Dynamically sculpturing plasmonic vortices: from integer to fractional orbital angular momentum

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.

Fractional Optical Angular Momentum and Multi-Defect-Mediated Mode Renormalization and Orientation Control in Photonic Crystal Microring Resonators

Whispering gallery modes (WGMs) in circularly symmetric optical microresonators exhibit integer quantized angular momentum numbers due to the boundary condition imposed by the geometry. Here, we show that incorporating a photonic crystal pattern in an integrated microring can result in WGMs with fractional optical angular momentum. By choosing the photonic crystal periodicity to open a photonic band gap with a band-edge momentum lying between that of two WGMs of the unperturbed ring, we observe hybridized WGMs with half-integer quantized angular momentum numbers (m∈Z+1/2). Moreover, we show that these modes with fractional angular momenta exhibit high optical quality factors with good cavity-waveguide coupling and an order of magnitude reduced group velocity. Additionally, by introducing multiple artificial defects, multiple modes can be localized to small volumes within the ring, while the relative orientation of the delocalized band-edge states can be well controlled. Our Letter unveils the renormalization of WGMs by the photonic crystal, demonstrating novel fractional angular momentum states and nontrivial multimode orientation control arising from continuous rotational symmetry breaking. The findings are expected to be useful for sensing and metrology, nonlinear optics, and cavity quantum electrodynamics.

Circular polarization analyzer based on surface plasmon polariton interference

The determination of chirality of circularly polarized light (CPL) is of great significance to the development of various optical techniques. In this paper, a miniature circular polarization analyzer (CPA) based on surface plasmon polariton (SPP) interference is proposed. The proposed CPA consists of a micron scale long sub-wavelength slit and two groups of spatially arranged periodic sub-wavelength rectangular groove pairs, which are etched in a metal layer. Under the illumination of a CPL with a given chirality, the proposed CPA is capable of forming SPP-mediated interference fringes with different periods in far field. The chirality of CPL can be directly and quantitatively differentiated by the frequency value of the far field SPP-mediated interference fringes. Different from the existing SPP-based CPAs, the proposed CPA can directly image the chirality information in far field, avoiding near-field imaging of the SPP field.

Generation of light beams with custom orbital angular momentum and tunable transverse intensity symmetries

We introduce a novel and simple modulation technique to tailor optical beams with a customized amount of orbital angular momentum (OAM). The technique is based on the modulation of the angular spectrum of a seed beam, which allows us to specify in an independent manner the value of OAM and the shape of the resulting beam transverse intensity. We experimentally demonstrate our method by arbitrarily shaping the radial and angular intensity distributions of Bessel and Laguerre-Gauss beams, while their OAM value remains constant. Our experimental results agree with the numerical and theoretical predictions.

Generating a plasmonic vortex field with arbitrary topological charges and positions by meta-nanoslits

A novel plasmonic vortex lens (PVL) consisting of an array of gold film nanoslits for vortex-field generation with arbitrary topological charges and positions is proposed. The performance of the PVL is analyzed theoretically and demonstrated numerically by the finite-element method. By utilizing a symmetric and antisymmetric phase, the plasmonic vortex generated at the center of the PVL can carry arbitrary topological charges (integer or fraction). Two circularly polarized illuminations with opposite spins can excite a composite plasmonic vortex field with symmetry-breaking distribution, in which the breaking point rotates with the phase difference between two spins. In addition, we can shift the center of the plasmonic vortex to any desired position such that a flexible location manipulation of the plasmonic vortex is achieved. The designed PVL can also work as a miniaturized polarimeter for characterizing the state of polarization of the incident light.

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.

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.

On-chip generation of broadband high-order Laguerre-Gaussian modes in a metasurface

With experimental results, we demonstrate the generation of high-order Laguerre-Gaussian modes with non-zero radial indices using a metal meta-surface, which is composed of a series of rectangle nanoholes with different orientation angles. The phase shift after transmission through the metasurface is determined by the orientation angle of the nanohole. This device works over a broad wavelength band ranging from 700 to 1000 nm. Moreover, we achieve a LG mode with a radial mode index of 10. Our results provide an integrated method to obtain high-order LG modes, which can be used to enhance the capacity in optical communication and manipulation.

Extreme Ultraviolet Fractional Orbital Angular Momentum Beams from High Harmonic Generation

We investigate theoretically the generation of extreme-ultraviolet (EUV) beams carrying fractional orbital angular momentum. To this end, we drive high-order harmonic generation with infrared conical refraction (CR) beams. We show that the high-order harmonic beams emitted in the EUV/soft x-ray regime preserve the characteristic signatures of the driving beam, namely ringlike transverse intensity profile and CR-like polarization distribution. As a result, through orbital and spin angular momentum conservation, harmonic beams are emitted with fractional orbital angular momentum, and they can be synthesized into structured attosecond helical beams –or “structured attosecond light springs”– with rotating linear polarization along the azimuth. Our proposal overcomes the state of the art limitations for the generation of light beams far from the visible domain carrying non-integer orbital angular momentum and could be applied in fields such as diffraction imaging, EUV lithography, particle trapping, and super-resolution imaging.