Tailoring the dispersion of plasmonic nanorods to realize broadband optical meta-waveplates

Dual-Functional Tunable Metasurface for Meta-Axicon with a Variable Depth of Focus and Continuous-Zoom Metalens

Optical metasurfaces have been widely investigated for their versatile ability to manipulate wavefront and miniaturize traditional optical components into ultrathin planar devices. The integration of metasurfaces with multifunctionality and tunability has fundamentally transformed optics with unprecedented control over light propagation and manipulation. This study introduces a pioneering framework for the development of tunable metasurfaces with multifunctionality, and an example of a tunable metasurface of dual functionalities is proposed and numerically verified as one of the tunable meta-axicon for generating Bessel beams with a variable depth of focus (DOF) and a continuous-zoom metalens. Specifically, this design achieves dual-functional phase modulation by helicity-multiplexing from the combination of the geometric phase as well as the propagation phase and realizes tunability for both functionalities through rotational actuation between double metasurface layers. As a result, dual functionalities with continuous tunability of the proposed TiO2 metasurface are enabled independently for the left and right circularly polarized (LCP and RCP) incidences at 532 nm. Specifically, LCP light triggers the metasurface to function as a tunable axicon, generating non-diffracting Bessel beams with variable numerical apertures (NA) and DOFs. Conversely, the RCP incidence induces it to operate as a continuous-zoom metalens and generates variable spherical wavefront focusing on diverse focal lengths. This study not only initially implements the design of tunable meta-axicon, but also achieves the integration of such a tunable meta-axicon and continuous-zoom metalens within a single metasurface configuration. The proposed device could find potential applications in biological imaging, microscopic measurement, laser fabrication, optical manipulation, multi-plane imaging, depth estimation, optical data storage, etc.

Metasurface-enabled polarization-independent LCoS spatial light modulator for 4K resolution and beyond

With the distinct advantages of high resolution, small pixel size, and multi-level pure phase modulation, liquid crystal on silicon (LCoS) devices afford precise and reconfigurable spatial light modulation that enables versatile applications ranging from micro-displays to optical communications. However, LCoS devices suffer from a long-standing problem of polarization-dependent response in that they only perform phase modulation on one linear polarization of light, and polarization-independent phase modulation-essential for most applications-have had to use complicated polarization-diversity optics. We propose and demonstrate, for the first time, an LCoS device that directly achieves high-performance polarization-independent phase modulation at telecommunication wavelengths with 4K resolution and beyond by embedding a polarization-rotating metasurface between the LCoS backplane and the liquid crystal phase-modulating layer. We verify the device with a number of typical polarization-independent application functions including beam steering, holographical display, and in a key optical switching element - wavelength selective switch (WSS), demonstrating the significant benefits in terms of both configuration simplification and performance improvement.

Near-flat top bandpass filter based on non-local resonance in a dielectric metasurface

Localized light matter interaction at a resonant nanostructure facilitates spectrally selective transmission of light, which has led to demonstrations of ultrathin metasurface based optical filters. Unfortunately, due to the nature of Lorentzian spectral line shape in such resonances, it is inevitable to suffer significant spectral crosstalk. In this work, we demonstrate a conceptually new type of spectral filter which exhibits near flat-top bandpass with minimized spectral overlaps. To realize this, we leverage the recent development of non-local resonance in dielectric nanostructure to design a double-layered optical filter with performance comparable to the ideal spectral filters. The designed metasurface shows averaged transmission of more than 90% across the target spectral band and suppressed transmission of less than 10% out of the spectral band.

Study of a High-Index Dielectric Non-Hermitian Metasurface and Its Application in Holograms

We demonstrate that a high-index dielectric Si metasurface with a designed chiral unit structure possesses an exceptional point (EP) when it is described by a non-Hermitian Hamiltonian associated with the transmission matrix. By encircling any path in the parameter space around the EP, topologically protected 2π-phase accumulation occurs. These typical non-Hermitian properties are ascribed to complex scattering phenomena related to the coupling between electric and magnetic dipolar modes from the high-index dielectric Si metasurface. The topologically guaranteed entire 2π-phase accumulation and chiral distinction around the EP open up many promising possibilities in nanophotonic device designing; for instance, phase-only and polarization multiplexing holograms are realized in this work.

Broadband Achromatic Metalens in the Visible Light Spectrum Based on Fresnel Zone Spatial Multiplexing

Metalenses composed of a large number of subwavelength nanostructures provide the possibility for the miniaturization and integration of the optical system. Broadband polarization-insensitive achromatic metalenses in the visible light spectrum have attracted researchers because of their wide applications in optical integrated imaging. This paper proposes a polarization-insensitive achromatic metalens operating over a continuous bandwidth from 470 nm to 700 nm. The silicon nitride nanopillars of 488 nm and 632.8 nm are interleaved by Fresnel zone spatial multiplexing method, and the particle swarm algorithm is used to optimize the phase compensation. The maximum time-bandwidth product in the phase library is 17.63. The designed focal length can be maintained in the visible light range from 470 nm to 700 nm. The average focusing efficiency reaches 31.71%. The metalens can achieve broadband achromatization using only one shape of nanopillar, which is simple in design and easy to fabricate. The proposed metalens is expected to play an important role in microscopic imaging, cameras, and other fields.

