Freeform Metagratings Based on Complex Light Scattering Dynamics for Extreme,High Efficiency Beam Steering

Conventional phased‐array metasurfaces utilize subwavelength‐scale nanoparticles or nanowaveguides to specify spatially‐dependent amplitude and phase responses to light. An alternative design strategy is based on freeform inverse optimization, in which wavelength‐scale elements are designed to produce devices that possess exceptionally high efficiencies. In this report, we theoretically analyze the physical mechanisms enabling high efficiency in freeform‐based periodic metasurfaces, i.e., metagratings. An in‐depth coupled mode analysis of ultra‐wide‐angle beam deflectors and wavelength splitters shows that the extraordinary performance of these designs originates from the large number of propagating modes supported by the metagrating, in combination with complex multiple scattering dynamics exhibited by these modes. We also apply our coupled mode analysis to conventional nanowaveguide‐based metagratings to understand and quantify the factors limiting the efficiencies of these devices. We envision that freeform metasurface design methods will open new avenues towards high‐performance, multi‐functional optics by utilizing strongly coupled nanophotonic modes and elements.     

A new numerical technique for the analysis of parametrically excited nonlinear systems

A new computational scheme using Chebyshev polynomials is proposed for the numerical solution of parametrically excited nonlinear systems. The state vector and the periodic coefficients are expanded in Chebyshev polynomials and an integral equation suitable for a Picard-type iteration is formulated. A Chebyshev collocation is applied to the integral with the nonlinearities reducing the problem to the solution of a set of linear algebraic equations in each iteration. The method is equally applicable for nonlinear systems which are represented in state-space form or by a set of second-order differential equations. The proposed technique is found to duplicate the periodic, multi-periodic and chaotic solutions of a parametrically excited system obtained previously using the conventional numerical integration schemes with comparable CPU times. The technique does not require the inversion of the mass matrix in the case of multi degree-of-freedom systems. The present method is also shown to offer significant computational conveniences over the conventional numerical integration routines when used in a scheme for the direct determination of periodic solutions. Of course, the technique is also applicable to non-parametrically excited nonlinear systems as well.     

Transmission Reflection Integrated Programmable Metasurface for Real-Time Beam Control and High Efficiency Transmission Polarization Conversion

The functional versatility and flexibility of multifunctional metasurfaces are of great significance for the application of electromagnetic (EM) metasurfaces in military stealth, holographic imaging, base station antennas, and other fields. Nevertheless, the functions of active programmable coding multifunctional metasurfaces are mostly reflective or transmissive, and the research on active multifunctional metasurfaces with transmission and reflection integration is still in its infancy. Here, a transmission reflection integrated active programmable coding metasurface is proposed to manipulate the transmission and reflection of different polarized electromagnetic waves independently in real time. As a proof of concept, a metasurface that can implement diverse programmable functionalities is designed. Numerical and experimental results indicate that the strategy has excellent performance in beam control and polarization conversion, which provides tremendous potential for extending highly integrated multifunctional devices.     

Anisotropic Plasmonic Metasurfaces with Linear Polarization-Dependent Focusings

As a burgeoning approach to modulate electromagnetic waves, metasurfaces fascinate many researchers. However, it is a persistent problem to realize independent wavefront tailoring in different polarization states. In this work, anisotropic plasmonic metasurfaces are presented with linear polarization-dependent focusings. The designed plasmonic meta-atoms consist of cross-shaped gold, silica spacer, and gold substrate. Phase modulation in different polarization channels is independently achieved by adjusting the dimensions of cross-shaped gold in x- and y-directions. Three metasurfaces are presented to verify linear polarization-dependent focusings at 200 THz. First, a focusing metasurface is designed with focal lengths of 5.2 µm under x-polarized incidence and 6.5 µm under y-polarized incidence. Second, by applying convolution operation, the second metasurface exhibits different focusings with different deflection angles at opposite directions under orthogonally polarized incidences. Finally, a multi-focal metasurface is demonstrated, which switches between dual- and quad-focal points depending on polarization state. The work may provide a novel platform for near-infrared integrated photonics.     

A double‐lined metasurface for plasmonic complex‐field generation

Since the surface plasmon polariton (SPP) has received a great deal of attention because of its capability of guiding light within the subwavelength scale, finding methods for arbitrary SPP field generation has been a significant issue in the area of integrated optics. To achieve such a goal, it will be necessary to generate a plasmonic complex field. In this paper, we propose a novel method for generating a plasmonic complex field propagating with arbitrary curvatures by using double‐lined distributed nanoslits. As a unit cell, two facing nanoslits are used for tuning both the amplitude and the phase of excited SPPs as a function of their tilted angles. For verification of the proposed design rule, the authors experimentally demonstrate some plasmonic caustic curves and Airy plasmons.

