Metallic Grating Structures Research Articles

Organic-based plasmonic emitters for sensing applications

Plasmonic emissions generated by excitation of an organic layer on a metal grating structure are demonstrated. The emissions correspond to the resonant condition of surface plasmon (SP) modes on the Alq(3)/Au interface, and the grating structure is coupled to the Au/air interface to provide light emissions. Experimental variations in pitch to control plasmonic bandgap obtained highly directional plasmonic emissions with enhanced intensity. This method is readily applicable for detecting refractive index changes by using SP-coupled fluorophores to obtain emissions of varying wavelengths and viewing angles. The calculations showed that the wavelength of the plasmonic emitter changed from 480 to 680 nm at certain viewing angles, while the concentration of contacting glucose increased from 10% to 40%. Accordingly, a device with a pitch size of 500 nm had a sensitivity of Δθe/Δn=37.76° and Δn/Δ=1.681×10(-4) RIU (refractive index unit). Therefore, the proposed approach has potential applications in low-cost, disposable, point-of-care biosensors.

ON THE USE OF METAL GRATINGS TO REDUCE DIFFRACTION FROM A FINITE GROUND PLANE IN CIRCULARLY-POLARIZED MICROSTRIP ARRAYS

It is shown that metal gratings can be used to improve the cross polarization of circularly-polarized aperture-coupled microstrip antennas. The metal gratings reduce edge difiraction from the flnite- size grounded dielectric slab on which the antennas are printed. The edge difiraction is due to surface waves that can propagate in the grounded dielectric slab. The design of the metal grating is based on an analytical model, which results in a flrst-order estimation for the design of the metal grating structure. The model provides physical insight and appears to be accurate enough for the application. Using this model, a prototype was developed, consisting of a circularly-polarized 2 £ 2 microstrip array with associated feeding network. Measurements show that the axial ratio can be reduced down to 1.75dB within the beam width of the antenna.

Electrical source of surface plasmon polaritons based on hybrid Au–GaAs QW structures

In this paper, the electrical excitation of surface plasmon polaritons (SPPs) based on a hybrid metal-semiconductor quantum well (QW) structure is investigated by finite-difference time-domain (FDTD) simulations and experiments. The metal-QW hybrid structure is made of a metal grating structure deposited on a semiconductor QW with small separation between them. When electron-hole pairs are excited using current injection into the QW, SPPs are generated and coupled out by a metal grating. The spectral and imaging measurements were performed to confirm the electrical excitation of SPPs. The hybrid structure could serve as a plasmonic source for integrated plasmonic circuits.

Theoretical analysis on shear horizontal polarized surface acoustic waves propagation in periodic metal grating using the variational method

A new theoretical method is presented to analyze dispersion characteristics of SH-type SAWs propagating on periodic metallic grating structures using the variational theory and coupling-of-modes (COM) equations. The characteristics of SH-type SAWs propagating in short-circuited Al gratings on various crystals are studied and the calculated results agree well with those of typical Hashimoto's method. Also, some involved problems like dispersion parameters of SH-SAW along the ST-90oX quartz with Al electrode were first described. The advantages of this method are that the complicate Green's function is avoided, resulting in simple, clear and fast theoretical analysis.

Directional terahertz beams realized by depth-modulated metallic surface grating structures

We propose a subwavelength metal slit with surrounding depth-modulated surface grating structures to realize directional beams in the terahertz regime. The surface gratings consist of two sets of grooves of different depths. The shallow grooves are designed to support the spoof surface plasmons, and the deep grooves are utilized to diffract the terahertz surface wave into free space. Theoretical analysis and numerical simulations based on the finite element method confirm that various beaming effects including splitting, on-axis beaming, and off-axis beaming can be realized by controlling the distributions of the deep grooves.

Broadband slow-wave systems of subwavelength thickness excited by a metal wire

We propose a broadband slow-wave system based on metallic graded grating structures of subwavelength thickness. A metal wire is used to excite the slow waves propagating along the gratings. The analysis and full-wave simulations show that the electromagnetic waves could be stopped at designed positions under different frequencies. Experiments are conducted in the microwave frequencies to verify the proposed system. The measured results show very good agreements to the full-wave simulations. We have further modeled a broadband slow-wave system of subwavelength thickness in the terahertz (THz) frequencies to demonstrate its validity.

Perfect extinction in subwavelength dual metallic transmitting gratings

We investigate the strong electromagnetic coupling that settles in dual metallic grating structures. This coupling is evidenced to lead to a perfect optical extinction in the transmission spectrum. The behavior of this perfect extinction that strongly depends on the longitudinal space and the lateral displacement between the two gratings can be explained by a simple model that describes the interference between a propagating mode and a couple of evanescent modes. The results show that the electromagnetic transmission of the structure can be tuned by controlling the position of this perfect transmission extinction and thus pave the way to new types of infrared tunable filters.

