Selective trapping or rotation of isotropic dielectric microparticles by optical near field in a plasmonic archimedes spiral

Archimedes spiral beam: composite of a helical-axicon generated Bessel beam and a Gaussian beam

This paper introduces a structured beam with Archimedes spiral intensity distribution. The Archimedes spiral (AS) beam is the composite of a helical-axicon generated (HAG) Bessel beam and a Gaussian (GS) beam. We observed the spiral intensity patterns using computational holography, achieving the tuning over spiral arms number and spiral spacing. Analyzing the propagation dynamics of AS beams, we present that the spiral intensity will reverse beyond the maximum diffraction-free distance. Before and after the beam reverse, the spiral spacing remains constant, but the spiral direction is opposite. In addition, we obtain the Archimedes spiral equations to describe the spiral intensity patterns. Unlike the beams with Fermat and hyperbolic spiral patterns, the intensity distributions of AS beams are isometrically spiral. The isometric spiral intensity makes it possible to form particle isometric channels. AS beams have potential application prospects in particle manipulation, microscopic imaging, and laser processing.

Ultrafast Time Dynamics of Plasmonic Fractional Orbital Angular Momentum

The creation and manipulation of optical vortices, both in free space and in two-dimensional systems such as surface plasmon polaritons (SPPs), has attracted widespread attention in nano-optics due to their robust topological structure. Coupled with strong spatial confinement in the case of SPPs, these plasmonic vortices and their underlying orbital angular momentum (OAM) have promise in novel light-matter interactions on the nanoscale with applications ranging from on-chip particle manipulation to tailored control of plasmonic quasiparticles. Until now, predominantly integer OAM values have been investigated. Here, we measure and analyze the time evolution of fractional OAM SPPs using time-resolved two-photon photoemission electron microscopy and near-field optical microscopy. We experimentally show the field's complex rotational dynamics and observe the beating of integer OAM eigenmodes at fractional OAM excitations. With our ability to access the ultrafast time dynamics of the electric field, we can follow the buildup of the plasmonic fractional OAM during the interference of the converging surface plasmons. By adiabatically increasing the phase discontinuity at the excitation boundary, we track the total OAM, leading to plateaus around integer OAM values that arise from the interplay between intrinsic and extrinsic OAM.

Manipulating plasmonic vortex based on meta-atoms with four rectangular slits

In this paper, four rectangular slits with the same size and regular rotation angle are regarded as the meta-atom, arranged on circular contours, to create plasmonic vortex lenses (PVLs) solely based on the geometric phase. These PVLs can achieve the same purpose of exciting surface plasmon polariton (SPP) vortices with arbitrary combinations of topological charge (TC) when illuminated by circularly polarized (CP) light with different handedness as the traditional PVLs. Furthermore, they can generate SPP vortices with different TCs and specific constant or varying electric-field intensities when excited by linearly polarized (LP) light, which marks the first instance of this phenomenon solely through geometric phase manipulation. The TC can be dynamically altered by controlling the polarization order of the incident vector beam. These PVLs not only possess advantages in terms of device miniaturization and the creation of a more uniform vortex field, as compared to PVLs based on the transmission phase, but also offer a more straightforward design process in comparison to traditional structures that rely solely on the geometric phase.

High-efficiency plasmonic vortex generation with near-infrared bifunctional metasurfaces

Plasmonic vortices have shown a wide range of applications in on-chip photonics due to their fascinating properties of the orbital angular momenta (OAM) and phase singularity. However, conventional devices to generate them suffer from issues of low efficiencies and limited functionalities. Here, we establish a systematic scheme to construct high-efficiency bifunctional metasurfaces that can generate two plasmonic vortices exhibiting distinct topological charges, based on a series of reflective meta-atoms exhibiting tailored reflection-phases dictated by both resonant and geometric origins. As a benchmark test, we first construct a meta-coupler with meta-atoms exhibiting geometric phases only, and experimentally demonstrate that it can generate a pre-designed plasmonic vortex at the wavelength of 1064 nm with an efficiency of 27% (56% in simulation). Next, we design/fabricate two bifunctional metasurfaces with meta-atoms integrated with both resonant and geometric phases, and experimentally demonstrate that they can generate divergent (or focused) or convergent (or defocused) plasmonic vortices with district OAM as shined by circularly polarized light with opposite helicity at 1064 nm wavelength. Our work provides an efficient platform to generate plasmonic vortices as desired, which can find many applications in on-chip photonics.

Selective plasmonic trapping of nano-particles by Archimedes metalens

Optical tweezer is a non-invasive method for optical force tool applied in various fields like biology, physics, and lab on chip manipulation. The Archimedean helix shape is ideal for creating chiral nanostructures, and being able to generate plasmonic focused hotspot field for optical trapping. Here we design a metal disk with the Archimedean shape to own the ability of selective trapping nanoparticles based on the spin-orbit interactions with circularly polarized light. The plasmonic near field on the metalens can be designed by adjusting the geometric parameter flexibly. We numerically analyze the optimal size and screw pitch of the metal disk to realize the switch modulation of hotspot generation, and then demonstrate the novel switchable optical trapping ability in the view of optical force and potential well analysis under the circularly polarized light excitation by a 532 nm laser. The work shows significant potential for on-chip optical trapping in various fields.

