Finite-difference And Time-domain Research Articles

Dual layer chessboard metasurface sandwiched by a spin-on-carbon for spectral modulation.

Metasurfaces, composed by metals and dielectrics in periodical order with subwavelength pitches, are of great importance for their unique ability to abruptly manipulate optical fields. So far, all the reported metasurfaces are constructed by thermally deposited metals and dielectric films, based on semiconductor processes which are expensive and time-consuming. Inspired by the outstanding dry etch property of spin-on-carbon (SOC) as the interlayer material in CMOS technology, this paper proposes to utilize the SOC as the dielectric layer in a chessboard metasurface with dual layer of gold to form an array of local surface plasmonic resonators (localized surface plasmon resonance). Finite difference and time domain (FDTD) method is used to investigate the spectral characteristics in reflectance of the metasurface in both visible and short wavelengths of infrared light. Electron beam lithography is applied to generate the nanoscale chessboard pattern on ZEP520A, followed by a conventional oxygen-based plasma etch to form high aspect ratio nanopillar arrays in SOC with the feature width under 50 nm, and ended by a thermal deposition of gold to form self-aligned dual layer local surface plasmonic resonators (LSPRs). The measured reflectance spectra agree with the simulated. A wealth of optical properties, such as coupling induced modulations of spectra by LSPRs, are revealed and analyzed. These special modes result in tunable structural colors and wavelength-selective antireflection ability. To the best of our knowledge, this is the first time that SOC is applied in the construction of metasurfaces, which has great potential for next generation nanophotonic devices.

Study of structural effects on the polarization characteristics of subwavelength metallic gratings in short infrared wavelengths.

Applications of subwavelength grating based-polarizers for polarimetric detections are being hindered due to the limited extinction ratio. In this work, the structural effect, including the line edge roughness (LER), of the gratings on the polarizing characteristics was studied by both numerical simulations using finite difference and time domain (FDTD) method and experiments, aiming to figure out the optimal grating profile for achieving high transmittance as well as high extinction ratio. Two different configurations of the gratings, one is dual layer Au lines and the other is parabolic shaped Al lines on structured spin-on-carbon (SOC) films were systematically studied and compared. Nanofabrication of the gratings by electron beam lithography without lift-off process were conducted and optical measurements of their polarization properties demonstrate superior performance of the developed polarizers. The origin of the structural effect was explained by the local surface plasmonic modes, existing in the nano-slits in metallic gratings, which is instructive for further enhancement of the polarization performance.

Preparation and characteristic analysis of nanofacula array

The development of nanofacula array is an effective methods to improve the performance of Near-field Scanning Optical Microscopy (NSOM) and achieve high-throughput array scanning. The nanofacula array is realized by preparing metal nanopore array through the "two etching-one development" method of double-layer resists and the negative lift-off process after metal film coating. The shading property of metal film plays important rules in nanofacula array fabrication. We investigate the shading coefficient of three kinds of metal films (gold–palladium alloy (Au/Pd), platinum (Pt), chromium (Cr)) under different coating times, and 3.5 min Au/Pd film is determined as the candidate of the nanofacula array fabrication for its lower thickness (about 23 nm) and higher shading coefficient (≥ 90%). The nanofacula array is obtained by irradiating with white light (central wavelength of 500 nm) through the metal nanopore array (250/450 nm pore diameter, 2 μm pore spacing and 7 μm group spacing). Moreover, the finite difference and time domain (FDTD) simulation proves that the combination of nanopore array and microlens array achieves high-energy focused nanofacula array, which shows a 3.2 times enhancement of electric field. It provides a new idea for NSOM to realize fast super-resolution focusing facula array.

Study of Hot Spots Probing Based on Meta-Pillars Array

Inspired by imaging principle of near-field scanning optical microscope (NSOM), meta-pillars array is designed and analyzed on the basis of microscopic imaging application with high resolution. Finely focused spots acting as tiny secondary sources for illumination at near-field can be derived under supporting of the meta-pillars for the purpose of increasing imaging resolution. Numerical calculation is carried out on the basis of finite difference and time domain (FDTD) algorithm. Our calculation results demonstrate that the meta-pillars are capable of supporting the microscopic imaging at sub-wavelength resolution.

Near-field visualization of plasmonic lenses: an overall analysis of characterization errors.

Many factors influence the near-field visualization of plasmonic structures that are based on perforated elliptical slits. Here, characterization errors are experimentally analyzed in detail from both fabrication and measurement points of view. Some issues such as geometrical parameter, probe–sample surface interaction, misalignment, stigmation, and internal stress, have influence on the final near-field probing results. In comparison to the theoretical ideal case of near-field probing of the structures, numerical calculation is carried out on the basis of a finite-difference and time-domain (FDTD) algorithm so as to support the error analyses. The analyses performed on the basis of both theoretical calculation and experimental probing can provide a helpful reference for the researchers probing their plasmonic structures and nanophotonic devices.

