Polarization-independent longitudinal multi-focusing metalens

A Multi-foci Sparse-Aperture Metalens

Multi-foci lenses are essential components for optical communications, virtual reality display and microscopy, yet the bulkiness of conventional counterparts has significantly hindered their widespread applications. Benefiting from the unprecedented capability of metasurfaces in light modulation, metalenses are able to provide multi-foci functionality with a more compact footprint. However, achieving imaging quality comparable to that of corresponding single-foci metalenses at each focal point poses a challenge for existing multi-foci metalenses. Here, a polarization-independent all-dielectric multi-foci metalens is proposed and experimentally demonstrated by spatially integrating single-foci optical sparse-aperture sub-metalenses. Such design enables the metalens to generate multiple focal points, while maintaining the ability to capture target information comparable to that of a single-foci metalens. The proposed multi-foci metalens is composed of square-nanohole units array fabricated by two-photon polymerization. The focusing characteristic and imaging capability are demonstrated upon the illumination of an unpolarized light beam. This work finds a novel route toward multi-foci metalenses and may open a new avenue for dealing with the trade-off between multi-foci functionality and high-quality imaging performance.

Manipulation of the polarization state of the focus based on a slab plasmon waveguide

A focusing nanostructure with tailored polarization properties based on a metal-dielectric slab waveguide combined with plasmonic slits and gratings is proposed. The polarization state of the focus light can be controlled with overlapping a transverse magnetic (TM) focus and a transverse electric (TE) focus, which are formed by focusing the waveguide modes into free space via grating coupling, extraordinary transmission, and plasmonic beaming. We demonstrated that it is possible to achieve either multiple foci or a single focal spot of the transmitted light with tailored polarization states by judicious design of the structure parameter and the polarization state of the incident light.

Directional terahertz holography with thermally active Janus metasurface

Dynamic manipulation of electromagnetic (EM) waves with multiple degrees of freedom plays an essential role in enhancing information processing. Currently, an enormous challenge is to realize directional terahertz (THz) holography. Recently, it was demonstrated that Janus metasurfaces could produce distinct responses to EM waves from two opposite incident directions, making multiplexed dynamic manipulation of THz waves possible. Herein, we show that thermally activated THz Janus metasurfaces integrating with phase change materials on the meta-atoms can produce asymmetric transmission with the designed phase delays. Such reconfigurable Janus metasurfaces can achieve asymmetric focusing of THz wave and directional THz holography with free-space image projections, and particularly the information can be manipulated via temperature and incident THz wave direction. This work not only offers a common strategy for realizing the reconfigurability of Janus metasurfaces, but also shows possible applications in THz optical information encryption, data storage, and smart windows.

Scannable Dual-Focus Metalens with Hybrid Phase

Metalenses with two foci in the longitudinal or transverse direction, called bifocal or dual-focus metalenses, are promising building blocks in tomography techniques, data storage, and optical tweezers. For practical applications, relative movement between the beam and specimen is required, and beam scanning is highly desirable for high-speed operation without vibration. However, dual-focus metalenses employ a hyperbolic phase that experiences off-axis aberrations, which is not suitable for beam scanning. Here, we demonstrated a scannable dual-focus metalens by employing a new phase called "hybrid phase". The hybrid phase consists of a hyperbolic phase inside and a quadratic phase outside to reduce off-axis aberrations while maintaining a high numerical aperture. We show that the two foci of the scannable dual-focus metalens move together without severe distortion for incident angles of up to 2.5°. Our design easily extends to the case of multifocusing, which is essential for various applications ranging from imaging to manipulation.

