1. Introduction

The past decade has witnessed a sustaining wave of interest and effort in developing optical metamaterials that exhibit exotic electromagnetic properties. These metamaterials are typically constructed with subwavelength building blocks that are artificially arranged in 3D configurations. The demonstration of these desired phenomena often times requires light to travel through the 3D structure and one undesirable consequence is always the significant attenuation of light intensity at the output. The strong attenuation is the direct result of the ohmic losses associated with the plasmonic metals typically required for their negative dielectric functions as subwavelength constituents for the fabrication of metamaterials. A viable pathway to circumvent this roadblock is naturally to minimize the penetration depth of the light yet still allowing strong light-matter interaction. This concept was first introduced in 2003 where the term, metafilm, as a 2D category of the metamaterial was brought up [1–3]. The experimental demonstration came about in 2011 [4,5] and since then, the term “metasurface” was widely used in the community. The well-known optical phenomena of reflection and refraction that occur at the interface of two media can be altered with subwavelength elements artificially arranged at the 2D interface. Such metasurfaces operating in either reflective or transmissive mode employ tunable phase discontinuity, resulting in higher efficiency with much less intensity attenuation, because light is not required to propagate through a large distance in order to accumulate sufficient phase change before emerging on either side of the metasurfaces. Another benefit is obviously the simplification in fabrication. Needless to say, the degree of complexity in fabricating 2D structures is orders of magnitude less than those of 3D which has been a major reason why many of the nicely designed optical properties from 3D metamaterials are always hard to realize in practice.

Propelled by ingenuity and innovation from many groups around the world, metasurfaces today have shown great potential for practical applications. Some of the metasurfaces are made of metal structures while others involve only dielectric. Metasurfaces demonstrated in early years were all made of noble metal nanostructures utilizing their plasmonic properties such as multi-resonance, gap-plasmon or hyperbolic dispersion. The first proposed and studied metasurface is of the multi-resonance type involving V-shaped antennas that produce a phase shift covering the range of 0 to 2$\pi$ [4–7]. However, the low efficiency and weak cross-polarization conversion severely limited its application. The gap-plasmon metasurfaces, on the other hand, use the metal-insulator-metal structure to achieve 2$\pi$ phase modulation by generating strong near-field coupling between the top antenna array and the bottom metallic plane [8–14]. This type of metasurfaces has the benefit of allowing for strong co-polarization conversion, but can only be used in reflective applications. Hyperbolic metasurfaces possess a metal-like behavior in one direction and a dielectric-like property in the other, supporting surface plasmon polaritons (SPPs) that propagate at the interface between the metal and dielectric [15–19]. The SPP propagation preserves hyperbolic dispersion below a critical wavelength and becomes elliptically dispersed beyond it. One significant benefit is its ability to induce strong field confinement due to the high local density of states for the SPPs as demonstrated in hyperbolic metamaterials.

These metasurfaces typically operate in reflective mode because their transmission efficiency is low due to the strong metal loss. In order to achieve high transmission, all-dielectric metasurfaces have emerged and are rapidly gaining momentum [20–27]. These metasurfaces are constructed of 2D patterns of near-wavelength dielectric structures on a flat surface. One such type is of Huygens featuring electric and magnetic dipole resonances that are spectrally overlapped with comparable strength capable of phase modulation of 0 to 2$\pi$ and near-unity transmission [28–30]. Metasurfaces of this type comprise of aperiodic or multilayered structures in order to engineer surface impedance to minimize reflection. Their transmission efficiencies in the microwave and near-infrared regimes are high but deteriorate rapidly at visible frequencies. The other type is the so-called high-contrast metasurfaces consisting of 2D dielectric arrays of high refractive-index contrast that allow for excitation of quadrupoles and high-order multiples in addition to electric and magnetic dipoles [31–33]. The dielectric nanostructures in such metasurfaces act as truncated waveguides that strongly concentrate optical energy around the nanostructures. The advantage of the high-contrast metasurfaces is the ability to exhibit high diffraction efficiencies over a broad range of steering angles of incident light.

The Pancharatnam-Berry metasurfaces are primarily developed for their strong ability to realize full-phase control of circularly polarized light [34–39]. The mechanism for phase control is provided by rotating the orientation of the nanostructures of identical dimensions in the 2D plane instead of adjusting the geometry of nano-resonators that are employed in the aforementioned metasurfaces. Such metasurfaces follow simple design rules and are capable of broadband operation but limited only to the circularly-polarized incident light.

There is yet another type of metasurfaces synthesized by depositing a highly absorbing ultrathin film on a substrate whose purpose, quite different from all the above, is to fully absorb incident light over a broad spectrum [40–45]. These thin-film metasurfaces do not require any specific patterning of nanostructures and the perfect absorption occurs when the difference in complex refractive indices between the film and substrate is properly chosen. Other thin-film metasurfaces have been applied to ultrathin nano-structural coloring [46], broadband reflectors [47] and absorbers [48], polarization beam splitters, converters and analyzers [49], as well as superchirality [50].

