Plasmonic Lens Research Articles

Tip-Enhanced Raman Spectroscopy Based on Spiral Plasmonic Lens Excitation

In this study, we proposed the idea of replacing the traditional objective lens in bottom-illumination mode with a plasmonic lens (PL) to achieve tip-enhanced Raman spectroscopy (TERS). The electric field energy of surface plasmon polaritons (SPPs) of the spiral PL was found to be more concentrated at the focal point without any sidelobe using the finite-difference time domain (FDTD) method compared with that of a symmetry-breaking PL. This property reduces far-field background noise and increases the excitation efficiency of the near-field Raman signal. The disadvantage of only the near-field Raman scattering of samples at the center of the structure being detected when using an ordinary PL in TERS is overcome by using our proposed method of changing only the polarization of the incident light.

High-Efficiency Plasmonic Lens Based on Archimedes-Spiral with Cross Section of an Asymmetric Slot

A high-efficiency plasmonic lens composed of a single Archimedes-spiral slot with a cross section of an asymmetric slot is proposed. By adding an auxiliary nanocavity under the primary spiral slot, unidirectional plasmonic waves can be efficiently transmitted in the inward direction and focused on a hot spot in the center. Due to the asymmetric slot, the finite-difference time-domain (FDTD) method is used to numerically optimize the geometric parameters of the single spiral slot, which can achieve high-intensity unidirectional inward focusing. The proposed structure can decrease background noises and prevent cross-talk of nearby components in optical networks, which significantly improves the integration level of nanophotonic circuits and devices.

Two-dimensional biocompatible plasmonic contact lenses for color blindness correction

Color blindness, or color vision deficiency (CVD), is an ocular disease that suppresses the recognition of different colors. Recently, tinted glasses and lenses have been studied as hopeful devices for color blindness correction. In this study, 2D biocompatible and flexible plasmonic contact lenses were fabricated using polydimethylsiloxane (PDMS) and a low-cost, and simple design based on the soft nano-lithography method and investigated for correction of red–green (deuteranomaly) color blindness. In addition, the stability test of the fabricated plasmonic contact lenses was investigated into the phosphate buffered saline (PBS) solution and the proposed lens offers an excellent stability into the PBS solution. The plasmonic contact lens proposed herein is based on the plasmonic surface lattice resonance (SLR) phenomenon and offers a good color filter for color blindness correction. The biocompatibility, low cost, stability, and simple fabrication of these contact lenses can offer new insights for applications of color blindness correction.

Far-field position-tunable trapping of dielectric particles using a graphene-based plasmonic lens.

In this report, a graphene-based plasmonic lens is designed for far-field position-tunable trapping of dielectric particles at a wavelength of 1550 nm, in which target particles can be floated at a variable z-position, using a variable gate voltage applied to the graphene ribbons. Preventing proximity of the trapped particle and the metallic lens structure, we can diminish general thermal issues in plasmonic tweezers, while realizing higher degrees of freedom in studying target characteristics of the particles by achieving position-tunable 3D trapping. These advantageous aspects are impossible in conventional plasmonic tweezers, because of the highly evanescent nature of the plasmonic field at the metal interface. The proposed structure is comprised of two concentric circular slit-sets (S1, S2), each capable of sending a directive beam, which can lead to a constructive interference, and forming a subwavelength focal spot in the far-field. Taking advantage of the epsilon-near-zero (ENZ) behavior of graphene, each of the radiating slit-sets can be switched ON/OFF, with a radiation switching ratio of about 49, by applying a small electric pulse of 80 meV to change the Fermi energy of the corresponding graphene ribbon from 0.535 eV to 0.615 eV. Hence, inverting the radiation state of the designed lens, from (S1:ON, S2:OFF) to (S1:OFF, S2:ON), we can change the z-position of the focal trapping site from 5000 nm to 9800 nm. This configuration can be proposed as a new generation of long-range, electrostatically tunable 3D plasmonic tweezing, without the need for any external bulky optomechanical equipment.

Plasmonic meniscus lenses

Controlling and manipulating the propagation of surface plasmons has become a field of intense research given their potential in a wide range of applications, such as plasmonic circuits, optical trapping, sensors, and lensing. In this communication, we exploit classical optics techniques to design and evaluate the performance of plasmonic lenses with meniscus-like geometries. To do this, we use an adapted lens maker equation that incorporates the effective medium concepts of surface plasmons polaritons travelling in dielectric-metal and dielectric-dielectric-metal configurations. The design process for such plasmonic meniscus lenses is detailed and two different plasmonic focusing structures are evaluated: a plasmonic lens with a quasi-planar output surface and a plasmonic meniscus lens having a convex-concave input–output surface, respectively. The structures are designed to have an effective focal length of 2λ0 at the visible wavelength of 633 nm. A performance comparison of the two plasmonic lenses is shown, demonstrating improvements to the power enhancement, with a 22% and 16.5% increase when using 2D (ideal) or 3D (realistic plasmonic) meniscus designs, respectively, compared to the power enhancement obtained with convex-planar lenses. It is also shown that the depth of focus of the focal spot presents a 19.8% decrease when using meniscus lenses in 2D and a 34.3% decrease when using the proposed 3D plasmonic meniscus designs. The broadband response of a plasmonic meniscus lens (550–750 nm wavelength range) is also studied along with the influence of potential fabrication errors on the generated effective focal length. The proposed plasmonic lenses could be exploited as alternative focusing devices for surface plasmons polaritons in applications such as sensing.

