Polarization-Controlled Tunable Multi-focal Plasmonic Lens

A polarization-controlled tunable plasmonic lens which can generate different multi-focal combinations with exciting sources of left and right circular polarizations is proposed in this paper. Both position and intensity of each focal point can be adjusted by modulating the structure of the plasmonic lens. It is believed that the polarization-controlled tunable plasmonic multi-focal lens can be potentially used for optical switches and multi-channel couplers in future logic photonic and plasmonic systems.     

Plasmonic Lens Based on Rectangular Holes

A compact plasmonic lens is proposed in this paper. This plasmonic lens consists of rectangular holes etched on the silver film and arranged on one straight line and possesses the characteristics of short focus length, ultrathin thickness, and strong focus ability. The theoretical design for the plasmonic lens abides by the constructive interference theorem, and the surface plasmon polaritons excited by the holes with linearly polarized light illumination focuses effectively. The plasmonic lenses with single and double focus spots are provided, and the simulation experiment gives the powerful verification. The distinct structure feature and the excellent focusing characteristic of this plasmonic lens are benefit for its applications in optical integration.     

Multiple-Wavelength Focusing and Demultiplexing Plasmonic Lens Based on Asymmetric Nanoslit Arrays

A multiple-wavelength focusing and demultiplexing plasmonic lens based on asymmetric nanoslit arrays is designed. The nanoslit arrays are perforated in a gold film and act as metal–insulator–metal plasmonic waveguides. By manipulating the widths of the slit arrays, the plasmonic lens can concentrate two incident plane wave beams to two separated focal points corresponding to their wavelengths. The full wave simulation is performed to verify the designed lens. This work provides a way to design more compact and integrated wavelength-division multiplexing plasmonic devices for nanophotonic communication and spectral imaging.     

Simulation and Analytical Study of Optical Complex Field in Nano-corral Slits Plasmonic Lens

Although spiral plasmonic lens has been proposed as circular polarization analyzer, there is no such plasmonic nanostructure available for linear polarization. In the current work, we have designed nano-corral slits (NCS) plasmonic lens, which focuses the x- and y-polarized light into spatially distinguished plasmonic fields. We have calculated analytically and numerically the electric field intensity and phase of the emission from nano-corral slits plasmonic lens with different pitch lengths under various polarizations of the illumination. It has been shown that one can control the wave front of the output beam of these plasmonic lenses by manipulating the illumination of both circular and linear polarization. Our theoretical study in correlation with FDTD simulation has shown that NCS plasmonic lens with pitch length equal to λ_spp produces scalar vortex beam having optical complex fields with helical wave front and optical singularity at the center under circular polarization of light. When NCS lens (pitch = λ_spp) is illuminated with linearly polarized light, it exhibits binary distribution of phase with same electric field intensity around the center. However, with pitch length of 0.5λ_spp, NCS shows linear dichroism under linearly polarized illumination unlike spiral plasmonic lens (SPL) eliminating the use of circularly polarized light. Optical complex fields produced by these NCS plasmonic lenses may find applications for faster quantum computing, data storage, and telecommunications.

     

Plasmonic Lens with Multiple-Turn Spiral Nano-Structures

In this paper, we investigate the focusing properties of a plasmonic lens with multiple-turn spiral nano-structures, and analyze its field enhancement effect based on the phase matching theory and finite-difference time-domain simulation. The simulation result demonstrates that a left-hand spiral plasmonic lens can concentrate an incident right-hand circular polarization light into a focal spot with a high focal depth. The intensity of the focal spot could be controlled by altering the number of turns, the radius and the width of the spiral slot. And the focal spot is smaller and has a higher intensity compared to the incident linearly polarized light. This design can also eliminate the requirement of centering the incident beam to the plasmonic lens, making it possible to be used in plasmonic lens array, optical data storage, detection, and other applications.     

An Integrated Multistage Nanofocusing System

We demonstrate an integrated multistage nanofocusing system which combines a conventional objective, a surface plasmonic lens, and a center-positioned rounded-tip cone nanoparticle. The surface plasmonic lens, fabricated on the cover glass which has been mounted on the biological microscopic objective, is composed of several concentric annular slits for exciting propagating surface plasmonic wave. The rounded-tip cone nanoparticle is for further generating non-propagating localized surface plasmonic wave. It is revealed that the enhancement of the nanoscale optical field can be improved by carefully choosing the appropriate numerical aperture of the objective to match the specific nanostructure of the surface plasmonic lens and choosing the relatively big cone angle of the nanoparticle. The investigation shows that a highly confined electric field as small as 20 nm and an enhancement factor of 5 orders of magnitude can be achieved through this multistage nanofocusing system when the system is illuminated with a uniform radially polarized beam.     

Polarization Dependent of Plasmonic Lenses with Variant Periods on Superfocusing

A plasmonic lens with variant periods was investigated for optical behavior at near-field by means of numerical computational method. To study influence of incident light on different polarization modes, we considered linear polarization, circular polarization, elliptical polarization, radial polarization (RP), and azimuthally polarization in our computational analyses. A finite difference and time domain algorithm is employed in the numerical study. Our computational numerical calculation results demonstrate that focusing performance for the plasmonic lens illuminated under radial polarization is best in comparison to that of the illumination with the other four polarization states. The plasmonic lens with RP illumination can realize superfocusing with ultra-long depth of focus. It is possible to be used as an optical probe or a type of plasmonic lens for imaging with high resolution in the near future.     

