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.     

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.     

A functional probe with bowtie aperture and bull's eye structure for nanolithograph

The bowtie aperture surrounded by concentric gratings(the bull’s eye structure) integrated on the near-field scanning optical microscopy(NSOM) probe(aluminum coated fiber tip) for nanolithography has been investigated using the finite-difference time domain(FDTD) method.By modifying the parameters of the bowtie aperture and the concentric gratings,a maximal field enhancement factor of 391.69 has been achieved,which is 18 times larger than that obtained from the single bowtie aperture.Additionally,the light spot depends on the gap size of the bowtie aperture and can be confined to sub-wavelength.The superiority of the combination of the bowtie aperture and the bull’s eye structure is confirmed,and the mechanism for the electric field enhancement in this derived structure is analyzed.     

Field enhancement by semi-nanocapsule plasmonic antenna at the visible violet wavelength

We investigate, at the visible violet wavelength of 400 nm, the localized field-enhancement properties of an optical antenna consisting of two coupled metallic nanoparticles placed on the silica substrate. Compared to other shapes of optical antennas such as bowtie and coupled elliptical rods, the coupled semi-nanocapsules exhibit a stronger field-intensity enhancement in the gap and relatively weak field intensity at the outer-ends. Furthermore, the intensity enhancement of the semi-nanocapsules antenna can be further enhanced by choosing the suitable direction of illumination. These proposed structures can be used for the design and the applications of an optical antenna at the visible violet wavelength.     

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     

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.     

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.     

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.     

Fabrication of superhydrophobic surface from binary micro-/nano-structure of mullite-whisk-based films

The surface plasmon resonance (SPR) modes and near field gap enhancement of bowtie nanoantennas with triangle void defects are studied numerically. According to the location of the defects, we classify them into four categories: inner, edge, base and vertex defects. It is concluded that inner and base defects have little impact on both SPR modes and gap enhancement while edge and vertex defects which lead to mode splitting have great impact on the gap enhancement with symmetry breaking. Specifically, the size and location of edge defects have a remarkable effect on the resonant modes, especially for the low-energy resonant mode. When the edge defect gets close to the gap, the gap enhancement increases even above that of bowties with no defects. These properties are instructive to the evaluation of the fabrication of bowtie nanoantennas. And, by careful control of the defect location, we can get useful resonant modes and increase the gap enhancement for applications such as broadband light harvesting, ultra-fast wavelength-sensitive photodetection and fluorescent detection for two or more targets.     

Resonances of sandwiched optical antenna

We calculate the resonant properties of the sandwiched triangle and bowtie antennas using finite difference time domain technique and compare with one-layer structures. The sandwiched antennas possess two tunable resonances corresponding to the symmetric and antisymmetric modes for dipole excitation, which can be understood by the hybridization of the plasmons supported by the two golden layers of the antennas. We obtain a giant field enhancement and a full width at half maximum as larger as 385 nm for the sandwiched bowtie antenna.     

Fabrication of periodical structure and shape-induced modulating spectroscopy of Au nanoparticles

This paper presents optical property of the periodical bowties structure of Au nanoparticles on glass substrate fabricated by high-resolution electron beam lithography. Numerical calculation is used to examine the shape-induced surface plasmon enhancement and extinction spectroscopy by using Finite-Difference Time Domain algorithm and Lorentz model of surface plasmon. The calculated extinction spectra of the periodical nanoparticles are consistent with the experimental results. The influence of the geometrical shape on the spectral properties of bowtie nanoparticles is discussed. This investigation may help one to design plasmonic sub-wavelength devices with desired spectral properties.     

Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas

Metallic bowtie nanoantennas should provide optical fields that are confined to spatial scales far below the diffraction limit. To improve the mismatch between optical wavelengths and nanoscale objects, we have lithographically fabricated Au bowties with lengths approximately 75 nm and gaps of tens of nm. Using two-photon-excited photoluminescence of Au, the local intensity enhancement factor relative to that for the incident diffraction-limited beam has been experimentally determined for the first time. Enhancements >10(3) occur for 20 nm gap bowties, in good agreement with theoretical simulations.     

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.     

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.     

Near-field plasmonic coupling for enhanced nonlinear absorption by femtosecond pulses in bowtie nanoantenna arrays

Plasmonic bowtie nanoantennas (BNAs) can exhibit a strong enhancement of optical field, leading to large nonlinear effects. We investigated the nonlinear optical absorption of an array of BNA by femtosecond pulses, using the open-aperture Z-scan technique. The BNA array composed of paired gold nanotriangles was fabricated by nanosphere lithography. We experimentally demonstrated that upon decreasing the gap width, nonlinear absorption is enhanced due to both the enhancement of near-field coupling of nanoantennas and the minimum of the spectral detuning between the center wavelength of the laser for excitation and the localized surface plasmon resonances. The role of near-field resonant plasmonic coupling in BNA is analyzed theoretically and confirmed by our simulations.     

Large enhancement and uniform distribution of optical near field through combining periodic bowtie nanoantenna with rectangular nanoaperture array

We present a novel (to the best of our knowledge) composite nanostructure composed of bowtie nanoantennas (BNAs) and rectangular nanoapertures, which provides a new way to improve the ability of the nanostructure to enhance the optical near field and obtain uniform near-field distribution in the z direction. It is specifically engineered to not only confine the incident light in the nanoscale but also to generate large localized near-field enhancement about 25 times larger than that of solitary BNAs. It also shows a more uniform near-field distribution in the z direction than that of solitary BNAs. The mechanisms of the large enhancement and the uniform near field are also discussed.     

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.     

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.     

Resonance tuning and broadening of bowtie nanoantennas on graphene

Metallic bowtie antennas are used in nanophotonics applications in order to confine the electromagnetic field into volumes much smaller than that of the incident wavelength. Electrically controllable carrier concentration of graphene opens the door to the use of plasmonic nanoantenna structures with graphene so that the resonant nature of nanoantennas can be tuned. In this study, we demonstrated with the Fourier transform infrared (FTIR) spectroscopy and the Finite Difference Time Domain (FDTD) method that the intensity and resonance peak of bowtie nanoantennas on monolayer graphene can be tuned at mid-infrared (MIR) wavelength regime by applying a gate voltage, since the optical properties of graphene change by changing the carrier concentration.     

Optical nanolithography with λ/15 resolution using bowtie aperture array

We report optical parallel nanolithography using bowtie apertures with the help of the interferometric-spatial-phase-imaging (ISPI) technique. The ISPI system can detect and control the distance between the bowtie aperture, and photoresist with a resolution of sub-nanometer level. It overcomes the difficulties brought by the light divergence of bowtie apertures. Parallel nanolithography with feature size of 22 ± 5 nm is achieved. This technique combines high resolution, parallel throughput, and low cost, which is promising for practical applications.