Detection of cylindrical vector beams with chiral plasmonic lens

The cylindrical vector beam (CVB) has been extensively studied in recent years, but detection of CVBs with on-chip photonic devices is a challenge. Here, we propose and theoretically study a chiral plasmonic lens structure for CVB detection. The structure illuminated by a CVB can generate single plasmonic focus, whose focal position depends on the incident angle and the polarization order of CVB. Thus, the incident CVB can be detected according to the focal position and incident angle and with a coupling waveguide to avoid the imaging of the whole plasmonic field. It shows great potential in applications including CVB-multiplexing integrated communication systems.

Numerical simulation of a plasmonic lens for laser light focusing

In this paper, the design of a plasmonic lens in gold and silver thin films for focusing the light with radial polarization is presented. Using the finite difference time domain method the optimal parameters of the plasmonic lens design are found. It was shown that the silver plasmonic lens produces a tight focal spot with a full width at half maximum of 0.38 of the incident light wavelength.

Efficient plasmonic functional lens constructed via a nano-dichroic element

The plasmonic functional lens can realize an efficient and functional focusing of surface plasmon polaritons (SPPs), making it have great potential in applications including nano-electron point sources, smart pixels, and particle manipulation. Here, we report for the first time a novel plasmonic functional lens constructed via a nano-dichroic element. The results show that the wavelength-selective nano-focusing of SPPs in the plasmonic functional lens can be achieved. Different from the conventional plasmonic lens that equally splits SPP power to propagate inward and outward from the boundaries, the plasmonic functional lens directs all of the SPP power of the matched wavelength inward for the focusing, resulting in higher collecting efficiency and SPP focusing intensity. Furthermore, we have demonstrated that a much higher focal spot intensity can be obtained even under the condition of a smaller inner radius. These findings provide a new way for the design of plasmonic functional lenses and can be facilitated to develop high-efficiency miniaturized focusing devices.

Web-above-a-Ring (WAR) and Web-above-a-Lens (WAL): Nanostructures for Highly Engineered Plasmonic-Field Tuning and SERS Enhancement.

Synthetic strategies of web-above-a-ring (WAR) and web-above-a-lens (WAL) nanostructures are reported. The WAR has a controllable gap between the nanoring core and a nanoweb with nanopores for the effective confinement of electromagnetic field in the nanogap and subsequent surface-enhanced Raman scattering (SERS) of Raman dyes inside the gap with high signal reproducibility, which are attributed to the generation of circular 3D hot zones along the rim of Pt@Au nanorings with wrapping nanoweb architecture. More specifically, Pt@Au nanorings are adopted as a plasmonic core for structural rigidity and built porous nanowebs above them through a controlled combination of galvanic exchange and the Kirkendall effect. Both nanoweb and nanolens structures are also formed on Pt@Au nanoring, which is WAL. structure. Remarkably, plasmonic hot zone, nanopores, and hot lens are formed inside a single WAL nanostructure, and these structural components are orchestrated to generate stronger SERS signals.

Plasmonic contact lens materials for glucose sensing in human tears

Surface-enhanced Raman scattering (SERS) contact lens materials (SERS-LM) have been developed for selective glucose detection in human tears. SERS-LM is composed of a silk fibroin (SF) layer, a layer of silver nanowires (AgNWs) coated with 4-mercaptophenyl boronic acid (MPBA), and a protection film (PF) layer in a layer-by-layer structure. SF offers a biocompatible interface and filters tear proteins larger than nanopores in the SF domain. The MPBA-coated AgNWs (AgNWs-MPBA) formed on SF creates a high density of hotspots for Raman signal enhancement, and MPBA functions as a chemical receptor that induces cis-diol complexation with a glucose molecule. The developed SERS-LM exhibits excellent glucose sensing performance in the concentration range of 500 nM - 1 mM, and the limit of detection is 211 nM. Then, the sensing capability of the SERS-LM is demonstrated using human tears before and after a meal. The tear glucose levels measured by SERS-LM represent trends of corresponding blood glucose level, confirming the feasibility of the developed SERS-LM for noninvasive glucose sensing in human tears. The developed SERS-LM is expected to be applied for various wearable-type personal health monitoring systems.

