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Convolution Operation on Pancharatnam-Berry Coding Metasurfaces in Visible Band
Abstract
Coding metasurfaces bring photonic design into a new era for their convenience in manipulating electromagnetic waves in a programmable way. Different coding patterns can be further convoluted to give rise to nontrivial effects such as multiple beam steering, simultaneous control of surface and space waves, and the integration of multiple functionalities. However, previous experimental works have been limited to low frequencies. Extending convoluted coding metasurfaces to optical frequency can significantly reduce the size of structures, and therefore facilitate their applications in optical circuits. Here, we experimentally demonstrate a Pancharatnam-Berry metasurface designed by convoluting two distinct coding patterns. Such a metasurface exhibits integrated functionalities: the optical spin Hall effect and spin to orbital angular momentum conversion, which are robust over a broad visible band and a wide range of incident angles. Our work represents an important step towards multifunctional coding metasurfaces at optical frequency based on convolution operation.
Vortex beams, as beams carrying orbital angular momentum (OAM), exhibit unique donut-shaped intensity distributions and helical wavefronts. They are widely applied in fields such as optical communication, nanoparticle manipulation, and quantum information. Traditional vortex beam generation methods, such as those based on Pancharatnam–Berry phase design, can effectively generate vortex beams, but the conversion efficiency and design flexibility are limited by polarization states and incident angles. In addition, the generated and propagated vortex beams require separate metasurface for wavefront deflection and refocusing for practical applications. This work proposes a novel metasurface design approach based on resonant phase, where phase coverage of 2 π is achieved by varying the radius of the nanocylinders. In addition to the efficient vortex beam generation in the visible regime, we have tackled the challenge of simultaneous control of vortex beam’s anomalous deflection and refocusing, through different encoding sequences superimposed based on the principle of Fourier convolution and metalens design. This all-in-one multifunctional metasurface design offers new technological pathways for secure optical communication and quantum manipulation applications.
Due to the physically unrestricted set of orthogonally helical modes of orbital angular momentum (OAM), it has contributed significantly to wireless communication and information capacity. Meanwhile, focusing has important applications in fields such as super-resolution microscopic imaging and optical integration. Plasmonic metasurfaces have a powerful ability to modulate electromagnetic (EM) waves, and diversified functionalities in them are strongly desired. As of today, few plasmonic metasurfaces are reported which have multi-function in a single flat device. Herein, by fine-tuning the geometric dimensions and orientation angle of the meta-atom, the geometric phase is combined with the propagation phase to produce an independent phase response when left-handed circular polarization (LCP) and right-handed circular polarization (RCP) waves illuminate the metasurface. This paper presents three plasmonic metasurfaces, and each of them implements multiple functions on a single plasmonic metasurface. Firstly, normal reflection of OAM and a focused beam is achieved. Secondly, we realize anomalous reflection of OAM by convolving a gradient sequence and implement computational focusing at any point. Finally, addition theorem is adopted to implement the above two functions, and this design contains normal and inclined output beams. Our work provides novel approaches for the integration of multifunctional EM modulation.
The optical spin Hall effect, as an optical analogue of spin-orbit coupling of fermionic electrons, has attracted increasing interest in fundamental physics and nanophotonic applications. In addition to spin, charge of particles, as the other degree of freedom, plays a significant role in spin-orbit interaction as well. However, the optical counterpart of charge conjugation has been absent due to the charge-free nature of photons. Here, in a one-dimensional photonic crystal, we report reversible optical spin currents from the opposite band edges of the photonic band gap, which can provide an optical analogue of charge conjugation in spin-orbit coupling. The effective magnetic field acting on the photons is controlled by the tunable splitting of the transverse electric and transverse magnetic optical modes, which is confirmed by both experiment and simulation. Based on the tunable effective magnetic field, the optical spin current can be controlled by the tunable cavity length in an open cavity. Our results will facilitate the development of optical spintronic devices and guide the understanding of analogies and generalizations involving quantum and classical wave theories.
Metalenses as miniature flat lenses exhibit a substantial potential in replacing traditional optical component. Although the metalenses have been intensively explored, their functions are limited by poor active ability, narrow operating band and small depth of field (DOF). Here, we show a dielectric metalens consisting of TiO2 nanofins array with ultrahigh aspect ratio to realize active multiband varifocal function. Regulating the orbital angular momentum (OAM) by the phase assignment covering the 2π range, its focal lengths can be switched from 5 mm to 35 mm. This active optical multiplexing uses the physical properties of OAM channels to selectively address and decode the vortex beams. The multiband capability and large DOFs with conversion efficiency of 49% for this metalens are validated for both 532 nm and 633 nm, and the incidence wavelength can further change the focal lengths. This non-mechanical tunable metalens demonstrates the possibility of active varifocal metalenses.
Recent progress in space‐time‐coding digital metasurface (STCM) manifests itself a powerful tool to engineer the properties of electromagnetic (EM) waves in both space and time domains, and greatly expands its capabilities from the physical manipulation to information processing. However, the current studies on STCM are focused under the synchrony frame, namely, all meta‐atoms follow the same variation frequency. Here, an asynchronous STCM is proposed, where the meta‐atoms are modulated by different time‐coding periods. In the proposed asynchronous STCM, the phase discontinuities on traditional metasurface are replaced with the frequency discontinuities. It is shown that dynamic wavefronts can be automatically realized for both fundamental and high‐order harmonics by elaborately arranging the spatial distribution of meta‐atoms with various time‐coding periods. The physics insight is due to the accumulated rapidly changing phase difference with time, which offers an additional degree of freedom during the wave‐matter interactions. As a proof‐of‐principle example, an asynchronous STCM for automatic spatial scanning and dynamic scattering control is investigated. From the theory, numerical simulations, and experiments, it can be found that the proposed STCM exhibits significant potentials for applications in radars and wireless communications.