Polarization-switchable focal vortex beam by an Archimedean array

Focal position control of vortex beams has tremendous applications in optical field. Herein, non-classical Archimedean arrays were proposed for optical devices with bifocal length and polarization-switchable focal length. The Archimedean arrays were constructed by rotational elliptical holes in a silver film, which were followed by two one-turned Archimedean trajectories. The elliptical holes in this Archimedean array provide the freedom of polarization control for the optical performance by their rotation status. The rotation of elliptical hole can provide additional phase to affect the shape of vortex beam (converged or diverged) under the illumination of circular polarization. The geometric phase of Archimedes trajectory will also determine the focal position of vortex beam. This Archimedean array can produce a converged vortex beam at the specific focal plane according to the handedness of the incident circular polarization and geometrical arrangement of array. The Archimedean array was also demonstrated by experiment and numerical simulation for its exotic optical performance.

Dynamic single microparticle manipulation in the far-field region using plasmonic vortex lens multiple arms with a circular groove

Recent development of particle manipulation has led to high demand for dynamic optical tweezer structures. However, confining and rotating a single microparticle in the far-field region with a uniform potential distribution remains a complicated task. A plasmonic vortex lens (PVL) has been proven to easily rotate the dielectric particle owing to its effect on orbital angular momentum (OAM). Here we propose and demonstrate PVL multiple arms with a circular groove (CG). The device consists of a multiple arm spiral slit that generates a plasmonic vortex (PV) and a circular groove to bring the PV from the surface to the far-field region. Numerical simulations are performed to calculate the intensity distribution of the primary ring, the optical force and potential. The primary ring size can be adjusted using different polarization directions. PVL 2-arms with a CG has primary ring sizes of 1082 nm under right-handed circular polarization (RCP) and 517 nm under left-handed circular polarization (LCP). Based on these primary ring sizes, a 1 μm polystyrene (PS) bead can be rotated under RCP with a minimum required power of 7.45 mW and trapped under LCP with a minimum required power of 11.84 mW. For PVL 4-arms with a CG under RCP illumination, we optimize the uniform potential distribution by carefully selecting the radius of the groove. Using a groove radius of 1050 nm, we obtain the potential difference between the smallest and largest depth along the x- and y-directions of only 70 k _B T/W with a minimum required power of 14.86 mW. The method and design discussed here offer an efficient way to manipulate microparticles for micro-rotors, cell dynamic analysis, etc.

Plasmonic vortices for tunable manipulation of target particles, using arrays of elliptical holes in a gold layer

Here, we numerically prove that light with linear polarization can be coupled to surface plasmon polaritons at an elliptical hole perforated in a gold layer to generate plasmonic vortex (PV). Benefiting from the smooth variation of the minor to major ellipse axes, a gradual variation in the phase profile of the generated PV is achieved. Regarding this, three types of independent arrays of elliptical holes are presented, which can produce uniform and high quality PVs with different topological charges at the center of the arrays. The first array can produce PV with topological charges of + 1 and - 1, depending on the polarization orientation of the incident light. In the second one, the topological charge of the PV can be switched between 0 and + 2, by switching the polarization direction of the incident light. In the third array, a robust PV with topological charge of + 1 is generated independent of possible tolerances in the polarization orientation. In order to use the generated PVs for plasmonic tweezing application, there are side fringes around the central vortex of the arrays that should be eliminated. To produce a single vortex, we propose metal-insulator-metal (MIM) structures, screening excessive fringes and allowing the central PVs to leak out. It is also demonstrated by simulation that target particles, such as gold and polystyrene spheres of subwavelength dimensions, can be efficiently manipulated by our MIM designs, suitable for different applications including local mixing, and applying switchable torque or force to target particles to explore their complete elastic characteristics.

Ultra-compact Archimedes spiral plasmonic lens with a circular groove for low power optical trapping in the far-field region

Particle levitation is crucial in optical trapping considering contamination and alteration of the character of the particle due to physical contact with the structure. A strong field gradient along the optical axis is required in this case. To manipulate the particle at a distance from the surface, we propose an Archimedes spiral plasmonic lens with a circular groove (CG-ASPL). The optical properties and parameters influencing the trapping performance of CG-ASPL are fully analyzed and discussed. By illuminating the structure with circular polarization and structure optimization, we can reduce the required optical power down to 2.4 mW for trapping particle of 1 µm in diameter with groove width and height of 100 and 125 nm, respectively. The particle can be stably trapped with trapping potential of 4138 kBT/W in the far-field region (1.1λ) owing to constructive interference of the scattered SPP waves. Furthermore, this structure is ultra-compact with a size of about 6.7 µm in diameter. We believe the results demonstrated in this work would be very useful for lab-on-a-chip applications and many others.

Arbitrary-Order and Multichannel Optical Vortices with Simultaneous Amplitude and Phase Modulation on Plasmonic Metasurfaces

The highly localized and uneven spatial distribution of the subwavelength light field in metal metasurfaces provides a promising means for the generation of optical vortices (OVs) with arbitrary topological charges. In this paper, a simple and reliable way for generating multichannel OVs on gold nanoporous metasurfaces is reported. The instantaneous field of arbitrary-order OVs can be regulated and concentrated on the same focal surface by adapting photonic spin-orbit interaction (SOI) and geometric phase. The focal ring energy distribution of OVs along the conical propagation path is accurately calculated, and the double phase of units induced by spin rotation is confirmed. Based on the parameter optimization of the nanohole arrangement, the simultaneous amplitude and phase modulation of multichannel OVs has been realized. Furthermore, the average multichannel signal-to-noise ratio exceeds 15 dB, which meets the requirements of high resolution and low crosstalk. Our study obtains broadband and efficient OVs, which can contribute to improving the capacity storage and security of optical information and possess great application prospects in beam shaping, optical tweezers, and communication coding.

