Complex Polarization Response in Plasmonic Nanospirals

Archimedean nanospirals exhibit many far-field resonances that result from the lack of symmetry and strong intra-spiral plasmonic interactions. Here, we present a computational study, with corroborating experimental results, on the plasmonic response of the 4π Archimedean spiral as a function of incident polarization, for spirals in which the largest linear dimension is less than 550 nm. We discuss the modulation of the near-field structure for linearly and circularly polarized light in typical nanospiral configurations. Computational studies of the near-field distributions excited by circularly polarized light illustrate the effects of chirality on plasmonic mechanisms, while rotation of linearly polarized light provides a detailed view of the effects of broken symmetry on nanospiral fields in any given direction in the plane of the spiral. The rotational geometry exhibits a preference for circular polarization that increases near-field enhancement compared to excitation with linearly polarized light and exchanges near-field configurations and resonant modes. By analyzing the effects of polarization and wavelength on the near-field configurations, we also show how the nanospiral could be deployed in applications such as tunable near-field enhancement of nonlinear optical signals from chiral molecules.     

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.     

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.     

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.     

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.     

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.     

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.     

A Universal Plasmonic Polarization State Analyzer

We propose a universal plasmonic polarization state analyzer consisting of rectangular holes arranged along an Archimedes spiral in silver film. The analyzer can detect different polarization states of light including linear, circular, radial and azimuthal polarizations. The theoretical analysis of its transmitted field is performed on the basis of the dipole radiations, and the analytic expressions of the electric field distributions under different polarized illuminations are provided. The numerical simulations of the near-field transmissions are also conducted to verify the analytic results. The significant differences between the field distributions predict the practicability of the universal plasmonic polarization state analyzer in determining the incident light polarization states.     

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.     

A Novel Plasmonic Zone Plate Lens Based on Nano-Slits with Refractive Index Modulation

Conventionally, plasmonic lenses introduce a phase delay distribution across their surfaces by modulating the dimensions of nanostructures within a metal film. However, there is very limited modulation of the phase delay due to the small dependence of the mode propagation constant on the structure dimensions. In this paper, a novel design of plasmonic zone plate lenses (PZPL) with both slit width and refractive index modulation is proposed to enable integrating more slits in a fixed lens aperture with the extended phase delay range and, therefore, greatly enhance the performance of the devices. More than three-time enhancement of the light intensity at the focus is achieved compared to the structure with only slit width modulation. Like a conventional immersion system, a PZPL embedded in a dielectric is found to have a further improved focusing performance, where light is focused down to a 0.44λ spot using a PZPL with an aperture of 12λ and a focal length of 6λ. Dispersive light-focusing behaviour is also analysed and the modulation of the focal length by colour has a potential application in stacked image sensors and multi-dimensional optical data storage.     

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.     

An Annular Plasmonic Lens Under Illumination of Circularly Polarized Light

In this paper, we consider a circular central aperture surrounded with annular depth-tuned grooves and investigate the beaming effect of the structure under illumination of a circularly polarized (CP) plane wave. As a CP plane wave is equivalent to the superposition of two linearly polarized plane waves (TM and TE) with a phase difference of π/2, the superposition of the electric field intensity, ( | E_x |² + | E_y |² ) \left( {{{\left| {E_x} \right|}^2} + {{\left| {E_y} \right|}^2}} \right) , is observed in the transmission field. In addition, two plasmonic modes are found at the resonant wavelengths λ ₁ and λ ₂ with each consisting of multiple wavelengths. At the wavelength λ ₁ = 420 nm, the significant near-field collimation is formed along the direction z, having a long propagation distance up to 1.75 μm (≈4λ) away from the exit plane of the new plasmonic lens.     

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.     