On-chip continuous position control of phase singularities in nanoscale.

In this paper, continuous position control of plasmonic phase singularities on a metal-air interface is achieved based on the misaligned coupling between the optical axis of vortex beam and nano ring plasmonic lens. The formula of surface plasmon polaritons field distribution in this case is derived. The offset distance and direction between the optical axis of the vortex beam and the center of the nano ring is used to control the distance and the angular distribution of the phase singularities in nanoscale, respectively. This can promote the accurate positioning of phase singularities in practical applications and provide a deeper understanding of the misaligned coupling between vortex beams and nano ring plasmonic lens.

High-Speed Parallel Plasmonic Direct-Writing Nanolithography Using Metasurface-Based Plasmonic Lens

Simple and efficient nanofabrication technology with low cost and high flexibility is indispensable for fundamental nanoscale research and prototyping. Lithography in the near field using the surface plasmon polariton (i.e., plasmonic lithography) provides a promising solution. The system with high stiffness passive nanogap control strategy on a high-speed rotating substrate is one of the most attractive high-throughput methods. However, a smaller and steadier plasmonic nanogap, new scheme of plasmonic lens, and parallel processing should be explored to achieve a new generation high resolution and reliable efficient nanofabrication. Herein, a parallel plasmonic direct-writing nanolithography system is established in which a novel plasmonic flying head is systematically designed to achieve around 15 nm minimum flying-height with high parallelism at the rotating speed of 8–18 m·s−1. A multi-stage metasurface-based polarization insensitive plasmonic lens is proposed to couple more power and realize a more confined spot compared with conventional plasmonic lenses. Parallel lithography of the nanostructures with the smallest (around 26 nm) linewidth is obtained with the prototyping system. The proposed system holds great potential for high-freedom nanofabrication with low cost, such as planar optical elements and nano-electromechanical systems.

A plasmonic lens based on coordinate transformation

With the development and maturity of nanofabrication technology, many nanostructures with specific electromagnetic characteristics and optical functions have been designed and demonstrated. Artificial materials with these special optical properties are known as metamaterials. In this paper, a metamaterial lens is proposed and characterized. After using the space transformation approach based on the coordinate transformation principles, the lens becomes anisotropic and inhomogeneous, thus realizing the regulation of electromagnetic waves. The electromagnetic wave transformation characteristics of the superlens are numerically analysed by irradiating a plane transverse wave with a wavelength of 300 nm onto a rectangular slab. The results show that the waveform of the incident light changes from plane waves to cylindrical after passing through the superlens. The validity of the superlens is verified by finite element method calculations.

Extraordinary spin-orbit interaction in the plasmonic lens with negative index material.

Spin-orbit interactions are inherent in many basic optical processes in anisotropic and inhomogeneous materials, under tight focusing or strong scattering, and have attracted enormous attention and research efforts. Since the spin-orbit interactions depend on the materials where they occur, the study of the effects of materials on the spin-orbit interactions could play an important role in understanding and utilizing many novel optical phenomena. Here, we investigate the effect of negative-index material on the spin-orbit interactions in a plasmonic lens structure in the form of a circular slot in silver film. Numerical simulations are employed to study the influence of the negative-index material on the plasmonic vortex formation and the plasmonic focusing in the structure under circularly polarized excitations bearing different orbital angular momentum. We reveal that the presence of negative-index material leaves the plasmonic vortex field distribution and the corresponding topological charge unaltered during the spin-to-orbital angular momentum conversion, whereas reverses the rotation direction of in-plane energy flux of the plasmonic vortex and shifts the surface plasmon polariton focus position to the opposite direction compared to the case without negative-index material. This work will help further the understanding of the regulation of optical spin-orbital interactions by material properties and design optical devices with novel functions.

Deuterogenic Plasmonic Vortices.

The optical vortex on a chip is of extreme importance for many applications in nanoscience, and as well-known, the chiral metallic nanostructures like plasmonic vortex lenses (PVLs) can produce a spin-dependent plasmonic vortex (PV) which is governed by plasmonic spin-orbit coupling. The well-established nanophotonic theory and various experimental demonstrations all show a single PV mode in one PVL, when the excitation is fixed. Here, counterintuitively, we report the existence of the nontrivial deuterogenic PVs, besides the one predicted previously. We theoretically reveal a general spin-to-orbit coupling and experimentally demonstrate the surprising existence of multiple PVs in a single PVL even when excited by a fixed circularly polarized vortex beam. This work provides a deeper fundamental understanding of the dynamics and the near-field spin-orbit coupling in nanophotonics, which promises to flexibly manipulate the PV for emerging optical vortex-based nanotechnologies and quantum optical applications on a chip.