Flat lenses constructed by graded negative index-based photonic crystals with tuned configurations

Flat lenses are designed by means of graded negative refractive index-based photonic crystals (PCs) constructed using air-holes tuned with different shapes. By gradually modifying the filling factor along the transverse direction, we obtain the graded negative index-based lenses for the purpose of focusing an incident plane wave. The finite-difference and time-domain (FDTD) algorithm is adopted for numerical calculation. Our calculation results indicate that these lenses can finely focus incident plane waves. Moreover, for the same size of air-holes, the focusing properties of the lens with rectangular air-holes are better than those with the other shaped air-holes. The graded negative index PCs lenses could possibly enable new applications in optoelectronic systems.

Tunability of graded negative index-based photonic crystal lenses for fine focusing

Graded negative refractive index-based photonic crystal (PC) lenses are designed by gradually modifying the sizes of air holes along the transverse direction for focusing the incident plane wave. To study the tunability of the graded negative index-based PC, we introduce filling factor A, gradually tune the filling factor, and use the finite-difference and time-domain (FDTD) algorithm for numerical calculation. Our calculation results indicate that the focal length and the spot size increase with A increasing. For the same A value, the focal length of a PC with elliptical air holes is the longest, and those of PC with square and rectangular air holes are the shortest. Moreover, when the focal length is greater than 1 μm, the focal parameters of the PC are highly insensitive to the variation of A. When the focal length is less than 1 μm, the PC lenses have higher transmittances and all well focus with a beam spot size breaking the diffraction limit. This feature possibly makes the graded negative index-based PC lenses have some new applications in optoelectronic systems.

Electron beam lithography designed silver nano-disks used as label free nano-biosensors based on localized surface plasmon resonance

We present a label-free, optical nano-biosensor based on the Localized Surface Plasmon Resonance (LSPR) that is observed at the metal-dielectric interface of silver nano-disk arrays located periodically on a sapphire substrate by Electron-Beam Lithography (EBL). The nano-disk array was designed by finite-difference and time-domain (FDTD) algorithm-based simulations. Refractive index sensitivity was calculated experimentally as 221-354 nm/RIU for different sized arrays. The sensing mechanism was first tested with a biotin-avidin pair, and then a preliminary trial for sensing heat-killed Escherichia coli (E. coli) O157:H7 bacteria was done. Although the study is at an early stage, the results indicate that such a plasmonic structure can be applied to bio-sensing applications and then extended to the detection of specific bacteria species as a fast and low cost alternative.

The Talbot effect of plasmonic nanolenses

The Talbot effect of an Ag nanolens with five periodic concentric rings that are illuminated by the radially polarized light was numerically studied by means of rigorous finite-difference and time-domain (FDTD) algorithm. It was found that the Talbot effect occurs only when the incident wavelength is at the scale of less than half of period of the grating structures of the nanolenses. Specifically, in this work, the nanolenses with a 500 nm period grating structures has five focal points due to Talbot effect for the incident wavelength of λ = 248 nm. The diameter of the first focal spot after the exit plane in free space is 100 nm. In contrast, we analyzed the corresponding focal points on the basis of Talbot self-imaging by scalar diffraction theory. It was found that the scalar Talbot effect cannot interpret the Talbot effect phenomenon for the metallic nanolenses. It may attribute to the paraxial approximation applied in the Talbot effect theory in far-field region. However, the approximation does not hold in our nanolenses structures during the light propagation. In addition, the Talbot effect appears at the short-wavelength regime only, especially in the ultraviolet wavelength region.

Investigation of Negative Refraction Phenomenon for Au Nanowires Array Tuning with Index of Filling Material

Negative refraction performance of Au nanowires arrays-based metamaterials was explored by means of finite difference and time domain (FDTD) algorithm for the purpose of providing flexible design freedom of the negative index metamaterials (NIMs) working in visible regime from nanofabrication point of view. Tuning performance of the nanowires for negative refraction was analyzed by use of varying refractive index of filling materials among the metallic nanowires. Computational numerical simulation and analyses were carried out. The performance of negative refraction was compared by optimization of the structures. By optimizing the nanowires radius, E-field intensity was calculated in the case that the refractive index of filling material is changeable. The calculated refraction angles illustrate a relationship between the refraction angle and the index of filling material. Our computational results demonstrate that effective value of the negative refractive index strongly depends on the refractive index of the filling material when other parameters are fixed.

Design Approach of Metallic Nanoparticles Array for Biosensing: Calculation of Enhanced Electronic-Field

We present calculation of enhanced electronic field in a rhombic Ag nanoparticles array induced by localized surface plasmon resonance (LSPR) for the purpose of improving detection resolution of the extinction efficiency for biosensing. The engineered rhombic Ag nanostructures array was designed by means of finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation through both spectrum and electromagnetic field analyses. The extinction efficiency was obtained by theoretical numerical calculation and experimental detection. Moreover, the LSPR-induced enhancement of the electronic field distribution is calculated, and compared to that of the silicon medium. The results show that the rhombic Ag nanostructures array can enhance the localized electric fields near the surface of the metallic array. To verify our calculation results, biotins binding and detection were carried out on the basis of the calculated metallic arrays. It is in agreement to that of the reported results before. This design approach may be a guideline for designing the engineered metallic particles arrays. Copyright © 2010 American Scientific Publishers All rights reserved.