Due to the strong ability of recognizing electromagnetic (EM) environment and adaptively control of EM waves, the intelligent metasurfaces have received great attention recently. However, the intelligent metasurface with frequency recognition for adaptive manipulation of the EM waves has not been studied. Here, we propose a frequency-recognition intelligent metasurface to precisely control the spatial EM waves under the agile frequencies with the help of a real-time radio-frequency sensor and an adaptive feedback control system. An active meta-atom is presented to reach 2 bit phase coding and 1 bit amplitude coding capacities to control the amplitude and phase independently. Experimental results demonstrate that the metasurface can recognize different frequency of the incoming wave with very high resolution, and can adaptively realize the self-defined multiple frequency agilities to manipulate the reflected EM waves without any human participation. As example, the intelligent metasurface with frequency recognition can adaptively operate wave absorption at 5.36 GHz, reflection to normal direction at 5.38 GHz, deflection to −30° at 5.40 GHz, random diffusion at 5.42 GHz, and deflection to +33° at 5.44 GHz by detecting the incoming frequency at the resolution of 0.02 GHz.
From metamaterials to metasurfaces, optical nano-structure has been widely investigated for novel and high efficiency functionalities. Apart from the intrisinsic properties of composite material, rich capabilities can be derived from the judicious design of metasurfaces, which enable more excellent and highly integrated optical devices than traditional bulk optical elements. In the meantime, the abundant manipulation abilites of light in the classical domain can be carried over into quantum domain. In this review, we highlight recent development of quantum optics based on metasurfaces, ranging from quantum plasmonics, generation, manipulation and appplication of quantum light to quantum vaccum engineering etc. Finally, some promising avenues for quantum optics with the help of optical metasurface are presented.
Metasurface-mediated bound states in the continuum (BIC) provides a versatile platform for light manipulation at the subwavelength dimension with diverging radiative quality factor and extreme optical localization. In this work, we theoretically propose the magnetic dipole quasi-BIC resonance in asymmetric silicon nanobar metasurfaces to realize giant Goos-Hänchen (GH) shift enhancement by more than three orders of wavelength. In sharp contrast to GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift here is located at the reflection peak with unity reflectance, which can be conveniently detected in the experiment. By adjusting the asymmetric parameter of metasurfaces, the Q-factor and GH shift can be modulated accordingly. More interestingly, it is found that GH shift exhibits an inverse quadratic dependence on the asymmetric parameter. Furthermore, we theoretically design an ultrasensitive environmental refractive index sensor based on the quasi-BIC enhanced GH shift, with a maximum sensitivity of 1.5×10⁷ μ m/RIU. Our work not only reveals the essential role of BIC in engineering the basic optical phenomena but also suggests the way for pushing the performance limits of optical communication devices, information storage, wavelength division de/multiplexers, and ultrasensitive sensors.
Vectorial optical fields (VOFs) exhibiting arbitrarily designed wavefronts and polarization distributions are highly desired in photonics. However, current methods to generate them either require complicated setups or exhibit limited functionalities, which is unfavorable for integration-optics applications. Here, we propose a generic approach to efficiently generate arbitrary VOFs based on metasurfaces exhibiting full-matrix yet inhomogeneous Jones-matrix distributions. We illustrate our strategy with analytical calculations on a model system and an experimental demonstration of a meta-device that can simultaneously deflect light and manipulate its polarization. Based on these benchmark results, we next experimentally demonstrate the generation of a far-field VOF exhibiting both a vortex wavefront and an inhomogeneous polarization distribution. Finally, we design/fabricate a meta-device and experimentally demonstrate that it can generate a complex near-field VOF—a cylindrically polarized surface plasmon wave possessing orbital angular momentum—with an efficiency of ~34%. Our results establish an efficient and ultracompact platform for generating arbitrary predesigned VOFs in both the near- and far-fields, which may find many applications in optical manipulation and communications.
This research presents a comprehensive examination of the effect of concentration on graphene oxide (GO) photoluminescence (PL). It was found that GO exhibits a concentration‐induced redshift and white light PL for GO aqueous dispersions, and displays visible‐near infrared PL for a three‐dimensional GO foam (3DGOF) corresponding to the high concentration‐limit of GO. It is deduced that the effect of the coupling between the GO sheets gives rise to the observed redshift, and hopping and tunneling effects result in the saturation of the PL redshift and then the PL quenching. Owing to the intense interactions and coupling of the GO sheets, the modification of the GO surface state increases the energy of the π* state and/or makes the high energy level appeared and therefore gives rise to the blue light component. Consequently, high concentrations of GO aqueous dispersion lead to the appearance of combined white light. For 3DGOF, electron‐hole radiation recombination in laser‐induced plasma and GO bandgap may be responsible for the observed visible‐near infrared PL. Based on its tunable PL in the visible‐near infrared region of the spectrum and the induced white light PL, GO has huge potential for applications in graphene‐based optoelectronics and biomedicine.
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The recent development of metasurfaces offers a unique technique for the arbitrary control of electromagnetic waves by spatially manipulating their amplitudes and phases. However, real time control in metasurfaces typically depends solely on dynamical phase modulation, but lacks of the amplitude modulation. In this paper, the dynamical amplitude modulation is tried to combine with phase coding metasurface, for the dynamical manipulation of electromagnetic scattering at microwave frequencies. Such an active metasurface is achieved by placing a patterned graphene layer on binary metallic resonators. When applying bias voltages on the graphene layer, continuous reflection amplitude modulation can be achieved for the selected resonators while the others are not affected. By this merit, two degrees of freedom, preset phase coding and dynamical amplitude modulation, are obtained to control the electromagnetic wave. As an example, a metasurface that can dynamically and continuously control the scattering pattern from mirror reflection to diffusion scattering is shown. The above concepts and physical phenomena are confirmed by numerical simulations and experiments.