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.

Ultrafast demagnetization in a ferrimagnet under electromagnetic field funneling

The quest to improve the density, speed and energy efficiency of magnetic memory storage has led to the exploration of new ways of optically manipulating magnetism at the ultrafast time scale, in particular in ferrimagnetic alloys. While all-optical magnetization switching is well-established on the femtosecond timescale, lateral nanoscale confinement and thus the potential significant reduction of the size of the magnetic element remains an outstanding challenge. Here we employ resonant electromagnetic energy funneling through plasmon nanoantennas to influence the demagnetization dynamics of a ferrimagnetic TbCo alloy thin film. We demonstrate how Ag nanoring-shaped antennas under resonant optical femtosecond pumping reduce the overall demagnetization in the underlying films up to three times compared to non-resonant illumination. We attribute such a substantial reduction to the nanoscale confinement of the demagnetization process. This is qualitatively supported by the electromagnetic simulations that strongly evidence the resonant optical energy-funneling to the nanoscale from the nanoantennas into the ferrimagnetic film. This observation is an important step for reaching deterministic ultrafast all-optical magnetization switching at the nanoscale in such systems, opening a route to develop nanoscale ultrafast magneto-optics.

Orbital angular momentum multiplication in plasmonic vortex cavities

Orbital angular momentum of light is a core feature in photonics. Its confinement to surfaces using plasmonics has unlocked many phenomena and potential applications. Here, we introduce the reflection from structural boundaries as a new degree of freedom to generate and control plasmonic orbital angular momentum. We experimentally demonstrate plasmonic vortex cavities, generating a succession of vortex pulses with increasing topological charge as a function of time. We track the spatiotemporal dynamics of these angularly decelerating plasmon pulse train within the cavities for over 300 femtoseconds using time-resolved photoemission electron microscopy, showing that the angular momentum grows by multiples of the chiral order of the cavity. The introduction of this degree of freedom to tame orbital angular momentum delivered by plasmonic vortices could miniaturize pump probe-like quantum initialization schemes, increase the torque exerted by plasmonic tweezers, and potentially achieve vortex lattice cavities with dynamically evolving topology.

Orbital-Angular-Momentum-Controlled Hybrid Nanowire Circuit

Plasmonic nanostructures can enable compact multiplexing of the orbital angular momentum (OAM) of light; however, strong dissipation of the highly localized OAM-distinct plasmonic fields in the near-field region hinders on-chip OAM transmission and processing. Superior transmission efficiency is offered by semiconductor nanowires sustaining highly confined optical modes, but only the polarization degree of freedom has been utilized for their selective excitation. Here we demonstrate that incident OAM beams can selectively excite single-crystalline cadmium sulfide (CdS) nanowires through coupling OAM-distinct plasmonic fields into nanowire waveguides for long-distance transportation. This allows us to build an OAM-controlled hybrid nanowire circuit for optical logic operations including AND and OR gates. In addition, this circuit enables the on-chip photoluminescence readout of OAM-encrypted information. Our results open exciting new avenues not only for nanowire photonics to develop OAM-controlled optical switches, logic gates, and modulators but also for OAM photonics to build ultracompact photonic circuits for information processing.

Novel silicon bipodal cylinders with controlled resonances and their use as beam steering metasurfaces

Metasurfaces have paved the way for high performance wavefront shaping and beam steering applications. Phase-gradient metasurfaces (PGM) are of high importance owing to the powerful and relatively systematic tool they offer for manipulating electromagnetic wave fronts and achieving various functionalities. Herein, we numerically present a novel unit cell known as bipodal cylinders (BPC), made of Silicon (Si) and placed on a Silicon dioxide (SiO2) substrate to be compatible with CMOS fabrication techniques and to avoid field leakage into a high index substrate. Owing to its geometrical structure, the BPC structure provides a promising unit cell for electromagnetic wave manipulation. We show that BPC offers a way to shift the electric dipole mode to a frequency higher than that of the magnetic dipole mode. We investigate the effect of varying different geometrical parameters on the performance of such unit cell. Building on that, a metasurface is then presented that can achieve efficient electromagnetic beam steering with high transmission of 0.84 and steering angle of 15.2°; with very good agreement with the theoretically predicted angle covering the whole phase range from 0 to 2[Formula: see text].

Incoherent Optical Tweezers on Black Titanium

Optical tweezers enable the manipulation of micro- and nanodielectric particles through entrapment using a tightly focused laser. Generally, optical trapping of submicron size particles requires high-intensity light in the order of MW/cm². Here, we demonstrate a technique of stable optical trapping of submicron polymeric beads on nanostructured titanium surfaces (black-Ti) without the use of lasers. Fluorescent polystyrene beads with a diameter d = 20-500 nm were successfully trapped on black-Ti by low-intensity focused illumination of incoherent light at λ = 370 m from a Hg lamp. Light intensity was 5.5 W/cm², corresponding to a reduced light intensity of 6 orders of magnitude. Upon switching off illumination, trapped particles were released from the illuminated area, indicating that trapping was optically driven and reversible. Such trapping behavior was not observed on nonstructured Ti surfaces or on nanostructured silicon surfaces. Thus, the Ti nanostructures were demonstrated to play a key role.

