Subwavelength Metallic Gratings Research Articles

Ultra-broadband wide-angle unidirectional plasmonic coupler based on joint effects of plasmonic critical angles and subwavelength metallic gratings.

A unidirectional plasmonic coupler with ultra-broadband and wide-angle coupling efficiency is proposed and demonstrated. Employing the plasmonic critical angle (PCA) phenomenon in conjunction with the effect of subwavelength metallic gratings, a wide incident angular full-width-half-maximum (AFWHM >28°) coupling efficiency larger than 50% and extinction ratio better than 15dB over an ultrabroad wavelength range 725-985nm (bandwidth >260 nm) are achieved. The operation principle and performance of the structure are clarified and theoretically illustrated.

Plasmonic metal-dielectric-metal stack structure with subwavelength metallic gratings for improving sensor sensitivity and signal quality.

In this study, we investigated the performance improvement of a localized surface plasmon resonance (LSPR) biosensor by incorporating a metal-dielectric-metal (MDM) stack structure and subwavelength metallic nanograting. The numerical results showed that the LSPR substrate with a MDM stack can provide not only a better sensitivity by more than five times but also a notably improved signal quality. While the gold nanogratings on a gold film inevitably lead to a broad and shallow reflectance curve, the presence of a MDM stack can prevent propagating surface plasmons from interference by locally enhanced fields excited at the gold nanogratings, finally resulting in a strong and deep absorption band at resonance. Therefore, the proposed LSPR structure could potentially open a new possibility of enhanced detection for monitoring biomolecular interactions of very low molecular weights.

A metal cascaded resonant cavity structure to enhance responsivity of photodetctor

A cascaded resonant F-P cavity structure is proposed to improve the responsivity of high-speed photodetector, which consists of subwavelength metal grating and metal film. Responsivity enhancement can be achieved by high light transmittance and high quantum efficiency, simultaneously. High transmission arises from slit cavity mode resonance of subwavelength metal grating, and high quantum efficiency is due to adopting resonant cavity enhanced structure. In our proposed photodetctor structure, subwavelength metal grating plays a very important role in performance enhancement. The results show that responsivity can be increased about 31 times compared to the conventional photodetector by optimizing the structure parameters.

Design study of nanograting-based surface plasmon resonance biosensor in the near-infrared wavelength

A key issue with surface plasmon resonance (SPR) biosensors, which are the focus of many researchers, is improving their sensitivity to detect lower amounts of analyte in a solution. Most SPR developments have focused on the grating-based sensitivity-enhancement approach. In addition to sensitivity, a substantial enhancement of other sensor characteristics such as resolution and signal-to-noise ratio (SNR) is desired for designing a practical sensor. So, in this paper, the characteristics of surface plasmon polaritons sustained by 1D subwavelength metallic gratings on a thin metal slab (under the Krestchmann configuration) have been investigated numerically for the analyte-ligand interactions detection. Effects of different structural parameters, such as grating period, grating depth, metal film thickness, and fill factor have been evaluated on the sensor sensitivity as well as resolution and SNR. Numerical results indicate that the sensor working in the near-infrared wavelength has a better performance than that in the visible one. The result of numerical investigation has been used to design an optimized sensor with the best figure of merit.

High-Transmittance Subwavelength Metal Grating with Relief Structure Composed of Multiple Steps

A new kind of subwavelength metal grating with relief structure is designed and analyzed, in which the shape of the grating lines is no longer a single rectangle, but a relief structure with multiple steps. GsolverV52 was used to determine the optimal values of the grating period, groove depth, and the number of steps. The optical performance of the novel structure is evaluated and compared in terms of the transmission efficiency and extinction ratio over the visible and near-infrared wavelength spectrum. It is shown that, in the near-infrared band, the maximum transmittance can be increased about 15% compared to the traditional metal grating under the same parameters. With the unique characteristics, the metal grating is expected to find applications in liquid crystal display fields, polarization imaging, optical communication, and so on.

Increase of spatial coherence by subwavelength metallic gratings

We study the coherence changes in partially coherent beams transmitted through binary metallic gratings. The interaction of Gaussian Schell-model beams with grating structures supporting surface plasmons is rigorously modeled using the Fourier modal method and the coherent mode representation of partially coherent fields. Our numerical results show that, by choosing suitable parameters for the grating, the degree of coherence of the beam can be significantly increased. The studied approach offers new possibilities to alter the coherence properties of fields using nanophotonic components.

Deep subwavelength optical imaging using correlated nano-torches

The authors propose and numerically demonstrate an ultra-high resolution (wavelength/50∼40 nm at wavelength λ=2.08 μm), high-throughput (∼66%), and non-destructive optical lens with a large contrast-to-noise ratio, based on the notion of correlated nano-torches formed in a subwavelength metallic grating. The correlations between the torches also allow the determination of the complex refractive index of the sample.

