Beaming of Helical Light from Plasmonic Vortices via Adiabatically Tapered Nanotip

Tailored Fabrication of 3D Nanopores Made of Dielectric Oxides for Multiple Nanoscale Applications

Solid-state nanopores are a key platform for single-molecule detection and analysis that allow engineering of their properties by controlling size, shape, and chemical functionalization. However, approaches relying on polymers have limits for what concerns hardness, robustness, durability, and refractive index. Nanopores made of oxides with high dielectric constant would overcome such limits and have the potential to extend the suitability of solid-state nanopores toward optoelectronic technologies. Here, we present a versatile method to fabricate three-dimensional nanopores made of different dielectric oxides with convex, straight, and concave shapes and demonstrate their functionality in a series of technologies and applications such as ionic nanochannels, ionic current rectification, memristors, and DNA sensing. Our experimental data are supported by numerical simulations that showcase the effect of different shapes and oxide materials. This approach toward robust and tunable solid-state nanopores can be extended to other 3D shapes and a variety of dielectrics.

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.

Experimental demonstration and investigation of vortex circular Pearcey beams in a dynamically shaped fashion

Optical vortex, typically characterized by a helical phase front, results in a possession of orbital angular momentum. In recent years, teleportation of the vortex mode using novel beams with peculiar features has gained great interest. Here, we experimentally demonstrate the propagation dynamics for a new class of the auto-focusing vortex circular Pearcey beam (VCPB), which is theoretically described by delivering the coaxial or off-axial spiral phases into the circular Pearcey beam (CPB), forming the crescent or bottle-like focal structure with self-rotation. Notably, such a hybrid beam with various types is experimentally obtained through a digital micromirror device (DMD) with the binary amplitude holography, and this DMD-based modulation scheme combined with controllable vortex modes enables dynamic switching among the VCPBs. We also measure the topological phase by interferometry and we explain the beam property on the basis of Poynting vector, showing a good agreement with the simulations. Further, the number, location and mode of embedded vortices could offer multiple dimensions of flexibility for target beam modulation, thus the experimentally controllable VCPBs will bring potential to high-speed optical communications and particle manipulations that require dynamic shaping.

Directional Plasmonic Excitation by Helical Nanotips

The phenomenon of coupling between light and surface plasmon polaritons requires specific momentum matching conditions. In the case of a single scattering object on a metallic surface, such as a nanoparticle or a nanohole, the coupling between a broadband effect, i.e., scattering, and a discrete one, such as surface plasmon excitation, leads to Fano-like resonance lineshapes. The necessary phase matching requirements can be used to engineer the light–plasmon coupling and to achieve a directional plasmonic excitation. Here, we investigate this effect by using a chiral nanotip to excite surface plasmons with a strong spin-dependent azimuthal variation. This effect can be described by a Fano-like interference with a complex coupling factor that can be modified thanks to a symmetry breaking of the nanostructure.

Plasmonic Helical Nanoantenna As a Converter between Longitudinal Fields and Circularly Polarized Waves

A wide variety of optical applications and techniques require control of light polarization. So far, the manipulation of light polarization relies on components capable of interchanging two polarization states of the transverse field of a propagating wave (e.g., linear to circular polarizations, and vice versa). Here, we demonstrate that an individual helical nanoantenna is capable of locally converting longitudinally oriented confined near-fields into a circularly polarized freely propagating wave, and vice versa. To this end, the nanoantenna is coupled to cylindrical surface plasmons bound to the top interface of a thin gold layer. Helices of constant and varying pitch lengths are experimentally investigated. The reciprocal conversion of an incoming circularly wave into diverging cylindrical surface plasmons is demonstrated as well. Interconnecting circularly polarized optical waves (carrying spin angular momentum) and longitudinal near-fields provides a new degree of freedom in light polarization control.