     

Plasmonic Metasurfaces Enabled Ultra‐Compact Broadband Waveguide TM‐Pass Polarizer

Silicon waveguide polarizers offer a simple yet robust approach to address the polarization‐dependent issue of silicon‐based optical components, and hence have found numerous applications in silicon photonics. However, the available silicon waveguide polarizers suffer from the issue of large device footprint, high insertion loss (IL), and/or fabrication complexities. Here, a silicon waveguide transverse magnetic (TM)‐pass polarizer is constructed by coating a silicon waveguide with an ultra‐thin plasmonic metasurface structure that is capable of guiding slow surface wave (SW) mode. The transverse electric (TE) waveguide mode can be converted into SW mode with the involvement of metasurfaces, and hence is intrinsically absorbed and forbidden to pass, while the TM waveguide mode can be well guided due to little influence. A typical metasurface polarizer with an ultra‐short length of 2.4 µm enables the IL of 28.16 dB for the TE mode, and that of 0.53 dB for the TM mode at 1550 nm. Multiple‐band TM‐pass polarizers can be obtained by cascading two or more different metasurface‐coated silicon waveguides along the propagation direction, and a dual‐band TM‐pass polarizer is demonstrated with the IL being of 19.21 and 29.09 dB for the TE mode at 1310 and 1550 nm, respectively.     

Metal–Graphene Hybrid Chiral Metamaterials for Tunable Circular Dichroism

In the field of optoelectronics, circular dichroism (CD) has caused great research interest because it is widely used in imaging and biosensing. A new method for dynamically controlling terahertz (THz) CD in metamaterials is proposed. By introducing chirality and graphene to metamaterials, a pair of chiral structures with completely opposite responses to left-handed circularly polarized (LCP) waves and right-handed circularly polarized (RCP) waves are designed. The influencing factors of CD are explored, including the gap of the structure, the linewidth of graphene, and the Fermi level of graphene. The largest CD (ΔR) is 77%. The CD can be actively modulated in a modulation range of 39–77% and the modulation depth is up to 38%. In addition, two-channel and four-channel chiral metasurfaces for near-field imaging are designed in this way. Good imaging effects and on (“1”) or off (“0”) effects of the multichannel metasurface are demonstrated. This work provides new ideas for the design of tunable metasurfaces and promotes the application of metasurfaces in THz dynamic imaging.     

Mathematical Operations of Transmissive Near Fields Controlled by Metasurface with Phase and Amplitude Modulations

Metasurfaces show great potential due to their powerful ability to manipulate electromagnetic waves as desired, which has been widely investigated in the microwave, terahertz, infrared, and visible regimes. Here, it is proposed to control near-field distributions through use of a metasurface with both amplitude and phase modulations. A C-shaped particle is designed to provide stable and continuous amplitude and phase profiles independently of transmitted waves by varying its opening and orientation angles. Benefiting from both amplitude and phase modulations on the metasurface, differentiation and integration on electric-field distributions can be achieved by changing the arrangements of complex coefficients on the metasurface. Besides, the differential operation can be utilized to show the edges of predesigned electric-field distributions. It is believed that the proposed method will accelerate the pace of metasurfaces towards many applications by engineering and operating the near-field distributions.     

Ultrawideband Spin-Decoupled Coding Metasurface for Independent Dual-Channel Wavefront Tailoring

Achieving distinct functionalities for electromagnetic (EM) waves with opposite handedness in a broad frequency range is highly desirable and essential for modern wireless communications and radar stealth. However, available functional meta-devices still suffer from the issues of locked functionalities or spin-decoupled properties but with limited bandwidth. Here, a spin-decoupled coding metasurface is presented for achieving independently spin-controlled functionalities with high efficiency in an ultrawide frequency band. By synthesizing the Jones matrix, it is predicted that two half-wave plates with a phase difference of 90° can form a 1-bit coding metasurface operating for orthogonal spins independently. As proofs of concept, two meta-devices are implemented by the metasurface in the microwave region. The first meta-device performs as a spin-decoupled beam deflector while the second one shows an ability to generate spin-decoupled multi-beams carrying desired orbital angular momentums. Both of the designed meta-devices can operate in the whole band of 7.5–18.5 GHz (with relative bandwidth of 80%), which, to the best knowledge, is so far the broadest bandwidth that can be achieved by spin-decoupled metasurfaces. This may trigger interest and open opportunities for advanced functional meta-devices in practical applications, for example, multichannel metasurface antennas, or multifunctional low-scattering devices in microwave region.     

Abundant periodic and aperiodic solutions of a discontinuous three-term recurrence relation

In this note, we study a discontinuous three-term recurrence relation which arises from seeking the steady states of a cellular neural network with step control function. Several collections of periodic solutions are found. A necessary and sufficient condition for a solution to be periodic is stated and aperiodic solutions are found as consequences. We also show that any periodic solution can be derived from a primary periodic solution with least period not divisible by 5. Although the periodic or aperiodic solutions we found are not exhaustive, they are quite abundant and may reflect some of the rich physical phenomena in true biological systems. Our method in this note may also provide a general approach to analyze the periodicity of solutions of similar recurrence relations.