formulation as the point of departure. Closed-form expressions are obtained for the electromagnetic flelds valid for all observation points. Sample numerical results demonstrate the accuracy and e-ciency of the method for both normal and oblique illuminations, including an evanescent wave. In the latter case, the results show a circulating power ∞ux and enhanced fleld conflnement near the aperture.

Mid‐infrared transmission through a subwavelength circular aperture coupled to a surrounding concentric metal grating by surface plasmon excitation

AbstractSurface plasmon polariton (SPP) coupling of mid‐infrared light to the electrons on the surface of a concentric metal grating integrated onto a GaSb substrate is simulated with 3‐dimensional finite difference time domain methods. The transmission through a subwavelength circular aperture at the center of the grating by this coupling is highly sensitive to the variation of the aperture diameter and shows the strongest SPP coupling when the diameter approximately becomes a half period of the grating. On the other hand, the simulation reveals that SPP coupling does not decay so fast for d larger than a half period and provides noticeable effects until the diameter becomes comparable to the grating period. It also shows that the coupling is ineffective when the diameter reaches a few grating periods. Transmission through the subwavelength aperture is remarkably enhanced by such SPP coupling, which can be very important in its practical applications. The simulation is focused on its application to mid‐infrared focal plane arrays. Some potential factors for additional enhancement of transmission are discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Coupling terahertz radiation onto a metal wire using a subwavelength coaxial aperture

We demonstrate a simple technique for coupling freely propagating broadband THz radiation to multi-cycle THz pulses on a cylindrical metal wire. This is accomplished by inserting the tapered end of a cylindrical wire into the center of a subwavelength circular aperture fabricated in a freestanding metal film, forming an effective coaxial waveguide. By doing so, we convert the transmission properties of THz pulses through the aperture from an evanescent mode to a propagating mode. By fabricating concentric annular grooves about the aperture, multicycle THz pulses are coupled to the wire. The individual groove geometry, number of grooves, and groove spacing surrounding the subwavelength aperture on the metal film determine the shape of THz pulses launched on the waveguide.

Plasmonic structures with extraordinary transmission and highly directional beaming properties

AbstractWe studied the grating–coupling phenomena between surface plasmons and electromagnetic waves in the microwave spectrum. We first present the experimental and theoretical results of an enhanced microwave transmission though a subwavelength circular annular aperture with and without metallic gratings. We demonstrate that a 145‐fold enhancement factor could be obtained with a subwavelength circular annular aperture that was surrounded by periodic metallic gratings. This was assisted by the guided mode of the coaxial waveguide and by coupling to the surface plasmons. We present the angular transmission distributions from circular annular apertures, and circular annular apertures surrounded by concentric periodic grooves. At the surface mode resonance frequency, the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. We demonstrate that only the output surface is responsible for the beaming effect. Furthermore, we present the field distributions and showed that there is no beaming effect at the off‐resonance frequency. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2491–2496, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22015

Plasmonic structures with highly directional beaming properties

We studied the diffraction of electromagnetic waves from subwavelength metallic circular apertures in the microwave spectrum. The metallic samples had a subwavelength hole with a diameter of 8 mm and had concentric grooves with a periodicity of 16 mm. We present the angular transmission distributions from circular annular apertures, and circular annular apertures surrounded by concentric periodic grooves. At the surface mode resonance frequency the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. This represents a fourfold reduction when compared to the angular divergence of the beam transmitted from the subwavelength circular aperture.

Beaming of electromagnetic waves emitted through a subwavelength annular aperture

We study the diffraction of electromagnetic waves from subwavelength metallic circular apertures in the microwave spectrum. The theoretical and experimental demonstration of the near- and far-field electromagnetic distributions for subwavelength circular annular apertures and circular annular apertures surrounded by concentric periodic grooves are reported here. The metallic samples had a subwavelength hole with a diameter of 8 mm and had concentric grooves with a periodicity of 16 mm. We present the angular transmission distributions from circular annular apertures, and circular annular apertures surrounded by concentric periodic grooves. At the surface-mode resonance frequency the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. This represents a fourfold reduction when compared with the angular divergence of the beam transmitted from the subwavelength circular aperture.

Extraordinary grating-coupled microwave transmission through a subwavelength annular aperture

We studied coupling phenomena between surface plasmons and electromagnetic waves in the microwave spectrum using circular apertures surrounded by array of grooves.We first present experimental and theoretical results of enhanced microwave transmission though a subwavelength circular aperture with concentric periodic grooves around the surface plasmon resonance frequency. This is followed by transmission studies through circular annular apertures and circular annular apertures surrounded by concentric periodic grooves. We demonstrated that 145 fold enhancement factor could be obtained with a subwavelength circular annular aperture surrounded by concentric periodic grooves. Our results show that, high transmission from a circular annular aperture with grooves is assisted by the guided mode of the coaxial waveguide and coupling to the surface plasmons.

Surface-topography-induced enhanced transmission and directivity of microwave radiation through a subwavelength circular metal aperture

Strongly enhanced transmission of microwave radiation (λ0∼5 mm) is observed through a single subwavelength circular aperture of diameter d=2.5 mm in a metallic plate. The phenomenon is caused by resonant excitation of electromagnetic surface waves supported by four concentric grooves surrounding the aperture on the illuminated side of the sample. It is also shown that similar surface patterning on the output face of the sample results in very strong angular confinement (directivity) of the transmitted beam. A finite element code is used to investigate the electromagnetic fields on both the illuminated and the exit side of the structure, the predictions from which show excellent agreement with the experimental results.

Optical transmission of a subwavelength aperture: size and fiber parameter dependence of near-field resolution

We present a theoretical calculation of the optical transmission of a subwavelength circular aperture in a thick and perfectly conducting screen coated upon an optical fiber. The theory is employed to study resolution of scanning near-field optical microscopy. In particular, we have studied the resolution dependence on the aperture size and the parameters of the fiber that is used to form the probe.