Plasmonic elliptical nanoholes for chiroptical analysis and enantioselective optical trapping

A simple yet effective achiral platform using elliptical nanoholes for chiroptical analysis is demonstrated. Under linearly polarized excitation, an elliptical nanohole in a thin gold film can generate a localized chiral optical field for chiroptical analysis and simultaneously serve as a near-field optical trap to capture dielectric and plasmonic nanospheres. In particular, the trapping potential is enantioselective for dielectric nanospheres, i.e., the hole traps or repels the dielectric nanoparticles depending on the sample chirality. For plasmonic nanospheres, the trapping potential well is much deeper than that for dielectric particles, rendering the enantioselectivity less pronounced. This platform is suitable for chiral analysis with nanoparticle-based solid-state extraction and pre-concentration. Compared to plasmonic chiroptical sensing using chiral structures or circularly polarized light, elliptical nanoholes are a simple and effective platform, which is expected to have a relatively low background because chiroptical noise from the structure or chiral species outside the nanohole is minimized. The use of linearly polarized excitation also makes the platform easily compatible with a commercial optical microscope.

Functional Meta Lenses for Compound Plasmonic Vortex Field Generation and Control

Surface plasmon polaritons carrying orbital angular momentum are of great fundamental and applied interest. However, common approaches for their generation are restricted to having a weak dependence on the properties of the plasmon-generating illumination, providing a limited degree of control over the amount of delivered orbital angular momentum. Here we experimentally show that by tailoring local and global geometries of vortex generators, a change in helicity of light imposes arbitrary large switching in the delivered plasmonic angular momentum. Using time-resolved photoemission electron microscopy we demonstrate pristine control over the generation and rotation direction of high-order plasmonic vortices. We generalize our approach to create complex topological fields and exemplify it by studying and controlling a "bright vortex", exhibiting the breakdown of a high-order vortex into a mosaic of unity-order vortices while maintaining the overall angular momentum density. Our results provide tools for plasmonic manipulation and could be utilized in lab-on-a-chip devices.

Optical Force-Induced Chemistry at Solution Surfaces

When an intense 1,064-nm continuous-wave laser is tightly focused at solution surfaces, it exerts an optical force on molecules, polymers, and nanoparticles (NPs). Initially, molecules and NPs are gathered into a single assembly inside the focus, and the laser is scattered and propagated through the assembly. The expanded laser further traps them at the edge of the assembly, producing a single assembly much larger than the focus along the surface. Amino acids and inorganic ionic compounds undergo crystallization and crystal growth, polystyrene NPs form periodic arrays and disklike structures with concentric circles or hexagonal packing, and Au NPs demonstrate assembling and swarming, in which the NPs fluctuate like a group of bees. These phenomena that depend on laser polarization are called optically evolved assembling at solution surfaces, and their dynamics and mechanisms are elucidated in this review. As a promising application in materials science, the optical trapping assembly of lead halide perovskites, supramolecules, and aggregation-induced emission enhancement-active molecules is demonstrated and future directions for fundamental study are discussed.

Review of optical sensing and manipulation of chiral molecules and nanostructures with the focus on plasmonic enhancements [Invited]

Chiral molecules are ubiquitous in nature; many important synthetic chemicals and drugs are chiral. Detecting chiral molecules and separating the enantiomers is difficult because their physiochemical properties can be very similar. Here we review the optical approaches that are emerging for detecting and manipulating chiral molecules and chiral nanostructures. Our review focuses on the methods that have used plasmonics to enhance the chiroptical response. We also review the fabrication and assembly of (dynamic) chiral plasmonic nanosystems in this context.

Plasmonic tweezers: for nanoscale optical trapping and beyond

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Characterisation and Manipulation of Polarisation Response in Plasmonic and Magneto-Plasmonic Nanostructures and Metamaterials

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

α spiral nanoslit and the higher order plasmonic vortex generation

In view of the conciseness of a spiral nanoslit and the limited order of the generated vortex, a kind of nanometer spiral, named α spirals, is proposed to generate a higher order plasmonic vortex. Theoretical analysis provides the basis for the advancement of an α spiral. The proposed spiral can generate the plasmonic vortex and the extreme order of the generated vortex depends on parameter α. The numerical simulations express the valid region of the plasmonic vortex generated by the α spiral. Discussions about the validity range of the α spiral nanoslit and the influence of the film material are beneficial to generate a high order vortex. This work builds a platform for the generation of the higher order plasmonic vortex using the simple spiral nanostructure and it can extend the potential applications of higher order plasmonic vortices.

Dynamically Modulating Plasmonic Field by Tuning the Spatial Frequency of Excitation Light

Based on the Fourier transform (FT) of surface plasmon polaritons (SPPs), the relation between the displacement of the plasmonic field and the spatial frequency of the excitation light is theoretically established. The SPPs’ field shifts transversally or longitudinally when the spatial frequency components fx or fy are correspondingly changed. The SPPs’ focus and vortex field can be precisely located at the desired position by choosing the appropriate spatial frequency. Simulation results are in good agreement with the theoretical analyses. Dynamically tailoring the plasmonic field based on the spatial frequency modulation can find potential applications in microparticle manipulation and angular multiplexed SPP focusing and propagation.

Two-state switchable plasmonic tweezers for dynamic manipulation of nano-objects

In this work, we present a plasmonic platform capable of trapping nano-objects in two different spatial configurations. The switch between the two trapping states, localized on the tip and on the outer wall of a vertical gold nanochannel, can be activated by varying the focusing position of the excitation laser along the main axis of the nanotube. We show that the switching of the trapping site is induced by changes in the distribution of the electromagnetic field and of the trapping force. The "inner" and "outer" trapping states are characterized by a static and a dynamic behavior respectively, and their stiffness is measured by analyzing the positions of the trapped specimens as a function of time. In addition, we demonstrate that the stiffness of the static state is high enough to trap particles with diameter as small as 40 nm. These results show a simple, controllable way to generate a switchable two-state trapping regime, which could be used as a model for the study of dynamic trapping or as a mechanism for the development of nanofluidic devices.

