Complementary bowtie aperture for localizing and enhancing optical magnetic field

Nanoscale bowtie antenna and bowtie aperture antenna have been shown to generate strongly enhanced and localized electric fields below the diffraction limit in the optical frequency range. According to Babinet's principle, their complements will be efficient for concentrating and enhancing magnetic fields. In this Letter, we discuss the enhancement of magnetic field intensity of nanoscale complementary bowtie aperture as well as complementary bowtie aperture antenna, or diabolo nanoantenna. We show that the complementary bowtie antenna resonates at a smaller wavelength and thus is more suitable for applications near visible wavelengths. The near-field magnetic intensity can be further enhanced by the addition of groove structures that scatter surface plasmon.     

Optimizing bowtie structure parameters for specific incident light

We investigate optical properties of a bowtie-shaped aperture using the finite difference time domain method to optimize its geometric parameters for specific incident lights.The influence of the parameters on local field enhancement and resonant wavelength in the visible frequency range is numerically analysed.It is found that the major resonance of the spectrum is exponentially depended on the bowtie angle but independent of the whole aperture size.The simulation also demonstrates that increasing the aperture size raises the local field intensity on the exit plane due to an enlarged interaction area between the light and the metal medium.And the near-field spot size is closely related to the gap.Based on these results,the design rules of the bowtie structure can be optimized for specific wavelengths excited.     

Geometrical phase and surface plasmon focusing with azimuthal polarization

Owing to a geometric phase effect, an isosceles triangular aperture etched into thin metal film leads to constructive or destructive interference of surface plasmons excited at the two equal sides under linearly polarized illumination. Through appropriate spatial arrangement of an array of triangles, a highly confined focal spot beyond the diffraction limit can be achieved at the geometric center under azimuthally polarized excitation with field enhancement comparable to a bull's eye plasmonic lens under radially polarized illumination. Through simply rotating the orientation of each triangle aperture by 90°, the plasmonic structure defocuses the same azimuthal polarization illumination due to destructive interference caused by a geometric π-phase difference between the two sides of the triangle and between the adjacent triangles.     

Enhanced UV/blue fluorescent sensing using metal-dielectric-metal aperture nanoantenna arrays

Subwavelength aperture antenna arrays are designed and fabricated for potential applications in fluorescence sensing in the near UV/blue range. They are designed using finite-difference time-domain (FDTD) simulation, fabricated using focused ion beam etching and characterised using angular Fourier spectroscopy. The aperture arrays are formed in the top layer of an aluminum-silica-aluminum trilayer and produce a maximum simulated field intensity enhancement of 5.8 times at 406 nm and highly directive emission with a beamwidth of 8.3 deg. The normal incidence reflection response has been measured and shows reasonable agreement with modelled results. In addition, to investigate higher field intensity enhancements, bowtie aperture arrays are simulated and the influence of parameters such as dielectric gap, position of dipole source, and aperture shape and size are discussed and show enhancements up to 67 times are possible.     

Numerical analysis of the spectral response of an NSOM measurement

Near-field Scanning Optical Microscopy (NSOM) is a powerful tool for investigating optical field with resolution greater than the diffraction limit. In this work, we study the spectral response that would be obtained from an aperture NSOM system using numerical calculations. The sample used in this study is a bowtie nanoaperture that has been shown to produce concentrated and enhanced field. The near- and far-field distributions from a bowtie aperture are also calculated and compared with what would be obtainable from a NSOM system. The results demonstrate that it will be very difficult to resolve the true spectral content of the near-field using aperture NSOM. On the other hand, the far-field response may be used as a guide to the near-field spectrum.     