Time-frequency joint mappings of a terahertz metasurface for multi-dimensional analysis of biological cells

Traditional fast Fourier transform is used to extract the frequency component at the cost of losing the time domain, which is critical for metasurface biosensing. In this Letter, a more comprehensive algorithm, continuous wavelet transform (CWT), to process signals from THz time-domain spectroscopy is introduced. By comparing the metasurface-enhanced 2D time-frequency mappings (TFMs) of HaCaT and HSC3 cells, the two types of biological cells can be clearly differentiated, showing the great potential of CWT in the label-free recognition of biological cells. Also, the 2D TFMs serve as effective visualization indicators, successfully detecting the concentration of cancer cells characterized by being label free and low cost. In addition, the 2D TFMs of different metasurfaces under the same cell concentration reveal the correlation of TFMs and localized fields. Such a feature provides evidence of an interaction between biological cells and electromagnetic waves, implying the absorption of THz radiation by biological cells can be effectively controlled by properly designing split ring resonators (SRRs) of metasurfaces.

Twisted Two-Dimensional Material Stacks for Polarization Optics

The ability to control the light polarization state is critically important for diverse applications in information processing, telecommunications, and spectroscopy. Here, we propose that a stack of anisotropic van der Waals materials can facilitate the building of optical elements with Jones matrices of unitary, Hermitian, non-normal, singular, degenerate, and defective classes. We show that the twisted stack with electrostatic control can function as arbitrary-birefringent wave-plate or arbitrary polarizer with tunable degree of non-normality, which in turn give access to plethora of polarization transformers including rotators, pseudorotators, symmetric and ambidextrous polarizers. Moreover, we discuss an electrostatic-reconfigurable stack which can be tuned to operate as four different polarizers and be used for Stokes polarimetry.

Broadband polarization-insensitive metalens integrated with a charge-coupled device in the short-wave near-infrared range

The performance of a charge-coupled device is important in detection accuracy for terminal sensitivity of a short-wave near-infrared spectrometer. The sizes of pixel pitch and pixel itself are reducing with the development of CCD technologies. However, the fill factor of CCD pixels has not been significantly improved due to the limits of the shift registers, which makes a lower utilization rate of incident light of CCD. In recent years, the advance of metasurface optics provides a new solution for solving this problem. In this paper, a polarization-insensitive metalens is experimentally demonstrated to increase the fill factor of short-wave near-infrared CCD pixels by 4 times, and the simulated results show that the designed metalens has an excellent optical crosstalk (≤0.8%). It proves that the fill factor of CCD pixels can be further improved by the proposed approach which would pave the way for the overall integration of metalens array and photodetectors, as well as the development of CCD miniaturization and lightweight.

Functional Metasurface Quarter-Wave Plates for Simultaneous Polarization Conversion and Beam Steering

The capability to manipulate the polarization state of light at the nanoscale is of paramount importance in many emerging research areas ranging from optical communication to quantum information processing. Gap-surface plasmon (GSP) metasurfaces, which provide advantages and abilities of molding reflected fields, have been demonstrated excellently suited for integration of multifunctional polarization optics into a single device. Here, we establish a versatile GSP metasurface platform based on nanoscale quarter-wave plates (nano-QWPs) that enable efficient circular-to-linear polarization conversion along with the complete phase control over reflected fields. Capitalizing on the nano-QWP design, we demonstrate, both theoretically and experimentally, how resonance and geometric phases can be used in concert to achieve independent and simultaneous phase modulation of both co- and cross-polarized circularly polarized (CP) waves by realizing arbitrary beam steering of co- and cross-polarized CP channels in a broadband near-infrared range. The GSP metasurface platform established in our work provides versatile and flexible solutions to enrich multiple functionalities for diversified metasurface-based polarization optics exploited in modern integrated photonic devices and systems.

Broadband anomalous reflective metasurface for complementary conversion of arbitrary incident polarization angles

The abrupt phase changes at the interface can modulate the polarization and wavefront of electromagnetic waves, which is the physical mechanism of the plasmonic metasurfaces. Conventional polarization converters are difficult to obtain pure polarized light, and most of the anomalously reflecting metasurfaces are limited by the specific angle of incident polarization. Here, we present a high-efficient polarization-independent metasurface for broadband polarization conversion and anomalous reflection when a plane wave with an arbitrary polarization angle is incident vertically. We vary the dimensions of the polarization conversion unit cells and arrange them periodically to cover the full 2π phase range of cross-polarized light in two orthogonal directions. The simulation results show that the pure anomalous cross-polarization efficiency is over 80% over a wavelength range from 1400nm to 1800nm. In particular, the metasurface can realize the complementary conversion of polarization angle for incident light at any polarization angle, and deflect it to a specific angle. Our design provides strategies for miniaturization and integration of polarization conversion devices and systems.

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.