Polarization-modulated broadband achromatic bifunctional metasurface in the visible light

Achromatic bifunctional metasurface is of great significance in optical path miniaturization among advanced integrated optical systems. However, the reported achromatic metalenses mostly utilize a phase compensate scheme, which uses geometric phase to realize the functionality and uses transmission phase to compensate the chromatic aberration. In the phase compensation scheme, all the modulation freedoms of a nanofin are driven at the same time. This makes most of the broadband achromatic metalenses restricted to realizing single function. Also, the phase compensate scheme is always addressed with circularly polarized (CP) incidence, leading to a limitation in efficiency and optical path miniaturization. Moreover, for a bifunctional or multifunctional achromatic metalens, not all the nanofins will work at the same time. Owing to this, achromatic metalenses using a phase compensate scheme are usually of low focusing efficiencies. To this end, based on the pure transmission phase in the x-/y- axis provided by the birefringent nanofins structure, we proposed an all-dielectric polarization-modulated broadband achromatic bifunctional metalens (BABM) in the visible light. Applying two independent phases on one metalens at the same time, the proposed BABM realizes achromatism in a bifunctional metasurface. Releasing the freedom of nanofin's angular orientation, the proposed BABM breaks the dependence on CP incidence. As an achromatic bifunctional metalens, all the nanofins on the proposed BABM can work at the same time. Simulation results show that the designed BABM is capable of achromatically focusing the incident beam to a single focal spot and an optical vortex (OV) under the illumination of x- and y-polarization, respectively. In the designed waveband 500 nm (green) to 630 nm (red), the focal planes stay unchanged at the sampled wavelengths. Simulation results prove that the proposed metalens not only realized bifunctional achromatically, but also breaks the dependence of CP incidence. The proposed metalens has a numerical aperture of 0.34 and efficiencies of 33.6% and 34.6%. The proposed metalens has advantages of being flexible, single layer, convenient in manufacturing, and optical path miniaturization friendly, and will open a new page in advanced integrated optical systems.

Ultra-Thin, Short-Focus, and High-Aperture Metalens for Generating and Detecting Laser Optical Vortices

A combined high-aperture metalens in a thin silicon nitride film that consists of two tilted sectored metalenses is considered. Each sector of the metalens consists of a set of binary subwavelength gratings. The diameter of the metalens is 14 μm. Using a time-domain finite difference method, we show that the metalens can simultaneously detect optical vortices with two topological charges −1 and −2, almost over the entire spectrum of visible wavelengths. The metalens can distinguish several wavelengths that are focused at different points in the focal plane due to a 1-nm change in wavelength resulting in a focal spot shift of about 4 nm. When the metalens is illuminated by a Gaussian beam with left-handed circular polarization, two optical vortices with topological charges 1 and 2 are simultaneously formed 6-μm apart at the focal distance of 6 μm.

Research on Fabrication Techniques and Focusing Characteristics of Metalens

Metalenses have recently attracted increased attention due to their remarkable characteristics. The fabrication technology of metalenses has also become an important research direction. In this study, we propose a metalens structure based on Au–MgF2–Au in infrared waveband. The preparation process of the metalens included magnetron sputtering, electron beam evaporation, and electron beam exposure. A dose test was performed during the exposure process, adjusting the exposure dose to minimize the proximity effect after exposure. Then, SEM was used to measure the processed metalens structure, and FDTD software was used to build a model based on the metalens, simulating and analyzing its focusing characteristics. The results show that the size deviation produced during the processing has little effect on the functionality of the metalens. The processed metalens can also focus different polarized light incidences at different spatial positions: The metalens can focus at 4.97 μm for x-polarized light and focus at 13.5 μm for y-polarized light. Additionally, the metalens has good focusing effects with different working wavelengths. We believe that the processing method of metalens proposed in this paper provides guidance for the preparation of subwavelength metasurface structures, and our findings are beneficial in developing new methods of near-infrared regime manipulation.

