Shengtao Mei

Hybrid bilayer plasmonic metasurface efficiently manipulates visible light

Two highly coupled plasmonic metasurfaces exhibit much higher conversion efficiency and extinction ratio than individual ones. Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs, as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.’s study), although the metal’s ohmic loss is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~λ/6), is found to obey generalized Snell’s law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a broad bandwidth and can be realized with a facile one-step nanofabrication process.

Visible-Frequency Metasurface for Structuring and Spatially Multiplexing Optical Vortices

A multifocus optical vortex metalens, with enhanced signal-to-noise ratio, is presented, which focuses three longitudinal vortices with distinct topological charges at different focal planes. The design largely extends the flexibility of tuning the number of vortices and their focal positions for circularly polarized light in a compact device, which provides the convenience for the nanomanipulation of optical vortices.

Spiniform phase-encoded metagratings entangling arbitrary rational-order orbital angular momentum

Quantum entanglements between integer-order and fractional-order orbital angular momentums (OAMs) have been previously discussed. However, the entangled nature of arbitrary rational-order OAM has long been considered a myth due to the absence of an effective strategy for generating arbitrary rational-order OAM beams. Therefore, we report a single metadevice comprising a bilaterally symmetric grating with an aperture, creating optical beams with dynamically controllable OAM values that are continuously varying over a rational range. Due to its encoded spiniform phase, this novel metagrating enables the production of an average OAM that can be increased without a theoretical limit by embracing distributed singularities, which differs significantly from the classic method of stacking phase singularities using fork gratings. This new method makes it possible to probe the unexplored niche of quantum entanglement between arbitrarily defined OAMs in light, which could lead to the complex manipulation of microparticles, high-dimensional quantum entanglement and optical communication. We show that quantum coincidence based on rational-order OAM-superposition states could give rise to low cross-talks between two different states that have no significant overlap in their spiral spectra. Additionally, future applications in quantum communication and optical micromanipulation may be found.

Evanescent vortex: Optical subwavelength spanner

Conventional optical spanners based on free-space focused vortex beams are very difficult to manipulate subwavelength objects due to the diffraction limit, while optical subwavelength spanners are not explored. Evanescent wave is one potential tool to realize subwavelength trapping. By combining vortex with evanescent field, we find that the evanescent vortex can function as an optical subwavelength spanner. We investigate the factors that will affect the generation/function of this subwavelength spanner, including numerical aperture and topological charge. Further, by calculating the optical force and potential on the illuminated objects, we have demonstrated that the evanescent optical vortex field is able to trap 200 nm polystyrene spherical particles and to rotate them around the ring-shaped field at the same time, making it a subwavelength optical spanner. This mechanism can be used as a tool to study the behaviour of very small objects in physics and biology.

High-efficiency hybrid plasmonic metasurfaces

There has been a growing amount of attention to 2D metasurfaces since their introduction for manipulating the propagation of electromagnetic waves. Indeed, many researchers have explored various designs for the deflection of an impinging beam into anomalous refraction channels (according to the generalized Snell’s law) by imparting a controlled gradient of phase discontinuities along the metasurface.1–3 The thickness of these structures is far smaller than the operational wavelength, which allows the miniaturization and integration of various optical components and systems. In general, all these promising metasurfaces can be categorized as operating either in reflection mode or transmission mode. Although metasurfaces that operate in reflection mode exhibit relatively higher efficiencies than those in transmission mode,4, 5 the reflective metasurfaces also introduce inconveniences in many applications. In addition, there has been particular interest in ultrathin metasurfaces operating in transmission mode, but these metasurfaces are still in their infancy because of their low manipulation efficiency and extremely complex fabrication methods (especially for visible light manipulation).6–8 The theoretical limit of cross-polarized transmitted light through 2D metasurfaces is only 25% of the total impinging energy.9 Realistic implementations of these surfaces have exhibited even lower efficiencies, on the order of a few percent. Meanwhile, it has also been reported that high efficiency is possible for transmission mode metasurfaces in the microwave or IR regions.10, 11 These designs, however, generally require sophisticated fabrication processes and unit cells. Such complicated designs result in great challenges for their application at visible wavelengths. Improved approaches to fill the wide gap between laboratory and practical applications are therefore still required and are being strongly pursued. Figure 1. Efficient manipulation of visible light is achieved with a V-shaped bilayer metasurface. The polarization state of the impinging beam (Ey) can be converted into the cross-polarized (Ex) component and deflected into the anomalous direction, although part of the impinging beam also travels unaffected through the structure (normal beam, Ey). The light spots on the screen in the foreground are combined photoimages of the transmitted light beams for different wavelengths. The subunit cell of the bilayer metasurface is shown in the bottom right. It consists of a top layer of gold nanoantennas and a bottom layer of its Babinet-inverted pattern. These layers are separated by conformal hydrogen silsesquioxane (HSQ) pillars. L: Length of the nanoantenna arms. W: Width of the nanoantennas. : Angle between two arms of the V-shape. T: Thickness of the gold film. H: Height of the HSQ pillars. D: Nominal space between top and bottom layers.

Multi-foci metalens for spin and orbital angular momentum interaction

The development of metasurface, capable of controlling wave-fronts through interfacial phase discontinuity, offers a fascinating methodology for designing two dimensional miniaturized optical devices. Owing to an additional advantage of the enhanced useful transmission via Bainet-inverted matasurface, we exploit them to demonstrate an intriguing concept of merging the phase-profiles of two distinct optical devices, a lens and a spiral phase plate, to realize an ultrathin nanostructured optical vortex lens. The proposed device can has multiple focal planes along the longitudinal direction; whereas the number of focal plans, corresponding topological charges and focal lengths can readily be tailored to meet any desired requirements. Meanwhile, the dual-polarity feature of the optical vertex metalens exhibits spin controlled real and virtual focal plans, while dispersionless aptitude of nanobars enables its working over the broadband. The concept unveils a novel way of employing metasurface, to engraft the phase profiles of multiple bulk devices, to achieve unique functionalities for promising applications in integrated photonics.