Spectral resonance of nanoscale bowtie apertures in visible wavelength

We report spectroscopic measurements of transmitted field through bowtie-shaped nanoscale apertures in visible wavelength region. Resonance in these apertures and its relation with the aperture geometry are investigated. The near-field spectral response is also investigated using finite difference time domain (FDTD) computation and compared with the spectroscopic measurements. The dependences of the peak wavelength and peak amplitude on the geometry of the bowtie aperture are illustrated. Design rules are proposed to optimize the bowtie aperture for producing a sub-wavelength, high transmission field. PACS 81.07.-b; 07.79.Fc; 71.36.+c; 78.66Bz; 42.79.Gn; 42.79.Vb     

Optical characteristics of rounded silver nanoprisms

We analyzed numerical optical characteristics of silver nanoprisms with rounded corners using the three-dimensional finite-difference time-domain method. The enhancement of the electric field was decreased from 240 to 13 times by introducing a large radius of curvature at the nanoprism corners such that it became a cylinder. This caused the optical multi-mode to change to single dipole mode. In the largest local electric field enhancement using the bowtie structure, which consisted of a pair of nanoprisms with rounded corners (the curvature radius and the gap distance were 8.66 and 1 nm, respectively), the electric field was enhanced by a factor of 360 at the hotspot. The bowtie structure that has non-zero curvature radii produces a larger electric field enhancement than does the single nanoprism without a curvature radius. Furthermore, the numerical simulation elucidates that the change of the curvature radius and the change of the gap distance have the same influence on the electric field enhancement.     

Impact of process parameters on pattern formation in the maskless plasmonic computational lithography

The extraordinary optical transmission through a sub-wavelength size metal-aperture and metamaterials has been tremendous interests for the untilization of the surface plasmon polariton (SPP). Its technology, however, is hard to apply for the optical lithography process. In this study, a maskless plasmonic lithography (MPL) is modeled and simulated for 15-nm critical dimension (CD). The near-field intensity with the plasmonic phenomena of aperture shapes is described due to aperture parameters by using a scattering matrix (S-matrix) analysis method and the finite difference time domain (FDTD) method. MPL parameters of bowtie structures are optimized and improved for the imperfection of the resist pattern. The most dominant parameter on CD is gap size of bowtie by Taguchi method.     

Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture

In this paper, the enhanced optical transmission through a special type of aperture of a bowtie shape is investigated through near-field imaging and finite-difference numerical analysis. Under linear polarizations in two orthogonal directions, the optical near fields of the bowtie aperture and comparable square and rectangular apertures made in gold and chromium thin films are measured and compared. The bowtie aperture is able to provide a nanometer-sized optical spot when the incident light is polarized across the bowtie gap and delivers a considerable amount of light. Localized surface plasmons are clearly observed in the near-field images for both bowtie and rectangular apertures in gold, but invisible in chromium. Finite-difference time-domain calculations reveal that, depending on the polarization of the incident light, the unique optical properties of the bowtie aperture are a result of either the optical waveguide and the coupled surface plasmon polariton modes existing in the bowtie gap or the coupling between the two open arms of the bowtie aperture. PACS 81.07.-b; 07.79.Fc; 71.36.+c; 78.66.Bz; 42.79.Gn; 42.79.Vb     

Design of a sector bowtie nano-rectenna for optical power and infrared detection

We designed a sector bowtie nanoantenna integrated with a rectifier (Au−TiO_x−Ti diode) for collecting infrared energy. The optical performance of the metallic bowtie nanoantenna was numerically investigated at infrared frequencies (5−30 μm) using three-dimensional frequency-domain electromagnetic field calculation software based on the finite element method. The simulation results indicate that the resonance wavelength and local field enhancement are greatly affected by the shape and size of the bowtie nanoantenna, as well as the relative permittivity and conductivity of the dielectric layer. The output current of the rectified nano-rectenna is substantially at nanoampere magnitude with an electric field intensity of 1 V/m. Moreover, the power conversion efficiency for devices with three different substrates illustrates that a substrate with a larger refractive index yields a higher efficiency and longer infrared response wavelength. Consequently, the optimized structure can provide theoretical support for the design of novel optical rectennas and fabrication of optoelectronic devices.     

