Spoof Plasmonic Metasurfaces with Catenary Dispersion for Two-Dimensional Wide-Angle Focusing and Imaging

Lightwave nano-converging enhancement by an arrayed optical antenna based on metallic nano-cone-tips for CMOS imaging detection

A kind of gold-coated glass nano-cone-tips (GGNCTs) is developed as an arrayed optical antenna for highly receiving and converging incident lightwaves. A local light field enhancement factor (LFEF) of ~ 2 × 10⁴ and maximum light absorption of ~ 98% can be achieved. The near-field lightwave measurements at the wavelength of 633 nm show that the surface net charges over a single GGNCT make a typical dipole oscillation and the energy transmits along the wave vector orientation, thus leading to a strong local light field enhancement. An effective detection method by near-field coupling an arrayed GGNCT and complementary metal-oxide-semiconductor (CMOS) sensor for highly efficient imaging detection is proposed. The lightwave detection at several wavelengths, including typical 473 nm, 532 nm, 671 nm, and 980 nm, shows a notable characteristic that a better capability of the net charge distribution adjusting and localized aggregating can be obtained at the absorption peak of the GGNCT developed and a stronger signal detection achieved. The research lays a foundation for further developing a light detector with an ideal optoelectronic sensitivity and broad spectral suitability, which is based on integrating GGNCTs as an arrayed optical antenna with common sensors.

Multi-functional high-efficiency light beam splitter based on metagrating

Inspired by the concept of phase-gradient metasurfaces (PGMs), we present a way to design a multi-functional PGM-based light beam splitter (LBS) operating in the optical regime by engineering the anomalous diffraction properties. As an example of a proof of concept, the designed LBS is a purely metallic slit array with gradient slit width, termed metagrating. It is shown that the designed LBS can simultaneously achieve high-efficiency light beam splitting on both energy and polarization, and it has broadband and wide-angle response. In addition, we also show that the Ohmic loss of metals plays an important role in determining the diffraction efficiency of each diffraction order, which is the physics for designing the LBS that can deliver the incident energy equally into the reflection and refraction sides. Our work enriches the existing methods of designing LBSs and particularly provides a route for the design of multi-functional LBSs with high performance.

Kramers-Kronig Relation for Attenuated Total Reflection from a Metal-Dielectric Interface Where Surface Plasmon Polaritons Are Excited

The applicability of the Kramers–Kronig relation for attenuated total reflection (ATR) from a metal–dielectric interface that can excite surface plasmon polaritons (SPP) is theoretically investigated. The plasmon-induced attenuation of reflected light can be taken as the resonant absorption of light through a virtual absorptive medium. The optical phase shift of light reflected from the SPP-generating interface is calculated using the KK relation, for which the spectral dependence of ATR is used at around the plasmonic resonance. The KK relation-calculated phase shift shows good agreement with that directly obtained from the reflection coefficient, calculated by a field transfer matrix formula at around the resonance. This indicates that physical causality also produces the spectral dependence of the phase of the leakage field radiated by surface plasmons that would interfere with the reflected part of light incident to the interface. This is analogous with optical dispersion in an absorptive medium where the phase of the secondary field induced by a medium polarization, which interferes with a polarization-stimulating incident field, has a spectral dependence that stems from physical causality.

Optical sensing based on multimode Fano resonances in metal-insulator-metal waveguide systems with X-shaped resonant cavities

A plasmonic metal-insulator-metal (MIM) waveguide system is proposed, which is composed of a symmetrical X-shaped resonant cavity and a bus waveguide with a baffle, and its Fano resonance and optical sensing characteristics are investigated by using the finite element method (FEM). The results show that the system allows easy implementation of up to four Fano resonances, and the maximum refractive index sensitivity and figure of merit are 1303 nm/RIU and 3113, respectively. The influences of the geometric parameters of the system on the Fano resonances are also investigated, and further the independent adjustments of the Fano resonance line shape and wavelength are realized. Moreover, when an additional X-shaped resonant cavity is added to the system, more ultrasharp Fano resonances with considerable performances are obtained, which may enhance the parallel processing capability of the system. The proposed plasmonic MIM waveguide system may have potential applications in integrated photonic devices and nanoscale optical sensing.

Spin-decoupled metasurface for simultaneous detection of spin and orbital angular momenta via momentum transformation

With inherent orthogonality, both the spin angular momentum (SAM) and orbital angular momentum (OAM) of photons have been utilized to expand the dimensions of quantum information, optical communications, and information processing, wherein simultaneous detection of SAMs and OAMs with a single element and a single-shot measurement is highly anticipated. Here, a single azimuthal-quadratic phase metasurface-based photonic momentum transformation (PMT) is illustrated and utilized for vortex recognition. Since different vortices are converted into focusing patterns with distinct azimuthal coordinates on a transverse plane through PMT, OAMs within a large mode space can be determined through a single-shot measurement. Moreover, spin-controlled dual-functional PMTs are proposed for simultaneous SAM and OAM sorting, which is implemented by a single spin-decoupled metasurface that merges both the geometric phase and dynamic phase. Interestingly, our proposed method can detect vectorial vortices with both phase and polarization singularities, as well as superimposed vortices with a certain interval step. Experimental results obtained at several wavelengths in the visible band exhibit good agreement with the numerical modeling. With the merits of ultracompact device size, simple optical configuration, and prominent vortex recognition ability, our approach may underpin the development of integrated and high-dimensional optical and quantum systems.

