Metasurfaces have become a rapidly growing field of research in recent years due to their exceptional abilities in light manipulation and versatility in ultrathin optical applications. They also significantly benefit from their simplified fabrication process compared to metamaterials and are promising for integration with on-chip nanophotonic devices owing to their planar profiles. The recent progress in metasurfaces is reviewed and they are classified into six categories according to their underlying physics for realizing full 2π phase manipulation. Starting from multi-resonance and gap-plasmon metasurfaces that rely on the geometric effect of plasmonic nanoantennas, Pancharatnam–Berry-phase metasurfaces, on the other hand, use identical nanoantennas with varying rotation angles. The recent development of Huygens' metasurfaces and all-dielectric metasurfaces especially benefit from highly efficient transmission applications. An overview of state-of-the-art fabrication technologies is introduced, ranging from the commonly used processes such as electron beam and focused-ion-beam lithography to some emerging techniques, such as self-assembly and nanoimprint lithography. A variety of functional materials incorporated to reconfigurable or tunable metasurfaces is also presented. Finally, a few of the current intriguing metasurface-based applications are discussed, and opinions on future prospects are provided.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/smtd.201600064

Fundamentals and Applications of Metasurfaces

The control of polarization and wavefront plays an important role in many optical systems. In this work, a monolayer metasurface is proposed to simultaneously realize circular asymmetric transmission (AT) and wavefront shaping based on asymmetric spin–orbit interactions. Circularly polarized incidence, accompanied with arbitrary wavefront modulation, experiences spin-selected destructive or constructive interference. An extinction ratio of ≈10:1 and an AT parameter of ≈0.69 at 9.6 µm, as well as a full width half-maximum of ≈2.9 µm (≈30% of the peak wavelength), are measured with the designed metasurface. These measured results are more than four times of those achieved with previous monolayer chiral structures. As far as it is known, this is the first report on the realization of simultaneous giant AT and arbitrary wavefront modulation with only one metasurface. Due to its fabrication simplicity and the multifunctionality of the designed metasurface, this work may provide a promising route to replace bulky cascading optical components with only one ultrathin metasurface for chiroptical spectroscopy, chiral imaging, optical communication, and so forth.

All-Dielectric Metasurfaces for Simultaneous Giant Circular Asymmetric Transmission and Wavefront Shaping Based on Asymmetric Photonic Spin–Orbit Interactions

Geometric-phase metasurfaces, recently utilized for controlling wavefronts of circular polarized (CP) electromagnetic waves, are drastically limited to the cross-polarization modality. Combining geometric with propagation phase allows to further control the co-polarized output channel, nevertheless addressing only similar functionality on both co-polarized outputs for the two different CP incident beams. Here we introduce the concept of chirality-assisted phase as a degree of freedom, which could decouple the two co-polarized outputs, and thus be an alternative solution for designing arbitrary modulated-phase metasurfaces with distinct wavefront manipulation in all four CP output channels. Two metasurfaces are demonstrated with four arbitrary refraction wavefronts, and orbital angular momentum modes with four independent topological charge, showcasing complete and independent manipulation of all possible CP channels in transmission. This additional phase addressing mechanism will lead to new components, ranging from broadband achromatic devices to the multiplexing of wavefronts for application in reconfigurable-beam antenna and wireless communication systems.

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Independent phase modulation for quadruplex polarization channels enabled by chirality-assisted geometric-phase metasurfaces

Holography is one of the most attractive approaches for reconstructing optical images, due to its capability of recording both the amplitude and phase information on light scattered from objects. Recently, optical metasurfaces for manipulating the wavefront of light with well-controlled amplitude, phase, and polarization have been utilized to reproduce computer-generated holograms. However, the currently available metasurface holograms have only been designed to achieve limited colors and record either amplitude or phase information. This fact significantly limits the performance of metasurface holograms to reconstruct full-color images with low noise and high quality. Here, we report the design and realization of ultrathin plasmonic metasurface holograms made of subwavelength nanoslits for reconstructing both two- and three-dimensional full-color holographic images. The wavelength-multiplexed metasurface holograms with both amplitude and phase modulations at subwavelength scale can faithfully produce not...

