Visible-frequency meta-gratings for light steering, beam splitting and absorption tunable functionality

Metasurface-based triple-band beam splitter with large spatial separation at visible wavelengths

The dual-function of a wavelength beam splitter and a power beam splitter is desired in both classical optics and quantum optics. We propose a triple-band large-spatial-separation beam splitter at visible wavelengths using a phase-gradient metasurface in both the x- and y-directions. Under x-polarized normal incidence, the blue light is split in the y-direction into two equal-intensity beams owing to the resonance inside a single meta-atom, the green light is split in the x-direction into another two equal-intensity beams owing to the size variation between adjacent meta-atoms, while the red light passes directly without splitting. The size of the meta-atoms was optimized based on their phase response and transmittance. The simulated working efficiencies under normal incidence are 68.1%, 85.0%, and 81.9% at the wavelengths of 420 nm, 530 nm, and 730 nm, respectively. The sensitivities of the oblique incidence and polarization angle are also discussed.

Dual-encryption freedom via a monolayer-nanotextured Janus metasurface in the broadband visible

As an emerging category of two-faced 2D architecture, the Janus metasurface aims to explore another universal optical property, that is, the wavevector direction (k-direction), and to enable the asymmetric transmission between the opposite directional incidences. It exhibits significant potential in creating versatile multiplexing metasurfaces and an optical isolator in optical communication applications. However, most previous asymmetric functionality shows merely one-way functionality with the other-way simply muted or demands multilayered nanostructure fabrication and alignment. Hence, it remains a great challenge to make a monolayer-nanotextured Janus metasurface with dual-encryption freedom and conquering the difficulty for multilayer alignment and practical operation bandwidth. In this work, we have proposed and experimentally demonstrated a new strategy of a dual-encryption Janus metasurface design with a simple monolayer-nanotextured metasurface coupled with a commercialized film of the half-wave plate. Utilizing the hybridization from two independent geometrical dimensions of rectangular-antennas, our approach ingeniously transforms the polarization-multiplexing into the dual-directional channels. A series of calculations and experimental results demonstrate that our asymmetric approach simultaneously constructs completely independent imaging encryptions for both forward and backward directions. Additionally, our proposed approach becomes a practical scheme with broadband visible-frequency operation and great simplicity in design and nanofabrication. We believe the universal scheme could facilitate to increase the information encoding capacity and holographic multiplexing channels by expanding the illumination wavevector to the full-space (+/-), and it paves the route toward the potential applications in on-chip integration, telecommunications, encryption, information processing, and communication.

3D Meta-Prisms for Versatile Beam Steering by Hybridizing Plasmonic and Diffractive Effect in the Broadband Visible Regime

As two independent optical sub-fields, diffraction optics and plasmonics both have been used for wavefront shaping and beam steering. However, the two separate concepts have always been developing as two parallel directions, which have not met for studying their structural hybridization to discover new potentials. For instance of the flat metasurfaces, even though the geometric parameters including shape, size, and periodicity have been studied, it remains mostly unexplored for the 3D spatial height variation. Here, a new type of all-metallic 3D meta-prism is proposed and experimentally demonstrated by hybridizing the localized surface plasmonic resonances (LSPR) and the blazed grating diffraction, which enables strong polarization-dependent behaviors to steer broadband visible light to drastically inverse directions. The nanofabrication of 3D meta-prism is achieved by nanostencil lithography with electron-beam evaporation. Such meta-prism could also enable to split different visible light (green, blue, and red) with high-efficiency contrast (≈10). By the mirror-symmetry arrangement, a multifunctional surface is demonstrated with polarization-/wavelength-multiplexing wavefront-shaping functions (concave, convex, or flat mirror). This unique 3D meta-prism enjoys great simplicity and versatility in broadband beam steering through the incorporation of plasmonic and diffractive effects and can be utilized in various applications including dichroic-prism splitters, multifunctional meta-mirrors, etc.