There have been several excellent review papers focusing on metasurfaces of different kinds and functionalities [51–67]. In this review article, we choose to review recent progress made in fabricating these optical metasurfaces and survey their cutting-edge applications.

2. Fabrication techniques for metasurfaces

In this section, we begin to review various methods and technologies developed for the fabrication of optical metasurfaces. These methods can be generally classified into four categories as shown schematically in Fig. 1. Three of them are well-established fabrication methods: direct-write, pattern transfer, and hybrid patterning lithography. The fourth is the so-called alternative techniques that only began to emerge as fabrication methods for metasurfaces. These four categories are primarily employed to produce passive optical metasurfaces. For actively tunable metasurfaces, however, novel fabrication methods need to be developed.

 

Fig. 1 The schematics of fabrication technologies for optical metasurfaces.

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2.1 Direct-write lithography

As one of the most matured technologies developed and adapted in semiconductor manufacturing, photolithography should also be one of the most promising technologies for optical metasurface fabrication considering its advantage in high yield, large area and large volume mass production. Traditional photolithography uses light to go through a focusing lens in order to transfer a geometric pattern from a photomask to a light-sensitive chemical “photoresist”. Shorter-wavelength light sources are preferred in fabricating the optical metasurfaces with fine features, and such sources are known to cause damages to the lens and mask. To circumvent this technical difficulty, the maskless direct-write lithography has been developed (Table 1) which includes three different methods employing either particle beam, mechanics or laser to pattern nanostructures.

Table 1. The properties of Direct-Write Lithography

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2.1.1 Particle beam lithography

Particle beam lithography uses a focused Gaussian particle beam to generate a highly concentrated spot being exposed to one pixel at a time. Two well-established particle beam methods for the fabrication of optical metasurfaces are electron-beam lithography (EBL) and focused-ion-beam (FIB) lithography. EBL along with subsequent pattern transfer techniques can be roughly categorized into either bottom-up or top-down approach depending the way with which the nanostructures are made. The bottom-up approach employs the evaporation or deposition of atoms or molecules together with the lift-off process to build up nanoscale structures, while the top-down refers to creating nanostructures by using the etching process.

Our group has developed a bottom-up approach that produced a broadband optical metasurface operating in the infrared region [68] as shown in Fig. 2(a). The process starts with coating a resist layer on a substrate followed by exposing the concentrated electron beam directly onto the top surface of the resist-coated substrate. The subsequent process builds up the nanostructures by evaporating atoms after developing the exposed resist-coated substrate. Figure 2(b) is the top-view of the scanning electron microscope (SEM) image of an optical metasurface after the lift-off process. In a parallel approach, our group also reported using a top-down method to fabricate the optical meta-lens as illustrated in Fig. 2(c) [69]. It begins with the spin-coated resist layer being exposed to the electron beam followed by an etching process to reduce its lateral dimensions to form the nanostructures. The top-view of the SEM image of the optical meta-lens is shown in Fig. 2(d). Challenges of EBL for large-scale manufacturing are its high cost and low speed operation that requires a highly stable environment. The resolution of EBL is limited by the electron scattering in the resist due to the proximity effect [70–72].

 

Fig. 2 (a) Schematic of the fabrication process for the bottom-up EBL method. A gold (Au) mirror is first deposited on a Si substrate by e-gun evaporator. A silicon dioxide (SiO₂) dielectric spacer is then deposited using plasma-enhanced chemical vapor deposition (PECVD). A resist layer is then spin-coated on the prepared substrate and baked on a hot plate. Subsequently, an Espacer layer is spin-coated on the resist layer. Espacer is an organic polymer with high conductivity to reduce the positional error during the e-beam exposure process. The structural profile of metasurfaces is defined by the e-beam exposure and development process. The final sample will be obtained after the Au deposition and lift-off process. (b) Zoom-in SEM image of the fabricated achromatic converging meta-device which successfully eliminated the chromatic aberration over a continuous wavelength region from 1200 to 1680 nm for circularly-polarized incidence in a reflection scheme. (c) Schematic illustrating the fabrication process of the top-down EBL method. An undoped GaN layer is first grown on a c-plane sapphire substrate by metal−organic chemical vapor deposition (MOCVD). A SiO₂ hard mask layer is then deposited using PECVD. A resist layer is spin-coated on the prepared substrate and then baked on a hot plate. The structural profile of the metasurface is defined by the e-beam exposure and the development process. After that, a Cr layer as an etching hard mask is coated on the substrate by e-gun evaporator and followed by a lift-off process. The patterns are transferred to the SiO₂ layer by reactive ion etching (RIE). The substrate with the patterned SiO₂ hard mask layer is etched by inductively coupled-plasma reactive ion etching (ICP-RIE) using BCl₃/Cl₂ chemistry. The final sample is obtained after the removal of the patterned SiO₂ hard mask layer with the buffered oxide etch (BOE) solution. (d) The SEM image of the on-axis focusing GaN-based metalens, designed with diameter of 100 μm and focal length of 300 μm. The metalens works in a transmission window with extremely high operation efficiency of 91.6% at visible light. (b) Reprinted with permission from [68]. Copyright 2017, Springer Nature. (d) Reprinted with permission from [69]. Copyright 2017, American Chemical Society.