Steering the optical response with asymmetric bowtie 2-color controllers in the visible and near infrared range

We have proposed an asymmetric bowtie 2-color controller and analyzed its resonance frequency spectra and temporal responses. The results show improved optical properties of the asymmetric bowtie 2-color controller as compared to symmetric bowties. The improved optical properties are a broad bandwidth of the plasmonic spectrum consisting of two resonant peaks, a high field enhancement in the gap of the bowtie structure, and a large effective enhancement volume. The system might have applications in the generation of XUV light via high-harmonic generation as well as in ultrabroadband sensors and multicolor optoelectronic filters.     

Design of plasmonic bowtie nanoring array with high sensitivity and reproducibility for surface‐enhanced Raman scattering spectroscopy

A metallic bowtie nanoring array is designed to gain high sensitive and reproducible substrate for surface‐enhanced Raman scattering (SERS) spectroscopy. The localized surface plasmon resonance (LSPR), the electric field enhancement factors (EFs) and the electric field distribution of the bowtie and bowtie nanoring array are numerically investigated by means of the finite‐difference time domain (FDTD) method. After the optimization of the particle size and the array period, the maximum electromagnetic field EF approaches 153, and the corresponding SERS electromagnetic enhancement factor (EMEF) reaches 5.4 × 10⁸. This highly sensitive and reproducible substrate can be a good candidate for SERS applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Design of high-efficiency diffraction gratings based on total internal reflection for pulse compressor

We present designs of high-efficiency compression grating based on total internal reflection (TIR) for picosecond pulse laser at 1053 nm. The setup is devised by directly etching gratings into the bottom side of a prism so that light can successfully enter (or exit) the compression grating. Dependence of the −1 order diffraction efficiencies on the constructive parameters is analyzed for TE- and TM-polarized incident light at Littrow angle by using Fourier modal method in order to obtain optimal grating structure. The electric field enhancement within the high-efficiency TIR gratings is regarded as another criterion to optimize the structure of the TIR gratings. With the criterion of high diffraction efficiency, low electric field enhancement and sufficient manufacturing latitude, TIR compression gratings with optimized constructive parameters are obtained for TE- and TM-polarized incident light, respectively. The grating for TE-polarized light exhibits diffraction efficiencies higher than 0.95 within 23 nm bandwidth and relatively low square of electric field enhancement ratio of 5.7. Regardless of the internal electric field enhancement, the grating for TM-polarized light provides diffraction efficiencies higher than 0.95 within 42 nm bandwidth. With compact structure, such TIR compression gratings made solely of fused silica should be of great interest for application to chirped pulse amplification (CPA) systems.     

Enhanced nonlinear optical conversion from a periodically nanostructured metal film

We demonstrate an approximately10(4) increase in conversion efficiency for optical second-harmonic generation (SHG) from a periodically nanostructured metal structure consisting of a single subwavelength aperture in a thin silver film surrounded by a set of concentric surface grooves. The forward-transmitted second-harmonic radiation from this structure is measured relative to that from an identical aperture with no surrounding surface periodicity. We explain the observed SHG enhancement quantitatively in terms of a measured 120x increase in the strength of the fundamental radiation in the vicinity of the aperture resulting from the periodic nanostructuring.     

Investigation of plasmonics resonance infrared bowtie metal antenna

An approximate resonance wavelength equation that varies with metal antenna structure size is developed to design a bowtie gold metal antenna working at near-infrared (IR) wavelength. Bowtie antenna structures with resonance wavelength of 1.06 μm, 1.55 μm and 10.6 μm are designed based on this equation. A finite-difference time domain (FDTD) algorithm with total field scattered field (TFSF) source simulation shows the resonance wavelength of the designed structures being precisely in agreement with the expected wavelengths from the equation. Planar integration of the metal bowtie antennas is discussed as well. Gold nanohole bowtie antenna arrays are fabricated and the near-field optical transmission properties of the nanohole array are investigated with a near-field scanning optical microscope (NSOM). Our experimental results verify the near-field optical transmission performance and further demonstrate that they are in agreement with the theoretical calculation results. The high enhancement efficiency and integration of the metal bowtie antennas open the possibility of a wide application in IR optoelectronics detection and imaging.     