Direct Metal Transfer Lithography for Fabricating Wire-Grid Polarizer on Flexible Plastic Substrate

This paper reports on the fabrication of polymer-based metallic wire-grid polarizers using a direct metal contact printing method. The proposed method can directly transfer a metal pattern from a silicon mold to a soft plastic substrate simply by roller pressing and infrared (IR) heating. With properly chosen loading contact pressure and heating temperature, metallic linear grating structures with a linewidth of 60 nm, a period of 170 nm, and an area size of 2.5 ×2.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are successfully formed on the surface of a polyethylene terephtalate (PET) substrate. The optical performance of the fabricated flexible polarizers is experimentally measured and numerically analyzed in the near-IR spectrum with wavelength from 800 to 1500 nm. Inspired by the numerical simulation results, the optical characteristics of the flexibe polarizers are further enhanced by reactive ion etching on the PET substrate. Good agreements between experiments and theoretical simulation are obtained. Future developments and potential applications of this metal contact transfer lithography method will be addressed.

Nano-patterned visible wavelength filter integrated with an image sensor exploiting a 90-nm CMOS process

A highly efficient visible wavelength filter enabling a homogeneous integration with an image sensor was proposed and manufactured by employing a standard 90-nm CMOS process. A one dimensional subwavelength Al grating overlaid with an oxide film was built on top of an image sensor to serve as a low-pass wavelength filter; a microlens was then formed atop the filter to achieve beam focusing. The structural parameters for the filter were: a grating pitch of 300nm, a grating height of 170nm, and a 150-nm thick oxide overlay. The overall transmission was observed to reach up to 80% in the visible band with a decent roll-off near ∼700nm. Finally, the discrepancy between the observed and calculated result was accounted for by appropriately modeling the implemented metallic grating structure, accompanying an undercut sidewall.

Electric Field Tuning of Surface Plasmon Resonance Using Vertical Alignment Liquid Crystals on a Silver Grating Structure

We report electric field tuning of surface plasmon resonance (SPR) by controlling the polarization state of incident light using vertical alignment liquid crystal (VA-LC) layer. Excitation of SPR depends on the polarization state of incident light and incident angle. We fabricated a metallic grating structure in order to excite SPR, and then we investigated the applied voltage dependence of reflection spectrum. Continuous tuning of SPR is achieved as a result of the polarization change of incident light by controlling the birefringence of the LC layer.

Electrically controlled broadband THz switch based on liquid-crystal-filled multi-layer metallic grating structures

We propose an electrically controlled broadband THz switch based on the liquid-crystal-filled multilayer metallic grating (MMG) structures. During the THz wave passing thought the device, it is multiple reflected among the MMG layers and suffers loss due to the absorption of the LC. The on and off state of the switch is realized by the different absorption intensity of the LC when its complex refractive index is switched between ordinary no and extraordinary ne value in the presence of an electric field. Simulation results show the proposed device exhibits a respectable switching effect for the THz wave over a wide frequency range.

Realization of integrated polarizer and color filters based on subwavelength metallic gratings using a hybrid numerical scheme

This study realizes integrated polarizer and RGB (red, green, and blue) color filters using single- and multiple-layered subwavelength metallic grating structures. A hybrid numerical scheme based on the rigorous coupled-wave analysis method and a genetic algorithm is used to determine the optimal values of the grating period, filling factor, and grating thickness of three different grating structures, namely, a single-layer grating, a double-layer grating, and a double-layer grating with a lateral shift. The optical performance of the various structures is evaluated and compared in terms of the transmission efficiency at the center wavelengths 700.0 nm, 546.1 nm, and 435.8 nm of red, green, and blue light, respectively, and the extinction ratio over the visible wavelength spectrum (380-780 nm). It is shown that the double-layer grating achieves a transmission efficiency of about 50% and an extinction ratio of around 60 dB. Thus, this grating structure provides a convenient and effective means of achieving the polarizing and filtering functions in LCD panels using a single device.

Reflex optical sensor based on dual metal gratings with high angular sensitivity

An interesting reflection phenomenon in a dual metal grating (DMG) structure is studied, which is related to the competition between Fabry-Perot (F-P) resonance effect and evanescent-field coupling effect inside the gap between the two composing single metal gratings. This competition leads to high angular sensitivity in response to the refractive index variation of the sample solution in the gap. A reflex optical sensor with high sensitivity based on DMG for detecting the change in refractive index is proposed and its performance theoretically is discussed.

Bidirectional surface wave splitter at visible frequencies

We experimentally demonstrate a metal-film bidirectional surface wave splitter for guiding light at two visible wavelengths in opposite directions. Two nanoscale gratings were patterned on opposite sides of a subwavelength slit. The metallic surface grating structures were tailored geometrically to have different plasmonic bandgaps, enabling each grating to guide light of one wavelength and prohibit propagation at the other wavelength. The locations of the bandgaps were experimentally confirmed by interferometric measurements. Based on these design principles, a green-red bidirectional surface wave splitter is demonstrated, and the observed optical properties are shown to agree with theoretical predictions.