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FIB lithography is a one-step etching process in which a beam of ions instead of electrons mills the sample surface via a sputtering process with nanometer precision. This process becomes more powerful when it is equipped with the SEM function to form the so-called dual-beam system. With this system we can simultaneously observe and mill the sample surface at any designated spot facilitated by direct visualization. Figures 3(a) and 3(b) show SEM images of the FIB-fabricated metasurfaces for visible hologram and optical vortex generation and focusing [73,74]. FIB lithography is unsuitable for large-area manufacturing because of high cost and low throughput. In addition, it faces fabrication challenges such as aspect ratio limitation, ion doping, long milling time that leads to spatial drift of sample images, and sample damage during imaging and milling.

 

Fig. 3 (a) and (b) are SEM images of the FIB-fabricated metasurfaces. (a) A metasurface hologram of the letter ‘P’ with the 5-μm scale bar. The inset is a zoomed-in view (scale bar, 500 nm). The hologram is designed for the operational wavelength of 676 nm, and the thickness of the sample is only about 1/23 the size of the wavelength. (b) A planar chiral metasurface to produce optical vortex from a circularly polarized light. Optical vortex can enhance data capacity for its extra degree of freedom of angular momentum. The metasurface has the ability to focus the incident light into a tiny point, which greatly increases the power intensity of the generated optical vortex and has potential application in highly integrated optical communication systems. (c) The morphology characterization of the negative refractive index metasurfaces obtained by AFM, which is fabricated with a scanner movement nanolithography on the thin silver film. The double split ring resonators with additional capacitive gaps can compensate for the inertial inductance. (d) Photograph of the fabricated metasurface using LDW in print circuit board (PCB) technology. (e) SEM image of a metasurface fabricated by orthogonal LIL capable of manipulating linear polarization. The near-infrared reflective linear polarization converter is composed of ellipse-shaped plasmonic planar resonators. A polarization conversion ratio in power larger than 91.1% is achieved from 730 to 1870 nm. (f) SEM image of the metasurface hologram film based on bilayered metallic nanowire gratings at different scales, produced by LIL. The bilayered metallic gratings behave as an ideal polarized beam splitter, producing strong negative reflection for transverse-magnetic (TM) light and efficient reflection for transverse-electric (TE) light. (a) Reprinted with permission from [73]. Copyright 2013, Springer Nature. (b) Reprinted with permission from [74]. Copyright 2015, Springer Nature. (c) Reprinted with permission from [75]. Copyright 2006, Elsevier B.V. (d) Reprinted with permission from [76]. Copyright 2016, Springer Nature. (e) Reprinted with permission from [77]. Copyright 2015, AIP Publishing LLC. (f) Reprinted with permission from [78]. Copyright 2014, Springer Nature.

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2.1.2 Probe scanning lithography

The probe scanning lithography (PSL) is another technique used in nanofabrication. Its ultimate resolution is reached by employing the technique of atomic force microscopy (AFM). It utilizes a probe to scratch out nano-patterns in resist or to arrange nano-particles in patterns. Figure 3(c) shows the surface morphologies of the negative refractive index metasurfaces in mid-infrared, fabricated by the PSL under normal atmospheric pressure, at room temperature and humidity [75]. The main problem of AFM-based PSL is its poor aspect ratio — “scratches” are relatively wide and shallow. Other techniques such as scanning tunneling microscope (STM) and scanning near field optical microscope (SNOM) are developed to solve the problem of poor aspect ratio. However, all of them are low throughput and expensive. Further improvement of PSL is necessary for large area patterning in order for it to be used in large-scale manufacturing.

2.1.3 Laser lithography

It is obviously desirable to develop large-area fabrication techniques with better flexibility, high precision and good uniformity. There are two maskless laser-based approaches that are capable of rapid and low-cost processing of micro- and nanostructures: laser-direct-write (LDW) lithography and laser-interference lithography (LIL).

The LDW lithography utilizes computer-controlled optics to project the desired nano-patterns directly onto the photoresist by holding the mask in software. It not only enables nano-fabrication of materials that are hard to machine mechanically but also generates 3D surface profiles by writing with variable laser dosage. As a result, the phase noise generated by discrete systems can be suppressed through the use of a continuously shaped metasurface [Fig. 3(d)] [76]. The LDW lithography effectively offers a compromise between the feature size and processing cost. One drawback of this method is that it is not a batch process.

The idea of LIL based on the interference of two or more coherent laser beams was put forward to produce large-area periodic nanostructures. As shown in Fig. 3(e), the metasurface acting as a linear polarization converter composed of ellipse-shaped plasmonic planar resonator can be fabricated by two-beam LIL [77]. Advanced LIL with multiple exposures and non-co-planar beams can produce complex periodic nanostructures [79]. As illustrated in Fig. 3(f), a metasurface composed of 2D complex nanostructures is realized by an orthogonal LIL to achieve a broadband and high-efficiency reflective linear polarization converter [78]. Indeed, LIL is a large-area, high-efficiency, inexpensive, maskless technique - a batch process that is limited to periodic patterns only.