Phase synthesis of a holographic metrological diffraction grating of unlimited length

A breakthrough in the technology of measuring systems of submicrometre and subsecond precision was made possible by the invention of holographic phase aperture synthesis of highly regular interference structures. The practical fulfilment of holographic phase aperture synthesis requires development of the following new optical methods: (1) synthesis of the aperture by out of phase auxiliary holographic diffraction gratings; (2) high-precision analysis of the phase structure of the interference field; (3) automatic control of the aperture synthesis process using two-frequency modulation of the interference field; (4) an adaptive interferometer for aperture synthesis. This paper is devoted to the above methods.     

Spatial polarization sensitivity of single Au bowtie nanostructures

Metallic bowtie nanostructures as plasmonic nanoantennas can create highly enhanced local fields when resonating with the incident light. With Au bowtie nanostructures fabricated by lithography method, we experimentally observed that the photoluminescence (PL) spatial profile from a single Au bowtie nanoantenna was strongly dependent on the excitation light polarization. While varying the incident light polarization, the spatial distribution of the PL intensity in the nanogap of an Au bowtie changed as predicted by the simulation results on the electromagnetic field enhancement distribution. The polarization feature of the PL intensity relative to the polarization direction of incident excitation light was also discussed. The study may find application in the design of polarization sensitive plasmonic sensors.     

不同周期数牛眼结构对基于伪表面等离子激发的太赫兹透射的影响

为研究不同周期数的牛眼结构对太赫兹透射的影响,本文利用微机械加工方法,在铝板上分别制作5个圆环凹槽和15个圆环凹槽两种牛眼结构,并利用时域太赫兹波谱系统,对比了这两种不同周期数的牛眼结构的太赫兹时域信号和频域信号.实验结果显示,0.1~2.7THz宽频太赫兹参考信号可以被两种不同周期数的牛眼结构滤成窄带信号,中心峰值均在约0.53THz处,在该值处,5个圆环凹槽的透射率约为55.7%,15个圆环凹槽的透射率约为68.3%,1THz以上高频信号都被基本滤除,小周期数样品与大周期数样品太赫兹透射谱信号带宽与幅值的差别主要是由于周期性展开不足及边缘泄漏所引起.本文利用伪表面等离子理论解释了滤波效果,并通过有限元方法模拟仿真了宽频太赫兹信号通过不同周期数的牛眼结构后的样品信号,模拟结果与实验结果符合得较好,证实了不同周期数的牛眼结构对太赫兹电磁波的增透效应是不同的.     

High-intensity bowtie-shaped nano-aperture vertical-cavity surface-emitting laser for near-field optics

We report a high-intensity nano-aperture vertical-cavity surface-emitting laser (VCSEL) utilizing a bowtie-shaped aperture. A maximum power of 188 microW is achieved from a 180 nm bowtie aperture at a wavelength of 970 nm. The near-field full width at half-maximum intensity spot size 20 nm away from the bowtie aperture is 64 nm x 66 nm from simulation, and the peak near-field intensity is estimated to be as high as 47 mW/microm(2). This intensity is high enough to realize near-field optical recording, and the small spot size corresponds to storage densities up to 150 Gbits/in(2). The bowtie-aperture VCSEL also enables other applications, such as compact high-intensity probes for ultrahigh-resolution near-field imaging and single molecule fluorescence and spectroscopy.     

Optimization of near-field optical field of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric (MLD) gratings for pulse compressors in high-energy laser systems should provide high diffraction efficiency as well as high laser induced damage thresholds (LIDT). Nonuniform optical near-field distribution is one of the important factors to limit their damage resistant capabilities. Electric field distributions in the gratings and multi-layer film region are analyzed by using Fourier modal method. Optimization of peak electric field in the gratings ridge is performed with a merit function, including both diffraction efficiency and electric field enhancement when the top layer material is HfO₂ and SiO₂, respectively. A set of optimized gratings parameters is obtained for each structure, which reduce the peak electric field within the gratings ridge to being respective 1.39 and 1.84 times the value of incident light respectively. Finally, we also discuss the effects of gratings refractive index, gratings sidewall angle and incident angle on peak electric field in the gratings ridge.