Plasmonic Emission of Bullseye Nanoemitters on Bi2Te3 Nanoflakes

Topological insulators, such as Bi₂Te₃, have been confirmed to exhibit plasmon radiation over the entire visible spectral range. Herein, we fabricate bullseye nanoemitters, consisting of a central disk and concentric gratings, on the Bi₂Te₃ nanoflake. Due to the existence of edge plasmon modes, Bi₂Te₃ bullseye nanostructures are possible to converge light towards the central disk. Taking advantage of the excellent spatial resolution of cathodoluminescence (CL) characterization, it has been observed that plasmonic behaviors depend on the excitation location. A stronger plasmonic intensity and a wider CL spectral linewidth can be obtained at the edge of the central disk. In order to further improve the focusing ability, a cylindrical Pt nanostructure has been deposited on the central disk. Additionally, the finite element simulation indicates that the electric-field enhancement originates from the coupling process between the plasmonic emission from the Bi₂Te₃ bullseye and the Pt nanostructure. Finally, we find that enhancement efficiency depends on the thickness of the Pt nanostructure.

Particle trapping and beaming using a 3D nanotip excited with a plasmonic vortex

Recent advances in nanotechnology have prompted the need for tools to accurately and noninvasively manipulate individual nano-objects. Among the possible strategies, optical forces have been widely used to enable nano-optical tweezers capable of trapping or moving a specimen with unprecedented accuracy. Here, we propose an architecture consisting of a nanotip excited with a plasmonic vortex enabling effective dynamic control of nanoparticles in three dimensions. The structure illuminated by a beam with angular momentum can generate an optical field that can be used to manipulate single dielectric nanoparticles. We demonstrate that it is possible to stably trap or push the particle from specific points, thus enabling a new, to the best of our knowledge, platform for nanoparticle manipulation.

Spin-locking metasurface for surface plasmon routing

Nanophotonic circuitry requires an ability to externally control and analyze optical signals tightly confined in subwavelength volumes. Various schemes of surface plasmon (SP) routing have been presented using active and passive metasurfaces. One of the most appealing approaches is the use of plasmonic spin-orbit interaction where the incident light spin state is efficiently coupled to an orbital degree of freedom of the surface wave. Recently, a major attention has been drawn to an additional plasmonic degree of freedom - the transverse spin and some application for near-field plasmonic manipulations have been presented. Here we propose a spin-locking metasurface incorporating a transverse spin of the SP wave to selectively route the near-field beams. Owing to the combination of the oblique incidence of circularly polarized light with the accurately designed momentum matching of the grating we achieve a precise directional control over the plasmonic distributions. The experimental verification of the directional launching is performed by a time-resolved leakage radiation measurements allowing one to visualize the shape and the dynamics of the excited beam.

Site-Selective Integration of MoS2 Flakes on Nanopores by Means of Electrophoretic Deposition

Here, we propose an easy method for site-selective deposition of two-dimensional (2D) material flakes onto nanoholes by means of electrophoretic deposition. This method can be applied to both simple flat nanostructures and complex three-dimensional structures incorporating nanoholes. The deposition method is here used for the decoration of large ordered arrays of plasmonic structures with either a single or few layers of MoS₂. In principle, the plasmonic field generated by the nanohole can significantly interact with the 2D layer leading to enhanced light-material interaction. This makes our platform an ideal system for hybrid 2D material/plasmonic investigations. The engineered deposition of 2D materials on plasmonic nanostructures is useful for several important applications such as enhanced light emission, strong coupling, hot-electron generation, and 2D material sensors.

Visible-broadband Localized Vector Vortex Beam Generator with a Multi-structure-composited Meta-surface

We demonstrate a vortex beam generator meta-surface that consists of silver structures and graphene layers. The miniature material is just a few microns in size and the working part is only a few hundred nanometers thick. With the incidence of the linearly polarized beam, the meta-surface generates high-localized vector vortex beam with a high proportion of the longitudinal component. Being compared with the constituent part of the meta-surface, the multi-structure-combined meta-surface increases the localization by 250% and the longitudinal component proportion by 200%. Moreover, the above artificial material can generate vortex beams in broadband within the visible light range. These novel optical properties have the potential to improve the precision and sensitivity of nanoparticle manipulation. The study serves as a foundation in optical miniaturization and integration, nanoparticle manipulation, high-efficiency optical and quantum communication, and light-driven micro-tools.

