Full 2π Phase Control Research Articles

Thickness-designable acoustic metamaterial for passive phased arrays

Passive phase-controlled acoustic arrays based on metamaterial surfaces artificially manipulate acoustic waves, which can be used in biomedical imaging and nondestructive testing. One of the most representative applications is ultrasonic therapy using a passive phased array within a complete 2π phase. In contrast, the surface thickness is 1/10 the wavelength of the operating frequency. However, conventional design methods feature a fixed array thickness for a specific frequency. This becomes necessary for different application scenarios where different thicknesses are needed. This study proposed a labyrinth resonant cavity metascreen phased array based on Helmholtz resonators and convoluted labyrinth acoustic metamaterials. The thickness of this array can be optimized to accommodate a specific operating frequency to achieve full 2π phase control by adjusting only one parameter. Theoretical simulations and experimental results verified the acoustic wave manipulation effect. The proposed method may find utility in applications for noninvasive ultrasonic therapy, including nondestructive testing, acoustic communication, and biomedical imaging.

Dielectric Polarization‐Filtering Metasurface Doublet for Trifunctional Control of Full‐Space Visible Light

AbstractMultilayered plasmonic metasurfaces have been previously shown to enable multifunctional control of full‐space electromagnetic waves, which are of great importance to the development of compact optical systems. While this structural configuration is practical for acquiring metasurfaces working in microwave frequency, it will inevitably become lossy and highly challenging to fabricate when entering the visible band. Here, an efficient yet facile approach to address this issue by resorting to a dielectric metasurface doublet (DMD) based on two vertically integrated polarization‐filtering meta‐atoms (PFMs) is presented. The PFMs exhibit polarization‐dependent high transmission and reflection, as well as independent and full 2π phase control characteristics, empowering the DMD to realize three distinct incidence‐direction and polarization‐triggered wavefront‐shaping functionalities, including anomalous beam deflection, light focusing, vortex beam generation, and holographic image projection as it is investigated either numerically or experimentally. The presented DMD undoubtedly holds several salient features compared with the multilayered metallic metasurfaces in aspects of design complexity, efficiency, and fabrication. Furthermore, as dielectric meta‐atoms with distinct polarization responses can be deployed to construct the DMD, it is anticipated that diverse full‐space metasurfaces equipped with versatile functionalities can be demonstrated in the future, which will greatly advance the development of multifunctional meta‐optics.

Complete 2π phase control by photonic crystal slabs

Photonic crystal slabs are the state of the art in the studies of the light confinement, optical wave modulating and guiding, as well as nonlinear optical response. Previous studies have shown abundant real-world implementations of photonic crystals in planar optics, metamaterials, sensors, and lasers. Here, we report a novel full 2π phase control method in the reflected light beam over the interaction with a photonic crystal resonant mode, verified by the temporal coupled-mode analysis and S-parameter simulations. Enhanced by the asymmetric coupling with the output ports, the 2π phase shift can be achieved with the silicon photonics platforms such as Silicon-on-Silica and Silicon-on-Insulator heterostructures. Such photonic crystal phase control method provides a general guide in the design of phase-shift metamaterials, suggesting a wide range of applications in the field of sensing, spatial light modulation, and beam steering.

Cooperative optical wavefront engineering with atomic arrays

AbstractNatural materials typically interact weakly with the magnetic component of light which greatly limits their applications. This has led to the development of artificial metamaterials and metasurfaces. However, natural atoms, where only electric dipole transitions are relevant at optical frequencies, can cooperatively respond to light to form collective excitations with strong magnetic, as well as electric, interactions together with corresponding electric and magnetic mirror reflection properties. By combining the electric and magnetic collective degrees of freedom, we show that ultrathin planar arrays of atoms can be utilized as atomic lenses to focus light to subwavelength spots at the diffraction limit, to steer light at different angles allowing for optical sorting, and as converters between different angular momentum states. The method is based on coherently superposing induced electric and magnetic dipoles to engineer a quantum nanophotonic Huygens’ surface of atoms, giving full 2π phase control over the transmission, with close to zero reflection.

Broadband cross-slots fractal nano-antenna and its extraordinary optical transmission characteristics

To overcome the disadvantages of narrow frequency band and low transmittance for traditional nano-antenna, a nano-antenna structure based on cross-slots fractal was designed. The extraordinary optical transmission characteristics of the cross-slots fractal nano-antenna and the differences between the cross-slots fractal nano-antenna and the uniform cross-slots nano-antenna were analyzed by the finite difference time domain method. Meanwhile, the influence of physical parameters on the extraordinary optical transmission characteristics of the cross-slots fractal nano-antenna and the relationship of transmission spectrum of the nano-antenna between the fractal size and the non-fractal size were discussed. The results show that the fractal cross-slots structure is more miniaturized, and realizes extraordinary optical transmission and full 2π phase control of transmission beam, and the transmittance is higher than the uniform cross-slots structure, the full width at half maximum (FWHM) is wider, and the highest transmittance is up to 99.51%. By adjusting the physical parameters, the transmission spectrum exhibits red-shift or blue-shift characteristics, achieving controllability of the transmission spectrum. When h=50 nm, the full width at half maximum is about 356 nm, and the transmittance is still as high as 95.66%, which is generally higher than traditional structures, and the peak transmittance is still greater than 74% at large incident angles (70 degrees). In short, the cross-slots fractal nano-antenna has the characteristics of wide frequency, controllable and adjustable, and more miniaturized structure compared with other nano-antenna structures, and realizes extraordinary optical transmission.

