Two-dimensional transition-metal dichalcogenides (TMDCs) exhibit extraordinary nonlinearities and direct bandgaps at K (K') valleys. Those valleys could be optically manipulated through the plasmon-valley-exciton coupling, for example, with spin-dependent photoluminescence. However, the weak coherence between the pumping and emission due to intervalley scattering poses formidable challenges in exploring the valley-contrasting physics and applications. Here we show that a synthetic metasurface entangling the phase and spin of light can simultaneously enhance and manipulate the nonlinear valley-locked chiral emission in monolayer tungsten disulfide (WS 2) at room temperature. The second-harmonic valley photons, accessed and coherently pumped by the light with spin-related geometric phase imparted by Au-metasurface, are separated and routed to predetermined directions in free space. Besides, the nonlinear photons with the same spin of incident light can be steered into any predefined direction thanks to the nonlinear optical selection rule of WS 2 in our synthetic metasurface. Our synthetic TMDCs-metasurface interface may facilitate advanced roomtemperature and free-space nonlinear, quantum and valleytronic nanodevices.

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We report on the design, fabrication, and measurement of a triple-band absorber enhanced from a planar two-dimensional artificial metamaterial transmission line (TL) concept. Unlike previous multiband absorbers, this implementation incorporates fractal geometry into the artificial TL framework. As a consequence of the formed large LC values, the utilized element is compact in size, which approaches λ 0 /15 at the lowest fundamental resonant frequency. For independent control and design, a theoretical characterization based on a circuit model analysis (TL theory) is performed and a set of design procedures is also derived. Both numerical and experimental results have validated three strong absorption peaks across the S, C, and X bands, respectively, which are attributable to a series of self-resonances induced in the specific localized area. The absorber features near-unity absorption for a wide range of incident angles and polarization states and a great degree of design flexibility by manipulating the LC values in a straightforward way.

Since the first demonstration, remarkable progress has been made in the theoretical analysis, structural design, numerical simulation, and potential applications of metamaterial absorbers (MAs). With the continuous advancement of novel materials and creative designs, the absorption of MAs is significantly improved over a wide frequency spectrum from microwaves to the optical regime. Further, the integration of active elements into the MA design allows the dynamical manipulation of electromagnetic waves, opening a new platform to push breakthroughs in metadevices. In the last several years, numerous efforts have been devoted to exploring innovative approaches for incorporating tunability to MAs, which is highly desirable because of the progressively increasing demand on designing versatile metadevices. Here, a comprehensive and systematical overview of active MAs with adaptive and on‐demand manner is presented, highlighting innovative materials and unique strategies to precisely control the electromagnetic response. In addition to the mainstream method by manipulating periodic patterns, two additional approaches, including tailoring dielectric spacer and transforming overall structure are called back. Following this, key parameters, such as operating frequency, relative tuning range, and switching speed are summarized and compared to guide for optimum design. Finally, potential opportunities in the development of active MAs are discussed.