Efficient transmission-type meta-holograms have been demonstrated using high-index dielectric nanostructures based on Huygens' principle. It is crucial that the geometry size of building blocks be judiciously optimized individually for spectral overlap of electric and magnetic dipoles. In contrast, reflection-type meta-holograms using the metal/insulator/metal scheme and geometric phase can be readily achieved with high efficiency and small thickness. Here, we demonstrate a general platform for design of dual magnetic resonance based meta-holograms based on the geometric phase using silicon nanostructures that are quarter wavelength thick for visible light. Significantly, the projected holographic image can be unambiguously observed without a receiving screen even under the illumination of natural light. Within the well-developed semiconductor industry, our ultrathin magnetic resonance-based meta-holograms may have promising applications in anticounterfeiting and information security.

Helicity-multiplexed metasurfaces based on symmetric spin–orbit interactions (SOIs) have practical limits because they cannot provide central-symmetric holographic imaging. Asymmetric SOIs can effectively address such limitations, with several exciting applications in various fields ranging from asymmetric data inscription in communications to dual side displays in smart mobile devices. Low-loss dielectric materials provide an excellent platform for realizing such exotic phenomena efficiently. In this paper, we demonstrate an asymmetric SOI-dependent transmission-type metasurface in the visible domain using hydrogenated amorphous silicon (a-Si:H) nanoresonators. The proposed design approach is equipped with an additional degree of freedom in designing bi-directional helicity-multiplexed metasurfaces by breaking the conventional limit imposed by the symmetric SOI in half employment of metasurfaces for one circular handedness. Two on-axis, distinct wavefronts are produced with high transmission efficiencies, demonstrating the concept of asymmetric wavefront generation in two antiparallel directions. Additionally, the CMOS compatibility of a-Si:H makes it a cost-effective alternative to gallium nitride (GaN) and titanium dioxide (TiO2) for visible light. The cost-effective fabrication and simplicity of the proposed design technique provide an excellent candidate for high-efficiency, multifunctional, and chip-integrated demonstration of various phenomena.

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Metasurfaces, two dimensional (2D) metamaterials comprised of subwavelength features, can be used to tailor the amplitude, phase and polarisation of an incident electromagnetic wave propagating at an interface. Though many novel metasurfaces have been explored, the hunt for cost-effective, highly efficient, low-loss and polarisation insensitive applications is ongoing. In this work, we utilise an efficient and cost-effective dielectric material, hydrogenated amorphous silicon (a-Si:H), to create a ultra-thin transmissive surface that simultaneously controls phase. This material exhibits significantly lower absorption in the visible regime compared to standard amorphous silicon, making it an ideal candidate for various on-chip applications. Our proposed design, which works on the principle of index waveguiding, integrates two distinct phase profiles, that of a lens and of a helical beam, and is versatile due to its polarisation-insensitivity. We show how this metasurface can lead to highly concentrated optical vortices in the visible domain, whose focused ring-shaped profiles carry orbital angular momentum at the miniaturised scale.

Ao ne-pot synthesis of bimetallic metal-organic frameworks( Co/Fe-MOFs)w as achieved by treating stoichiometric amounts of Fe and Co salts with 2-aminoterephthalic acid (NH 2 -BDC). Monometallic Fe (catalystA )and Co (catalyst F) were also prepared along with mixed-metal Fe/Co catalysts (B-E) by changing the Fe/Co ratio. For mixed-metal catalysts (B-E) SEM energy-dispersive X-ray (EDX) analysisc onfirmed the incorporation of both Fe and Co in the catalysts. However,aspindle-shaped morphology,t ypicallyk nown for the Fe-MIL-88Bs tructure and confirmed by PXRD analysis, was only observed for catalysts A-D. To test the catalytic potential of mixed-metal MOFs, reduction of nitroarenes was selected as ab enchmark reaction. Incorporation of Co enhanced the activity of the catalysts compared with the parentN H 2 -BDC-Fe catalyst. These MOFs werea lso testeda s electrocatalysts for the oxygen evolution reaction( OER) and the best activity was exhibited by mixed-metal Fe/Co-MOF (Fe/Cob atch ratio = 1). The catalystp rovided ac urrent density of 10 mA cm À2 at 410 mV overpotential, whichi sc omparable to the benchmarkO ER catalyst (i.e.,R uO 2 ). Moreover,i t showedl ong-term stability in 1 m KOH. In at hird catalytic test, dehydrogenation of sodium borohydride showedh igh activity (turnover frequency = 87 min À1 )a nd hydrogen generation rate (67 Lmin À1 g À1 catalyst). This is the first example of the synthesis of bimetallic MOFs as multifunctional catalysts particularly for catalytic reductiono fn itroarenes and dehydrogenation reactions. Figure 7. Catalytichydrolysis of NaBH 4 .a )Effect of monometallic and bimetallic catalysts on the dehydrogenation reaction;b )effect of temperature; c) Arrhenius plot (see Table S6 in the SupportingI nformation).

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Development of efficient and cost-effective transition metalbased catalysts for overall water splitting is desired. Herein, a facile synthesis procedure for the development of FeCo bimetallic alloy nanoparticles (NPs) located on the tips of multiwalled carbon nanotubes (CNTs) supported over N-doped carbon nanofibers (CNFs) is presented. The CNTs, not only prevent FeCo NPs from agglomeration by encapsulating them at the tip but also provide an efficient electron pathway. The materials exhibited excellent performance in oxygen evolution reaction (OER) and decent activity in hydrogen evolution reaction (HER) with long-term durability of up to 48 hours on glassy carbon electrode. The best OER activity with a overpotential of 283 mV@10 mAcm À 2 and Tafel slope of 38 mV/dec was achieved with a hierarchically porous catalyst having Fe : Co molar ratio of 1 : 2. This synthesis approach is promising for growing well-dispersed CNTs over N-doped carbonaceous support using bimetallic alloys for electrocatalytic applications.catalyst's intrinsic resistance and facilitating the charge transfer between catalyst and the electrolyte interface. This work might introduce a new and cost effective way for the homogenously distributed in-situ growth of CNTs over N doped carbonaceous support that have potential applications as electrocatalyst in metal air batteries, water splitting and fuel cells.

Polarization insensitive metasurface axicons of hydrogenated amorphous silicon are proposed generating highly concentrated Bessel beams with desired orders. The metasurfaces are designed by index waveguiding and experimentally verified.