Tunable fiber-tip lens based on thermo-optic effect of amorphous silicon

We theoretically demonstrate a compact all-dielectric metasurface fiber-tip lens composed of sub-wavelength amorphous silicon on the end face of a multimode fiber. The full 2π phase control was realized by varying the widths of resonant units. The tunable focal length is achieved by using the thermal-optic effect of amorphous silicon. The focal length increases from 309 μm to 407 μm when the temperature changes by 300 K. The temperature controlled all-fiber integrated lens is compact and with high efficiency and provides an excellent platform of a fiber-tip lab. Meanwhile, the proposed fiber lens does not have any structural changes during dynamic tuning, which improves the durability and repeatability of the devices.

High-Efficiency Visible Light Manipulation Using Dielectric Metasurfaces

The development of a miniaturised device that provides efficient beam manipulation with high transmittance is extremely desirable for the broad range of applications including holography, metalens, and imaging. Recently, the potential of dielectric metasurfaces has been unleashed to efficiently manipulate the beam with full 2π-phase control by overlapping the electric and magnetic dipole resonances. However, in the visible range for available materials, it comes with the price of higher absorption that reduces efficiency. Here, we have considered dielectric amorphous silicon (a-Si) nanodisk and engineered them in such a way which provides minimal absorption loss in the visible range. We have experimentally demonstrated meta-deflector with high transmittance which operates in the visible wavelengths. The supercell of proposed meta-deflector consists of 15 amorphous silicon nanodisks numerically shows the transmission efficiency of 95% and deflection efficiency of 95% at operating wavelength of 715 nm. However, experimentally measured transmission and deflection efficiencies are 83% and 71%, respectively, having the experimental deflection angle of 8.40°. Nevertheless, by reducing the supercell length, the deflection angle can be controlled, and the value 15.50° was experimentally achieved using eight disks supercell. Our results suggest a new way to realise the highly transmittance metadevice with full 2π-phase control operating with the visible light which could be applicable in the imaging, metalens, holography, and display applications.

Deflecting transmissive light beams with metasurfaces based on crystalline silicon high-contrast grating

Dielectric metasurfaces have emerged and expanded rapidly for their success in realizing high-efficiency wavefront control in the optical and infrared ranges. However, devices have either been demonstrated at near-infrared regime using high-index semiconductors such as silicon, or they use lower index dielectric materials such as TiO2 or Si3N4 and operate in the visible wavelength regime to avoid the optical loss of dielectric materials. It seems that dielectric metasurfaces could not achieve either high index or high device efficiency, simultaneously. Here, we provide a crystalline silicon metasurface scheme to efficiently modulate transmitted light beams at the wavelengths as short as to 532 nm. The unit cell of the metasurfaces consists of high-contrast grating (HCG), which provides full 2π phase control for circular polarized incident beams in thin film. The demonstration of HCG deflector shows that the device enables high deflecting efficiency as high as those made by transparent materials. Our results provide a paradigm to functionalize transmitted light beams based on crystalline silicon in the visible wavelength regime.

Multilayer Noninteracting Dielectric Metasurfaces for Multiwavelength Metaoptics.

Metasurfaces provide a versatile platform for manipulating the wavefront of light using planar nanostructured surfaces. Transmissive metasurfaces, with full 2π phase control, are a particularly attractive platform for replacing conventional optical elements due to their small footprint and broad functionality. However, the operational bandwidth of metasurfaces has been a critical limitation and is directly connected to either their resonant response or the diffractive dispersion of their lattice. While multiwavelength and continuous band operation have been demonstrated, the elements suffer from either low efficiency, reduced imaging quality, or limited element size. Here, we propose a platform that provides for multiwavelength operation by employing tightly spaced multilayer dielectric metasurfaces. As a proof of concept, we demonstrate a multiwavelength metalens doublet (NA = 0.42) with focusing efficiencies of 38% and 52% at wavelengths of 1180 and 1680 nm, respectively. We further show how this approach can be extended to three-wavelength metalenses as well as a spectral splitter. This approach could find applications in fluorescent microscopy, digital imaging, and color routing.

High-Efficiency All-Dielectric Metasurfaces for Broadband Polarization Conversion

We have presented two polarization convertors based on amorphous silicon metasurfaces. Results show that the cross-polarized transmission of the first polarization convertor is over 90% with over 95% polarization conversion efficiency across the 300-nm bandwidth and maintains high-efficiency performance with big incident angles in this bandwidth. To steer cross-polarized light with an angle according to the generalized Snell’s law, we have created another polarization convertor by choosing several resonators with different geometries in one supercell, achieving full 2π phase control, guiding the co-polarized and cross-polarized transmitted light spatially separated efficiently with broadband operation.