Metalens Eyepiece for 3D Holographic Near-Eye Display

Near-eye display (NED) systems for virtual reality (VR) and augmented reality (AR) have been rapidly developing; however, the widespread use of VR/AR devices is hindered by the bulky refractive and diffractive elements in the complicated optical system as well as the visual discomfort caused by excessive binocular parallax and accommodation-convergence conflict. To address these problems, an NED system combining a 5 mm diameter metalens eyepiece and a three-dimensional (3D), computer-generated holography (CGH) based on Fresnel diffraction is proposed in this paper. Metalenses have been extensively studied for their extraordinary capabilities at wavefront shaping at a subwavelength scale, their ultrathin compactness, and their significant advantages over conventional lenses. Thus, the introduction of the metalens eyepiece is likely to reduce the issue of bulkiness in NED systems. Furthermore, CGH has typically been regarded as the optimum solution for 3D displays to overcome limitations of binocular systems, since it can restore the whole light field of the target 3D scene. Experiments are carried out for this design, where a 5 mm diameter metalens eyepiece composed of silicon nitride anisotropic nanofins is fabricated with diffraction efficiency and field of view for a 532 nm incidence of 15.7% and 31°, respectively. Furthermore, a novel partitioned Fresnel diffraction and resample method is applied to simulate the wave propagations needed to produce the hologram, with the metalens capable of transforming the reconstructed 3D image into a virtual image for the NED. Our work combining metalens and CGH may pave the way for portable optical display devices in the future.

Large bandwidth and high-efficiency plasmonic quarter-wave plate

A large bandwidth and high-efficiency subwavelength quarter-wave plate (QWP) is an indispensable component of an integrated miniaturized optical system. The bandwidth of existing plasmonic quarter-wave plates with a transmission efficiency of more than 50% is less than 320 nm in the near-infrared band. In this paper, a metallic quarter-wave plate with a bandwidth of 600 nm (0.95-1.55 µm) and an average transmittance of more than 70% has been designed and shows excellent potential to be used in miniaturized optical polarization detection systems and as an optical data storage device. For TE mode incident waves, this miniaturized optical element can be equivalent to a Fabry-Pérot (FP) resonator. Meanwhile, for the TM mode incident wave, the transmission characteristics of this structure are controlled by gap surface plasmon polaritons (G-SPPs) existing in the symmetric metal/insulator/metal (MIM) configuration.

None sharp corner localized surface plasmons resonance based ultrathin metasurface single layer quarter wave plate

We report on a non-sharp-corner quarter wave plate (NCQW) within the single layer of only 8 nm thickness structured by the Ag hollow elliptical ring array, where the strong localized surface plasmons (LSP) resonances are excited. By manipulating the parameters of the hollow elliptical ring, the transmitted amplitude and phase of the two orthogonal components are well controlled. The phase difference of π/2 and amplitude ratio of 1 is realized simultaneously at the wavelength of 834 nm with the transmission of 0.46. The proposed NCQW also works well in an ultrawide wavelength band of 110 nm, which suggests an efficient way of exciting LSP resonances and designing wave plates, and provides a great potential for advanced nanophotonic devices and integrated photonic systems.

Multiplexing multifoci optical metasurfaces for information encoding in the ultraviolet spectrum

Recently, optical metasurfaces have attracted much attention due to their versatile features in manipulating phase, polarization, and amplitude of both reflected and transmitted light. Because it controls over four degrees of freedom: phase, polarization, amplitude, and wavelength of light wavefronts, optical cryptography is a promising technology in information security. So far, information encoding can be implemented by the metasurface in one-dimensional (1D) mode (either wavelength or polarization) and in a two-dimensional (2D) mode of both wavelength and polarization. Here, we demonstrate multiplexing multifoci optical metasurfaces for information encoding in the ultraviolet spectrum both in the 1D and 2D modes in the spatial zone, composed of high-aspect-ratio aluminum nitride nanorods, which introduce discontinuous phases through the Pancharatnam-Berry phase to realize multifoci in the spatial zone. Since the multiplexed multifocal optical metasurfaces are sensitive to the helicity of the incident light and the wavelength is within the ultraviolet spectrum, the security of the information encrypted by it would be guaranteed.

Tunable wave plates based on phase-change metasurfaces

Wave plates based on metasurfaces have attracted intensive attention over the past decade owing to their compactness and design flexibility. Although various wave plates have been designed, their working wavelengths are fixed once they are made. Here we present a study on tunable wave plates based on phase-change metasurfaces made of Ge₂Sb₂Te₅ nanopillar structures. The Ge₂Sb₂Te₅ nanopillars can work as a high-efficiency transmissive half- or quarter-wave plate depending on their structural parameters. The working wavelength of wave plate can be tuned via the phase transition of Ge₂Sb₂Te₅. Moreover, the polarization state of the transmitted light at a fixed wavelength can be modified by changing the crystallinity of Ge₂Sb₂Te₅. The features suggest that tunable wave plates may have applications in optical modulators, molecular detection, and polarimetric imaging.

Planar multi-angle retro-reflectors based on the wave-vector-reversion of spoof surface plasmon polaritons

Spoof surface plasmon polariton (SSPP) is kind of sub-wavelength electromagnetic (EM) mode, which is favorable for miniaturization and thinning of EM devices. In this paper, we propose a method of designing thin planar retro-reflector that can operate under multiple incidence angles at the same frequency. The retro-reflector is composed of a transmissive phase gradient metasurface (TPGM) placed above a metallic patch array (MPA), where the former couples and decouples SSPPs while the latter supports eigen-mode propagation of SSPPs. Under oblique incident angles, the TGPM can impart 0 and π Pancharatnam-Berry (P-B) phases alternatively, producing P-B phase gradients along its surface. Incident waves can be coupled as SSPPs propagating on the MPA which will be reflected at the borders of the MPA, thus the wave-vector of SSPPs is reversed. In this way, retro-reflection can be realized under the two incidence angles θ=±45.0°. Moreover, due to mode mismatch between the TPGM and MPA under normal incidence, the retro-reflector acts like a planar metallic plate under θ=0°. To verify this method, a prototype was designed, fabricated and measured. Both the simulation and measurement results verify significant backscattering enhancement under θ=±45.0° and 0° at 10.0 GHz. This work provides an alternative method of designing planar retro-reflectors and may find applications in wireless communication, target tracking, etc.