Investigation of Physical Limits of Plasmonic Focusing Lens in Different Regions

We present theoretical studies of three regions for plasmonic focusing, which are surface plasmon-dominating, Fresnel, and Fraunhoffer regions. The boundaries of the three regions are defined and the physical behaviors of plasmonic lenses in terms of focal length and focus size in these regions are investigated. A plasmonic lens that renders a subdiffraction-limit focus in the Fresnel region is presented and the lens performance with respect to the design parameters is studied by using finite-difference time-domain simulations. This work can serve as a basis for understanding plasmonic-focusing phenomenon and designing plasmonic lenses for various applications.     

Far-Field Focusing of Spiral Plasmonic Lens

In this paper, we study the nanoscale-focusing effect in the far field for a spiral plasmonic lens with a concentric annular groove by using finite-difference time domain simulation. The simulation result demonstrates that a left-hand spiral plasmonic lens can concentrate an incident right-hand circular polarization light into a focal spot at the exit surface. And this spot can be focused into far field due to constructive interference of the scattered light by the annular groove. The focal length and the focal depth can be adjusted by changing the groove radius and number of grooves within a certain range. These properties make it possible to probe the signal of spiral plasmonic lens in far field by using conventional optical devices.     

Robustly Efficient Superfocusing of Immersion Plasmonic Lenses Based on Coupled Nanoslits

We report plasmonic lenses consisting of coupled nanoslits immersed in a high-index medium to obtain the robustly efficient superfocusing. Based on the geometrical optics and the wavefront reconstruction theory, an array of nanoslits perforated in a gold film and a series of spacings between adjacent nanoslits are optimally designed to realize the desired phase modulation for light focusing. The numerical results verify the design of each plasmonic lens in excellent agreement. For the given total phase difference of 2π, the immersion plasmonic lenses with smaller lens aperture can have much better focusing performance than the non-immersion one. A superfocusing spot of λ/4.39 is achieved using an oil immersion plasmonic lens with an aperture size of 4.97λ, resulting in a resolution improvement of 68.9 % compared with the non-immersion lens. Moreover, such superfocusing performance can be still well kept when the structural parameters of the lens, e.g., nanoslit width and metal film thickness, are deviated from the original design, making the final implementation of the superfocusing lenses much easier.     

A Super Lens System for Demagnification Imaging Beyond the Diffraction Limit

A super lens system is proposed to achieve subdiffraction limit demagnification imaging. The super lens system consists of a hyperlens with planar input and output surfaces, a metal superlens, and a plasmonic reflector. By employing the hyperlens to transform evanescent waves into propagating waves and employing the metal superlens and the plasmonic reflector to amplify evanescent waves, the super lens system can produce a subdiffraction limit image with relatively high electric field intensity. The reduction factor of the super lens system depends on the geometric parameters of the hyperlens. Simulation results show that an image with a half-pitch resolution of about one tenth the operating wavelength and a reduction factor of about 2.2 can be produced by the super lens system. The proposed super lens system has potential applications in nanolithography.     

A Design Method for a Micron-Focusing Plasmonic Lens Based on Phase Modulation

A design method of a micron-focusing plasmonic lens is proposed, which consists of a nanoaperture surrounded by concentric annular grooves with fixed width and depth. The phase modulation of the radiation lights decoupled from surface plasmon polariton waves by the annular grooves is realized by altering the radii of the grooves. Based on the principle of the constructive interference, a design formula of a micron-focusing plasmonic lens is deduced. The transmitted fields through the designed plasmonic lenses are numerically simulated with finite-difference time-domain method, and the results show that a circular focusing spot is generated where the focal length can be controlled in several micrometers, which agree with our theoretical analysis.     

Polarization Effect on Superfocusing of a Plasmonic Lens Structured with Radialized and Chirped Elliptical Nanopinholes

Tuning effect of different polarization states was presented in this paper. It can be realized by a plasmonic lens constructed with elliptical pinholes ranging from submicron to nanoscales distributed in variant period along radial direction. Propagation properties of the lens illuminated under four different polarization states: linear, elliptical, radial, and cylindrical vector beam, were calculated and analyzed combining with finite-difference time-domain algorithm. Different focusing performances of the lens were illustrated while the polarized light passes through the pinholes. Our calculation results demonstrate that polarization effect of the elliptical pinholes-based plasmonic lens can generate high transmission intensity and sharp focusing for our proposed specific structures. Beam focal region, position, and transmission intensity distribution can be tailored by the four polarization states.     