Dual-Wavelength Polarization-Dependent Bifocal Metalens for Achromatic Optical Imaging Based on Holographic Principle

Recently, ultrathin metalenses have attracted dramatically growing interest in optical imaging systems due to the flexible control of light at the nanoscale. In this paper, we propose a dual-wavelength achromatic metalens that will generate one or two foci according to the polarization of the incident. Based on geometric phase modulation, two unit cells are attentively selected for efficient operation at distinct wavelengths. By patterning them to two divided sections of the metalens structure plane, the dual-wavelength achromatic focusing effect with the same focal length is realized. In addition, the holographic concept is adopted for polarization-dependent bifocal generation, in which the objective wave is originated from two foci that are respectively formed by two orthogonal polarization states of circularly polarized light, namely Left-handed circularly polarized (LCP) light and Right-handed circularly polarized (RCP) light. The incident light is considered as the reference light. The achromatic focusing and polarization-dependent bifocusing are numerically verified through simulations. The proposed design opens the path for the combination of multi-wavelength imaging and chiral imaging, which may find potential applications, such as achromatic optical devices and polarization-controlled biomedical molecular imaging systems.

Performance Analysis of Metalenses Based on Three Kinds of Phase Compensation Techniques

The phase delays introduced by anisotropic nanounits include propagation phase delay, resonant phase delay and geometric phase delay. Various phase devices can be formed based on the metasurfaces consisting of anisotropic nanounits and the phase devices of the same kind function have different performances because of different working modes. In this paper, metalenses and vortex metalenses are chosen as examples to compare the optical performance of metasurface phase devices based on three kinds of phase compensation techniques. We design separately three kinds of metalenses and vortex metalenses using the cross nanoholes, L-shaped nanohole and V-shaped nanoholes and simulate numerically their intensity and phase distributions. Additionally, the results show the differences among these elements in structure complexity, polarization dependence, working efficiency and phase uniformity. The comparison for three kinds of metalenses clearly shows the merits of different phase compensation techniques and this work must be helpful for expanding the practical applications of metasurfaces.

Continuous-zoom bifocal metalens by mutual motion of cascaded bilayer metasurfaces in the visible

Metalens, a subcategory of metasurfaces, has been widely investigated by virtue of its miniature and ultrathin characteristics as well as versatile functionalities. In this study, a tunable bifocal metalens with two continuous-zoom foci is proposed and numerically verified. This design utilizes two cascaded layers of metasurfaces, and different phase profiles for incidences of opposite helicities are imparted on each layer by the combination of geometric phase and propagation phase. When two layers of metasurfaces are actuated laterally, focal lengths of both foci are tuned continuously, with the difference of both focal lengths increasing or decreasing. Additionally, the zoom range for each focus can be designed at will, and the relative intensity of both foci can be modulated by altering the ellipticity of incidence, with the focusing efficiency of the bifocal metalens varying from 19.8% to 32.7% for numerical apertures in a range from 0.53 to 0.78. The proposed device is anticipated to find applications in multi-plane imaging, optical tomography technique, optical data storage, and so on.

High-Efficiency Spin-Related Vortex Metalenses

In this paper, one spin-selected vortex metalens composed of silicon nanobricks is designed and numerically investigated at the mid-infrared band, which can produce vortex beams with different topological charges and achieve different spin lights simultaneously. Another type of spin-independent vortex metalens is also designed, which can focus the vortex beams with the same topological charge at the same position for different spin lights, respectively. Both of the two vortex metalenses can achieve high-efficiency focusing for different spin lights. In addition, the spin-to-orbital angular momentum conversion through the vortex metalens is also discussed in detail. Our work facilitates the establishment of high-efficiency spin-related integrated devices, which is significant for the development of vortex optics and spin optics.

Liquid crystal bifocal lens with adjustable intensities through polarization controls

In this Letter, transverse and longitudinal liquid crystal bifocal lenses (LCBLs) are proposed to continuously control the relative intensity of two foci through a simple polarization control. The modulation of a LCBL comes from the geometric phase control and is designed through the principle of holography, where the object wave is a light field from two foci respectively formed by the left-circular polarized (LCP) and right-circular polarized (RCP) light, and the reference wave is the incident plane wave. Constructed millimeter-scale LCBLs are verified experimentally, and the foci are precisely formed at the preset plane. Besides, the relative intensity can be easily controlled with different weights of LCP and RCP light. The proposed strategy overcomes the shortcomings of previous bifocal lenses, such as a complex design method, a long optimization time, and an unchangeable relative intensity, and it is expected to find potential applications in parallel optical processing and optical interconnections.

