Outcoupling surface plasmons into propagation modes using a subwavelength dielectric grating for device applications

Coupling of surface plasmon polaritons to radiation modes by use of a one-dimensional subwavelength dielectric grating on a thin metal slab is discussed. The surface plasmon waves obtained in Kretschmann configuration are resonant outcoupled to radiation modes by using a subwavelength dielectric grating. A peak outcoupling efficiency is predicted to be 74.57% with rigorous coupled-wave analysis. In addition, potential applications of these results in the design and improvement of various optoelectronic devices, such as polarizers, wavelength filters and biochemical sensors are discussed.     

Beam focusing from double subwavelength metallic slits filled with nonlinear material surrounded by dielectric surface gratings

Based on the radiation properties of surface plasmon polaritons (SPPs) can be controlled by adjusting the refractive indexes of dielectric materials in the metallic slits, a novel plasmonic focusing structure formed by two subwavelength metal apertures filled with Kerr nonlinear material surrounded by surface dielectric gratings is proposed and demonstrated numerically. Directions of radiation fields are determined by the phase difference of the surface waves at the exit interface and resonance property of each surface grating. Numerical simulations using two-dimensional (2D) Finite-Difference Time-Domain (FDTD) method verify that the deflection angle and focal length can be controlled easily by changing the intensity of incident light, dynamically tunable on-axis and off-axis focusing effects can be achieved.     

Comparison of subwavelength focusing properties of diffraction and Fresnel zone plate plasmonic planar lenses

The subwavelength focusing properties of diffractive plasmonic planar lenses (DPLs) and Fresnel zone plate plasmonic planar lenses (FZPs) have been compared in this paper. To that end, we use the same lens material, incident wavelength, thickness and focal length for comparison. Both DPLs and FZPs consist of central circular slits surrounded by transparent and opaque zones and can get clear focusing performances. By using the rigorous electromagnetic numerical method, the fields in the focal region are analyzed in detail, and our results demonstrate that FZP can generate higher transmission efficiency, and higher peak field intensity at the focal plane. Focusing polarization properties of the lens illuminated by linearly polarized state, are calculated and analyzed also. The numerical results show that both the DPL and the FZP produce asymmetrical focal spot distributions with a low f-number. To the FZP, the full-width at half-maximum (FWHM) varies from 256 nm (along y-axis) to 516 nm(along x-axis) and to the DPL, the FWHM varies from 256 nm(along y-axis) to 580 nm(along x-axis), respectively. But for a high f-number, the asymmetrical performance of the focal spot will be reduced due to complicated electromagnetical field interferences and micro waveguide effect. Otherwise, the DPL can get a higher depolarization effect than the FZP does.     

Beam bending through compound structure with two subwavelength metallic slits in each period

To provide a more practical, easy-to-implement method to achieve directional modulation with a plasmonic lens, beam manipulation method via compound metallic gratings with two subwavelength slits filled with different dielectrics inside each period is proposed and numerically investigated by finite-difference time-domain (FDTD) method. Compared with conventional metal-grating based structures, phase retardation is tuned by the Fabry-Pérot (FP) resonant condition and light bending is achieved by constructing a carefully designed, curved phase front for the plasmonic lenses. Our designs have advantages in ease of fabrication and capability to perform in the far field. With these advantages, the designs are expected to be valuable in applications such as plasmonic circuits and photonic communication.     

1D Bessel-like beam generation by highly directive transmission through a sub-wavelength slit embedded in periodic metallic grooves

A new method to generate a quasi-one dimensional (1D) Bessel-like beam whose non-diffracting length extends to macroscopic scale is proposed by utilizing highly directive transmission through a sub-wavelength single slit embedded in periodic metallic grooves. We employed finite-difference time-domain methods for full-vectorial diffraction and spectral analysis. In optimal conditions, unique quasi-1D Bessel-like beams were generated in transmission for the incident p-polarized plane wave such that highly asymmetric far-field distribution was achieved; Bessel-like beam along the slit axis and a flat-top super-Gaussian beam along the other perpendicular axis. Detailed parametric studies for the proposed structure are reported in terms of the operating spectral range and a general window of optimal conditions.     