Optimization on Plasmonic Lenses Based on Generation Efficiency of Surface Plasmon Polaritons at Metallic Nanoslit

The generation efficiency of surface plasmon polaritons at metallic nanoslit is theoretically analyzed, and a novel plasmonic lens with two semiannular nanoslits is proposed in this paper. Based on the analysis results, the focusing performance of the proposal is optimized with a maximum field intensity enhancement factor of 7.69 and the full width at half maximum is 132 nm (~0.2λ _i), far beyond theoretical diffraction limit. Meanwhile, some other classical plasmonic lenses are also optimized through improving generation efficiency of surface plasmon polaritons at nanoslit and the focusing performances are consequently greatly enhanced.     

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.     

Field Enhancement of Tip with Spiral Nanostructure

Recently, tips suitable for illuminating from top to bottom have received great attraction due to high energy utilization and easiness of constituting array, which can increase scan efficiency to realize large-area rapid microprobe scan. In this work, we study the field enhancement on a cone tip with a planar Archimedes’ spiral nanostructure under an illumination of circularly polarized light. By means of a simulation based on phase matching theory and finite-difference time-domain, it is demonstrated that electric field intensity in close proximity to the tip, which is contributed by evanescent mode (EM) and propagating mode (PM) of surface plasmon polaritons (SPPs) induced from Archimedes’ spiral slits, increases 3 orders of magnitude than that of the incoming beam. The beam induced by the tip has a full width at half maximum of 10 nm (~0.015λ₀).     

Plasmonic Focusing in Nanostructures

In this review, we show that by designing the metallic nanostructures, the surface plasmon (SP) focusing has been achieved, with the focusing spot at a subwavelength scale. The central idea is based on the principle of optical interference that the constructive superposition of SPs with phase matching can result in a considerable electric-field enhancement of SPs in the near field, exhibiting a pronounced focusing spot. We first reviewed several new designs for surface plasmon focusing by controlling the metallic geometry or incident light polarization: We made an in-plane plasmonic Fresnel zone plates, a counterpart in optics, which produces an obvious SP focusing effect; We also fabricated the symmetry broken nanocorrals which can provide the spatial phase difference for SPs, and then we propose another plasmon focusing approach by using semicircular nanoslits, which gives rise to the phase difference through changing refractive index of the medium in the nanoslits. Further, we showed that the spiral metallic nanostructure can be severed as plasmonic lens to control the plasmon focusing under a linearly polarized light with different angles.

     

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.     

Extended High Circular Polarization in the Orion Massive Star Forming Region: Implications for the Origin of Homochirality in the Solar System

We present a wide-field (∼6′ × 6′) and deep near-infrared (K _s band: 2.14 μm) circular polarization image in the Orion nebula, where massive stars and many low-mass stars are forming. Our results reveal that a high circular polarization region is spatially extended (∼0.4 pc) around the massive star-forming region, the BN/KL nebula. However, other regions, including the linearly polarized Orion bar, show no significant circular polarization. Most of the low-mass young stars do not show detectable extended structure in either linear or circular polarization, in contrast to the BN/KL nebula. If our solar system formed in a massive star-forming region and was irradiated by net circularly polarized radiation, then enantiomeric excesses could have been induced, through asymmetric photochemistry, in the parent bodies of the meteorites and subsequently delivered to Earth. These could then have played a role in the development of biological homochirality on Earth.     

Development of a polarization imaging method to detect paraffin-embedded pathology tissues before applying other techniques

The present article describes the development of a technique, applied to paraffin-embedded tissues, which uses three different wavelengths of monochromatic light (λ1 = 445 nm, λ2 = 540 nm and λ3 = 660 nm) for the measures of the degree of polarization, degree of linear polarization, degree of circular polarization and birefringence, all obtained from measurements of Stokes parameters by using polarized light. The goal of this study was to detect changes in developing embryonic mouse eye when pregnant mice fed diets without folic acid for variable periods compared with a healthy control group. We present a biomedical diagnostic technique based on polarized light detection applied to paraffin-embedded tissues to visualize the structural damage to aid us in the diagnosis before applying other techniques. Through this method, we can visualize and identify which parts of the tissue were altered with respect to the control group.