Generation of subdiffraction longitudinal bifoci by shaping a radially polarized wave

Lenses with two or more foci along the longitudinal direction exhibit immense potential in several optical applications. In this study, we propose an approach for generating subdiffraction longitudinal bifoci by binary-phase bifocal super-oscillatory lenses (SOLs), which are realized by simple AND operation between two single-foci SOLs with different focal lengths. Three bifocal SOLs with radiusRlens=70λ are designed at an operating wavelength of λ=118.8µm. Simulation results demonstrate that the minimum full width at half maximum (FWHM) is 0.397λ, and the maximum FWHM is 0.449λ, which is still smaller than the Abbe diffraction limit of 0.510λ, while all the sidelobe ratios are small (<15.1%). By properly choosing the focal length of the single-foci SOLs in the design process, the distance between the two foci can be easily controlled. Significantly, the generated bifoci with relatively uniform intensity contain a strong longitudinal electric field, which indicates their excellent prospects in optical imaging, particle acceleration, and other optical applications. In addition, the proposed bifoci-SOLs are based on the binary phase modulation, which facilitates easy fabrication compared with other approaches. These outstanding properties indicate the wide application prospects of bifocal SOLs.

All-dielectric bifocal isotropic metalens for a single-shot hologram generation device

Planar metalenses are regarded as promising functional nanodevices because of their lightweight, nano-resolution properties, and, therefore, they can serve as versatile platforms for imaging and Fourier transforming. Here, we demonstrate a meta-device that functions as an isotropic bifocal all-dielectric Huygens' metalens to realize nanoscale real-time coaxial digital hologram generation. We design an isotropic bifocal metalens for micro/nano hologram recording, and the metalens utilizes the complete region compared to a previously reported interleaved multifocal metalens scheme. In addition, the hologram generation does not depend on complex polarization conversion, thereby improving the practical efficiency. For high-fidelity reconstruction, compressive reconstruction is utilized to remove twin-image and zero-order items and to suppress noise. Such concept would be extended to white-light achromatic meta-holography and three-dimensional micro/nano in vivo incoherent super-resolution imaging under subwavelength modulation.

Polarization Controlled Dual Functional Reflective Planar Metalens in Near Infrared Regime

The metalens has been a hotspot in scientific communications in recent years. The polarization-controlled functional metalens is appealing in metalens investigation. We propose a metalens with dual functions that are controlled by polarization states. In the first design, when applied with x- and y-polarized light, two focal spots with different focal lengths are acquired, respectively. The proposed metalens performs well when illuminated with adjacent wavelengths. In the second design, the reflected light is focused when applied with x-polarized light, and when applied with y-polarized light, the reflected light is split into two oblique paths. We believe that the results will provide a new method in light manipulation.

Electrically modulated varifocal metalens combined with twisted nematic liquid crystals

Focus-tunable lenses are indispensable to optical systems. This paper proposes an electrically modulated varifocal metalens combined with twisted nematic liquid crystals. In our design, a metalens is employed to focus on different points depending on the polarization state of incident light. We demonstrated that the varifocal metalens has a sub-millisecond response time. Furthermore, the numerical aperture of both the first and second focal points can be customized to achieve a wide range of 0.2-0.7. Moreover, the full width at half maximum approached the diffraction limit at multiple focal points. Because of the advantages of our proposed electrically modulated metalens, it has the potential for application in optical technology and biomedical science, both of which require high image quality and a rapid response time.

High-Efficiency and Broadband Near-Infrared Bi-Functional Metasurface Based on Rotary Different-Size Silicon Nanobricks

Several novel spin-dependent bi-functional metasurfaces consisting of different-sized rotary silicon nanobricks have been proposed and numerically investigated based on the Pancharatnam-Berry phase and structural phase simultaneously. Here, a transmission mechanism is strictly deduced, which can avoid crosstalk from the multiplexed bi-functional metasurface. Four kinds of high-efficiency bi-functional devices have been designed successfully at infrared wavelengths, including a spin-dependent bi-functional beam deflector, a spin-dependent bi-functional metalens, a bi-functional metasurface with spin-dependent focusing and deflection function, and a spin-dependent bi-functional vortex phase plate. All of the results demonstrate the superior performances of our designed devices. Our work opens up new doors toward building novel spin-dependent bi-functional metasurfaces, and promotes the development of bi-functional devices and spin-controlled photonics.