Chiral Second-Harmonic Generation from Monolayer WS2/Aluminum Plasmonic Vortex Metalens

Two-dimensional spiral plasmonic structures have emerged as a versatile approach to generate near-field vortex fields with tunable topological charges. We demonstrate here a far-field approach to observe the chiral second-harmonic generation (SHG) at designated visible wavelengths from a single plasmonic vortex metalens. This metalens comprises an Archimedean spiral slit fabricated on atomically flat aluminum epitaxial film, which allows for precise tuning of plasmonic resonances and subsequent transfer of two-dimensional materials on top of the spiral slit. The nonlinear optical measurements show a giant SHG circular dichroism. Furthermore, we have achieved an enhanced chiral SHG conversion efficiency (about an order of magnitude greater than the bare aluminum lens) from monolayer tungsten disulfide (WS₂)/aluminum metalens, which is designed at the C-exciton resonance of WS₂. Since the C-exciton is not a valley exciton, the enhanced chiral SHG in this hybrid system originates from the plasmonic vortex field-enhanced SHG under the optical spin-orbit interaction.

Plasmonic Manipulation of DNA using a Combination of Optical and Thermophoretic Forces: Separation of Different-Sized DNA from Mixture Solution

We demonstrate the size-dependent separation and permanent immobilization of DNA on plasmonic substrates by means of plasmonic optical tweezers. We found that a gold nanopyramidal dimer array enhanced the optical force exerted on the DNA, leading to permanent immobilization of the DNA on the plasmonic substrate. The immobilization was realized by a combination of the plasmon-enhanced optical force and the thermophoretic force induced by a photothermal effect of the plasmons. In this study, we applied this phenomenon to the separation and fixation of size-different DNA. During plasmon excitation, DNA strands of different sizes became permanently immobilized on the plasmonic substrate forming micro-rings of DNA. The diameter of the ring was larger for longer DNA (in base pairs). When we used plasmonic optical tweezers to trap DNA of two different lengths dissolved in solution (φx DNA (5.4 kbp) and λ-DNA (48.5 kbp), or φx DNA and T4 DNA (166 kbp)), the DNA were immobilized, creating a double micro-ring pattern. The DNA were optically separated and immobilized in the double ring, with the shorter sized DNA and the larger one forming the smaller and larger rings, respectively. This phenomenon can be quantitatively explained as being due to a combination of the plasmon-enhanced optical force and the thermophoretic force. Our plasmonic optical tweezers open up a new avenue for the separation and immobilization of DNA, foreshadowing the emergence of optical separation and fixation of biomolecules such as proteins and other ncuelic acids.

Optical field tuning of localized plasmon modes in Ag microcrystals at the nanofemto scale

Nanoscale plasmonic field enhancement at sub-wavelength metallic particles is crucial for surface sensitive spectroscopy, ultrafast microscopy, and nanoscale energy transduction. Here, we demonstrate control of the spatial distribution of localized surface plasmon modes at sub-optical-wavelength crystalline silver (Ag) micropyramids grown on a Si(001) surface. We employ multiphoton photoemission electron microscopy (mP-PEEM) to image how the plasmonic field distributions vary with the photon energy, light polarization, and phase in coherent two-pulse excitation. For photon energy hυ > 2.0 eV, the mP-PEEM images show single photoemission locus, which splits into a dipolar pattern that straddles the Ag crystal at a lower energy. We attribute the variation to the migration of plasmon resonances from the Ag/vacuum to the Ag/Si interfaces by choice of the photon energy. Furthermore, the dipolar response of the Ag/Si interface follows the polarization state of light: for linearly polarized excitations, the plasmon dipole follows the in-plane electric field vector, while for circularly polarized excitations, it tilts in the direction of the handedness due to the conversion of spin angular momentum of light into orbital angular momentum of the plasmons excited in the sample. Finally, we show the coherent control of the spatial plasmon distribution by exciting the sample with two identical circularly polarized light pulses with delay defined with attosecond precision. The near field distribution wobbles at the pyramid base as the pump-probe delay is advanced due to interferences among the contributing fields. We illustrate how the frequency, polarization, and pulse structure can be used to design and control plasmon fields on the nanofemto scale for applications in chemistry and physics.

Plasmonic Nanolenses Produced by Cylindrical Vector Beam Printing for Sensing Applications