Plasmonic waves allow perfect transmission through sub-wavelength metallic gratings

We investigate the transmission properties of a metallic layer with narrow slits. Recent measurements and numerical calculations concerning the light transmission through metallic sub-wavelength structures suggest that an unexpectedly high transmission coefficient is possible. We analyze the time harmonic Maxwell's equations in the $H$-parallel case for a fixed incident wavelength. Denoting by $\eta>0$ the typical size of the complex structure, effective equations describing the limit $\eta\to 0$ are derived. For metallic permittivities with negative real part, plasmonic waves can be excited on the surfaces of the channels. When these waves are in resonance with the height of the layer, the result can be perfect transmission through the layer.

Ultraviolet laser writing system based on polar scanning strategy to produce subwavelength metal gratings for surface plasmon resonance

We demonstrate the use of ultraviolet (UV) laser lithography in the production of subwavelength metal gratings. A laser writing system with a 413-nm Kr laser is used to write patterns on a resist-coated fused silica substrate mounted on a rotating table with a linear slider. One- and two-dimensional patterns are written in the resist at a selected sampling pitch or grating period, and the substrate is dry etched and coated with Au to obtain metallized gratings. Surface plasmon resonance dips, which appear in the reflectance spectra of the gratings, shift depending on the orientation of the incident polarization, because the gratings lack perfect symmetry owing to a system-induced skew in the writing beam. This dip shift can be considered tolerable when the gratings are used as a signal enhancer in Raman sensing applications. We conclude that UV laser writing based on polar coordinates is a candidate method for surface structuring on submicron scales. Devising a method to attain an unskewed beam will be the subject of future work.

Optical bistability in subwavelength compound metallic grating

We have investigated the optical bistability behavior based on an electromagnetically induced reflection (EIR) effect in a compound metallic grating consisting of subwavelength slits and Kerr nonlinear nanocavities embedded in a metallic film. The theoretical and simulation results show that a narrow peak in the broad reflection dip possesses a red-shift with increasing the refractive index of coupled nanocavities. Importantly, we have obtained an obvious optical bistability with threshold intensity about ten times lower than that of metallic grating coated by nonlinear material. The results indicate that our structure may find excellent applications for nonlinear plasmonic devices, especially optical switches and modulators.

Exceptional points in extraordinary optical transmission through dual subwavelength metallic gratings

Complex metamaterials with multiple optical resonances in constituent elements possess many similarities with open quantum systems that can be described by non-Hermitian Hamiltonian. By analogy with a two-state open quantum system, we show that a classic analogue of exceptional points can be observed in the transmission spectra of dual subwavelength metallic gratings. Anti-crossing (crossing) between the two branches λ(R) of extraordinary optical transmission, with crossing (anti-crossing) of the corresponding widths Γ(R), is observed in the parameter space spanned by the lateral displacement L and the angle of incidence φ0. Exchanges of field patterns and phases, and the variation of field profile when circling the exceptional point are discussed. This work highlights the potential to transfer the concepts and applications from open quantum systems to optical metamaterials.

Optical characteristics of subwavelength metallic grating coupled porous film surface plasmon resonance sensor with high sensitivity

A high performance sub-wavelength metallic grating coupled surface plasmon resonance (SWMGCSPR) sensor with metal and porous composite layer is proposed. Rigorous coupled-wave analysis (RCWA) is conducted to prove the design feasibility, characterize the sensor's performance and determine geometric parameters of the structure, which is also employed to compute the electromagnetic (EM) field distributions at the resonant wavelengths. Parameters of sensing platform are optimized to achieve the best performance of the SPR sensor. Obtained results reveal that the proposed structure can excite SPR with negative diffraction order of SWMG. Both wavelength and angular sensitivities are greatly enhanced because surface plasmon wave (SPW) exhibits a large penetration depth which will enlarge the distance of interactions between SP and analytes. The detection sensitivities and quality parameters are estimated to be 700nm/RIU and 509°/RIU with full width at half maximum (FWHM) less than 2.5nm using the same optimized structure.