2.2 Pattern transfer lithography

Pattern transfer lithography is developed in order to meet the requirements for high throughput and large area fabrication (Table 2). Several techniques that fall into this category are discussed here including plasmonic, nano-imprint and self-assembly lithography.

Table 2. The unique properties of Pattern Transfer Lithography

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2.2.1 Plasmonic lithography

It is well known that the resolution of traditional photolithography suffers from diffraction limit. Plasmonic lithography has been developed to acquire deep subwavelength resolution beyond the diffraction limit. A dielectric spacer layer is sandwiched between the photomask with subwavelength nanostructures and the photoresist-coated substrate. Normal incident light excites free-electron oscillations at the interface between metal and dielectric, resulting in SPPs. The SPP waves are capable of confining optical fields in a scale that is much smaller than the wavelength of incident light. When they are used as light source to expose the photoresist-coated substrate, finer features of subwavelength can be achieved. Schematic of the reflective plasmonic lithography structure with a silver lens is illustrated in Fig. 4(a) and SEM image of the anisotropically-arrayed nano-slot metasurfaces fabricated with the technique [80] is shown in Fig. 4(b). The drawback of such plasmonic lithography is that the range of SPP propagation distance of is small because of the metal loss. A superlens is thus placed underneath the mask to improve the efficiency of projection onto the photoresist through the mask. This superlens-embedded plasmonic lithography bends light into a negative angle with respect to the normal direction of the surface due to the negative index of the superlens. Further improvement in resolution has been sought after from the superlens-based plasmonic lithography for the fabrication of metasurface holograms as shown in Figs. 4(c) and 4(d) [81]. Although plasmonic lithography has the benefit of being high throughput and low cost, mass production that requires large-area photomask remains an issue.

 

Fig. 4 (a) Schematic of the imaging reflective plasmonic lithography structure with a silver lens. The reflective lens amplifies and compensates evanescent waves, resulting in the production of nano resist patterns. (b) Corresponding SEM image of the anisotropically arrayed nano-slot metasurface using the reflective plasmonic lithography after RIE and ion beam etching processes. Zoomed area has a scale bar of 200 nm. The metasurface can focus a helicity-dependent plane wave into a spot. (c) Schematic of plasmonic cavity lithography system consisting of a Cr mask and a Ag/PR/Ag plasmonic cavity with an air separation layer sandwiched between them to avoid contamination and damage of mask patterns. The cavity can effectively amplify the evanescent waves and modulate the electric field components on imaging plane, resulting in greatly improved resolution and fidelity compared to near field and superlens lithography. (d) SEM image of the corresponding metasurface hologram by hydrogen fluoride (HF) wet etching and ion beam dry etching. The metasurface hologram exhibits a target object in the form of character “E”. (a) and (b) are reprinted with permission from [80]. Copyright 2015, Royal Society of Chemistry. (c) and (d) are reprinted with permission from [81]. Copyright 2017, John Wiley and Sons.

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2.2.2 Nano-imprint lithography

Nano-imprint lithography (NIL) is a technique that uses mechanical deformation to replicate nanostructures. Conventional NIL employs heat, called thermal NIL, to cure a polymer-coated substrate while a nano-structural master mold is being pressed against the substrate. After the mold is detached from the substrate, the imprinted pattern is transferred onto the polymer layer. Thermal-NIL has been used to fabricate ultrathin polarizing plasmonic metasurfaces [Fig. 5(a)] [82], hybrid perovskite metasurfaces [Figs. 5(b) and 5(c)] [83], and all-dielectric metasurfaces used as an efficient ultra-broadband reflector [Fig. 5(d)] [84].

 

Fig. 5 (a)-(d) SEM images of thermal-NIL-fabricated metasurfaces. (a) Metasurface quarter-wave plate constructed from an orthogonal array of nanorods of two different sizes on a glass substrate. It operates in the wide bandwidth of near-to-mid infrared and provides high polarization conversion efficiency. (b) and (c) Nanostripe and nanohole structures of perovskite metasurfaces, respectively. Insets show cross sections of the metasurfaces (scale bars are 300 nm). The metasurfaces exhibit a significant enhancement of both linear and nonlinear photoluminescence (up to 70 times) combined with advanced stability. It may pave the way toward highly efficient planar optoelectronic metadevices. (d) the heterogeneous all-dielectric metasurfaces as an ultra-broadband reflector. The over etching into the substrate is intentional since any residual a-Si will degrade optical efficiency. (e) and (f) SEM images of UV-NIL-defined metasurfaces. (e) The dual-band metasurface thermal emitters integrated with a resistive membrane heater. The heater pattern is seen on the right-hand side. This is because the bottom of the resist does not reach the substrate after the dry etching in the blank area outside the heater. Therefore, the metasurface is patterned only on the heater. (f) The metasurface with stacked subwavelength gratings. The metasurface has high-contrast and diode-like asymmetric optical transmittance in the visible-to-infrared wavelength range for TM-polarized light. (a) Reprinted with permission from [82]. Copyright 2015, American Chemical Society. (b) and (c) are reprinted with permission from [83]. Copyright 2017, American Chemical Society. (d) Reprinted with permission from [84]. Copyright 2017, John Wiley and Sons. (e) Reprinted with permission from [85]. Copyright 2015, National Institute for Materials Science. (f) Reprinted with permission from [86]. Copyright 2016, Optical Society of America.