Chiral Nanoceramics

The study of different chiral inorganic nanomaterials has been experiencing rapid growth during the past decade, with its primary focus on metals and semiconductors. Ceramic materials can substantially expand the range of mechanical, optical, chemical, electrical, magnetic, and biological properties of chiral nanostructures, further stimulating theoretical, synthetic, and applied research in this area. An ever-expanding toolbox of nanoscale engineering and self-organization provides a chirality-based methodology for engineering of hierarchically organized ceramic materials. However, fundamental discoveries and technological translations of chiral nanoceramics have received substantially smaller attention than counterparts from metals and semiconductors. Findings in this research area are scattered over a variety of sources and subfields. Here, the diversity of chemistries, geometries, and properties found in chiral ceramic nanostructures are summarized. They represent a compelling materials platform for realization of chirality transfer through multiple scales that can result in new forms of ceramic materials. Multiscale chiral geometries and the structural versatility of nanoceramics are complemented by their high chiroptical activity, enantioselectivity, catalytic activity, and biocompatibility. Future development in this field is likely to encompass chiral synthesis, biomedical applications, and optical/electronic devices. The implementation of computationally designed chiral nanoceramics for biomimetic catalysts and quantum information devices may also be expected.

Characterisation and Manipulation of Polarisation Response in Plasmonic and Magneto-Plasmonic Nanostructures and Metamaterials

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

Constructing Metastructures with Broadband Electromagnetic Functionality

Electromagnetic metastructures stand for the artificial structures with a characteristic size smaller than the wavelength, which may efficiently manipulate the states of light. However, their applications are often restricted by the bandwidth of the electromagnetic response of the metastructures. It is therefore essential to reassert the principles in constructing broadband electromagnetic metastructures. Herein, after summarizing the conventional approaches for achieving broadband electromagnetic functionality, some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and several trade-off approaches are reviewed, followed by some perspectives for the future development of broadband metamaterials. It is anticipated that broadband metastructures will have even more substantial applications in optoelectronics, energy harvesting, and information technology.

Generation of E-band metasurface-based vortex beam with reduced divergence angle

Vortex beams carrying orbital angular momentum (OAM) have attracted considerable attention for the development of high-capacity wireless communication systems due to their infinite sets of orthogonal modes. However, the practical applications of Laguerre-Gaussian type vortex beams are limited due to the fact that the divergence angle increases as the order of the OAM mode increases. In this work, we present metasurfaces that generate vortex beams carrying OAM modes with reduced divergence angles in the E-band frequency range. The metasurfaces were designed using eight different meta-atom phase elements, including a spiral phase distribution for OAM modes l = 1 and 2, a phase gradient array to avoid interference with the source beam, and a lens pattern array to reduce the divergence angle. Through simulation and experimental measurement, it was confirmed that the divergence angle of the vortex beam generated by the metasurface with the lens pattern was reduced from 13° to 9° and 14° to 11° for OAM modes l = 1 and 2, respectively, in comparison with the metasurface without the lens pattern. Our results provide new design methods for various applications based on OAM multiplexing especially in high frequency E-band range.

Design of AlN ultraviolet metasurface for single-/multi-plane holography

The metasurface promises an unprecedented way for manipulating wavefronts and has strengths in large information capacity for the hologram. However, strong absorption loss for most dielectric materials hinders the realization of such a metasurface operating in the ultraviolet (UV) spectrum. Herein, aluminum nitride (AlN) with an ultrawide bandgap has been utilized as the material of the UV metasurface for multi-plane holography, increasing the information capacity and security level of information storage simultaneously. The metasurface for multi-plane holography achieving a correlation coefficient of over 0.8 with three reconstructed images has been investigated, and also the single-plane holography at an efficiency of 34.05%. Our work might provide potential application in UV nanophotonics.

Stretchable All-Dielectric Metasurfaces with Polarization-Insensitive and Full-Spectrum Response

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO₂ metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

Generation of polarization singularities with geometric metasurfaces

The polarization singularities are directly generated by using plasmonic metasurfaces with the geometric phase profiles designed to form the Poincaré beams. Different morphologies of polarization topological structures of lemon, star, monstar, spiral, dipole and quadrupole are created by the superpositions of Laguerre–Gauss modes with different orders under orthogonal circular or linear polarization basis. The polarization ellipse patterns and topological features of the produced optical vector fields are analyzed to reveal the properties of the polarization singularities of C-points and L-lines, and the orbital angular momentum states are also measured. The demonstrated polarization singularities generated from the geometric metasurfaces will promise many potential applications related to optical polarization imaging, metrology, optical trapping and quantum information processing.