Wide-Angle Behaviors of Sub-Wavelength Metal Gratings as Polarizing Beam Splitters

A second-order effective medium theory (second-order EMT) and thin film theory have been used to design a polarizing beam splitter (PBS) composed of one-dimensional sub-wavelength metal grating, considering the material of metal to be copper (Cu). It was analyzed by the finite difference time domain method for the wavelength of 1500nm. A high extinction ratio in transmission (>55.71dB) over abroad interval of angles of incidence ([-60o, 60o]) is achieved with the depth and the filling factor of the grating chosen to be 350nm and 0.3762, which are selected to transmit TM polarized light and reflect TE polarized light as much as possible at normal incident. The loss of the TE polarized light for the Cu structures is approximately less than 5.55% due to the absorption of Cu in a large angle range. The desired property will enable this type of polarizing beam splitter to be used in diverging beams. When the PBS needs to transmit approximately 25% of the TE polarized light and as much as possible the TM polarized light, it can be used in magneto-optical data storage head. But the depth of grating has been changed (approximately d=40nm) and TM transmission is only 94.7%. But it can be further improved (from 94.7% to 97.97%) by etching into the substrate.

Hybrid plasmonic biosensor for simultaneous measurement of both thickness and refractive index

A novel hybrid plasmonic biosensor using a sub-wavelength reflection metal grating is proposed and analyzed. The simulation results show that the hybrid plasmonic-cavity (HPCM) are wavelength dependent on both the refractive index (RI) and thickness of the surrounding analyte layer, with a sensitivity of 448nm/RIU (refractive index unit) and 75nm/μm, respectively. However wavelength related to the cavity resonant mode (CRM) is only sensitive to the environmental refractive index changes. In principle it is possible to determine both RI and thickness of the coating layer simultaneously.

Fast modal method for crossed grating computation, combining finite formulation of Maxwell equations with polynomial approximated constitutive relations

We present a modal method for the fast analysis of 2D-layered gratings. It combines exact discrete formulations of Maxwell equations in 2D space with polynomial approximations of the constitutive equations, and provides a sparse formulation of the eigenvalue equations. In specific cases, the use of sparse matrices allows us to calculate the electromagnetic response while solving only a small fraction of the eigenmodes. This significantly increases computational speed up to 100×, as shown on numerical examples of both dielectric and metallic subwavelength gratings.

Investigation of inverse polarization transmission through subwavelength metallic gratings in deep ultraviolet band

Inverse polarization transmission of light, with the TE transmittance ηTE largely exceeding the TM transmittance ηTM, through subwavelength metallic gratings at certain wavelength in deep ultraviolet band is theoretically investigated. By using the Fourier modal method and the planar waveguide theory, we show that the light transmission involves both the surface plasmon and the low-loss waveguide modes. Strong coupling of the incident wave to surface plasmon polariton results in the minimum in TM transmittance, whereas the coupling to the low-loss mode leads to the TE transmittance maximum. The established physical mechanism is sufficient to explain the inverse polarizing phenomenon observed in aluminum grating at the wavelength of 193nm, which may lead to important applications such as effective and compact polarizer used in DUV lithography.

Theoretical Study of Transmission Characteristics of Sub-Wavelength Nano-Structured Metallic Grating

Some unique optical properties of sub-wavelength nanometer-sized metallic grating are presented in this paper. We propose a model to predict and explain the transmission of transverse-magnetic polarized light through a metallic grating with no substrate in the visible and near-infrared region. The model is based on a theoretical combination of surface plasmon polariton resonance and Fabry-Perot (FP)-like resonance that was excited by the nano-slit located in the grating. The finite difference time domain method is employed for modeling and simulation, and the minimum and maximum characteristics are analyzed, which shows its extraordinary enhanced transmission that differs from the conventional micrometer-sized metallic grating. This model can also be used to optimize the structural parameters in the design of the sub-wavelength wire-grid polarizer.

Plasmonic waves allow perfect transmission through sub-wavelength metallic gratings

We investigate the transmission properties of a metallic layer with narrow slits. Recent measurements and numerical calculations concerning the light transmission through metallic sub-wavelength structures suggest that an unexpectedly high transmission coefficient is possible. We analyze the time harmonic Maxwell's equations in the $H$-parallel case for a fixed incident wavelength. Denoting by $\eta>0$ the typical size of the complex structure, effective equations describing the limit $\eta\to 0$ are derived. For metallic permittivities with negative real part, plasmonic waves can be excited on the surfaces of the channels. When these waves are in resonance with the height of the layer, the result can be perfect transmission through the layer.

二维亚波长金属光栅多波长透射滤光片

二维亚波长金属光栅多波长透射滤光片

亚波长金属光栅结构的制备与矢量衍射理论分析

We fabricated a subwavelength metallic grating using nanoimprint technology and mea-sured the reflection spectrum using ultraviolet-visible-near-infrared spectrophotometer.Based on the theory of conventional rigorous coupled wave analysis,we used a new method to analyze the diffraction problems of subwavelength metallic gratings.We used fast Fourier factorization(FFF) method to derive the coupled wave equations,then each space harmonic can be expanded in terms of Legendre polynomials in grating region.Using this modified vector diffraction theory,we calculated the diffraction efficiency and the field distribution.All calculated results show great agreement with the experimental results.