Experimental Study of Metallic Elliptical Nano-Pinhole Structure-Based Plasmonic Lenses

An elliptical nano-pinhole structure-based plasmonic lens was designed and investigated experimentally by means of focused ion beam nanofabrication, atomic force microscope imaging, and scanning near-field optical microscope (NSOM). Two scan modes, tip scan and sample scan, were employed, respectively, in our NSOM measurements. Both the scan modes have their characteristics while probing the plasmonic lenses. Our experimental results demonstrated that the lens can realize subwavelength focusing with elongated depth of focus. This type of lens can be used in micro-systems such as micro-opto-electrical–mechanical systems for biosensing, subwavelength imaging, and data storage.     

Plasmonic Lenses: A Review

Four types of plasmonic lenses for the purpose of superfocusing designed on the bases of approximate negative refractive index concept, subwavelength metallic structures, waveguide mode were introduced, and curved chains of nanoparticles, respectively, were introduced. Imaging mechanism, fabrication, and characterization issues were presented. Theoretical analyses of the illumination with different polarization states on focusing performance of the plasmonic lenses were given also. In addition, a hybrid Au-Ag plasmonic lens with chirped slits for the purpose of avoiding oxidation of Ag film was presented.     

Super-Resolution Long-Depth Focusing by Radially Polarized Light Irradiation Through Plasmonic Lens in Optical Meso-field

In this paper, we propose a novel plasmonic lens design consisting of an annular slit and concentric grooves. The simulation results show that under radially polarized illumination, a super-resolution long depth of focus (DOF) spot can be achieved in optical meso-field due to the constructive interference of scattered light by the concentric grooves. We also analyze the influence of depth-tuned annular grooves on focusing performance, including focal length, DOF, and full-width half-maximum. Moreover, focusing efficiency can be enhanced (～350 %) by introducing a circular metallic grating which surrounds the annular slit. This plasmonic lens has potential applications in nano-imaging and nano-photolithography.     

Active Focal Length Control of Terahertz Slitted Plane Lenses by Magnetoplasmons

Active plasmonic devices are mostly designed at visible frequencies. Here, we propose an active terahertz (THz) plasmonic lens tuned by an external magnetic field. Unlike other tunable devices where the tuning is achieved by changing the plasma frequency of materials, the proposed active lens is tuned by changing the cyclotron frequency through manipulating magnetoplasmons (MPs). We have theoretically investigated the dispersion relation of MPs of a semiconductor?Cinsulator?Csemiconductor structure in the Voigt configuration and systematically designed several lenses realized with a doped semiconductor slab perforated with sub-wavelength slits. It is shown through finite?Cdifference time?Cdomain simulations that THz wave propagating through the designed structure can be focused to a small size spot via the control of MPs. The tuning range of the focal length under the applied magnetic field (up to 1?T) is ??3??, about 50% of the original focal length. Various lenses, including one with two focal spots and a tunable lens for dipole source imaging, are realized for the proposed structure, demonstrating the flexibility of the design approach. The proposed tunable THz plasmonic lenses may find applications in THz science and technology such as THz imaging.     

Numerical Study of a Nonplanar Two-Stage Surface Plasmonic Lens Illuminated by a Radially Polarized Beam

We design and fabricate a nonplanar two-stage surface plasmonic lens composed of concentric circular slits for exciting propagating surface plasmonic wave and a center-positioned cone-like nanoparticle for generating localized surface plasmonic waves. The numerical investigation based on the finite difference in time domain method is performed. It is found that, when a radially polarized beam illumination is applied, a highly confined electric field with full width half maximum of as small as 6 nm and the transmission enhancement factor of six orders higher than the incident beam is achievable. The optimization design is conducted through comparison of different conic angles and different materials of the cone-like nanoparticles.     

Influence of V-Shaped Metallic Subwavelength Slits with Variant Periods for Superfocusing

In this paper, we discussed the influence of a plasmonic lens with V-shaped metallic subwavelength slits and variant periods on transmission properties. In order to analyze the influence, a finite-difference time-domain numerical algorithm was adopted for computational numerical simulation of the plasmonic structures. The structures are flanked with the penetrated slits through a metal (Ag) film which is coated on a quartz substrate. Our simulation results demonstrated that different cone angles originated from the V-shaped slits generate different influences on the beam propagation. The width variation affects the intensity significantly. The cone angles formed by the V-shaped slits can change the focusing performance. These results are very encouraging for future study of the plasmonic lens-based applications.     

Coupling Between Silicon Waveguide and Metal-Dielectric-Metal Plasmonic Waveguide with Lens-Funnel Structure

Various photonic integrated components have been implemented by ultra-thin silicon-on-insulator (SOI) waveguides; therefore, it is desirable to couple ultra-thin SOI waveguides to plasmonic waveguides. In this paper, we present an ultra-thin SOI waveguide to a metal-dielectric-metal plasmonic waveguide based on a lens-funnel structure consisting of truncated Luneburg lens and metallic parabolic funnel. The lens is implemented by varying the guiding layer thickness. The effect of different parameters of the coupler’s geometry is studied using the finite-difference time-domain method. The 1.13-μm-long coupler improves the average coupling efficiency in the C-band from 66.4 to 82.1%. The numerical simulations indicate that the coupling efficiency is higher than 69% in the entire O, E, S, C, L, and U bands of optical communication.