Proposal of an electrically controlled terahertz switch based on liquid-crystal-filled dual-metallic grating structures

We propose an electrically controlled terahertz switch based on metallic grating–liquid crystal–metallic grating (MG-LC-MG) structures. The switching mechanism is realized by modifying the effective refractive of the LC using different bias electric fields. In our design, the MGs not only support supertransmittance at certain frequencies, but they also act as the electrodes. Simulation results show the proposed structure has the potential to realize an electrically controlled terahertz switch with a high extinction ratio and low insertion loss. A prototype design is also proposed for practical implementation.

Unidirectional SPP excitation at asymmetrical two-layered metal gratings

A new configuration of a reflective one-dimensional (1D) surface-relief metallic-grating structure for unidirectional excitation of surface plasmon polaritons (SPPs) is proposed and numerically explored for plane wave normal incidence. The structure is embedded between two different dielectric media and composed of two layers, each of them consisting of periodically placed rectangular metal stripes. It is shown that even a small horizontal shift between these two layers, or a change in dielectric contrast of the grating fillings, may redirect energy flow propagation in the vicinity of the grating structure. From these two cases of counter propagating energy flows, only one shows the field behaviour typical of SPP excitation. This evident directivity phenomenon exists together with a high concentration of the electromagnetic field at the periodic structure. The configurations analysed may be useful in designing optical devices like photodetectors or, in general, in any case where efficient control of energy propagation directivity is of primary importance.

Surface-enhanced Raman scattering on dual-layer metallic grating structures

Dual-layer Metallic grating (DMG) structures as surface-enhanced Raman scattering (SERS) substrates are studied using benzenethiol as the probe analyte. The DMG structure consists of a SiO2 grating and 100-nm-thick gold coating layers. An enhancement factor of 105 is achieved by optimizing the SiO2 grating height within the range from 165 to 550 nm. The enhancement factor dependence on the SiO2 grating height is due to the surface plasmon excitation, which is dependent on the polarization of the incident light, and confirmed by finite difference time domain simulations. This study demonstrates the advantages of high uniformity, reproducibility and sensitivity in the DMG structures for SERS applications.

FDTD analysis of infrared radiative properties of microscale structure aluminum surfaces

Metallic triangular grating structures have a wide range of applications. This study focuses on the radiative properties of triangular grating structures of aluminum in the mid-infrared wavelength to determine if we can tailor the infrared radiative properties by designing special geometrical details into triangular grating surfaces. Fabrication of triangular gratings within the range of micron scale is very difficult. Therefore, the influences of slight geometry modification, generated by fabrication errors, on radiative properties of triangular gratings are mainly investigated. The electromagnetic wave scattering from such surfaces is predicted by solving Maxwell's equations using the finite-difference time-domain (FDTD) method. The results show that the spectral reflectance varies with the dimensions of triangular gratings, which is due to the excitation of surface wave by confirmation of the EM fields. It is found that the spectral reflectance can also be reduced by increasing the height of slight geometry modification. The effect of oblique incident wave state is also investigated. An optical vortex is found due to the coupling of surface waves, the oblique incident wave and the scattering waves. This study helps to gain a better understanding of the radiative properties of metallic triangular gratings with slight geometry modification and will have an impact on triangular grating processing.

Role of surface plasmon polaritons on optical transmission through double layer metallic hole arrays

We study analytically and numerically the far-field extraordinary optical transmission (EOT) through double-layer metallic grating structures patterned with subwavelength hole arrays. In addition to EOT phenomena due to the well-known surface plasmon polaritons (SPPs) on the outer surfaces such as those on a single layer hole array, further EOT peaks are observed. The separation between the metallic layers is small enough to allow SPPs propagating through the inner interfaces to couple and form an internal SPP, with a different dispersion relation from the outer one and so giving rise to EOT peaks at different frequencies. We propose a relatively simple model to predict the frequencies of those EOT peaks. Internal SPPs show certain unique properties different from the external SPPs: they can give rise to a magnetic response and a negative effective permeability, and the transmission of their EOT peaks increases when no direct line of sight is allowed through the structure. All these findings may be utilized in wavelength tuning of extraordinary optical transmission in subwavelength optics.

Surface wave splitter based on metallic gratings with sub-wavelength aperture

We investigated the splitting of surface electromagnetic waves trapped at the output surface of a one-dimensional metallic grating structure. The output gratings of the structure asymmetrically such that the output surfaces at the different sides of the subwavelength aperture can support surface waves at different frequencies. The transmission amplitude as measured at the left side is 1,000 times of that at the right side at 16 GHz. At 24 GHz, the transmission measured at the right side is 20 times that of the left side of the structure. Therefore, surface waves are guided into the different sides of the aperture at different frequencies via metallic gratings. The experimental results are in agreement with the theoretical results.