Broadband Ultrathin Transmission Quarter Waveplate with Rectangular Hole Array Based on Plasmonic Resonances

The control of the polarization states of light plays an important role in modern optical systems. However, traditional polarization manipulating devices often have narrow bandwidth and their large size makes it difficult for them to achieve miniaturization and integration of optical systems. This work presents an ultrathin quarter waveplate with a periodic silver film 2 × 2 rectangular hole array with a thickness less than λ/50. Numerical simulation shows that the waveplate can efficiently transform a circular polarized wave into a linearly polarized one at the center of 1550 nm, and its bandwidth is 525 nm. Furthermore, the quarter waveplate can efficiently invert linear polarization into circular polarization at 1550 nm, which ellipticity is near unit. With an array of small holes on a metal film to enhance transmission, this structure can increase the transmission to 0.44. The broadband quarter waveplate can be used in communication system and near infrared band system, and be integrated with other optical devices at nanoscale to achieve polarization operation, detection, and sensing.

Spatial variation of vector vortex beams with plasmonic metasurfaces

The spatial variation of vector vortex beams with arbitrary polarization states and orbital angular momentum (OAM) values along the beam propagation is demonstrated by using plasmonic metasurfaces with the initial geometric phase profiles determined from the caustic theory. The vector vortex beam is produced by the superposition of deflected right- and left-handed circularly polarized component vortices with different helical phase charges, which are simultaneously generated off-axially by the single metasurface. Besides, the detailed evolution processes of intensity profile, polarization distribution and OAM value along the beam propagation distance is analyzed. The demonstrated arbitrary space-variant vector vortex beam will pave the way to many promising applications related to spin-to-orbital angular momentum conversion, spin-orbit hybrid entanglement, particle manipulation and transportation, and optical communication.

Orbital angular momentum transformation of optical vortex with aluminum metasurfaces

The orbital angular momentum (OAM) transformation of optical vortex is realized upon using aluminum metasurfaces with phase distributions derived from the caustic theory. The generated OAM transformation beam has the well-defined Bessel-like patterns with multiple designed topological charges from −1 to +2.5 including both the integer-order and fractional-order optical vortices along the propagation. The detailed OAM transformation process is observed in terms of the variations of both beam intensity and phase profiles. The dynamic distributions of OAM mode density in the transformation are further analyzed to illustrate the conservation of the total OAM. The demonstration of transforming OAM states arbitrarily for optical vortex beams will lead to many new applications in optical manipulation, quantum optics, and optical communication.

Modulating phase by metasurfaces with gated ultra-thin TiN films

Active control over the flow of light is highly desirable because of its applicability to information processing, telecommunication, and spectroscopic imaging. In this paper, by employing the tunability of carrier density in a 1 nm titanium nitride (TiN) film, we numerically demonstrate deep phase modulation (PM) in an electrically tunable gold strip/TiN film hybrid metasurface. A 337° PM is achieved at 1.550 μm with a 3% carrier density change in the TiN film. We also demonstrate that a continuous 180° PM can be realized at 1.537 μm by applying a realistic experiment-based gate voltage bias and continuously changing the carrier density in the TiN film. The proposed design of active metasurfaces capable of deep PM near the wavelength of 1.550 μm has considerable potential in active beam steering, dynamic hologram generation, and flat photonic devices with reconfigurable functionalities.

Approach of realizing arbitrary wavefront manipulation with continuous meander line structures

In the paper, we systematically present the approach of realizing arbitrary wavefront manipulation with continuous meander line structures that can send the outgoing light with the opposite polarization in respect to the incident light into desired directions. The manipulation behavior is found to be wavelength-dependent, analogous to the traditional grating. Propagation characteristic and filed analysis are used to understand the generation of the polarization conversed outgoing light. The results suggest that the surface mode of TM component mainly governs at long wavelength and the oscillating mode of TE component is dominant at short wavelength, resulting in a broad operation region. The backward process, i.e., deriving a special function based on the given irradiation pattern, verifies the generation of the approach. The finding gives an alternative to realize the wavefront manipulation of transmitted light with a thin metal device and an insight into the traditional meander line structure.

From Single-Dimensional to Multidimensional Manipulation of Optical Waves with Metasurfaces

Metasurfaces, 2D artificial arrays of subwavelength elements, have attracted great interest from the optical scientific community in recent years because they provide versatile possibilities for the manipulation of optical waves and promise an effective way for miniaturization and integration of optical devices. In the past decade, the main efforts were focused on the realization of single-dimensional (amplitude, frequency, polarization, or phase) manipulation of optical waves. Compared to the metasurfaces with single-dimensional manipulation, metasurfaces with multidimensional manipulation of optical waves show significant advantages in many practical application areas, such as optical holograms, sub-diffraction imaging, and the design of integrated multifunctional optical devices. Nowadays, with the rapid development of nanofabrication techniques, the research of metasurfaces has been inevitably developed from single-dimensional manipulation toward multidimensional manipulation of optical waves, which greatly boosts the application of metasurfaces and further paves the way for arbitrary design of optical devices. Herein, the recent advances in metasurfaces are briefly reviewed and classified from the viewpoint of different dimensional manipulations of optical waves. Single-dimensional manipulation and 2D manipulation of optical waves with metasurfaces are discussed systematically. In conclusion, an outlook and perspectives on the challenges and future prospects in these rapidly growing research areas are provided.