Manipulating circularly polarized (CP) light waves at will are highly important for photonic researches and applications. Recently, while Pancharatnam-Berry (PB) metasurfaces have shown unprecedented capabilities to control CP light, meta-devices constructed so far always suffer from the limitations of low-efficiency and narrow bandwidth. Here, we propose a scheme to construct PB metasurfaces with these two issues well addressed. To verify our idea, two PB meta-devices are designed and fabricated for achieving high-efficiency and broadband photonic spin Hall effect and focusing effect, respectively. Experimental results, in good agreement with full wave simulations, demonstrate the desired functionalities with efficiencies reaching 80% within an ultra-wide frequency band (8.2-17.3GHz). The proposed design scheme is generic and can be extended to high-frequency regimes. Our work can stimulate the realizations of high-performance and broadband PB meta-devices with diversified functionalities.
Shrinking conventional optical systems to chip-scale dimensions will benefit custom applications in imaging, displaying, sensing, spectroscopy, and metrology. Towards this goal, metasurfaces—planar arrays of subwavelength electromagnetic structures that collectively mimic the functionality of thicker conventional optical elements—have been exploited at frequencies ranging from the microwave range up to the visible range. Here, we demonstrate high-performance metasurface optical components that operate at ultraviolet wavelengths, including wavelengths down to the record-short deep ultraviolet range, and perform representative wavefront shaping functions, namely, high-numerical-aperture lensing, accelerating beam generation, and hologram projection. The constituent nanostructured elements of the metasurfaces are formed of hafnium oxide—a loss-less, high-refractive-index dielectric material deposited using low-temperature atomic layer deposition and patterned using high-aspect-ratio Damascene lithography. This study opens the way towards low-form factor, multifunctional ultraviolet nanophotonic platforms based on flat optical components, enabling diverse applications including lithography, imaging, spectroscopy, and quantum information processing.
We propose a polarization-reprogrammable digital coding metasurface to realize arbitrarily linear polarization editing for reflected electromagnetic (EM) waves and beams. We present a digital coding element integrated with a PIN diode for switching the polarization conversion, which matches the digital 0 and 1 states. Based on the coding element, we combine the digital states and physical states of EM waves successfully. By arranging distinct coding sequences, we show that the polarization of the EM reflected beam can be controlled flexibly in real time. Not only are two orthogonally linear polarizations switched, but also arbitrary polarization directions can be achieved with specific coding sequences in a programmable manner. Such a manner can be accessibly used in communication and imaging applications. As a proof of concept, six different polarization states are presented to validate the proposed method. A metasurface sample with 30 × 30 elements is fabricated and measured in microwave frequency. The experimental results have good agreement with simulations and theoretical analyses. We envision that this work will promote the research of active polarization manipulation and extend its perspective from traditional physical fields to the information world.
Time-domain digital coding metasurfaces have been proposed recently to achieve efficient frequency conversion and harmonic control simultaneously; they show considerable potential for a broad range of electromagnetic applications such as wireless communications. However, achieving flexible and continuous harmonic wavefront control remains an urgent problem. To address this problem, we present Fourier operations on a time-domain digital coding metasurface and propose a principle of nonlinear scattering-pattern shift using a convolution theorem that facilitates the steering of scattering patterns of harmonics to arbitrarily predesigned directions. Introducing a time-delay gradient into a time-domain digital coding metasurface allows us to successfully deviate anomalous single-beam scattering in any direction, and thus, the corresponding formula for the calculation of the scattering angle can be derived. We expect this work to pave the way for controlling energy radiations of harmonics by combining a nonlinear convolution theorem with a time-domain digital coding metasurface, thereby achieving more efficient control of electromagnetic waves.
Metasurfaces are ultrathin metamaterials consisting of planar electromagnetic (EM) microstructures (e.g., meta-atoms) with pre-determined EM responses arranged in specific sequences. Based on careful structural designs on both meta-atoms and global sequences, one can realize homogenous and inhomogeneous metasurfaces that can possess exceptional capabilities to manipulate EM waves, serving as ideal candidates to realize ultracompact and highly efficient EM devices for next-generation integration-optics applications. In this paper, we present an overview on the development of metasurfaces, including both homogeneous and inhomogeneous ones, focusing particularly on their working principles, the fascinating wave-manipulation effects achieved both statically and dynamically, and the representative applications so far realized. Finally, we also present our own perspectives on possible future directions of this fast-developing research field in the conclusion.
Lead halide perovskites have emerged as promising materials for photovoltaic and optoelectronic devices. However, their exceptional nonlinear properties have not been fully exploited in nanophotonics yet. Herein we fabricate methyl ammonium lead tri-bromide perovskite metasurfaces and explore their internal nonlinear processes. While both of third-order harmonic generation and three-photon luminescence are generated, the latter one is less affected by the material loss and has been significantly enhanced by a factor of 60. The corresponding simulation reveals that the improvement is caused by the resonant enhancement of incident laser. Interestingly, such kind of resonance-enhanced three-photon luminescence holds true for metasurfaces with a small period number of 4, enabling promising applications of perovskite metasurface in high-resolution nonlinear color nanoprinting and optical encoding. The encoded information ‘NANO’ is visible only when the incident laser is on-resonance. The off-resonance pumping and the single-photon excitation just produce a uniform dark or photoluminescence background. Lead halide perovskites attract high interest as semiconductor materials but their exceptional nonlinear properties have not been fully exploited. Here Fan et al. demonstrate third-order harmonic generation and 60-fold enhanced three-photon luminescence, enabling optical encoding applications.