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Another approach is the so-called UV NIL that employs a liquid-phase polymer spin-coated on a substrate. During the imprint step, the coated substrate is cured to form polymer crosslinking and generate solidified polymer by UV radiation. Therefore, the mold must be made of UV-transparent materials, and the process can be done with low imprint pressure at room temperature. Metasurface thermal emitters for infrared CO₂ sensing are manufactured with UV NIL, followed by monolayer lift-off based on a soluble UV resist, shown in Fig. 5(e) [85]. Figure 5(f) reveals that a UV-NIL-fabricated metasurface has superior extinction in the unidirectional optical transmission mode from visible to infrared [86].

NIL holds the advantages of high resolution, large area fabrication and low cost. Parallel fabrication can be implemented with NIL for high yield and mass production as well. However, this method still requires a mold to be fabricated with high-resolution equipment. Furthermore, oxygen or argon-oxygen plasma etching is a necessary step to remove the residual imprint polymer layer during which the imprinted polymer may be degraded.

2.2.3 Self-assembly lithography

When it comes to large-area nanostructures, self-assembly lithography is an efficient and facile method. One of the promising self-assembly techniques is nanosphere lithography (NL). The NL technique combines the low cost of colloidal self-assembled polystyrene (PS) spheres as a hard mask with the subsequent etching or deposition process. This process of generating regularly-arrayed nanospheres is simple and cheaper as demonstrated in Fig. 6(a) [87]. A monolayer of PS spheres is self-assembled in a compactly arrayed hexagonal lattice at the interface between air and water. The monolayer is then transferred onto a target substrate by slowly withdrawing the water out of a container. Figure 6(b) shows a dielectric metasurface that functions as a near-perfect mirror on a silicon-on-insulator (SOI) substrate which is manufactured with a self-assembled monolayer of the PS spheres as a hard mask for etching [87]. It should be noted that the reflectivity of the silicon metasurface is less sensitive to the residual disorder and the asymmetry of the PS technique. This can be attributed to the fact that the silicon cylinders possess magnetic dipolar resonances rather than the electric resonances. All-dielectric metasurfaces can also be fabricated on a flexible substrate via the NL process for sensing applications [Figs. 6(c) and 6(d)] [88]. The flexible substrates are made of polyethylene terephthalate (PET), a polymer commonly used for bearing complex electronic systems, because of its optical transparency at visible wavelengths.

 

Fig. 6 (a) Schematic of a large-scale colloidal self-assembly process. (b) SEM image of a dielectric metasurface acting as a near-perfect mirror in the telecommunications spectral window, based on NL with the PS spheres as a hard mask for the subsequent reactive ion etching of silicon. The optical measurements show an almost perfect reflectivity of 99.7% at 1530 nm with a good spectral tolerance. (c) The SEM images of Si-cylinder metasurface formed with NL and RIE. The inset in (c) is the 60° tilted view of a specially chosen defective area to better illustrate the spatial morphology. Scale bars represent 1 μm. The regularly arrayed Si cylinders with hexagonal lattice fabricated on PET flexible substrate are exploited to detect applied strain and surface dielectric environment by measuring transmission spectra. (d) Schematic of the main fabricating procedure for the flexible, all-dielectric metasurface. A thin layer of Si is deposited on PET substrate by electron beam evaporation. After that, a monolayer of PS spheres is self-assembled at the air/water interface. The size of PS spheres is further reduced with an isotropic oxygen plasma etching. The etched monolayer works as a hard mask for the subsequent etching. Finally, the sample is immersed in chloroform coupled with sonication to remove all remaining PS spheres. (e) Schematic of the multi-angled deposition for metasurface realization with the definition of free parameters relative to the crystal axis. It also shows an example composed of three different types of features: (Case 1) an interconnected line, (Case 2) an asymmetric bar, and (Case 3) a symmetric bar. (f) Left panel: the duplication of feature types from (Case 1) to (Case 3) at intervals of φ = 60 with the composition of six angles of projection. Right panel: images collected with SEM images of the fabricated versions of these patterns (Ag on Si). Inset: The six angles by which the features are reproduced. (g) Top panel: the fabrication schematic of moiré metasurfaces by the NL technique. θ indicates the angle of in-plane rotation between the two layers of self-assembled PS spheres. Bottom panel: SEM images of two representative moiré metasurfaces with θ ∼12° and θ ∼19°. The scale bars are 2 μm. (a) and (b) are reprinted with permission from [87]. Copyright 2015, Springer Nature. (c) and (d) are reprinted with permission from [88]. Copyright 2017, Optical Society of America. (e) and (f) are reprinted with permission from [89]. Copyright 2014, American Chemical Society. (g) Reprinted with permission from [90]. Copyright 2015, Royal Society of Chemistry.

