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We combine theory and experiment to demonstrate that a carefully designed gradient meta-surface supports high-efficiency anomalous reflections for near-infrared light following the generalized Snell's law, and the reflected wave becomes a bounded surface wave as the incident angle exceeds a critical value. Compared to previously fabricated gradient meta-surfaces in infrared regime, our samples work in a shorter wavelength regime with a broad bandwidth (750-900 nm), exhibit a much higher conversion efficiency (∼80%) to the anomalous reflection mode at normal incidence, and keep light polarization unchanged after the anomalous reflection. Finite-difference-time-domain (FDTD) simulations are in excellent agreement with experiments. Our findings may lead to many interesting applications, such as antireflection coating, polarization and spectral beam splitters, high-efficiency light absorbers, and surface plasmon couplers.

The terahertz region is a special region of the electromagnetic spectrum that incorporates the advantages of both microwaves and infrared light waves. In the past decade, metamaterials with effective medium parameters or gradient phases have been studied to control terahertz waves and realize functional devices. Here, we present a new approach to manipulate terahertz waves by using coding metasurfaces that are composed of digital coding elements. We propose a general coding unit based on a Minkowski closed-loop particle that is capable of generating 1-bit coding (with two phase states of 0 and 1806), 2-bit coding (with four phase states of 0, 906, 1806, and 2706), and multi-bit coding elements in the terahertz frequencies by using different geometric scales. We show that multi-bit coding metasurfaces have strong abilities to control terahertz waves by designing-specific coding sequences. As an application, we demonstrate a new scattering strategy of terahertz waves-broadband and wide-angle diffusion-using a 2-bit coding metasurface with a special coding design and verify it by both numerical simulations and experiments. The presented method opens a new route to reducing the scattering of terahertz waves.

geometrical-optics theory. [ 7 ] Although the intrinsic PSHE was found signifi cantly enhanced by the (spin-independent) phase gradients at carefully designed meta-surfaces (artifi cial ultra-thin metamaterials composed by planar units with tailored properties exhibiting extraordinary capabilities to control light propagations), [8][9][10][11][12][13][14][15][16][17] the measured ratio between the transverse displacement of spin-polarized photons and their traveling distance is still very small (≈10 −2 ). [ 12 ] In a parallel line, strong PSHE was discovered at a particular class of meta-surfaces that can scatter spin-polarized lights to different directions, [13][14][15][16] which is analogous to the extrinsic SHE discovered in electron systems. [ 3 ] The PSHE of this type can be very pronounced because the "transverse forces" acting on the spin-polarized photons come from the (spindependent) phase gradient (comparable to the wave vector of light in vacuum) on the meta-surface, which is realized at subwavelength scales in a fully controllable manner. [13][14][15][16] In sharp contrast to the intrinsic PSHE for which a semi-geometrical-optics theory is suffi cient, [ 12 ] the extrinsic PSHE can only be understood based on the full-wave Maxwell equations where wave interferences play very important roles. [13][14][15][16] However, wave interferences can also form unwanted zero-order modes after scatterings by meta-surfaces, so that the devices realized so far all suffer loweffi ciency problem: typically only a small portion (theoretical limit 25%) of incident spin-polarized photons can be anomalously defl ected by the meta-surfaces yielding the PSHE. [ 14,15,18,19 ] Here we show that in principle a giant PSHE with nearly 100% effi ciency can be realized at meta-surfaces satisfying certain criterion, which is derived from a general Jones matrix analysis. Such a criterion is approachable from two different routes, leading to two types of meta-surfaces with distinct symmetry properties. While the idea is realizable at general frequencies, as a proof of concept, here we design and fabricate two realistic microwave samples and perform experiments to demonstrate that both can realize PSHE with ≈90% effi ciency within a broad frequency bandwidth (≈10-14 GHz). Finally, we experimentally demonstrate that our meta-surfaces can work as effi cient and broadband polarization detectors as one illustration of many potential applications of our fi ndings. Results and Discussion Criterion to Realize PSHE with 100% Effi ciencyWe start from analyzing the electromagnetic (EM) properties of the building block (meta-atom) of our meta-surfaces. As shown in Figure 1 a, consider a generic slab, representing a 2D array Photonic spin Hall effect (PSHE; i.e., spin-polarized photons can be laterally separated in transportation) gains increasing attention from both science and technology, but available mechanisms either require bulky systems or exhibit very low effi ciencies. Here it is demonstrated that a giant PSHE with ≈100% effi ciency can...

As the basis of a diverse set of photonic applications, such as hologram imaging, polarization, and wave front manipulation, the local phase control of electromagnetic waves is fundamental in photonic research. However, currently available bulky, passive, range-limited phase modulators pose an obstacle in photonic applications. Here, we propose a new mechanism to achieve a wide phase modulation range, with graphene used as a tunable loss to drive an underdamped to overdamped resonator transition. Based on this mechanism, we present widely tunable phase modulation in the terahertz regime, realized in gate-tuned ultrathin reflective graphene metasurfaces. A one-port resonator model, supported by full-wave simulations, explains the underlying physics of the discovered extreme phase modulation and indicates general strategies for designing tunable photonic devices. As an example, we demonstrate a gate-tunable terahertz (THz) polarization modulator with a graphene metasurface. Our findings establish the possibility for photonic applications based on active phase manipulation.