Polarization independent dielectric metasurface for infrared beam steering applications

Over the past years, metasurfaces have been of great interest due to their ability manipulate optical wavefront by introducing a phase gradient across the transverse directions of the wave. This phase gradient was usually realized using plasmonic resonators which had high intrinsic losses. Here, we demonstrate, numerically, a proof of principle of an all-dielectric silicon based metasurface at the infrared (IR) range that manipulates the wave front and achieves beam steering with significantly high transmission. The proposed cross-shaped unit cell design shows high transmission with the ability to fully control the phase of the transmitted wave from 0 to 2π. The metasurface is made of silicon cross resonators, arranged to have a linear phase gradient, on SiO₂ substrate which makes the device compatible with most standard semiconductor fabrication techniques.

Spin-Selected Dual-Wavelength Plasmonic Metalenses

Several novel spin-selected dual-wavelength metalenses have been proposed and investigated based on the plasmonic metasurface consisting of two kinds of rotary rectangle gap nanoantennas (RGN), which are designed based on merging two or four polarity-inverse lenses corresponding to different wavelengths (765 and 1300 nm). The spin-selected dual-wavelength metalenses with two similar and two different vertical and lateral focal points have also been proposed respectively, which can focus two wavelengths with inverse spin states to arbitrary special positions. The three-dimensional metalens with four focal points have also been proposed, which can focus four beams with inverse spin states and different wavelengths to preset positions. Moreover, a spin-dependent achromatic metalens has also been proposed, which can focus left circularly polarized (LCP) incidence with different wavelengths to the same position. Our work opens up new avenues toward establishing novel spin-selected and wavelength-selected metadevices, and is significant for the development of spin-controlled photonics and particles manipulation. In addition, it provides a new idea for solving the problem of data transmission from optical fiber communication to visible light communication.

Dielectric longitudinal bifocal metalens with adjustable intensity and high focusing efficiency

Metalens recently attracts enormous attention due to its microscale figure and versatile functionalities. With the combination of geometric phase and propagation phase, we first wrote the phase equation of bifocal metalens that can high efficiently focus incidence into one or two foci in tandem along longitudinal direction, depending on the polarization of incidence. More importantly, the relative intensity of the two foci can be modulated conveniently by changing the ellipticity of incidence, which is different from previous bifocal metalenses need to be repatterned for each kind of relative intensity [Opt. Express23, 29855 (2015)]. Besides, the focusing efficiency of the proposed metalens is as high as 72%, and the separate distance between those two foci can be designed at will, which may find itself significant applications in optical tomography technique, optical data storage, and so on.

Dielectric Metasurface-Based High-Efficiency Mid-Infrared Optical Filter

Dielectric nanoresonantors may generate both electric and magnetic Mie resonances with low optical loss, thereby offering highly efficient paths for obtaining integrated optical devices. In this paper, we propose and design an optical filter with a high working efficiency in the mid-infrared (mid-IR) range, based on an all-dielectric metasurface composed of silicon (Si) nanodisk arrays. We numerically demonstrate that, by increasing the diameter of the Si nanodisk, the range of the proposed reflective optical filter could effectively cover a wide range of operation wavelengths, from 3.8 μm to 4.7 μm, with the reflection efficiencies reaching to almost 100%. The electromagnetic eigen-mode decomposition of the silicon nanodisk shows that the proposed optical filter is based on the excitation of the electric dipole resonance. In addition, we demonstrate that the proposed filter has other important advantages of polarization-independence and incident-angle independence, ranging from 0° to 20° at the resonance dip, which can be used in a broad range of applications, such as sensing, imaging, and energy harvesting.