Constructing multifunctional wave plates with stereo-metastructure arrays

Driven by the development of nanophotonics and integrated optics, manipulating polarization of light with metastructures has been extensively studied in recent decades. So far there is still a high demand for more efficient ways to control the polarization state of light with extraordinary performance. In this Letter, we report on constructing multifunctional wave plates with stereo-metastructure arrays (SMAs) by two-photon absorption polymerization. In one frequency range, the SMA can turn the polarization direction of incident linearly polarized (LP) light to its orthogonal direction, acting as a half-wave plate (HWP). In the other frequency range, it converts the LP incident light to circularly polarized (CP) light, acting as a quarter-wave plate (QWP). Such a multifunctional element is expected to possess an energy efficiency as high as 75%. By encoding SMAs with different rotation angles at different spatial areas, we show that SMAs can be applied in imaging and sensing, where the focal-plane-array (FPA) imaging demonstrates patterned contrast following different structural distribution.

Broadband circular polarizer for randomly polarized light in few-layer metasurface

Controlling the polarization state of light has been a significant issue for various integrated optical devices such as optical imaging, sensors, and communications. Recent advances in metamaterials enable the optical elements for controlling light to be miniaturized and to have various multi-functions in subwavelength scale. However, a conventional approach of a circular polarizer has an inherent limitation to eliminate the unwanted circular polarization, which means that the efficiency varies significantly depending on the polarization state of incident light. Here, we propose a novel concept of a circular polarizer by combining two functions of transmission and conversion for orthogonal circular polarizations with a total thickness of 440 nm. The proposed three-layer metasurface composed of rotating silver nanorods transmits the left-handed circularly polarized (LCP) light with maintaining its own polarization state, whereas the right-handed circularly polarized (RCP) light is converted into LCP light. Regardless of the polarization state of incoming light, the polarization of light in the last medium is LCP state in the broadband operating wavelength range from 800 nm to 1100 nm. The converted RCP and the transmitted LCP have efficiencies of up to 48.5% and 42.3%, respectively. Thus the proposed metasurface serves as a stable circular polarizer for a randomly polarized light. In addition, high-efficiency asymmetric transmission of about 0.47 is achieved at the same time due to the conversion characteristic of RCP component. The proposed metasurface has the significance as an ultra-thin optical element applicable to optical switching, sensors, and communications in unidirectional channel as well as a broadband circular polarizer for randomly polarized light.

Subwavelength polarization optics via individual and coupled helical traveling-wave nanoantennas

Light polarization control is a key factor in modern photonics. Recent advances in surface plasmon manipulation have introduced the prospect of more compact and more efficient devices for this purpose. However, the current plasmonic-based polarization optics remain much larger than the wavelength of light, which limits the design degrees of freedom. Here, we present a plasmonic traveling-wave nanoantenna using a gold-coated helical carbon nanowire end-fired with a dipolar aperture nanoantenna. Our nonresonant helical nanoantenna enables tunable polarization control by swirling surface plasmons on the subwavelength scale and taking advantage of the optical spin-orbit interaction. Four closely packed helical traveling-wave nanoantennas (HTNs) are demonstrated to locally convert an incoming light beam into four beams of tunable polarizations and intensities, with the ability to impart different polarization states to the output beams in a controllable way. Moreover, by near-field coupling four HTNs of opposite handedness, we demonstrate a subwavelength waveplate-like structure providing a degree of freedom in polarization control that is unachievable with ordinary polarization optics and current metamaterials.

Plasmonic metasurfaces with 42.3% transmission efficiency in the visible

Metasurfaces are two-dimensional nanoantenna arrays that can control the propagation of light at will. In particular, plasmonic metasurfaces feature ultrathin thicknesses, ease of fabrication, field confinement beyond the diffraction limit, superior nonlinear properties, and ultrafast performances. However, the technological relevance of plasmonic metasurfaces operating in the transmission mode at optical frequencies is questionable due to their limited efficiency. The state-of-the-art efficiency of geometric plasmonic metasurfaces at visible and near-infrared frequencies, for example, is ≤10%. Here, we report a multipole-interference-based transmission-type geometric plasmonic metasurface with a polarization conversion efficiency that reaches 42.3% at 744 nm, over 400% increase over the state of the art. The efficiency is augmented by breaking the scattering symmetry due to simultaneously approaching the generalized Kerker condition for two orthogonal polarizations. In addition, the design of the metasurface proposed in this study introduces an air gap between the antennas and the surrounding media that confines the field within the gap, which mitigates the crosstalk between meta-atoms and minimizes metallic absorption. The proposed metasurface is broadband, versatile, easy to fabricate, and highly tolerant to fabrication errors. We highlight the technological relevance of our plasmonic metasurface by demonstrating a transmission-type beam deflector and hologram with record efficiencies.

Superoscillation: from physics to optical applications

The resolution of conventional optical elements and systems has long been perceived to satisfy the classic Rayleigh criterion. Paramount efforts have been made to develop different types of superresolution techniques to achieve optical resolution down to several nanometres, such as by using evanescent waves, fluorescence labelling, and postprocessing. Superresolution imaging techniques, which are noncontact, far field and label free, are highly desirable but challenging to implement. The concept of superoscillation offers an alternative route to optical superresolution and enables the engineering of focal spots and point-spread functions of arbitrarily small size without theoretical limitations. This paper reviews recent developments in optical superoscillation technologies, design approaches, methods of characterizing superoscillatory optical fields, and applications in noncontact, far-field and label-free superresolution microscopy. This work may promote the wider adoption and application of optical superresolution across different wave types and application domains.