Controlling spin-to-orbital angular momentum (OAM) conversion plays an important role in many optical and wireless communication applications. While independent control of different circular-polarized (CP) wavefronts could offer an ultimate degree of freedom in designing advanced multifunctional spin devices, most metasurfaces only provide dependent wavefront control. We propose a reflective dual-helicity decoupled coding metasurface to completely realize independent control of OAM vortices for two orthogonal helicities. The element combines both the propagation phase and geometric phase, thus overcoming the inherent limitation encountered by conventional geometric phase elements whose phase responses are constrained to be opposite values for different CP wavefronts. Several design examples for independently generating OAM vortices in the microwave region are presented where a free combination of different OAM modes, helicity, as well as complex spatial beam editing can be fully achieved. Experiments show good agreements with the full-wave simulations, successfully verifying the design theory. The proposed method could offer an alternative platform for designing high-performance devices with independent CP wavefront manipulations, which may enhance the information capacity of metasurfaces and trigger versatile electromagnetic (em) wave function integrations for advanced compact systems.
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.
A new concept of the coding phase gradient metasurface (CPGM) is proposed, which is constructed using the phase gradient metasurface as the coding elements. Different from the previous coding metasurface (CM), both the coding sequences and gradient phases in the coding elements are designed to manipulate the electromagnetic (EM) wave for the CPGMs, and thus the manipulation will be more flexible. As an example, wide-band, wide-angle CPGMs with zero and non-zero phase gradient based on Pancharatnam-Berry (PB) phase are achieved using the co-polarization reflection unit cells under circularly polarized (CP) wave incidence. Both theoretically calculated and numerically simulated scattering patterns of the designed CPGMs demonstrate the expected manipulations. Additionally, two kinds of random CPGMs with different phase gradients are designed for radar cross section (RCS) reduction, and the measured RCS reveals a good accordance with the simulation.
Because of their exceptional capability to tailor the effective medium parameters, metamaterials have been widely used to control electromagnetic waves, which has led to the observation of many interesting phenomena, for example, negative refraction, invisibility cloaking, and anomalous reflections and transmissions. However, the studies of metamaterials or metasurfaces are mainly limited to their physical features; currently, there is a lack of viewpoints on metamaterials and metasurfaces from the information perspective. Here we propose to measure the information of a coding metasurface using Shannon entropy. We establish an analytical connection between the coding pattern of an arbitrary coding metasurface and its far-field pattern. We introduce geometrical entropy to describe the information of the coding pattern (or coding sequence) and physical entropy to describe the information of the far-field pattern of the metasurface. The coding metasurface is demonstrated to enhance the information in transmitting messages, and the amount of enhanced information can be manipulated by designing the coding pattern with different information entropies. The proposed concepts and entropy control method will be helpful in new information systems (for example, communication, radar and imaging) that are based on the coding metasurfaces.
The concept of coding metasurface makes a link between physically metamaterial particles and digital codes, and hence it is possible to perform digital signal processing on the coding metasurface to realize unusual physical phenomena. Here, this study presents to perform Fourier operations on coding metasurfaces and proposes a principle called as scattering-pattern shift using the convolution theorem, which allows steering of the scattering pattern to an arbitrarily predesigned direction. Owing to the constant reflection amplitude of coding particles, the required coding pattern can be simply achieved by the modulus of two coding matrices. This study demonstrates that the scattering patterns that are directly calculated from the coding pattern using the Fourier transform have excellent agreements to the numerical simulations based on realistic coding structures, providing an efficient method in optimizing coding patterns to achieve predesigned scattering beams. The most important advantage of this approach over the previous schemes in producing anomalous single-beam scattering is its flexible and continuous controls to arbitrary directions. This work opens a new route to study metamaterial from a fully digital perspective, predicting the possibility of combining conventional theorems in digital signal processing with the coding metasurface to realize more powerful manipulations of electromagnetic waves.
Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response, mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.
We apply the Pancharatnam-Berry phase approach to plasmonic metasurfaces
loaded by highly nonlinear multi-quantum well substrates, establishing a
platform to control the nonlinear wavefront at will based on giant localized
nonlinear effects. We apply this approach to design flat nonlinear metasurfaces
for efficient second-harmonic radiation, including beam steering, focusing, and
polarization manipulation. Our findings open a new direction for nonlinear
optics, in which phase matching issues are relaxed, and an unprecedented level
of local wavefront control is achieved over thin devices with giant nonlinear
responses.
The surface plasmon polariton is an emerging candidate for miniaturizing optoelectronic circuits. Recent demonstrations of polarization-dependent splitting using metasurfaces, including focal-spot shifting and unidirectional propagation, allow us to exploit the spin degree of freedom in plasmonics. However, further progress has been hampered by the inability to generate more complicated and independent surface plasmon profiles for two incident spins, which work coherently together for more flexible and tunable functionalities. Here by matching the geometric phases of the nano-slots on silver to specific superimpositions of the inward and outward surface plasmon profiles for the two spins, arbitrary spin-dependent orbitals can be generated in a slot-free region. Furthermore, motion pictures with a series of picture frames can be assembled and played by varying the linear polarization angle of incident light. This spin-enabled control of orbitals is potentially useful for tip-free near-field scanning microscopy, holographic data storage, tunable plasmonic tweezers, and integrated optical components.
The terahertz region is a special region of the electromagnetic spectrum that incorporates the advantages of both microwaves and infrared light waves. In the past decade, metamaterials with effective medium parameters or gradient phases have been studied to control terahertz waves and realize functional devices. Here, we present a new approach to manipulate terahertz waves by using coding metasurfaces that are composed of digital coding elements. We propose a general coding unit based on a Minkowski closed-loop particle that is capable of generating 1-bit coding (with two phase states of 0 and 1806), 2-bit coding (with four phase states of 0, 906, 1806, and 2706), and multi-bit coding elements in the terahertz frequencies by using different geometric scales. We show that multi-bit coding metasurfaces have strong abilities to control terahertz waves by designing-specific coding sequences. As an application, we demonstrate a new scattering strategy of terahertz waves—broadband and wide-angle diffusion—using a 2-bit coding metasurface with a special coding design and verify it by both numerical simulations and experiments. The presented method opens a new route to reducing the scattering of terahertz waves.