Polarization-insensitive broadband visible-light steering with tunable direction enabled by scalable plasmonics meta-gratings

As an emerging field in the discipline of optics, plasmonics and metasurfaces have been demonstrated to enable a new degree of freedom to manipulate light for arbitrary beam steering, spectral splitting as well as precise wavefront shaping. However, it has been mostly studied in parallel with the field of diffractive optics, and awaits the unveiling of how the hybridizations between plasmonic effect and diffraction effect interact and impact. Here, we have theoretically proposed a new type of polarization-insensitive meta-grating structure across the broadband visible regime. The structure design combines the width gradient (critical resonant length) from a trapezoid-nanoantenna with the height gradient from a blazed grating profile. The hybridized meta-grating creates both plasmonic effect and grating effect, which enables all the optical incident photons to be directed to the same orientation regardless of the light polarization. As we know, both metasurfaces and diffractive optical elements (such as gratings) are, more often than not, quite sensitive to the incident light polarization. Moreover, if placing our meta-grating on a flexible/stretchable substrate (such as polydimethylsiloxane), the outgoing angle can be effectively adjusted by tuning the period or density of meta-grating arrays. Such meta-grating architectures can be potentially manufactured by existing photolithography and nanoimprint techniques, and can easily find a wide range of practical polarization-insensitive applications, including broadband deflector and emitter, tunable display and imaging device, high signal-to-noise ratio spectrometer, polarization-insensitive plasmonic coupler, etc.

On-chip metalenses based on one-dimensional gradient trench in the broadband visible

Metasurfaces are composed of flat, ultrathin subwavelength nanoantennas with strong capability in manipulating light propagation by modulations on its phase, amplitude, and polarization. For instance, the invention of two-dimensional (2D) metalenses has enabled light focusing and imaging in three-dimensional (3D) free space with miniaturized thickness and device size at a planar surface. However, such inherent form of 2D arrays and focusing functionality at 3D optical free-space limits the degree of freedom for light propagation and manipulation along a 2D planar surface and eventually the possibility of on-chip photonic system integration. Here, we theoretically study and demonstrate a new type of planar on-chip metalens, which enables light focusing and strong localization at a 2D surface. The planar on-chip architecture design is based on the one-dimensional (1D) length or width gradient trench metalens (GTM), which could yield the elaborately engineered phase shift for propagating light within the on-chip waveguide at the visible wavelength of 500 nm. By generating 1D phase arrangement at the nanoscale, a miniature on-chip metalens with ∼3×0.5µm dimension could achieve light focusing on a 2D waveguide surface with the flexibility to design scalable focal lengths and ultra-high numerical aperture of up to ∼0.99. Additionally, GTM metalens designs could also exhibit overlapped high depth-of-focus, which consequently could behave as achromatic-like lensing at the selected focal plane. Furthermore, we manifest that the focusing functionality can also be subject to dynamically tuning and switching on-and-off with TE/TM polarization change or waveguide index alteration. We believe this new form of on-chip 1D metalens holds potential applications including on-chip light manipulation functionality of focusing and diverging, optical on-chip sensing, next-generation on-chip optical communication, signal processing as well as imaging devices, etc.

High-NA achromatic diffractive lensing for arbitrary dual-wavelengths enabled by hybridized metal-insulator-metal cavities

A new type of diffractive lens based on hybridized Fabry-Perot (FP) cavities with high-NA and achromatic features for arbitrary dual-wavelengths is theoretically proposed and demonstrated. We utilize the subwavelength-scale metal-insulator-metal nanocavity to form a Fresnel zone plate (MIM-FZP) that benefits from both spectral selectivity and high numerical aperture (NA > 0.9) to enable lensing functionality. By taking advantage of the different transmission orders from MIM, any arbitrary dual-wavelength achromatic focusing design is achieved. Using this approach, we merge two independent MIM-FZP designs and realize achromatic focusing performance at the selected dual-wavelength of 400/600 nm. Furthermore, the achromatic lens also exhibits a crucial potential for dynamically tuning of the operation wavelengths and focusing lengths as actively scaling the core layer thickness of MIM. The unique MIM-FZP design can be practically fabricated using a grayscale lithography technique. We believe such high-NA and achromatic optical devices enjoy great simplicity for structural design and can easily find applications including high-resolution imaging, new-generation integrated optoelectronic devices, confocal collimation, and achromatic lens, etc.