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The conventional NL has so far been limited to producing simple periodic patterns. To open up new possibilities for creating more complex periodic nanostructures, other modified NL techniques have been developed. The shadow NL exploits sequential deposition from multiple angles with multiple plasma-etched colloidal masks to fabricate more complex structures [Figs. 6(e) and 6(f)] [89]. Engineered shadows of spheres provide a new strategy to efficiently prototype periodic metasurfaces. Besides, the moiré NL utilizes two stacked layers of PS nanospheres as the masks for etching and metal deposition to create metasurfaces with moiré patterns [Fig. 6(g)] [90] that feature high rotational symmetry and support multiple surface plasmon modes, leading to broadband field enhancement. NL lithography is an inexpensive technique that can be applied to large-scale substrates, but uniformity has been a significant challenge.

Laser-induced, self-organizing technique was proposed to fabricate large-scale resonant metasurfaces on Si thin films [Fig. 7(a)] [91]. The desired surface morphology of a Si thin film is obtained through deformation induced by the incident femtosecond laser pulses at a given wavelength. This is a self-adjusting approach that eliminates the need for multiple lithography steps by imprinting the interference pattern of the incident and scattered laser pulse directly onto the film. Figure 7(b) shows the metal particle array that is self-assembled by the block copolymer (BCP) on a thermal shrinkage film — a polymeric substrate [92]. The subsequent pattern shrinkage allows for period and symmetry control. Figure 7(c) shows a highly tunable refractive index metasurface constructed using the BCP self-assembly that operates in a broad wavelength range spanning across the visible region [92]. The Langmuir–Blodgett trough shown in Fig. 7(d) is a robust and scalable assembly method that can be used to construct a metasurface with close-packed Ag nano-cube arrays [93]. Such fabrication process can introduce colloidal nanocrystals of various shapes onto metasurfaces for different purposes. Figure 7(e) reveals the SEM image of the close-packed Ag nano-cube metasurface that exhibits near-ideal electromagnetic absorbance which can be tuned from visible to mid-infrared. The wrinkled template can support self-assembly as well. Assembly occurs once the wrinkled template is withdrawn from a container with a solution full of nanorods [94]. AFM image of a magnetic metasurface fabricated with the self-assembly process by dip-coating the wrinkled template in a highly concentrated Au nanorod solution is shown in Fig. 7(f) along with the schematic of the process.

 

Fig. 7 (a) SEM image of a laser-induced self-organized Si metasurface. Insert: schematic sketch of the laser-induced nanostructuring on the Si film in upper left, and zoom-in of the metasurface in upper right. (b) Schematic of metal nanoparticle ensemble preparation by BCP self-assembly, substrate transfer and pattern shrinkage. (c) SEM images of hexagonal Au nanoparticle arrays as-prepared from BCP self-assembly. The precise manipulation of the distance between block copolymer nanopatterns via pattern shrinkage can increase the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1000 nm wavelength bandwidth. (d) Photo of the Langmuir–Blodgett trough apparatus as a robust and scalable assembly method that is used to form the metasurface constructed of close-packed Ag nano-cube array. The deposition process follows the assembly of Ag nano-cube array at an air–water interface. The array can be subsequently transferred onto substrates of different kinds including those of flexible and non-planar. (e) SEM image showing closely-packed Ag nanocubes after deposition. A measured spacing of 3 nm occurs due to polymer grafts at the Ag surface. Scale bar is 1 μm. (f) Left panel: schematic template-assisted self-assembly process by dip coating the wrinkled templates in a highly concentrated Au nanorod solution. Right panel: the AFM image of the self-assembled array of Au nanorods in the wrinkle template. (a) Reprinted with permission from [91]. Copyright 2016, Royal Society of Chemistry. (b) and (c) are reprinted with permission from [92]. Copyright 2016, Springer Nature. (d) and (e) are reprinted with permission from [93]. Copyright 2015, Springer Nature. (f) Reprinted with permission from [94]. Copyright 2016, Royal Society of Chemistry.

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2.3 Hybrid patterning lithography

Hybrid patterning techniques combining various aforementioned lithography methods are developed to realize metasurfaces with more complex nanostructures (Table 3). Two such methods are discussed here.

Table 3. The unique properties of Hybrid Patterning Lithography

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2.3.1 Micro-sphere projection lithography

Projection lithography offers new capabilities in rapid-prototyping of periodic and quasi-periodic metasurfaces. It uses self-assembled arrays of silica spheres as colloidal microlenses. Each microlens projects an image of a distant, macroscopic mask onto the coated substrate. Figure 8 (a) depicts the schematic of the fabrication process for aperiodic metasurfaces from the template in silicon to the final substrate [95]. The beads are selectively adhered inside the wells in the silicon wafer filled with polyethylenimine (PEI). The PEI is removed by thermal decomposition with a butane torch. The array of beads is picked up with a polymer slab and then placed in contact with the substrate. After the exposure and development, the pattern is transferred by metallization and lift-off or by etching [Fig. 8(b)] [95].