Metamaterials are artificial structures that are usually described by effective medium parameters on the macroscopic scale, and these metamaterials are referred to as ‘analog metamaterials’. Here, we propose ‘digital metamaterials’ through two steps. First, we present ‘coding metamaterials’ that are composed of only two types of unit cells, with 0 and π phase responses, which we name ‘0’ and ‘1’ elements, respectively. By coding ‘0’ and ‘1’ elements with controlled sequences (i.e., 1-bit coding), we can manipulate electromagnetic (EM) waves and realize different functionalities. The concept of coding metamaterials can be extended from 1-bit coding to 2-bit coding or higher. In 2-bit coding, four types of unit cells, with phase responses of 0, π/2, π, and 3π/2, are required to mimic the ‘00’, ‘01’, ‘10’ and ‘11’ elements, respectively. The 2-bit coding has greater freedom than 1-bit coding for controlling EM waves. Second, we propose a unique metamaterial particle that has either a ‘0’ or ‘1’ response controlled by a biased diode. Based on this particle, we present ‘digital metamaterials’ with unit cells that possess either a ‘0’ or ‘1’ state. Using a field-programmable gate array, we realize digital control over the digital metamaterial. By programming different coding sequences, a single digital metamaterial has the ability to manipulate EM waves in different manners, thereby realizing ‘programmable metamaterials’. The above concepts and physical phenomena are confirmed through numerical simulations and experiments using metasurfaces.
Recently, reflectionless or low-reflection surfaces made of subwavelength structures have been of broad interest in practical engineering. Here, a single-layer terahertz metasurface is proposed to produce ultralow reflections across a broad-frequency spectrum and wide incidence angles by controlling the reflection phases of subwavelength structures. To enable full control of the phase range in a continuous band, a combination of two different subwavelength elements are employed, both of which exhibit weak interactions with the incident terahertz waves, thereby showing high local reflectivities near the operating frequency. An optimization method is utilized to determine the array pattern with the minimum overall reflections under the illumination of plane waves. Both numerical simulations and experimental results demonstrate ultralow reflections of terahertz waves by the metasurface over a broad frequency band and wide incidence angles. By using the proposed metasurface, the far-field scattering patterns of metallic objects can be efficiently controlled, which opens up a new route for low-reflection surface designs in the terahertz spectrum.
Light beams with a helical phase-front possess orbital angular momentum along their direction of propagation in addition to the spin angular momentum that describes their polarisation. Until recently, it was thought that these two 'rotational' motions of light were largely independent and could not be coupled during light-matter interactions. However, it is now known that interactions with carefully designed complex media can result in spin-to-orbit coupling, where a change of the spin angular momentum will modify the orbital angular momentum and vice versa. In this work, we propose and demonstrate that the birefringence of plasmonic nanostructures can be wielded to transform circularly polarised light into light carrying orbital angular momentum. A device operating at visible wavelengths is designed from a space-variant array of subwavelength plasmonic nano-antennas. Experiment confirms that circularly polarised light transmitted through the device is imbued with orbital angular momentum of +/- 2 (h) over bar (with conversion efficiency of at least 1%). This technology paves the way towards ultrathin orbital angular momentum generators that could be integrated into applications for spectroscopy, nanoscale sensing and classical or quantum communications using integrated photonic devices.
Gradient metasurfaces are two-dimensional optical elements capable of manipulating light by imparting local, space-variant
phase changes on an incident electromagnetic wave. These surfaces have thus far been constructed from nanometallic optical
antennas, and high diffraction efficiencies have been limited to operation in reflection mode. We describe the experimental
realization and operation of dielectric gradient metasurface optical elements capable of also achieving high efficiencies
in transmission mode in the visible spectrum. Ultrathin gratings, lenses, and axicons have been realized by patterning a 100-nanometer-thick
Si layer into a dense arrangement of Si nanobeam antennas. The use of semiconductors can broaden the general applicability
of gradient metasurfaces, as they offer facile integration with electronics and can be realized by mature semiconductor fabrication
technologies.
Balancing the complexity and the simplicity has played an important role in
the development of many fields in science and engineering. As Albert Einstein
was once quoted to say: 'Everything must be made as simple as possible, but not
one bit simpler'. The simplicity of an idea brings versatility of that idea
into a broader domain, while its complexity describes the foundation upon which
the idea stands. One of the well-known and powerful examples of such balance is
in the Boolean algebra and its impact on the birth of digital electronics and
digital information age. The simplicity of using only two numbers of '0' and
'1' in describing an arbitrary quantity made the fields of digital electronics
and digital signal processing powerful and ubiquitous. Here, inspired by the
simplicity of digital electrical systems we propose to apply an analogous idea
to the field of metamaterials, namely, to develop the notion of digital
metamaterials. Specifically, we investigate how one can synthesize an
electromagnetic metamaterial with desired materials parameters, e.g., with a
desired permittivity, using only two elemental materials, which we call
'metamaterial bits' with two distinct permittivity functions, as building
blocks. We demonstrate, analytically and numerically, how proper spatial
mixtures of such metamaterial bits leads to 'metamaterial bytes' with material
parameters different from the parameters of metamaterial bits. We also explore
the role of relative spatial orders of such digital materials bits in
constructing different parameters for the digital material bytes. We then apply
this methodology to several design examples such as flat graded-index digital
lens, cylindrical scatterers, digital constructs for epsilon-near-zero (ENZ)
supercoupling, and digital hyperlens, highlighting the power and simplicity of
this methodology and algorithm.