Subwavelength polarization optics via individual and coupled helical traveling-wave nanoantennas

Light polarization control is a key factor in modern photonics. Recent advances in surface plasmon manipulation have introduced the prospect of more compact and more efficient devices for this purpose. However, the current plasmonic-based polarization optics remain much larger than the wavelength of light, which limits the design degrees of freedom. Here, we present a plasmonic traveling-wave nanoantenna using a gold-coated helical carbon nanowire end-fired with a dipolar aperture nanoantenna. Our nonresonant helical nanoantenna enables tunable polarization control by swirling surface plasmons on the subwavelength scale and taking advantage of the optical spin-orbit interaction. Four closely packed helical traveling-wave nanoantennas (HTNs) are demonstrated to locally convert an incoming light beam into four beams of tunable polarizations and intensities, with the ability to impart different polarization states to the output beams in a controllable way. Moreover, by near-field coupling four HTNs of opposite handedness, we demonstrate a subwavelength waveplate-like structure providing a degree of freedom in polarization control that is unachievable with ordinary polarization optics and current metamaterials.

Hybrid plasmonic nanostructures based on controlled integration of MoS2 flakes on metallic nanoholes

Here, we propose an easy and robust strategy for the versatile preparation of hybrid plasmonic nanopores by means of controlled deposition of single flakes of MoS2 directly on top of metallic holes. The device is realized on silicon nitride membranes and can be further refined by TEM or FIB milling to achieve the passing of molecules or nanometric particles through a pore. Importantly, we show that the plasmonic enhancement provided by the nanohole is strongly accumulated in the 2D nanopore, thus representing an ideal system for single-molecule sensing and sequencing in a flow-through configuration. Here, we also demonstrate that the prepared 2D material can be decorated with metallic nanoparticles that can couple their resonance with the nanopore resonance to further enhance the electromagnetic field confinement at the nanoscale level. This method can be applied to any gold nanopore with a high level of reproducibility and parallelization; hence, it can pave the way to the next generation of solid-state nanopores with plasmonic functionalities. Moreover, the controlled/ordered integration of 2D materials on plasmonic nanostructures opens a pathway towards new investigation of the following: enhanced light emission; strong coupling from plasmonic hybrid structures; hot electron generation; and sensors in general based on 2D materials.

Unusual polarizing effect of cylindrical plasmonic holes

We observe an unusual polarization state conversion in the light that passes through a cylindrical hole in a thick metal film. This phenomenon is related to the helicity locking of the guided mode due to the plasmonic transverse spin-an intrinsic angular momentum of the surface waves. We show how this effect is linked to the generation of the plasmonic vortex inside the hole and can be altered by varying the hole diameter. In addition, the total light transmission through the hole is shown to be partially contributed from the direct transmission, which can further modify the resulting light polarization state.

Interplay between topological phase and self-acceleration in a vortex symmetric Airy beam

Photons in an optical vortex usually carry orbital angular momentum, which boosts the application of the micro-rotation of absorbing particles and quantum information encoding. Such photons propagate along a straight line in free space or follow a curved trace once guided by an optical fiber. Teleportation of an optical vortex using a beam with non-diffraction and self-healing is quite challenging. We demonstrate the manipulation of the propagation trace of an optical vortex with a symmetric Airy beam (SAB) and found that the SAB experiences self-rotation with the implementation of a topological phase structure of coaxial vortex. Slight misalignment of the vortex and the SAB enables the guiding of the vortex into one of the self-accelerating channels. Multiple off-axis vortices embedded in SAB are also demonstrated to follow the trajectory of the major lobe for the SAB beam. The Poynting vector for the beams proves the direction of the energy flow corresponding to the intensity distribution. Hence, we anticipate that the proposed vortex symmetric Airy beam (VSAB) will provide new possibilities for optical manipulation and optical communication.