 

Fig. 8 (a) Schematic diagram of the process flow for the microlens projection lithography. Detailed description can be found in [95] (b) Left panel: a non-periodic metasurface composed of T-shaped nano-patterns that was first patterned in Au on silicon and then etched to yield micro-pillars using dry etching. Right panel: the zoomed-in view of T-shaped nano-patterns. The scale bars of the left and right panels are 20 and 5 μm, respectively. (c) Schematic illustration showing the fabrication process for tilted nano-pillars using hole-mask colloidal lithography and off-normal deposition. (d) SEM images for the fabricated samples with different tilting angles. The scale bar is 500 nm. (a) and (b) are reprinted with permission from [95]. Copyright 2016, American Chemical Society. (c) and (d) are reprinted with permission from [96]. Copyright 2016, American Chemical Society.

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The method features size in the range of 0.4-10 μm and is suitable for academic research on metasurfaces. This is because efficient verification between theory and experiment requires generation of new designs rapidly over large areas and at low cost.

2.3.2 Hole-mask colloidal and off-normal deposition

Several challenges remain to realize directional metasurfaces with tilted nanopillars that are compact, scalable and cost-effective to produce in large quantities. Figure 8(c) combines an inexpensive and high throughput hole-mask colloidal lithography method, and off-normal deposition to fabricate the directional metasurfaces [96]. At first, the PS spheres are randomly dispersed on a substrate coated with polymethyl methacrylate (PMMA) followed by the deposition of a thin metal film. The hole-mask is formed by removing the PS spheres through the use of tape stripping. Tilted nanopillars are created after the deposition at an off-normal angle and the removal of PMMA.

Figure 8(d) shows the plasmonic metasurfaces with an out-of plane asymmetrical, aligned, and tilted subwavelength Au nanopillars [96]. These structures produce directional optical response as a result of light scattering off these asymmetrical structures.

2.4 Alternative techniques

While the three categories of fabrication techniques reviewed above have all played important roles in demonstrating optical metasurfaces and their unconventional applications, they each have certain limitations that leave space for innovation and improvement. As the community continues its search for innovative techniques, it is important to realize that some existing fabrication techniques could be but not yet applied to the making of optical metasurfaces. These methods offer great potential in expanding the versatility and functionality of metasurface in the near future (Table 4).

Table 4. The unique properties of Alternative Techniques

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2.4.1 Laser-induced forward transfer lithography

Although the multi-exposure EBL technology has been used to fabricate multi-layer micro- and nanostructures, the method is rather complex to use because of the difficulty in alignment error in addition to being a time-consuming process. Laser-induced forward transfer (LIFT) is a versatile and high-throughput fabrication technique based on the LDW technology [Fig. 9(a)] [97,102]. The main idea behind LIFT is energy transformation where the pulsed laser beam passes through the supporting transparent substrate and gets focused on a donor material. The optical energy of the laser pulses is then transformed into the kinetic energy of the donor material. As a result, the illuminated material can be ablated forward and deposited on an opposite receiver substrate [Figs. 9(b) and 9(c)] [97,102].

 

Fig. 9 (a) Schematic of a novel method based on femtosecond LIFT for high-throughput and efficient fabrication of nano-structures. With the precise control of laser raster path applied on sputtered multilayer thin films, the laser-ablated materials can be transferred to another substrate leaving a fabricated multilayer structure on the original substrate. SEM images of (b) the fabricated multilayer split-ring resonator arrays on donor and (c) the corresponding transferred structures on receiver. (d) Close-up SEM image of recorded marks showing the void at the center of the mark and a ring surrounding the void. The laser ablation fabrication is realized in a 50-nm-thick phase-change film on a 130-nm-thick ZnS-SiO₂ dielectric layer deposited on a glass substrate. (e) SEM image of a single Au nanoparticle on a SiO₂ substrate fabricated by the laser-induced dewetting process of a 30-nm-thick Au film. Scale bar is 500 nm. (f) Illustration of multiphoton polymerization generated by a focused laser beam. A photopolymer absorbs two near-infrared photons simultaneously in a single quantum event whose collective energy corresponds to the UV region of the spectrum. The rate of two-photon absorption is proportional to the square of the light intensity, so that the near-infrared light is strongly absorbed only at the focal point within the photopolymer. SEM images of 3D microstructures with multi-photon polymerization for (g) a photonic crystal structure and (h) Venus. (a)-(c) are reprinted with permission from [97]. Copyright 2012, John Wiley and Sons. (d) Reprinted with permission from [98]. Copyright 2010, Optical Society of America. (e) Reprinted with permission from [99]. Copyright 2016, John Wiley and Sons. (f) Reprinted with permission from [100]. Copyright 2008, John Wiley and Sons. (g) and (h) are reprinted with permission from [101]. Copyright 2003, Optical Society of America.

