Surfaces covered by ultrathin plasmonic structures--so-called metasurfaces--have recently been shown to be capable of completely controlling the phase of light, representing a new paradigm for the design of innovative optical elements such as ultrathin flat lenses, directional couplers for surface plasmon polaritons and wave plate vortex beam generation. Among the various types of metasurfaces, geometric metasurfaces, which consist of an array of plasmonic nanorods with spatially varying orientations, have shown superior phase control due to the geometric nature of their phase profile. Metasurfaces have recently been used to make computer-generated holograms, but the hologram efficiency remained too low at visible wavelengths for practical purposes. Here, we report the design and realization of a geometric metasurface hologram reaching diffraction efficiencies of 80% at 825 nm and a broad bandwidth between 630 nm and 1,050 nm. The 16-level-phase computer-generated hologram demonstrated here combines the advantages of a geometric metasurface for the superior control of the phase profile and of reflectarrays for achieving high polarization conversion efficiency. Specifically, the design of the hologram integrates a ground metal plane with a geometric metasurface that enhances the conversion efficiency between the two circular polarization states, leading to high diffraction efficiency without complicating the fabrication process. Because of these advantages, our strategy could be viable for various practical holographic applications.

Metasurfaces have attracted large interest in recent years due to their relatively simple fabrication, compact design, and ability to control the wavefront of incident light. Ohmic loss attributed to bulk metal metamaterials are not a primary issue, whereby the meta-atom or plasmonic structure is typically only as thin as a fraction of the operation wavelength. Numerous novel applications have been demonstrated by metasurfaces, including an ultrathin metasurface flat lens, and 3D holography.Here, by combining the freedom of both the structural design and the orientation of split ring resonator antennas, we demonstrate Terahertz metasurfaces that are capable of controlling both the phase and amplitude profiles over a very broad bandwidth at~1THz under linearly-polarised incidence. As an example, we show that these phase-amplitude metasurfaces can be engineered to control the diffraction orders arbitrarily.

Synthetic fractals inherently carry spatially encoded frequency information that renders them as an ideal candidate for broadband optical structures. Nowhere is this more true than in the terahertz (THz) band where there is a lack of naturally occurring materials with valuable optical properties. One example are perfect absorbers that are a direct step toward the development of highly sought after detectors and sensing devices. Metasurface absorbers that can be used to substitute for natural materials suffer from poor broadband performance, while those with high absorption and broadband capability typically involve complex fabrication and design and are multilayered. Here, we demonstrate a polarization-insensitive ultrathin (∼λ/6) planar metasurface THz absorber composed of supercells of fractal crosses capable of spanning one optical octave in bandwidth, while still being highly efficient. A sufficiently thick polyimide interlayer produces a unique absorption mechanism based on Salisbury screen and antireflection responses, which lends to the broadband operation. Experimental peak absorption exceeds 93%, while the average absorption is 83% from 2.82 THz to 5.15 THz. This new ultrathin device architecture, achieving an absorption-bandwidth of one optical octave, demonstrates a major advance toward a synthetic metasurface blackbody absorber in the THz band.

physics, it is still a big challenge to achieve practical metamaterials for real-world applications-primarily due to their bulky size, unavoidable material losses, and fabrication difficulties. Metasurfaces, 2D counterparts of metamaterials that consist of 2D array of planar metallic or dielectric structures, have shown great promise for practical applications owing to their exceptional capability of controlling the wavefront of light. [17][18][19][20] With suitable design of the building blocks, metasurfaces are capable of generating phase discontinuities with in-plane gradient, leading to anomalously refracted beam in transmission and/or reflection. Recent progress in metasurfaces has led to various ultrathin optical devices including flat lenses, [21][22][23][24] vortex beam generators, [23][24][25] broadband quarter wave plates, [26,27] efficient surface plasmon couplers, [28] 3D and high-efficiency holograms. [29][30][31][32] The concept of metasurfaces has also been extended to nonlinear optics for manipulating the nonlinearity phase in harmonic generations. [33,34] Although metasurfaces have offered new degrees of freedom for controlling the propagation of light, the amplitude of anomalous refracted waves in metasurfaces is typically fixed by their structural geometry and dimensions, which limits their potential for various applications that require dynamical control over the electromagnetic waves, such as active focusing for lensing and dynamic holography. Active tuning of metasurface requires incorporation of active media whose electromagnetic properties can be changed in real time under external stimuli. Recently, it was shown that anomalous deflection can be dynamically controlled by means of various tuning schemes based on microelectromechanical system (MEMS) [35] and Schottky diode. [36] One suitable candidate for such purpose is graphene, a 2D form of carbon with the atoms arranged in a honeycomb lattice. Graphene has been studied extensively during the last decade due to its high carrier mobility and unique doping capability originated from its gapless and cone-shaped band structure at the Dirac point. Graphene also shows a gate-controllable lightmatter interaction by the shift of the Fermi level, which can be further enhanced by the electromagnetic resonance provided by suitably designed structures. [37,38] Particularly, in the terahertz (THz) regime, strong modulation has been achieved by electrically tuning the density of states available for intraband transitions. [39] Although significant effort has been devoted to various graphene-based metamaterials for active control of the amplitude and polarization of THz waves in direct transmission, [40][41][42][43] Although recent progress in metasurfaces has shown great promise for applications, optical properties in metasurfaces are typically fixed by their structural geometry and dimensions. Here, an electrically controllable amplitude of anomalously-refracted waves in a hybrid graphene/metasurface system are experimentally demonstrated, which cons...