Interaction of complex-shaped light fields with specially designed plasmonic nanostructures gives rise to various intriguing optical phenomena like nanofocusing of surface waves, enhanced nonlinear optical response and appearance of specific low-loss modes, which can not be excited with ordinary Gaussian-shaped beams. Related complex-shaped nanostructures are commonly fabricated using rather expensive and time-consuming electron- and ion-beam lithography techniques limiting real-life applicability of such an approach. In this respect, plasmonic nanostructures designed to benefit from their excitation with complex-shaped light fields, as well as high-performing techniques allowing inexpensive and flexible fabrication of such structures, are of great demand for various applications. Here, we demonstrate a simple direct maskless laser-based approach for fabrication of back-reflector-coupled plasmonic nanorings arrays. The approach is based on delicate ablation of an upper metal film of a metal-insulator-metal (MIM) sandwich with donut-shaped laser pulses followed by argon ion-beam polishing. After being excited with a radially polarized beam, the MIM configuration of the nanorings permitted to realize efficient nanofocusing of constructively interfering plasmonic waves excited in the gap area between the nanoring and back-reflector mirror. For optimized MIM geometry excited by radially polarized CVB, substantial enhancement of the electromagnetic near-fields at the center of the ring within a single focal spot with the size of 0.37λ2 can be achieved, which is confirmed by Finite Difference Time Domain calculations, as well as by detection of 100-fold enhanced photoluminescent signal from adsorbed organic dye molecules. Simple large-scale and cost-efficient fabrication procedure offering also a freedom in the choice of materials to design MIM structures, along with remarkable optical and plasmonic characteristics of the produced structures make them promising for realization of various nanophotonic and biosensing platforms that utilize cylindrical vector beam as a pump source.

Dynamical Manipulation of Surface Plasmon Polaritons

As the fundamental and promising branch of nanophotonics, surface plasmon polaritons (SPP) with the ability of manipulating the electromagnetic field on the subwavelength scale are of interest to a wide spectrum of scientists. Composed of metallic or dielectric structures whose shape and position are carefully engineered on the metal surface, traditional SPP devices are generally static and lack tunability. Dynamical manipulation of SPP is meaningful in both fundamental research and practical applications. In this article, the achievements in dynamical SPP excitation, SPP focusing, SPP vortex, and SPP nondiffracting beams are presented. The mechanisms of dynamical SPP devices are revealed and compared, and future perspectives are discussed.

Compensation of spin-orbit interaction using the geometric phase of distributed nanoslits for polarization-independent plasmonic vortex generation

A metasurface is a planar optical device that controls the phase, amplitude, and polarization of light through subwavelength-scale unit elements, called meta-atom. The tunability of plasmonic vortex lens (PVL) which generates surface plasmon polaritons (SPPs) carrying orbital angular momentum can be improved by using meta-atom. However, conventional PVLs exhibit nonuniform field profiles according to the incident polarization states owing to the spin-orbital interaction (SOI) effect observed during SPP excitation. This paper describes a method of compensating for SOI of PVL by using the geometric phase of distributed nanoslits in a gold film. By designing the orientation angles of slit pairs, the anti-phase of the SOI effect can be generated for compensatory effect. In addition, polarization-independent PVLs are designed by applying a detour phase based on the position of the slit pairs. PVLs for center-, off-center-, and multiple-focus cases are demonstrated and measured via a near-field scanning microscope.

Solenoidal optical forces from a plasmonic Archimedean spiral

The optical forces generated by a right-handed plasmonic Archimedean spiral (PAS) have been mapped and analyzed. By changing the handedness of the circularly polarized excitation, the structure can switch from a trapping force profile to a rotating force profile. The Helmholtz-Hodge decomposition method has been used to separate the solenoidal component and the conservative component of the force and quantify their relative magnitude. It is shown that the for right-hand circularly polarized excitation, the PAS creates a significant amount of solenoidal forces. Using the decomposed force components, an intuitive explanation of the motion of micro- and nanoparticles in the force field is presented. Vector field topology is used to visualize the force components. The analysis is found to be consistent with numerical and experimental results. Due to the intuitive nature of the analysis, it can be used in the initial design process of complex laboratory-on-a-chip systems where a rigorous analysis is computationally expensive.

Spin-Orbit Interaction of Light in Plasmonic Lattices

In the past decade, the spin-orbit interaction (SOI) of light has been a driving force in the design of metamaterials, metasurfaces, and schemes for light-matter interaction. A hallmark of the spin-orbit interaction of light is the spin-based plasmonic effect, converting spin angular momentum of propagating light to near-field orbital angular momentum. Although this effect has been thoroughly investigated in circular symmetry, it has yet to be characterized in a noncircular geometry, where whirling, periodic plasmonic fields are expected. Using phase-resolved near-field microscopy, we experimentally demonstrate the SOI of circularly polarized light in nanostructures possessing dihedral symmetry. We show how interaction with hexagonal slits results in four topologically different plasmonic lattices, controlled by engineered boundary conditions, and reveal a cyclic nature of the spin-based plasmonic effect which does not exist for circular symmetry. Finally, we calculate the optical forces generated by the plasmonic lattices, predicting that light with mere spin angular momentum can exert torque on a multitude of particles in an ordered fashion to form an optical nanomotor array. Our findings may be of use in both biology and chemistry, as a means for simultaneous trapping, manipulation, and excitation of multiple objects, controlled by the polarization of light.

Near-field optical trapping in a non-conservative force field

The force-field generated by a near-field optical trap is analyzed. A C-shaped engraving on a gold film is considered as the trap. By separating out the conservative component and the solenoidal component of the force-field using Helmholtz-Hodge decomposition, it was found that the force is non-conservative. Conventional method of calculating the optical potential from the force-field is shown to be inaccurate when the trapping force is not purely conservative. An alternative method is presented to accurately estimate the potential. The positional statistics of a trapped nanoparticle in this non-conservative field is calculated. A model is proposed that relates the position distribution to the conservative component of the force. The model is found to be consistent with numerical and experimental results. In order to show the generality of the approach, the same analysis is repeated for a plasmonic trap consisting of a gold nanopillar. Similar consistency is observed for this structure as well.