Full-Color Plasmonic Metasurface Holograms

Vector vortex beams (VVBs) possess ubiquitous applications from particle trapping to quantum information. Recently, the bulky optical devices for generating VVBs have been miniaturized by using metasurfaces. Nevertheless, it is quite challenging for the metasurface-generated VVBs to possess arbitrary polarization and phase distributions. More critical is that the VVBs' annular intensity profiles demonstrated hitherto are dependent on topological charges and are hence not perfect, posing difficulties in spatially shared co-propagation of multiple vortex beams. Here, a single-layer metasurface to address all those aforementioned challenges in one go is proposed, which consists of two identical crystal-silicon nanoblocks with varying positions and rotation angles (i.e., four geometric parameters throughout). Those four geometric parameters are found to be adequate for independent and arbitrary control of the amplitude, phase, and polarization of light. Perfect VVBs with arbitrary polarization and phase distributions are successfully generated, and the constant intensity profiles independent of their topological charges and polarization orders are demonstrated. The proposed strategy casts a distinct perception that a minimalist design of just one single-layer metasurface can empower such robust and versatile control of VVBs. That provides promising opportunities for generating more complex vortex field for advanced applications in structural light, optical micromanipulation, and data communication.

A Minimalist Single‐Layer Metasurface for Arbitrary and Full Control of Vector Vortex Beams

Nanoscale compact optical vortex generators promise substantially significant prospects in modern optics and photonics, leading to many advances in sensing, imaging, quantum communication, and optical manipulation. However, conventional vortex generators often suffer from bulky size, low vortex mode purity in the converted beam, or limited operation bandwidth. Here, we design and demonstrate gradient–rotation split-ring antenna metasurfaces as unique spin-to-orbital angular momentum beam converters to simultaneously generate and separate pure optical vortices in a broad wavelength range. Our proposed design has the potential for realizing miniaturized on-chip OAM-multiplexers, as well as enabling new types of metasurface devices for the manipulation of complex structured light beams.

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Generating and Separating Twisted Light by gradient-rotation Split-Ring Antenna Metasurfaces.

The Purcell effect of quantum dot (QD) spontaneous emission with Ag-SiO2 multilayer metamaterial nanostructures has been demonstrated in experiment and simulation. A broadband enhanced spontaneous emission rate of QDs is observed due to large local density of states in the epsilon-near-zero and hyperbolic regions of multilayer structures. Multilayer gratings are utilized to further enhance the QD spontaneous emission as the QDs located inside the grating grooves strongly interact with high-k coupled surface plasmon polariton modes. Photoluminescence decay measurements are in good agreement with both analytical treatment with a nonlocal effect and three-dimensional finite-element simulation. Detailed studies of QD position and polarization effects on emission rate enhancement for multilayer and multilayer grating nanostructures provide important insight for understanding the coupling mechanisms of emitter–multilayer interaction and the engineering of local density of states in metamaterial nanostructures. ...

https://par.nsf.gov/servlets/purl/10050587

Enhanced Quantum Dot Spontaneous Emission with Multilayer Metamaterial Nanostructures