Optical transmission through metal/dielectric multilayer films perforated with periodic subwavelength slits

The transmission of p-polarized plane wave through Ag/SiO₂ multilayer films perforated with periodic subwavelength air slits is investigated by using the finite-difference time-domain (FDTD) method. The results show that the optical transmission property is mediated by the interference among the propagating coupled-SPP modes along the lateral direction inside the SiO₂ layers and the conditions of Fabry-Pérot-like resonance along the longitudinal direction together. When some geometric parameters are suitably initialized, the high transmission peaks can split into more peaks as the functional layer (metal/dielectric/metal sandwich stack) number increases, and the wavelength of the same-order transmission peak exhibits a red shift as the grating period increases.     

Simulation investigation on waveguide properties of terahertz wave through subwavelength semiconductor gap

The waveguide propagation properties of terahertz wave through subwavelength semiconductor trench have been simulationally investigated. The effects of gap width, temperature, doping concentration and dielectric filling materials on waveguide property have been given and discussed. The results show that as temperature and doping concentration increases, the skin depth and the propagation constant decreases. In addition, the effective index increases and the propagation length decreases as the dielectric constant of filling materials increases.     

Localized surface plasmons-based transmission enhancement of terahertz radiation through metal aperture arrays

The extraordinary transmission spectrum of a copper film pierced by a periodic array of subwavelength rectangular holes is measured by the terahertz time-domain spectroscopy. The transmission coefficient is strongly dependent on the angle between the polarization of terahertz electric field and the latitudinal direction of the periodic apertures. When the angle increases from 0^o to 90^o, a peak becomes stronger and another peak reduces. The transmission is proposed to be the contributions of localized surface plasmons inside the apertures. The finite-difference-time-domain (FDTD) simulation results are in good agreement with experimental observations.     

Surface plasmon-assisted optical switching/bistability at telecommunication wavelengths in nonlinear dielectric gratings

In this paper, we numerically investigate nonlinear optical responses in dielectric gratings coated by flat metallic surface, where the structural parameters are chosen for wavelengths of operation at the telecom regime. The presented designs show the output light reflection with respect to the incident intensity of increasing, decreasing, and non-monotonic belt-shape functions. Nonlinear optical responses including switching and bistability appeared in the telecom region owing to assistance of surface plasmon resonance excited at the metal/dielectric interface, which enhances the diffraction efficiency and induces the momentum matching for the leaky modes of the dielectric grating. This results in electric intensity-effective refractive index dependence enhancement in the Kerr nonlinear media. Our design and investigation of the linear and nonlinear optical responses will provide a general guideline for a hybrid metallic-dielectric structure-based optical switching/bistability device design.     

表面等离子体亚波长光学前沿进展

目前表面等离子体激元(surface plasmon polaritons，SPPs)在光存储、光激发、显微术以及生物光子学等领域中的应用前景受到了广泛的关注．文章介绍了SPPs的基本性质和表面等离子体亚波长光学(surface plasmons subwavelength optics)研究中的热点问题及发展方向．     

Unidirectional manipulation of surface plasmon polariton by dual-nanocavity in a T-shaped waveguide

A structure of two dimensional T-shaped metal-insulator-metal waveguide with dual-nanocavity is proposed. The two nanocavities located at each side of the slit on the lower metallic surface, act as band rejection filters and are capable of stopping the surface plasmon polaritons (SPPs) at the resonant wavelengths. The Fabry-Perot interferometry theory and the Finite-Difference-Time-Domain method are utilized to investigate the proposed waveguide. The numerical results demonstrate the realization of miniaturized photonic devices for effectively switching the SPPs propagation between the left and right waveguides in one direction.     

TM-mode perturbation analysis of dielectric gratings

An improved perturbation procedure that was previously presented for TE modes guided by dielectric gratings is extended here to TM modes. In both cases, the fields are described in terms of a transverse transmission-line network, which lends considerable physical insight into the behavior of dielectric gratings. However, TM modes require equivalent sources of both the voltage and current varieties, in contrast to TE modes which involve only current sources. By comparing with an exact solution, the numerical results obtained by the present perturbation procedure are found to be very accurate. This work was supported by the U.S. Office of Naval Research, under Contract No. N 00014-67-A-0438-0014, and by the U.S. Joint Services Electronics Program, under Contract No. F 44620-69-C-0047.     