Phase Change Metasurfaces by Continuous or Quasi-Continuous Atoms for Active Optoelectronic Integration

In recent decades, metasurfaces have emerged as an exotic and appealing group of nanophotonic devices for versatile wave regulation with deep subwavelength thickness facilitating compact integration. However, the ability to dynamically control the wave-matter interaction with external stimulus is highly desirable especially in such scenarios as integrated photonics and optoelectronics, since their performance in amplitude and phase control settle down once manufactured. Currently, available routes to construct active photonic devices include micro-electromechanical system (MEMS), semiconductors, liquid crystal, and phase change materials (PCMs)-integrated hybrid devices, etc. For the sake of compact integration and good compatibility with the mainstream complementary metal oxide semiconductor (CMOS) process for nanofabrication and device integration, the PCMs-based scheme stands out as a viable and promising candidate. Therefore, this review focuses on recent progresses on phase change metasurfaces with dynamic wave control (amplitude and phase or wavefront), and especially outlines those with continuous or quasi-continuous atoms in favor of optoelectronic integration.

Surface wave manipulation by plasmonic metasurface based on mode resonance

We proposed a method to manipulate the surface waves with a deep subwavelength metasurface by applying resonators with interfering mode resonance. The simulation results demonstrate that a single deep subwavelength obstructed groove can effectively control the propagation of surface terahertz (THz) waves by a small step increase (< 1/20 λ) of the depth or a slight change of refractive index (Δn = 0.1). The surface waves transmitted and reflected by the single groove can be controlled periodically by increasing the groove depth or refractive index with a high efficiency owing to the mode resonance between surface spoof plasmonics modes and groove cavity modes. The generated circle resonance mode provides a new idea for the development of THz devices. Importantly, the transmitted or reflected intensity of the surface wave is also enhanced by the Mode resonance. It is a simple and effective method to operate surface THz waves and manufacture more compact integrated optical devices in deep subwavelength scale.

Broadband Spin-Dependent Directional Coupler via Single Optimized Metallic Catenary Antenna

With the rapid development of on-chip optics, integrated optical devices with better performance are desirable. Waveguide couplers are the typical integrated optical devices, allowing for the fast transmission and conversion of optical signals in a broad working band. However, traditional waveguide couplers are limited by the narrow operation band to couple the spatial light into the chip and the fixed unidirectional transmission of light flow. Furthermore, most of the couplers only realize unidirectional transmission under the illumination of the linear polarized light. In this work, a broadband polarization directional coupler based on a metallic catenary antenna integrated on a silicon-on-insulator (SOI) waveguide has been designed and demonstrated under the illumination of the circularly polarized light. By applying the genetic algorithm to optimize the multiple widths of the metallic catenary antenna, the numerical simulation results show that the extinction ratio of the coupler can be maintained larger than 18 dB in a wide operation band of 300 nm (from 1400 to 1700 nm). Moreover, the coupler can couple the spatial beam into the plane and transmit in the opposite direction by modulating the rotation direction of the incident light. The broadband polarization directional coupler might have great potential in integrated optoelectronic devices and on-chip optical devices.

Minimized two- and four-step varifocal lens based on silicon photonic integrated nanoapertures

Integration of optical waveguide and subwavelength structure may help address the problems of large footprint, low robustness, and small operation bandwidth, those of that are typically inborn in traditional integrated optical devices. Here, a design method of an ultra-compact small footprint lens is proposed. Combing particle swarm optimization (PSO) algorithm with spatial multiplexing technology, we successfully integrated two- and four-step varifocal lenses on SOIs chips with small footprint of 35×35 µm², non-mechanically leading to 2.5× and 3.4× zoom capacity, respectively. The proposed designed method may shed a new light on compact on-chip display devices and offer an alternative approach to design integrated optical communication with high information storage capacity.

Crosstalk reduction of integrated optical waveguides with nonuniform subwavelength silicon strips

Suppression of the crosstalk between adjacent waveguides is important yet challenging in the development of compact and dense photonic integrated circuits (PICs). During the past few years, a few of excellent approaches have been proposed to achieve this goal. Here, we propose a novel strategy by introducing nonuniform subwavelength strips between adjacent waveguides. In order to determine the widths and positions of nonuniform subwavelength strips, the particle swarm optimization (PSO) algorithm is utilized. Numerical results demonstrate that the coupling length between adjacent waveguides is increased by three (five) orders of magnitude in comparison with the case of uniform (no) subwavelength strips. Our method greatly reduces crosstalk and is expected to achieve a highly compact integrated density of PICs.