Simultaneous control of polarization and amplitude over broad bandwidth using multi-layered anisotropic metasurfaces

In this paper, the broadband transmissive modulation of polarization and amplitude is demonstrated with high efficiency and tunability using multi-layered aluminum metasurfaces. Broadband and nondispersive optical rotation in the optical frequency region is realized by using Fabry-Pérot-like cavity and phase compensation. Simultaneously, the transmission amplitude can be independently controlled by adjusting the twist angle of the anisotropic metasurfaces. The proposed polarization-amplitude modulators are numerically demonstrated to achieve large tunability with an amplitude modulation depth of 0.95 and maximum rotation angle of 180°.

Optical interconnects based on high-contrast all-dielectric nano-post arrays

In this paper, we present a compact solution for optical interconnects in optoelectronic integrated neural networks using high-contrast all-dielectric nano-post arrays. The nano-post arrays are made of amorphous silicon, which has a high refractive index and high transmittance in the near infrared. The radius of each post is changed to generate different phase delays. Deflection and convergence of the light are realized by proper design of the phase profile of the nano-post array. Connection efficiencies are calculated by numerical simulations and compared with those of zone plate interconnects. Simulation results show that the proposed nano-post arrays can have superior performance over zone plates in applications that require short focal length and high efficiency.

Generation of vector beams using spatial variation nanoslits with linearly polarized light illumination

Vector beams (VBs) that possess nonuniform polarization distributions in space have various applications. In view of the utilization of the circularly polarized light in generating VBs based on the metallic structures, this paper proposes an approach to generate VBs using metallic nanoslits with linearly polarized light illumination. These nanoslits are located on two concentric circular orbits, and the nanoslits on the inner circle are perpendicular to the ones in the outer circle. The linearly polarized light is effectively changed into the rotational symmetric VBs by rotating these orthogonal nanoslits, and the polarization order of the VBs can be adjusted by changing the rotation angles of nanoslits. The detailed theoretical analysis provides the basis for the conversion from the linearly polarized light to the VBs. Numerical simulations and experimental measurement demonstrate the performance of VB generators. This paper's proposed method has advantages that include ultrathin and compact structure, convenient operation and immediate conversion from linear polarization to VBs, and easier expansion of VB applications.

0.2 λ0 Thick Adaptive Retroreflector Made of Spin-Locked Metasurface

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength-thick reconfigurable C-shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ₀ down to only 0.2 λ₀ . The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)-polarized and transverse electric (TE)-polarized incidences. With the phase gradient being engineered, an in-plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro-reflected wave identical to that of the incident light. Such spin-locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin-locked metasurface could find promising applications in spin-based optical devices, communication systems, remote sensing, RCS enhancement, and so on.

Circular-polarization-sensitive absorption in refractory metamaterials composed of molybdenum zigzag arrays

Circularly polarized light (CPL) is utilized in various fields, including optical communication and biological imaging. To overcome the lack of circular-polarization-sensitive absorbers working at high temperature, a refractory and circular-polarization-sensitive absorber comprised of molybdenum zigzag arrays is proposed. At certain resonant wavelengths, one component of circular polarization is absorbed by confining electromagnetic field in the dielectric layer, while the other component is backscattered. The circular-polarization-sensitive absorber could be applied as a CPL thermal radiator as well as a reflective linear-to-circular polarizer. As a CPL thermal radiator, left-handed circular radiation and right-handed circular radiation are dominant at different temperatures, respectively. As a linear-to-circular polarizer, both perfect left-handed circularly polarized light and nearly perfect right-handed circularly polarized light are obtained.

Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials

Deep-learning framework has significantly impelled the development of modern machine learning technology by continuously pushing the limit of traditional recognition and processing of images, speech, and videos. In the meantime, it starts to penetrate other disciplines, such as biology, genetics, materials science, and physics. Here, we report a deep-learning-based model, comprising two bidirectional neural networks assembled by a partial stacking strategy, to automatically design and optimize three-dimensional chiral metamaterials with strong chiroptical responses at predesignated wavelengths. The model can help to discover the intricate, nonintuitive relationship between a metamaterial structure and its optical responses from a number of training examples, which circumvents the time-consuming, case-by-case numerical simulations in conventional metamaterial designs. This approach not only realizes the forward prediction of optical performance much more accurately and efficiently but also enables one to inversely retrieve designs from given requirements. Our results demonstrate that such a data-driven model can be applied as a very powerful tool in studying complicated light-matter interactions and accelerating the on-demand design of nanophotonic devices, systems, and architectures for real world applications.

Generation of three-dimensional optical cusp beams with ultrathin metasurfaces

Cusp beams are one type of complex structured beams with unique multiple self-accelerating channels and needle-like field structures owning great potentials to advance applications such as particle micromanipulation and super-resolution imaging. The traditional method to generate optical catastrophe is based on cumbrous reflective diffraction optical elements, which makes optical system complicated and hinders the nanophotonics integration. Here we design geometric phase based ultrathin plasmonic metasurfaces made of nanoslit antennas to produce three-dimensional (3D) optical cusp beams with variable numbers of self-accelerating channels in a broadband wavelength range. The entire beam propagation profiles of the cusp beams generated from the metasurfaces are mapped theoretically and experimentally. The special self-accelerating behavior and caustics concentration property of the cups beams are also demonstrated. Our results provide great potentials for promoting metasurface-enabled compact photonic devices used in wide applications of light-matter interactions.