High numerical aperture multifocal metalens based on Pancharatnam-Berry phase optical elements

A high numerical aperture multifocal metalens was proposed based on well-designed Pancharatnam-Berry phase optical elements. Both circularly and linearly polarized incident lights could be tightly focused into diffraction-limited focal spots. Right and left circularly polarized focal spots could be realized simultaneously by illuminating a linearly polarized beam. The highest numerical aperture reached to 0.84 with full width at half-maximum of 263 nm. Moreover, we also presented a metalens whose optical elements are hybrid arranged and the metalens can realize spin-independent focusing with a numerical aperture of 0.8. The presented metalens has significant potential applications in particles manipulation and high-resolution imaging.

Depth perception based 3D holograms enabled with polarization-independent metasurfaces

Metasurfaces consist of dielectric nanobrick arrays with different dimensions in the long and short axes can be used to generate different phase delays, predicting a new way to manipulate an incident beam in the two orthogonal directions separately. Here we demonstrate the concept of depth perception based three-dimensional (3D) holograms with polarization-independent metasurfaces. 4-step dielectric metasurfaces-based fan-out optical elements and holograms operating at 658 nm were designed and simulated. Two different holographic images with high fidelity were generated at the same plane in the far field for different polarization states. One can observe the 3D effect of target objects with polarized glasses. With the advantages of ultracompactness, flexibility and replicability, the polarization-independent metasurfaces open up depth perception based stereoscopic imaging in a holographic way.

Reflective metalens with sub-diffraction-limited and multifunctional focusing

We propose an ultra-thin planar reflective metalens with sub-diffraction-limited and multifunctional focusing. Based on the equal optical path principle, the specific phase distributions for multifunction focusing are derived. Following the formulas, on-center focusing with the characteristics of sub-diffraction-limited, high focusing efficiency (85%) and broadband focusing is investigated in detail. To demonstrate the flexibility of the reflective metalens, off-center and dual spots focusing (at the horizontal and longitudinal directions) are demonstrated. Note that all these focusings are sub-diffraction-limited due to the evanescent-filed enhancement mechanism in our elaborately designed structure. The designed reflective metalens will find important applications in super-resolution imaging, microscopes, and spectroscopic designs.

Enhanced optical performance of multifocal metalens with conic shapes

A multifocal metalens, which focuses incident light at multiple foci, has many applications in imaging systems and optical communications. However, the traditional design strategy of a multifocal metalens combines several lenses that have different focal points into a planar integrated unit, resulting in low imaging quality because of the high background noise. Here we show that the defects of the traditional method can be overcome by designing a metalens with conic shapes (the ellipse and the hyperbola); this approach could improve the imaging performance and substantially decrease the background noise of multifocal metalenses. These benefits arise from the intrinsic properties of the two conic curves, which can focus incident light constructively at all of the foci of the metalens. We further demonstrate that the proposed conic-shaped metalens can function well within a broadband operation wavelength that ranges from 600 to 900 nm with the dual polarity actively controlled by the incident circular polarized light. The great agreement between the experimental and simulation results demonstrates that our proposed metalens has significant potential for use in future integrated nanophotonic devices.

High-Order Dielectric Metasurfaces for High-Efficiency Polarization Beam Splitters and Optical Vortex Generators

In this paper, a high-order dielectric metasurface based on silicon nanobrick array is proposed and investigated. By controlling the length and width of the nanobricks, the metasurfaces could supply two different incremental transmission phases for the X-linear-polarized (XLP) and Y-linear-polarized (YLP) light with extremely high efficiency over 88%. Based on the designed metasurface, two polarization beam splitters working in high-order diffraction modes have been designed successfully, which demonstrated a high transmitted efficiency. In addition, we have also designed two vortex-beam generators working in high-order diffraction modes to create vortex beams with the topological charges of 2 and 3. The employment of dielectric metasurfaces operating in high-order diffraction modes could pave the way for a variety of new ultra-efficient optical devices.