In metal optics gold assumes a special status because of its practical importance in optoelectronic and nano-optical devices, and its role as a model system for the study of the elementary electronic excitations that underlie the interaction of electromagnetic fields with metals. However, largely inconsistent values for the frequency dependence of the dielectric function describing the optical response of gold are found in the literature. We performed precise spectroscopic ellipsometry measurements on evaporated gold, template-stripped gold, and single-crystal gold to determine the optical dielectric function across a broad spectral range from 300 nm to 25 μm (0.05–4.14 eV) with high spectral resolution. We fit the data to the Drude free-electron model, with an electron relaxation time τD=14±3 fs and plasma energy ℏωp=8.45 eV. We find that the variation in dielectric functions for the different types of samples is small compared to the range of values reported in the literature. Our values, however, are comparable to the aggregate mean of the collection of previous measurements from over the past six decades. This suggests that although some variation can be attributed to surface morphology, the past measurements using different approaches seem to have been plagued more by systematic errors than previously assumed.
We discuss screw dislocations of a phase surface as the one type of wavefront of a monochromatic wave. The simple method for construction of the optical wavefronts with an isolated screw dislocation is reported. Laser beams with the dislocations of different orders were experimentally achieved by using diffraction on computer-synthesized gratings.
As they travel through space, some light beams rotate. Such light beams have angular momentum. There are two particularly important ways in which a light beam can rotate: if every polarization vector rotates, the light has spin; if the phase structure rotates, the light has orbital angular momentum (OAM), which can be many times greater than the spin. Only in the past 20 years has it been realized that beams carrying OAM, which have an optical vortex along the axis, can be easily made in the laboratory. These light beams are able to spin microscopic objects, give rise to rotational frequency shifts, create new forms of imaging systems, and behave within nonlinear material to give new insights into quantum optics.
Infrared metasurface, especially that having a working range covering wavelengths from 0.75 to 25 μm, has been exploited as a revolutionary tool to manipulate the properties of electromagnetic waves owing to its potential applications in military and civilian fields. It owns the capacity to steer electromagnetic waves within subwavelength scale, with full degrees of freedom such as phase, amplitude and polarization, allowing the development of a number of planar meta-devices including the metalens, hologram, wave-plate and polarimeter. In particular, polarization, which determines the interaction of electromagnetic waves with matter, is important in almost every area of science. However, conventional materials for infrared polarization control inevitably introduce extra optical components and bulky configurations, hindering future miniaturization and integration. Moreover, compared with their short wavelength counterparts, polarization nanodevices in the infrared band and especially those in the long-wavelength infrared region have been far less explored due to the loss of material and immature fabrication techniques. Here, we review recent progress in the development of infrared metasurfaces in terms of generating, manipulating and detecting the polarization on standard and higher-order Poincaré spheres. The principles, typical strategies and emerging applications of these processes are introduced. We also discuss the challenges and outlook of future developments in this emerging field.
Acoustic metasurfaces display unprecedented potential for their unique and flexible capabilities of wave front manipulations. Although rapid progress and significant developments have been achieved in this field, it still remains a significant challenge to obtain an acoustic metasurface that can perform different functions in three-dimensional (3D) space, as desired. Here, a tunable acoustic metasurface composed of Helmholtz-resonator-like digital-coding meta-atoms is presented to overcome the limitation and realize 3D dynamic wave manipulations. The digital meta-atom is constructed from two cylindrical cavities controlled by a motor. By changing the depth of the bottom cavity with the motor automatically, the reflection phase of the meta-atom can be adjusted continuously over the range of nearly 360°, realizing the required digital states. To build the relationship between the digital-coding profiles and scattering patterns, convolution and addition operations are implemented. Based on such operations, various fascinating functionalities, such as arbitrary scattering-pattern shift and superposition of different scattering beams, are achieved. This study paves the way for studying acoustic metasurfaces from the digital perspective and provides efficient methods for realizing spatial acoustic wave control.
Coding metasurfaces are a kind of metasurface that can combine artificial structures with digital codes “0” and “1.” By digitally programming the arrangements of structural units, they can manipulate the transmitted or reflected waves into arbitrary patterns. Recently, coding metasurfaces have been demonstrated to realize fascinating functions in acoustics, including far-field modulation, focusing, and vortex beam generation. However, the previous acoustic coding metasurfaces work at a single frequency and have only one function under a specific arrangement, limiting the efficiency and capacity of manipulation to a low quality. To overcome these shortcomings, here we propose frequency-selected bifunctional coding acoustic metasurfaces. Dual-layer Helmholtz-like resonators are utilized to produce a 0 and π phase difference, corresponding to the “0” and “1” codes, respectively. Owing to the design of the dual-layer resonators, the two working frequencies are integrated on a single unit with a subwavelength thickness, which eliminates the long-wavelength limitations. Frequency-selected anomalous reflection, diffusion, and Airy-beam generation are further demonstrated to prove its availability. The results are shown through theoretical predictions, simulations, and experiments, which show good correspondence. The proposed frequency-selected bifunctional coding acoustic metasurfaces can be used in acoustic information preservation and provide an alternative way to achieve multispectral acoustic devices.
Vortex beam carrying orbital angular momentum (OAM) has been widely explored in optical and microwave regions attributed to its potential characteristics in communication systems. For circular polarization incidence, Pancharatnam-Berry (PB) phase is a direct resource to generate phase gradient along the azimuthal direction required by specific OAM mode. The main drawback of PB phase is that it only affects cross-polarized fields and keeps the copolarized part unmodulated. Here, a paradigm-shift perspective of noninterleaved metasurfaces is proposed, which can simultaneously generate separate multiple integer and fractional OAM modes occupying both copolarized and cross-polarized output channels. The scheme is validated by a series of experimental demonstrations in the microwave regime. By adjusting the polarization states of both input and receiving ends, different integer and fractional OAM modes are demonstrated in the full transmission channels. The results offer a unique recipe to enhance information capacity of metasurfaces and trigger versatile electromagnetic (EM) wave function integrations for advanced compact systems. A variety of applications can be readily expected in spin-selective optics, spin-Hall metadevices, and multitask metasurfaces operating in both reflection and transmission modes.