Angular-momentum nanometrology in an ultrathin plasmonic topological insulator film

Complementary metal–oxide–semiconductor (CMOS) technology has provided a highly sensitive detection platform for high-resolution optical imaging, sensing and metrology. Although the detection of optical beams carrying angular momentum have been explored with nanophotonic methods, the metrology of optical angular momentum has been limited to bulk optics. We demonstrate angular-momentum nanometrology through the spatial displacement engineering of plasmonic angular momentum modes in a CMOS-compatible plasmonic topological insulator material. The generation and propagation of surface plasmon polaritons on the surface of an ultrathin topological insulator Sb₂Te₃ film with a thickness of 100 nm is confirmed, exhibiting plasmonic figures of merit superior to noble metal plasmonics in the ultraviolet-visible frequency range. Angular-momentum nanometrology with a low crosstalk of less than −20 dB is achieved. This compact high-precision angular-momentum nanometrology opens an unprecedented opportunity for on-chip manipulation of optical angular momentum for high-capacity information processing, ultrasensitive molecular sensing, and ultracompact multi-functional optoelectronic devices.

Multiplexed vortex light beams are promising for optical communications, but efficient mode sorting is so far limited to bulk optics. Here the authors develop a scalable vortex beam sorter that uses a plasmonic topological insulator structure to spatially separate the modes to resolve them on a standard CMOS detector.

Nanoradiator-Mediated Deterministic Opto-Thermoelectric Manipulation

Optical manipulation of colloidal nanoparticles and molecules is significant in numerous fields. Opto-thermoelectric nanotweezers exploiting multiple coupling among light, heat, and electric fields enables the low-power optical trapping of nanoparticles on a plasmonic substrate. However, the management of light-to-heat conversion for the versatile and precise manipulation of nanoparticles is still elusive. Herein, we explore the opto-thermoelectric trapping at plasmonic antennas that serve as optothermal nanoradiators to achieve the low-power (∼0.08 mW/μm2) and deterministic manipulation of nanoparticles. Specifically, precise optical manipulation of nanoparticles is achieved via optical control of the subwavelength thermal hot spots. We employ a femtosecond laser beam to further improve the heat localization and the precise trapping of single ∼30 nm semiconductor quantum dots at the antennas where the plasmon-exciton coupling can be tuned. With its low-power, precise, and versatile particle control, the opto-thermoelectric manipulation can have applications in photonics, life sciences, and colloidal sciences.

Efficient modulation of subwavelength focusing via meta-aperture-based plasmonic lens for multifunction applications

Subwavelength focusing is crucial for many applications in photonics including super-resolution micro/nanoscopy, nanolithography, and optical trapping. However, most nanostructures exhibit poor ability to modulate focusing spot, which makes them hard to achieve ultra-small resolution. Here, we propose three kinds of plasmonic lens (PL) by utilizing different meta-aperture designs for efficient subwavelength focusing modulation. The shape of nanoaperture strongly influences the diffraction properties. Spatial modulation of focusing spot by employing a circular array of proposed nanoapertures is explored. The best focusing performance among these PLs is the design of T-shape nanoaperture, which has great resolution achieving ultra-small focusing spot of 0.14 λ² and 0.20 λ² (λ = 633 nm) for simulation and experiment respectively, better than lots of focusing devices especially by using linear polarization. Multiple-object trapping can be realized by using T-shape nanoaperture-based PL. Our designed PLs with different nanoapertures demonstrate the capability to broaden and integrate different functionalities for on-chip nanotechnologies development.

3D Optical Vortex Trapping of Plasmonic Nanostructure

3D optical vortex trapping upon a polystyrene nanoparticle (NP) by a 1D gold dimer array is studied theoretically. The optical force field shows that the trapping mode can be contact or non-contact. For the former, the NP is attracted toward a corresponding dimer. For the latter, it is trapped toward a stagnation point of zero force with a 3D spiral trajectory, revealing optical vortex. Additionally the optical torque causes the NP to transversely spin, even though the system is irradiated by a linearly polarized light. The transverse spin-orbit interaction is manifested from the opposite helicities of the spin and spiral orbit. Along with the growth and decline of optical vortices the trapped NP performs a step-like motion, as the array continuously moves. Our results, in agreement with the previous experiment, identify the role of optical vortex in the near-field trapping of plasmonic nanostructure.

Nanoscale chiral surface relief of azo-polymers with nearfield OAM light

An optical vortex with orbital angular momentum (OAM) can be used to induce microscale chiral structures in various materials. Such chiral structures enable the generation of a nearfield vortex, i.e. nearfield OAM light on a sub-wavelength scale, thereby leading to further nanoscale mass-transport. We report on the formation of a nanoscale chiral surface relief in azo-polymers due to nearfield OAM light. The resulting nanoscale chiral relief exhibits a diameter of ca. 400 nm, which corresponds to less than 1/5-1/6th of the original chiral structure (ca. 2.1 µm). Such a nanoscale chiral surface relief is established by the simple irradiation of uniform visible plane-wave light with an intensity of <500 mW/cm².

Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture Using 10 nm Gap

We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single coaxial apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with a laser power as low as 4.7 mW. Furthermore, the resonant coaxial nanoaperture enables real-time label-free detection of the trapping events via simple transmission measurements. Our fabrication technique is scalable and reproducible, since the critical nanogap dimension is defined by atomic layer deposition. Thus our platform shows significant potential to push the limit of optical trapping technologies.