Controllable design of super-oscillatory lenses with multiple sub-diffraction-limit foci

The conventional multifocal optical elements cannot precisely control the focal number, spot size, as well as the energy distribution in between. Here, the binary amplitude-type super-oscillatory lens (SOL) is utilized, and a robust and universal optimization method based on the vectorial angular spectrum (VAS) theory and the genetic algorithm (GA) is proposed, aiming to achieve the required focusing performance with arbitrary number of foci in preset energy distribution. Several typical designs of multifocal SOLs are demonstrated. Verified by the finite-difference time-domain (FDTD) numerical simulation, the designed multifocal SOLs agree well with the specific requirements. Moreover, the full-width at half-maximum (FWHM) of the achieved focal spots is close to λ/3 for all the cases (λ being the operating wavelength), which successfully breaks the diffraction limit. In addition, the designed SOLs are partially insensitive to the incident polarization state, functioning very well for both the linear polarization and circular polarization. The optimization method presented provides a useful design strategy for realizing a multiple sub-diffraction-limit foci field of SOLs. This research can find its potentials in such fields as parallel particle trapping and high-resolution microscopy imaging.

A Reconfigurable Active Huygens' Metalens

Metasurfaces enable a new paradigm to control electromagnetic waves by manipulating subwavelength artificial structures within just a fraction of wavelength. Despite the rapid growth, simultaneously achieving low-dimensionality, high transmission efficiency, real-time continuous reconfigurability, and a wide variety of reprogrammable functions is still very challenging, forcing researchers to realize just one or few of the aforementioned features in one design. This study reports a subwavelength reconfigurable Huygens' metasurface realized by loading it with controllable active elements. The proposed design provides a unified solution to the aforementioned challenges of real-time local reconfigurability of efficient Huygens' metasurfaces. As one exemplary demonstration, a reconfigurable metalens at the microwave frequencies is experimentally realized, which, to the best of the knowledge, demonstrates for the first time that multiple and complex focal spots can be controlled simultaneously at distinct spatial positions and reprogrammable in any desired fashion, with fast response time and high efficiency. The presented active Huygens' metalens may offer unprecedented potentials for real-time, fast, and sophisticated electromagnetic wave manipulation such as dynamic holography, focusing, beam shaping/steering, imaging, and active emission control.

Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays

Conventional metasurface reflector-arrays based on metallic resonant nanoantenna to control the wavefront of light for focusing always suffer from strong ohmic loss at optical frequencies. Here, we overcome this challenge by constructing a non-resonant, hybrid dielectric-metal configuration consisting of TiO₂ nanofins associated with an Ag reflector substrate that provides a broadband response and high polarization conversion efficiency in the visible range. A reflective flat lens based on this configuration shows an excellent focusing performance with the spot size close to the diffraction limit. Furthermore, by employing the superimposed phase distribution design to manipulate the wavefront of the reflected light, various functionalities, such as multifocal and achromatic focusing, are demonstrated for the flat lenses. Such a reflective flat lens will find various applications in visible light imaging and sensing systems.

Dielectric metasurface based high-efficiency polarization splitters

In this paper, a novel polarization splitter has been designed based on the dielectric metasurface consisted of silicon nanobricks array, which can generate two different wavefronts for two orthogonal input polarizations with over 90% transmitted efficiency.

Characteristics of Plasmonic Bragg Reflectors with Graphene-Based Silicon Grating

We propose a plasmonic Bragg reflector (PBR) composed of a single-layer graphene-based silicon grating and numerically study its performance. An external voltage gating has been applied to graphene to tune its optical conductivity. It is demonstrated that SPP modes on graphene exhibit a stopband around the Bragg wavelengths. By introducing a nano-cavity into the PBR, a defect resonance mode is formed inside the stopband. We further design multi-defect PBR to adjust the characteristics of transmission spectrum. In addition, through patterning the PBR unit into multi-step structure, we lower the insertion loss and suppress the rippling in transmission spectra. The finite element method (FEM) has been utilized to perform the simulation work.