The Airy beam, known to present particular unique features, such as diffraction-free, self-healing, and self-bending, has attracted considerable interest in recent years. Here, an electronically controlled reflective coding metasurface is exploited to generate and dynamically manipulate the Airy beam over a wide frequency band in the microwave regime. By judiciously controlling the external dc bias voltage applied to the programmable metasurface, two distinct control states, "0" and "1", are engineered as digital codes for −π /2 and +π /2 phase responses, respectively. The programmable binary-phase-coding metasurface enables the generation and active reshaping of the Airy beam to be numerically simulated and experimentally validated. Moreover, the metasurface allows the bandwidth limitation of passive counterparts to be overcome and is able to modulate electromagnetic waves over a broad frequency range, spanning from 9 to 12 GHz, to realize diffraction-free Airy beams.
Lenses are the most important components of portable and integrable optical systems. Due to the inherent limitation of system sizes, external accessories are required for realizing additional functions such as close‐up or telegraph imaging, making the entire system complicated and cumbersome. Herein, based on the developments of metalenses, a spin angular momentum controlled multifunctional camera system is reported. Two types of metalens with high efficiencies are designed and fabricated for circularly polarized and nonpolarized light, respectively. By combining them into a doublet, multiple functions are realized beyond a camera lens. For left‐handed circular polarization light, the doublet works as a compound microscope with a magnification factor of 20 and a resolution of 1.07 µm. The system switches to a Galileo telescope when the incident light turns to be right‐handed circularly polarized. This research shall extend and strengthen the practical applications of metasurfaces in portable imaging systems as well as the integrated sensing terminals. A spin angular momentum controlled multifunctional camera system is demonstrated. By combining two metalenses for different polarizations into a doublet, multiple functions are realized beyond a camera lens. For the left‐handed circularly polarized light, the doublet functions as a compound microscope. It switches to a Galileo telescope when the incident light is right‐handed circularly polarized.
Image encoding and decoding are significant for information security, authenticity verification, and anticounterfeiting. We propose and demonstrate a polarization multiplexed metasurface for multi-image hiding and seeking in the terahertz band. The designed metasurface is composed of rod slot antennas and C-shaped slot antennas operating for both linear and circular polarizations. Different hidden images are revealed by controlling the polarization states of the incident and detection light. Three channels for object image encoding are demonstrated, indicating that this approach can increase the degree of freedom for optical image hiding and verification. This work may open an alternative avenue for polarization-controlled metasurface design, product identification, optical image encoding, and optical information security.
We demonstrate room temperature chiral coupling of valley excitons in a transition metal dichalcogenide monolayer with spin-momentum locked surface plasmons. At the onset of the strong coupling regime, we measure spin-selective excitation of directional flows of polaritons. Operating under such conditions, our platform yields surprisingly robust intervalley contrasts (ca. 40%) and coherence (ca. 5-8 %) in comparison with the total absence of contrast and coherence of the uncoupled valley excitons at room temperature that open rich and easy to implement possibilities in the context of chiral optical networks. Key words: Chiral coupling, transition metal dichalcogenide monolayer, surface plasmons, valley polarization and coherence, optical spin-orbit interaction, plasmonic spin-momentum locking
Photonic metasurface, a 2D array of plasmonic or dielectric optical meta-atoms, provides a versatile and compact platform for manipulating the polarization, phase and amplitude of light. It has been used to design planar optical functional elements such as ultrathin metasurface lens, metasurface waveplates, polarization beam splitter and so on. More generally, metasurface is capable of shaping the general and complex wavefront of light through holography techniques. Compared with conventional techniques, optical holography based on metasurface provides much more flexibilities to control the polarization, view angle, dispersive color crosstalk of holographic images, and thus making the metasurface optics more practical for various applications. In this review, we firstly discuss the concept of metasurface holography based on various phase modulation mechanisms including resonant phase, geometric phase, and propagation phase, etc. Then various holographic multiplexing techniques through degrees of wavelength, polarization, spatial distribution and nonlinear optical processes are summarized. Finally, we envision the future applications of metasurface optical holography from the view of large area fabrication, reconfigurable and dynamic wavefront engineering for 3D displaying, high capacity data storage, information encryption and so on.
Diffusive scatterings of electromagnetic (EM) waves by a thin screen are important in many applications, but available approaches cannot ensure uniform angular distributions of low-intensity scatterings without time-consuming optimizations. Here, we propose a robust and deterministic approach to design metasurfaces to achieve polarization-independent diffusive scatterings of EM waves within an ultra-broad frequency band and for wide-range of incident angles. Our key idea is to use high-efficiency Pancharatnam–Berry meta-atoms to form subarrays exhibiting focusing reflection-phase profiles, that can guarantee nearly uniform diffusive scatterings for arbitrarily polarized EM waves. As an illustration, we design and fabricate two metasurfaces and experimentally characterize their wave-diffusion properties in C, X, and Ku bands. Theoretical, numerical and experimental results demonstrate that our approach can diffuse the incident energy much more uniformly than available approaches based on the uniform-phase subarrays, thanks to the intrinsic wave-diffusion capabilities of the focusing-phase subarrays. The -7 dB fractional bandwidth is measured as 88.3% and the diffusive scattering behavior can be preserved up to 60o off-normal incidence irrespective of incident polarizations. Our approach, simple and robust, can help realize stealth applications under bistatic detections.