Dynamic tailoring of surface plasmon polaritons through incident angle modulation

Dynamic tailoring of the propagating surface plasmon polaritons (SPPs) through incident angle modulation is proposed and numerically demonstrated. The generation and tailoring mechanism of the SPPs are discussed. The relationship formula between the incident angle and the generated SPP wave vector direction is theoretically derived. The correctness of the formula is verified with three different approaches using finite difference time domain method. Using this formula, the generated SPP wave vector direction can be precisely modulated by changing the incident angle. The precise modulation results of two dimensional Bessel-like SPP beam and SPP bottle beam array are given. The results can deepen the understanding of the generation and modulation mechanism of the SPPs.

Holographic Plasmonic Nanotweezers for Dynamic Trapping and Manipulation

We demonstrate dynamic trapping and manipulation of nanoparticles with plasmonic holograms. By tailoring the illumination pattern of an incident light beam with a computer-controlled spatial light modulator, constructive and destructive interference of plasmon waves create a focused hotspot that can be moved across a surface. Specifically, a computer-generated hologram illuminating the perimeter of a silver Bull's Eye nanostructure generates surface plasmons that propagate toward the center. Shifting the phase of the plasmon waves as a function of space gives complete control over the location of the focus. We show that 200 nm diameter nanoparticles trapped in this focus can be moved in arbitrary patterns. This allows, for example, circular motion with linearly polarized light. These results show the versatility of holographically generated surface plasmon waves for advanced trapping and manipulation of nanoparticles.

Radially polarized plasmonic vector vortex generated by a metasurface spiral in gold film

Vector vortices with spatially varying polarization are interesting phenomena and have motivated many recent studies. A vector vortex in the wavefield of a surface plasmon polariton (SPP) may be extended to the sub-wavelength scale, which would be more significant. However, the formation of vector vortices requires the polarization state to possess components parallel to the surface of metal films. In this study, we generated radially polarized vector plasmonic vortices using the metasurface spiral of orthogonal nanoslit pairs. We theoretically derived the x and y component expressions in the central point area of the spiral and obtained a doughnut-shaped intensity distribution with radial polarization. The Jones matrix of the metasurface spiral was generated to describe the polarization characteristics. The results were validated by performing finite-difference time-domain simulations. In addition, we used a Mach-Zehnder interferometer system to extract the intensity and phase distributions of different components of the SPP field. The experimental doughnut-shaped radially polarized vector vortex was consistent with the theoretical and simulated results.

Review of the Functions of Archimedes' Spiral Metallic Nanostructures

Here, we have reviewed some typical plasmonic structures based on Archimedes' spiral (AS) architectures, which can produce polarization-sensitive focusing phenomenon and generate plasmonic vortices (PVs) carrying controllable orbital angular momentum (OAM) because of the relation between the incident polarized states and the chiralities of the spiral structures. These features can be used to analyze different circular polarization states, which has been one of the rapidly developing researching topics in nanophotonics in recent years. Many investigations demonstrate that the multifunctional spiral-based plasmonic structures are excellent choices for chiral selection and generating the transmitted field with well-defined OAM. The circular polarization extinction ratio, as an evaluation criterion for the polarization selectivity of a designed structure, could be effectively improved by properly modulating the parameters of spiral structures. Such functional spiral plasmonic nanostructures are promising for applications in analyzing circular polarization light, full Stokes vector polarimetric sensors, near-field imaging, and so on.

Reflective metalens with sub-diffraction-limited and multifunctional focusing

We propose an ultra-thin planar reflective metalens with sub-diffraction-limited and multifunctional focusing. Based on the equal optical path principle, the specific phase distributions for multifunction focusing are derived. Following the formulas, on-center focusing with the characteristics of sub-diffraction-limited, high focusing efficiency (85%) and broadband focusing is investigated in detail. To demonstrate the flexibility of the reflective metalens, off-center and dual spots focusing (at the horizontal and longitudinal directions) are demonstrated. Note that all these focusings are sub-diffraction-limited due to the evanescent-filed enhancement mechanism in our elaborately designed structure. The designed reflective metalens will find important applications in super-resolution imaging, microscopes, and spectroscopic designs.

Plasmonic in-plane total internal reflection: azimuthal polarized beam focusing and application

Due to the characteristic of surface plasmon polaritons (SPP) excitation, radial polarized beams and circular polarized beams are widely used for plasmonic lens and plasmonic near field focusing. In this paper, a plasmonic lens based on in-plane total internal reflection (TIR) scheme is proposed and numerically demonstrated to achieve the simultaneous nanofocusing of azimuthal and radial polarized beams. By means of the in-plane TIR mechanism, the operation bandwidth of lens ranges from visible light to mid-infrared. The proposed structure has been utilized in the design of a plasmonic liquid refractive index sensor and is expected to find potential applications in near-field optical energy focusing, near-field imaging and sensing.

Polarization-independent metalens constructed of antennas without rotational invariance

We propose a novel approach to designing an ultrathin polarization-independent metalens (PIM) by utilizing antennas without rotational invariance. Two arrays of nanoblocks are elaborately designed to form the super cell of the PIM, which are capable of focusing right-handed circularly polarized and left-handed circularly polarized lights. With such a strategy, the PIM is able to achieve polarization-independent focusing, since the light with any polarization can be treated as a combination of the two orthogonal ones. A theoretical analysis based on the Jones vector is proposed to detailedly explore the underlying physics. The polarization-independent characteristic of the designed PIM is also demonstrated by utilizing finite difference time domain simulations. Moreover, polarization-independent focusing can be achieved within a wavelength range of 400 nm. These results can deepen our understanding of polarization-independent focusing and provide a new method for designing ultrathin polarization-independent devices.