Doughnut-shaped emission from vertical organic nanowire coupled to thin plasmonic film

Vertical nanowires facilitate an innovative mechanism to channel the optical field in the orthogonal direction and act as a nanoscale light source. Subwavelength, vertically oriented nanowire platforms, both of plasmonic and semiconducting variety, can facilitate interesting far-field emission profiles and potentially carry orbital angular momentum states. Motivated by these prospects, in this Letter, we show how a hybrid plasmonic-organic platform can be harnessed to engineer far-field radiation. The system that we have employed is an organic nanowire made of diaminoanthroquinone grown on a plasmonic gold film. We experimentally and numerically studied angular distribution of surface plasmon polariton mediated emission from a single, vertical organic nanowire by utilizing evanescent excitation and Fourier plane microscopy. Photoluminescence and elastic scattering from a single nanowire was analyzed individually in terms of inplane momentum states of the outcoupled photons. We found that the emission is doughnut-shaped in both photoluminescence and elastic scattering regimes. We anticipate that the discussed results can be relevant in designing efficient, polariton-mediated nanoscale photon sources that can carry orbital angular momentum states.

Scanning Probe Photonic Nanojet Lithography

The use of nano/microspheres or beads for optical nanolithography is a consolidated technique for achieving subwavelength structures using a cost-effective approach; this method exploits the capability of the beads to focus electromagnetic waves into subwavelength beams called photonic nanojets, which are used to expose the photoresist on which the beads are placed. However, this technique has only been used to produce regular patterns based on the spatial arrangement of the beads on the substrate, thus considerably limiting the pool of applications. Here, we present a novel microsphere-based optical lithography technique that offers high subwavelength resolution and the possibility of generating any arbitrary pattern. The presented method consists of a single microsphere embedded in an AFM cantilever, which can be controlled using the AFM motors to write arbitrary patterns with subwavelength resolution (down to 290 nm with a 405 nm laser). The performance of the proposed technique can compete with those of commercial high-resolution standard instruments, with the advantage of a one-order-of-magnitude reduction in costs. This approach paves the way for direct integration of cost-effective, high-resolution optical lithography capabilities into several existing AFM systems.

Simple method for efficient reconfigurable optical vortex beam splitting

In recent years, singular light beams with orbital angular momentum are one of the most striking examples of structured light that have been widely applied in modern science. The transition from the generation of a single vortex beam to the generation of multiple such beams progressed the development of singular optics. This paper presents a new efficient method of vortex laser beam splitting using a two-level pure-phase diffractive optical element. The proposed compact element, which can be easily implemented with a low-cost binary spatial light modulator or fabricated by electron beam lithography or photolithography, is a useful tool for the reconfigurable generation of multiple closed-packed vortex beams. Furthermore, the proposed splitter can efficiently operate in the wavelength range of approximately 8% of the central wavelength, thus providing an efficient method to generate optical vortex arrays with various potential applications in modern optics and photonics.

Tuning of nanofocused vector vortex beam of metallic granary-shaped nanotip with spin-dependent dielectric helical cone

We present the combined configuration of dielectric helical cone and metallic granary-shaped nanotip to produce three -dimensional vector vortex nanofocused optical field. The intensity and phase of the electric fields, and Povnting vector of the optical field generated by the combined configuration with linearly polarized illumination are studied with three-dimensional finite difference time-domain method. The localized vector electric field near the apex of the metallic granary-shaped nanotip is strongly depended on the chirality of the dielectric helical cone and the bottom radius of the metallic granary-shaped nanotip. The localized vector electric field is wavelength selective with the maximum intensity enhancement up to 10⁴ times and minimum size of about 900 nm², and the maximum radial electric field rotates 67.0° along z axis. This indicates the vector vortex beam generated by the combined configuration can be applied in nanofabrication, nano-sensing and nano-manipulation.

Helicity locking of chiral light emitted from a plasmonic nanotaper

Surface plasmon waves carry an intrinsic transverse spin, which is locked to its propagation direction. Apparently, when a singular plasmonic mode is guided on a conic surface this spin-locking may lead to a strong circular polarization of the far-field emission. Specifically, a plasmonic vortex excited on a flat metal surface propagates on an adiabatically tapered gold nanocone where the mode accelerates and finally beams out from the tip apex. The helicity of this beam is shown to be single-handed and stems solely from the transverse spin-locking of the helical plasmonic wave-front. We present a simple geometric model that fully predicts the emerging light spin in our system. Finally, we experimentally demonstrate the helicity-locking phenomenon by using accurately fabricated nanostructures and confirm the results with the model and numerical data.