We propose to design coding metasurfaces based on Pancharatnam-Berry (PB) phase. The proposed PB coding metasurface could control circularly polarized components of incident waves, by encoding geometric phase into the orientation angle of coding particles to generate 1-bit and multi-bit phase responses. We perform digital convolution operations on scattering patterns of the PB coding metasurface to reach flexible controls of the circularly-polarized waves, forming spin-controlled multiple beams with different polarizations in free space, such as pencil beams and vortex beams carrying orbital angular momentum. Both numerical and experimental results demonstrate the excellent performance of the PB coding metasurface, which opens a pathway to novel types of multi-beam generations and provides an effective way to expand the beam coverage for wireless communication applications.
Geometric phase has attracted considerable attention in recent years, due to its capability of arbitrary beam shaping in a most efficient and compact way. While, traditional geometric phases are usually limited to handling single structured beams and lack the capability of parallel manipulation. Here, we propose a digitalized geometric phase enabling parallel optical spin and orbital angular momentum encoding. The concept is demonstrated in inhomogeneous anisotropic media by imprinting a particularly designed binary phase into a space-variant geometric phase. We theoretically analyze its spin-orbit interaction of light, and experimentally created the higher-order Poincaré sphere beam lattices, the order number and symmetry of which can be flexibly manipulated. Special lattices of cylindrical vector beams and orbital angular momentum modes with square and hexagonal symmetry are presented. This work discloses a new insight in programming geometric phases for tailoring the optical field and inspires various photonics applications.
This schematic shows a facile metasurface approach to realize polarization-controllable multichannel superpositions of orbital angular momentum (OAM) states with various topological charges. By manipulating the polarization state of the incident light, four kinds of superpositions of OAM states are realized using a single metasurface consisting of space-variant arrays of gold nanoantennas.
Coding acoustic metasurfaces can combine simple logical bits to acquire sophisticated functions in wave control. The acoustic logical bits can achieve a phase difference of exactly π and a perfect match of the amplitudes for the transmitted waves. By programming the coding sequences, acoustic metasurfaces with various functions, including creating peculiar antenna patterns and waves focusing, have been demonstrated.
Photonic spin Hall effect (PSHE; i.e., spin-polarized photons can be laterally separated in transportation) gains increasing attention from both science and technology, but available mechanisms either require bulky systems or exhibit very low efficiencies. Here it is demonstrated that a giant PSHE with ≈100% efficiency can be realized at certain meta-surfaces with deep-subwavelength thicknesses. Based on rigorous Jones matrix analysis, a general criterion to design meta-surfaces that can realize 100%-efficiency PSHE is established. The criterion is approachable from two distinct routes at general frequencies. As a demonstration, two microwave meta-surfaces are fabricated and then experimentally characterized, both showing ≈90% efficiencies for the PSHE. The findings here pave the way for many exciting applications based on high-efficiency manipulations of photon spins, with a polarization detector experimentally demonstrated here as an example.
The optical constants n and k were obtained for the noble metals (copper, silver, and gold) from reflection and transmission measurements on vacuum-evaporated thin films at room temperature, in the spectral range 0.5-6.5 eV. The film-thickness range was 185-500 Å. Three optical measurements were inverted to obtain the film thickness d as well as n and k. The estimated error in d was ± 2 Å, and that in n, k was less than 0.02 over most of the spectral range. The results in the film-thickness range 250-500 Å were independent of thickness, and were unchanged after vacuum annealing or aging in air. The free-electron optical effective masses and relaxation times derived from the results in the near infrared agree satisfactorily with previous values. The interband contribution to the imaginary part of the dielectric constant was obtained by subtracting the free-electron contribution. Some recent theoretical calculations are compared with the results for copper and gold. In addition, some other recent experiments are critically compared with our results.
Metasurfaces with interfacial phase discontinuities provide a unique platform for manipulating light propagation both in free space and along a surface. Three-dimensional focusing of visible light is experimentally exhibited as a bi-functional phenomenon by controlling the radial orientation of identical plasmonic dipoles, generating a desired phase profile along the interface. With this technique, the in-plane and out-of-plane refractions are manipulated by an ultrathin flat lens such that a beam can be focused into a 3D spot either in a real or virtual focal plane, which can be reversed via manipulation of the circularly polarized status of the incident light. Both the inverted real image and the upright virtual image of an arbitrary object are experimentally demonstrated using the same flat lens in the visible range, which paves the way towards robust application of phase discontinuity devices.
Abstract . In 1955 Pancharatnam,showed,that,a cyclic,change,in the,state,of polarization,of light,is accompanied,by a phase,shift,determined,by the,geometry of the,cycle,as represented,on the,Poincare,sphere . The phase,owes,its,existence,to the,non-transitivityof,Pancharatnam's,connection,between,different,states,of polarization . Using the algebra of spinors and 2 x 2 Hermitian matrices, the precise,relation,is established,between,Pancharatnam's,phase,and,the,recently discovered,phase,change,for,slowly,cycled,quantum,systems . The polarization phase,is an,optical,analogue,of the,Aharonov-Bohm,effect . For slow,changes,of polarization, the connection leading to the phase is derived from Maxwell's equations,for,a twisted,dielectric . Pancharatnam'sphase,is contrasted,with,the phase,change,of circularly,polarized,light,whose,direction,is cycled,(e .g. when guided,in a,coiled,optical,fibre) .
Metamaterials, or engineered materials with rationally designed, subwavelength-scale building blocks, allow us to control
the behavior of physical fields in optical, microwave, radio, acoustic, heat transfer, and other applications with flexibility
and performance that are unattainable with naturally available materials. In turn, metasurfaces—planar, ultrathin metamaterials—extend
these capabilities even further. Optical metasurfaces offer the fascinating possibility of controlling light with surface-confined,
flat components. In the planar photonics concept, it is the reduced dimensionality of the optical metasurfaces that enables
new physics and, therefore, leads to functionalities and applications that are distinctly different from those achievable
with bulk, multilayer metamaterials. Here, we review the progress in developing optical metasurfaces that has occurred over
the past few years with an eye toward the promising future directions in the field.