单一亚波长孔径增强透射效应的传输线模型

为了研究亚波长孔径的增强透射效应,把传输线理论引入到了单一亚波长金属孔径的增强光学透射效应中。把金属孔径看成传输线,将电流在传输线上的传输和类Fabry-Pérot腔联系起来;把孔径的出射面看成是传输线的输出端,将出射面的电流看成是一种局域的衰逝电流,电流经传输线在传输线的输出端以天线辐射的形式辐射电磁波。解释了孔径透射的近场分布问题、孔径增强透射峰的位置问题和透射峰位置随金属板厚度的红移问题,得到了与其它理论和实验一致的结果,对开发基于增强透射的亚波长元件具有重要的参考价值。     

Genetic optimization of double subwavelength metal slits surrounded by surface dielectric gratings for directional beaming manipulation

In this work, a model of double subwavelength metal slits surrounded by surface dielectric gratings is proposed for the directional beaming based on surface plasmon polariton (SPP). A genetic algorithm (GA) in conjunction with the finite-difference time-domain (FDTD) method is employed to optimize this structure, which leads to the high-efficiency directional beaming at any angle in a broad range (from 0° to 70°). The results show that the furthest propagation distance of the beaming can reach more than 25 μm, the maximum deflection efficiency can reach 97.37%, and the enhancement factor of the beaming can reach 7.58 × 10². The acceptable perturbation range of optimum structures is also obtained, which indicates the feasibility and realization of this model.     

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 700 nm/RIU and 509°/RIU with full width at half maximum (FWHM) less than 2.5 nm using the same optimized structure.     

Critical process of extraordinary optical transmission through periodic subwavelength hole array: Hole-assisted evanescent-field coupling

We present a new explanation of the extraordinary optical transmission through subwavelength hole array in metal films. By using the classical coupled-wave theory, we analyzed the coupling process between light and the surface plasmons wave on metal films with hole array. The analysis shows clearly the physical mechanism of extraordinary optical transmission. The calculation demonstrates that, instead of energy flux directly passing through the holes, the electromagnetic modes could exchange energy by overlapping the evanescent fields under the assistance of hole array. The periodicity of the array provides the momentum-matching condition to present the transmission peaks. The theory exhibits a good agreement with the experimental results reported in every detail.     

Resonantly enhanced transmission of light through subwavelength apertures with dielectric filling

Using both analytical and numerical methods to study transmission of light through dielectric-filled subwavelength apertures in a real metal, we have found that a propagating mode can in principle exist inside a waveguide of arbitrary small size if a particular relationship between the dielectric constants of the cladding and filling materials at the incident frequency is satisfied. Practical transmission through a subwavelength aperture of finite depth can be enhanced when the depth is such that Fabry-Pérot-like resonances are excited. For 810 nm light incident on a silicon-filled 50-nm-diameter aperture in a 200-nm-thick gold film, we found that a normalized near-field intensity ratio of 1.6 at the exit can be achieved. This resonantly enhanced transmission phenomenon may be advantageously applicable to near-field scanning optical microscopy and single-molecule spectroscopy.     

Theory of highly directional emission from a single subwavelength aperture surrounded by surface corrugations

We present a theoretical foundation for the beaming of light displayed by a single subwavelength aperture in an appropriately corrugated metal film [H. J. Lezec, Science 297, 820 (2002)]]. Good agreement is found between calculations and experimental data. We show that beaming is due to the formation of electromagnetic surface resonances and that the beam direction, width, and wavelength at which it occurs can be selected by tuning geometrical parameters of the structure.     

Demagnifying far field focusing spot to deep subwavelength scale by truncated hyperlens for nanolithography

A truncated hyperlens composed of pairs of metal-dielectric cylindrical multilayers is proposed to demagnify the diffraction limited spot to achieve deep subwavelength imaging. The diffraction limited spot is focused by far field converging cylindrical wave. Numerical simulations demonstrate that full width at half maximum (FWHM) of the image down to 32 nm (∼λ/11) can be achieved from object (∼λ/3) by 365 nm light illumination. It is discussed that the influence of size and focusing shift of the spot on those of the demagnification image on photoresist. It is also demonstrated that multi-objects can be demagnified and projected on the photoresist.