A large numerical aperture structured lens formed by varying high-refractive-index-dielectric square holes

A structured lens with a large numerical aperture for high diffraction efficiency is proposed. This structured lens is formed by varying high-refractive-index-dielectric (HRID) square holes in a metallic film. Compared with previous structured lenses formed by varying air square holes, the modulated wavefront at the exit pupil of the structured lens is smoother and the diffraction efficiency is greatly improved. A dielectric film was introduced as a buffer layer to reduce the energy loss at the interface caused by the great impedance mismatching between air and the high refractive index dielectric. A lens with an aperture of 100 µm and a focal length of 50 µm was designed; its diffraction efficiency has been raised from 22.8% to 47.2% by the finite difference and time domain (FDTD) algorithm. This structured lens has the advantages of not only light weight and high compactness, but also large numerical aperture, high diffraction efficiency, and good imaging quality.

Hybrid Au-Ag subwavelength metallic structures with variant periods for superfocusing

Yongqi Fu School of Physical Electronics, University of Electronic Science and Technology of China, No. 4, Section 2, North Jianshe Road, Chengdu, Sichuan Province 610054 China Wei Zhou Precision Engineering and Nanotechnology Center, School of Mechanical and Aerospace Engineering, Nanyang Technological University and Institute of High Performance Computing, Singapore, 639798 Singapore A hybrid Au-Ag subwavelength metallic zone plate-like structure was put forth for the purpose of preventing oxidation and sulfuration of Ag film, as well as realizing superfocusing. The Au film acts as both a protector and modulator in the structure. Focusing performance is analyzed by means of three-dimensional finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation. It can be tuned by varying thicknesses of both Au and Ag thin films. Our calculation results show that thickness difference between the Au and Ag thin films plays an important role for transmission spectra. Ratio of Au to Ag film thicknesses, hAu/hAg is proportional to the relevant peak transmission intensity. In case of hAuhAg=50 nm, both transmission intensity and focusing performance are improved. In addition, the ratio hAu/hAg strongly influences position of peak wavelengths Au and Ag generated from beaming through the metallic structures.

Beam Splitter Achieved by Using Metallic Structure with Nanoslits

A design method of beam splitter is proposed on the basis of the particular properties of surface plasmon polarizations in metallic nanoslits. The nanoslits with different width and depth can lead to different phase retardations while the surface plasmons passing through the structure. We can achieve a beam splitting effect with the splitting angle of near 90 by adjusting the width and depth of the slits accordingly. The numerical simulations using two dimensional finite-difference and time-domain (FDTD) algorithm is presented here to analyze working performance of the designed structure. Our computational results demonstrate that the splitting effect of the beam can be realized successfully.

Localized surface plasmon resonance-based hybrid Au–Ag nanoparticles for detection of Staphylococcus aureus enterotoxin B

A triangular hybrid Au–Ag nanoparticles array was proposed for the purpose of biosensing in this paper. Constructing the hybrid nanoparticles, an Au thin film is capped on the Ag nanoparticles which are attached on glass substrate. The hybrid nanoparticles array was designed by means of finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation and optimization. Sensitivity of refractive index of the hybrid nanoparticles array was obtained by the computational calculation and experimental detection. Moreover, the hybrid nanoparticles array can prevent oxidation of the pure Ag nanoparticles from atmosphere environment because the Au protective layer was deposited on top of the Ag nanoparticles so as to isolate the Ag particles from the atmosphere. We presented a novel surface covalent link method between the localized surface plasmon resonance (LSPR) effect-based biosensors with hybrid nanoparticles array and the detected target molecules. The generated surface plasmon wave from the array carries the biological interaction message into the corresponding spectra. Staphylococcus aureus enterotoxin B (SEB), a small protein toxin was directly detected at nanogramme per milliliter level using the triangular hybrid Au–Ag nanoparticles. Hence one more option for the SEB detection is provided by this way.

Modulation of Main Lobe for Superfocusing Using Subwavelength Metallic Heterostructures

A subwavelength metallic heterostructure is put forth for the purpose of suppressing sidelobes and improving superfocusing at a quasi-far field region. Improvement has been made by means of optimization of the heterostructure composed of structured Au and Ag thin films. By tuning thicknesses of both the structured Au and Ag films, we can modulate propagation distance of the plasmonic lens and beam width of main lobe for the superfocusing. A finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation was carried out for analyzing the focusing performance and tuning ability of the metal films. Our computational calculation results show that the sidelobes which play negative role for the focusing can be suppressed significantly in the case of the metal film thicknesses of hAu = 50 nm and hAg = 10 nm. Theoretically, the metallic structure with smaller thicknesses of the structured Au and Ag films is helpful for improving the focusing performance. This heterostructure-based device is possible to be used as a superlens or nanoprobe in data storage, nanometrology/inspection, and biosensing etc.