Twisting phase and intensity of light with plasmonic metasurfaces

Twisting light in both phase and intensity has recently drawn great interests in various fields related to light-matter interactions such as optical manipulation of particles and quantum entanglement of photons. Conventionally, bulky optical components are required to produce such twisted optical beams, which significantly limits their applications in integrated photonics and optical chips. Here, we design and demonstrate aluminum plasmonic metasurfaces consisting of nanoslit antennas as ultracompact beam converters to generate the focused twisted beams in both phase and intensity across the visible wavelength range. The metasurface is encoded with the combined phase profile containing the helico-conical phase function together with a Fourier transform lens based on the Pancharatnam-Berry (PB) geometric phase. It is demonstrated that the created twisted beams simultaneously possess three-dimensional (3D) spiral intensity distribution around the propagation axis and complex phase structure containing both the central vortex and the peripheral vortex string. Moreover, the twisted beam exhibits an arithmetic intensity spiral at the focal plane with the maximum photon concentration located at the leading point of the spiral. Our results show the promising potential for advancing metasurface-based integrated devices in many applications of light-matter interactions.

Nonreciprocal Flat Optics with Silicon Metasurfaces

Metasurfaces enable almost complete control of light through ultrathin, subwavelength surfaces by locally and abruptly altering the scattered phase. To date, however, all metasurfaces obey time-reversal symmetry, meaning that forward and backward traveling waves will trace identical paths when being reflected, refracted, or diffracted. Here, we use full-field calculations to design a passive metasurface for nonreciprocal transmission of both direct and anomalously refracted near-infrared light over nanoscale optical path lengths. The metasurface consists of a 100 nm-thick, periodically patterned Si slab. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal anomalous refraction with appropriate patterning to form a phase gradient metasurface. Compared to existing schemes for breaking time-reversal symmetry, our platform enables subwavelength nonreciprocity for arbitrary free-space optical inputs and provides a straightforward path to experimental realization. The concept is also generalizable to other metasurface functions, providing a foundation for one-way lensing and holography.

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Polarization control using passive and active crossed graphene gratings

Graphene gratings provide a promising route towards the miniaturization of THz metasurfaces and other photonic devices, chiefly due to remarkable optical properties of graphene. In this paper, we propose novel graphene nanostructures for passive and active control of the polarization state of THz waves. The proposed devices are composed of two crossed graphene gratings separated by an insulator spacer. Because of specific linear and nonlinear properties of graphene, these optical metasurfaces can be utilized as ultrathin polarization converters operating in the THz frequency domain. In particular, our study shows that properly designed graphene polarizers can effectively select specific polarization states, their thickness being about a tenth of the operating wavelength and size more than 80× smaller than that of similar metallic devices. Equally important, we demonstrate that the nonlinear optical properties of graphene can be utilized to actively control the polarization state of generated higher harmonics.

Broadband wave plates made by plasmonic metamaterials

Although metamaterials wave-plates have been demonstrated previously, many of them suffer from the issue of narrow bandwidth since they typically rely on resonance principles and thus exhibit inevitable frequency dispersions. Here, we show that the dispersion of spoof surface plasmon (SSP) mode supported by a fishbone structure can be freely modulated by varying the structural parameters. This motivates us to establish a general strategy of building broadband wave-plates by cascading two fishbone structures with different propagation constants of SSP modes. We derive a criterion under which the cross-polarization phase-difference across the whole device can maintain at a nearly constant value over a wide frequency band, with frequency dispersions in the two fishbone structures cancelled out. As an illustration, we design and fabricate an efficient microwave quarter-wave plate and experimentally characterize its excellent polarization-control performances over a broad frequency band (7-9.2 GHz). Our findings can stimulate making dispersion-controlled high-performance optical functional devices in different frequency domains.

Nanoscale beam splitters based on gradient metasurfaces

Beam splitters are essential components in various optical and photonic applications, for example, interferometers, multiplexers, and so on. Present beam splitters based on cubes or plates are normally bulky. Realizing beam splitters in nanoscales is useful to reduce the total size of photonic devices. We demonstrate here a beam splitter with nanoscale thickness based on a gradient metasurface comprising lithium niobate cylinder arrays. Since one unit cell of metasurface comprising two cylinder rows shows two opposite phase gradients, the incident light is split into different directions according to the generalized Snell's law. The split ratio is proven to be effectively tunable.

Photoassisted bottom-up construction of plasmonic nanocity

Herein, photoassisted self-construction of nanocity as a novel plasmonic metasurface was achieved. It is an ensemble of nanobuildings, and the height of each nanobuilding is greater than its depth. Plasmonic nanocity exhibits a vertical resonance mode in addition to distal longitudinal and proximal longitudinal resonance modes, such that it can be applied to chromatic angular polarizers, sophisticated image recording, and high density data storage. Further growth of nanobuildings to bulky and tall nanocuboids leads to asymmetric and dichroic scattering, which can be applied in security printing.