NANOSATELLITES are defined as spacecraft with a mass of 1– 50 kg. The demand for and use of these and other small satellites iswidespread and is projected to continue to increase according to the 2015 SpaceWorks report on the nano/microsatellite market [1]. The SpaceWorks report also makes note that most nano/microsatellites are launched in large clusters and that certain companies such as SpaceX and OneWeb plan to use large constellations of small satellites for communications purposes. Satellites within sizeable constellationswill need the ability tomaneuver and orient themselves precisely in relation to the other satellites of the cluster. Coupling these requirements with the constraints of mass and volume demand the use of novel propulsion systems that are optimized for use on smallsats (a nano/micro type satellite defined in the SpaceWorks report). This propulsion system must also be adaptable for satellites that continue to decrease in size. Plasmonics is a subfield of optics that addresses the nanoscale interactions of light and metallic nanostructures. If a beam of light is allowed to strike the surface of a metal, then the oscillating electric field component of the light will cause the electrons in the metal to oscillate. A group of oscillating electrons is known as a plasmon. If the metal is restricted in dimensions to the nanoscale regime, the plasmons are able to resonate with the incident light, which creates a strong electromagnetic field about the location of the nanostructure. This electromagnetic field can be tuned by changing the size and shape of themetallic nanostructures and used to control themotion of particles within the vicinity of the nanostructure. The control of nanoparticles in the vicinity of a plasmon interaction is well known as “plasmon nano-optical tweezers” [2]. This technique demonstrates optical particle trapping coupled with the plasmon interaction to trap nanoparticles beyond the diffraction limit. Previous research [3] extended this idea of plasmonic nanoparticle manipulation from trapping to acceleration andwas specifically aimed at propulsion for smallsats. This work developed a thruster design that made use of the plasmonic force concept. Results predicted that, with plasmonic force propulsion, the relative position (distance) and angular orientation (degrees) between two spacecraft was able to be controlled to a resolution that was one to two orders of magnitude smaller than state of the art. Results for a conceptual design of a plasmonic thruster that had 35 layers, 86 array columns, a multistage length of 5 mm, a 5-cm-diam light focusing lens, and used 100 nm polystyrene nanoparticles expelled at a rate of 1 × 10 per second would have a thrust of 250 nN, a specific impulse of 10 s, and a minimum impulse bit of 50 pN · s. The thrustermass andvolumewere estimated at 100 g and 50 cm, respectively [3]. Previous numerical simulations have shown that asymmetric nanostructures can resonate strongly within the visible spectrum, but this has never been demonstrated in experiments until now. Here, we report the first time these types of nanostructures have been manufactured and optically characterized.We report on the first experiment that demonstrates this resonance, with a nanostructure designed to resonate at λ 770 nm, for use as a plasmonic space propulsion thruster.

Transmission Spectrum of Asymmetric Nanostructures for Plasmonic Space Propulsion

The emergence of multilayer metamaterials in the research field of enhancing spontaneous emission rates has recently received extensive attention. Previous research efforts mostly focus on periodic metal-dielectric multilayers in hyperbolic dispersion region; however, the influence of lattice order in subwavelength multilayers on spontaneous emission is rarely studied. Here, we observe the stronger Purcell enhancement of quantum dots coupled to the aperiodic metal-dielectric multilayer with Thue-Morse lattice order from elliptical to hyperbolic dispersion regions, compared to the periodic multilayer with the same metal filling ratio. This work demonstrates the potential of utilizing quasiperiodic metamaterial nanostructures to engineer the local density of states for various nanophotonic applications.

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Spontaneous emission rate enhancement with aperiodic Thue-Morse multilayer.

The objective of this research is to manufacture and investigate the characteristics and use of asymmetric, metallic, nanostructures for plasmonic force propulsion, a developing method of nano-/picosatellite thrust generation. Visible to near-infrared light is focused onto sub-wavelength nanostructures to generate polarized oscillations of electrons on the surface of the metallic nanostructures (surface plasmon polaritons). The surface plasmon polaritons accelerate nanoparticle propellant away from the nanostructure, creating thrust. Previous numerical simulations have shown that asymmetric nanostructures can resonate strongly within the visible spectrum. This is the first experiment ever attempted and first to successfully demonstrate this resonance where the resonance peak is λ = 830 nm. The resonance peak of the experimental optical characterization agrees well with our computed model, showing an 11.2% difference. However, the off resonance behavior exhibits peak broadening where the variation of intensity with wavelength, off resonance, has an experimental slope that is 3.7 times less steep than the computed model. Furthermore, the optical transmittance of the sample is 2.1 times higher than computationally modeled. It is shown that the nanostructures are thermodynamically stable in the projected environmental conditions and have an equilibrium temperature of 746.4 K. Upon review of the experimental optical setup, we conclude that thrust generation is not possible with continuous irradiation of light and propose a method of synchronous dynamic acceleration of nanoparticle propellant by use of a pulsed light beam.

Ling Li

Papers

Generating and Separating Twisted Light by gradient-rotation Split-Ring Antenna Metasurfaces.

Enhanced Quantum Dot Spontaneous Emission with Multilayer Metamaterial Nanostructures

Transmission Spectrum of Asymmetric Nanostructures for Plasmonic Space Propulsion

Spontaneous emission rate enhancement with aperiodic Thue-Morse multilayer.

Fabrication of Asymmetric Nanostructures for Plasmonic Force Propulsion