Enhancement of Resolution and Propagation Length by Sources with Temporal Decay in Plasmonic Devices

Highly lossy nature of metals has severely limited the scope of practical applications of plasmonics. The conventional approach to circumvent this limitation has been to search for new materials with more favorable dielectric properties (e.g., reduced loss), or to incorporate gain media to overcome the inherent loss. In this study, however, we turn our attention to the source and show that the wealth of new SPP modes with simultaneous complex frequencies and complex wave vectors that are otherwise unreachable can be excited by imposing temporal decay on the excitation. Therefore, to understand the possible implications of these new modes and how to be able to tune them for specific applications, we propose a framework of pseudo-monochromatic modes that are generated by introducing exponential decays into otherwise monochromatic sources. Within this framework, the dispersion relation of complex SPPs is re-evaluated and cast to be a surface rather than a curve, depicting all possible ω − k pairs (both complex in general) that are supported by the given geometry. To demonstrate the potentials of the complex modes and the use of the framework to study them selectively, we have chosen two important, and somewhat limiting, features of SPPs to investigate; resolution in plasmonic lenses and propagation length in SPP waveguides. While the former is mainly used to validate the proposed method and the framework on the recent improvement of resolution in plasmonic superlenses, the latter provides a novel approach to extend the propagation length of the SPP modes in planar waveguides significantly. Since the improvement in propagation length due to the introduction of temporal decay to the excitation is rather counter-intuitive, the dispersion-based theoretical predictions (the proposed approach) have been validated via the FDTD simulations of Maxwell’s equations in the same geometry without any a priori assumptions on the frequency or the wave vector.

Generating plasmonic vortex field with spin-dependent metananoslots

In the last decade, the plasmonic vortex field has been studied extensively due to intriguing properties such as high field enhancement, optical singularity, and orbital angular momentum. In this work, we propose metananoslots that consist of paired orthogonal nanoslots arranged in an Archimedes spiral distribution. The metananoslots work as a plasmonic vortex lens that enables the synthesis of a highly tunable plasmonic vortex via the strong interaction between the illumination and the slots etched on the gold film. By adjusting the orientation of the orthogonal nanoslots pair, the metananoslots exhibit strong and controllable spin-dependent effects. The topological charge of the plasmonic vortex is found to be determined by both the incident spin and the geometrical topological charge of the metananoslots, making it suitable for applications such as optical manipulation, optical trapping, and optical data storage.

Ultracompact Plasmonic Meta-pixel for Arbitrary Polarization Detection

Polarization of light is a fundamental characteristic which represents the oscillation of electric fields in electromagnetic optics. The utilization of polarization gives opportunities to analyze the light-matter interaction or improve the performance of optical devices. Miniaturization of polarimeter consisting of bulky optical components is an important challenge for applying to integrated photonic chips or sensors. In this letter, we propose a compact plasmonic polarimeter with six different plasmonic lenses for detecting Stokes parameters in near field. Each plasmonic lens, which focuses plasmonic fields under the specific polarization incidence, can be designed by pairs of nanoslit with different configurations. We theoretically and numerically show that the intensities of plasmonic focal spots can derive full Stokes parameter. We expect that the proposed work can contribute to miniaturization and integration of polarimeter.

Inverted plasmonic lens design for nanometrology applications

Planar plasmonic lenses have attracted a great deal of interest over the last few years for their super-resolution focusing capabilites. These highly compact structures with dimensions of only a few micrometres allow for the focusing of light to sub-wavelength-sized spots with focal lengths reaching into the far-field. This offers opportunities for new methods in nanometrology; for example, applications in microscopic Mueller matrix ellipsometry setups. However, the conventional plasmonic lens is challenging to fabricate. We present a new design for plasmonic lenses, which is called the inverted plasmonic lens, to accommodate the lithographic fabrication process. In this contribution, we used numerical simulations based on the finite element method in combination with particle swarm optimization to determine ideal parameter ranges and tolerances for the design of inverted plasmonic lenses for different wavelengths in the visible and near-infrared domain and focal lengths between 5 µm and 1 mm.

Detecting polarization state of arbitrary polarized light by chiral plasmonic lenses with distributed nanoslits

We devise three chiral plasmonic lenses (PLs) with distributed nanoslits for detecting the complete state of polarization of an arbitrary polarized light. A double-ring arrayed nanocrosses-based plasmonic lens is proposed to focus the surface plasmon polaritons excited by the two circularly polarized components of the incident light, thus enabling the phase difference between the two components to be retrieved. Two Archimedes-spiral arrayed nanoslits-based PLs with opposite chirality are designed to detect the relative strength of the two components. The focusing properties of the three structures are theoretically derived and numerically investigated, demonstrating the effectiveness of the proposed method.