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The setup and operation of the LIFT experiment are fairly simple and low-cost since LIFT is a kind of LDW process. Most LIFT-correlated processes only require ambient atmospheric conditions without the demand of a clean room, chemical compounds and vacuum chambers. This simple, fast, and one-step technique shows great potential in micro- and nano-device fabrication.

2.4.2 Laser ablation and laser-induced dewetting

Laser ablation is another interesting technique where materials are lifted off from the surface of a substrate with a focusing laser. The removed quantity depends on the material as well as the wavelength, intensity and pulse width of the excitation laser. Dense hotspots in Ag nanostructures have been demonstrated with the laser ablation method [103]. The structure of recorded mark on a phase-change thin film under the laser ablation process has been reported as shown in Fig. 9(d) [98].

Dewetting of a heated material is one type of self-assembly process. Laser radiation as a heat source can launch the dewetting process. Laser-induced dewetting is a single-step, lithography-free and cost-efficient method for large-scale fabrication of ordered and disordered structures [Fig. 9(e)] [99]. The method opens up new possibilities for creating nanostructures in metal, dielectric, semiconductors and multilayer substrates on large-scale.

2.4.3 Two photon lithography

3D micro- and nanostructures have long been sought for but their fabrication has faced significant technical challenges. The lithography techniques have severe limitations here because of their high cost, prohibiting their applications to the fabrication of such complex structures [100,101,104–107]. Two-photon lithography with ultrashort laser pulses is an effective and well-established method in polymerization of arbitrary 3D structures. Polymerization occurs once a molecule is excited from one state to a higher energy state by absorbing two photons of identical or different frequencies [Figs. 9(f)-9(h)] [100,101]. The advantages of two-photon method are good geometry control, scalable resolution and cleanroom free. At the same time, the low efficiency of two-photon absorption and the limitation of photosensitive materials remain significant challenges.

3. Applications of metasurfaces

Novel optical metasurfaces that promise ultrathin, lightweight, and ultra-compact advantages over the bulky and heavy conventional optical devices have revolutionized the ways with which electromagnetic waves can be controlled and manipulated. A wide variety of functionalities that were impossible to realize in conventional devices are now made possible with optical metasurfaces. In this section, we shall survey some of the applications that have arisen from these novel devices over the last couple of years. In particular, we shall focus on those applications that rely on wavefront shaping, polarization control and active tuning.

3.1 Wavefront shaping

Wavefront engineering for conventional optical elements relies on the spatial refractive-index distribution, surface topography and phase accumulation along the optical path. Some examples are illustrated here to show how these purposes are realized such as beam splitting, beam steering, metalens, meta-hologram and optical vortex generation. Two of the hot topics in metasurface development have been the control of their strong phase dispersion and the ability to extend their working bandwidth. We shall review metasurfaces designed and fabricated for monochromatic and broadband operations. We shall also discuss wavelength dispersion in these devices.

Beam steering as a unique function of metasurface was first proposed and demonstrated by Capasso’s group in 2011 [4]. With the abrupt phase change at the surface, the new wavefront with anomalous propagation direction can be created by orderly arranging the nano-resonators on a metasurface. The anomalous propagation direction has been found to follow the generalized Snell’s law that takes into account of the index gradient along the in-plane direction of metasurface which can be tuned by design in order to achieve manipulation of wavefront. Recently this law has been applied to metasurface composed of supercells with various phase differences by Ho et al. [108]. Metalens is another application of the wavefront shaping. The phase retardation distribution of a metalens can be described as:

 

Fig. 10 (a) Measured focusing efficiency of the metalenses for incident circularly polarized light as a function of wavelength. The two lenses are designed at two different wavelengths λ_d = 532 and 660 nm. The efficiency defined as the transmitted optical power with opposite helicity divided by the incident circularly polarized light. Insert: SEM micrograph of the fabricated metalens designed at the 660-nm wavelength with the scale bar of 300 nm. (b) Measured focal spot intensity distribution of the metalens designed at the wavelength of 660 nm. (c) Schematic of the imaging principle of the proposed chiral metasurface, which focuses into two spots with different helicity. The inlet in the figure comprises two images of beetle where left and right panels are revealed by focusing the reflected left-circularly polarized (LCP) and the right-circularly polarized (RCP) light from the beetle, respectively. (d) The diagram demonstrates a multiplex color router with the dielectric metalens capable of guiding individual primary colors into different spatial positions. (a) and (b) are reprinted with permission from [109]. Copyright 2016, The American Association for the Advancement of Science. (c) Reprinted with permission from [110]. Copyright 2016, American Chemical Society. (d) Reprinted with permission from [69]. Copyright 2017, American Chemical Society.

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In addition to shaping of symmetrical wavefront, metasurfaces can be designed to generate arbitrary wavefront. In comparison to holograms created by conventional methods, optical metasurfaces can enhance the hologram imaging performance. The computer generated hologram (CGH) technique has been used to calculate the phase distribution requirement for holographic images. Meta-holograms have been demonstrated recently to show high contrast, multiplexing and tunability. Shown in Figs. 11(a) and 11(b), Kivshar’s group has realized grayscale meta-holograms composed of a set of 36 different high-index Si nano-pillars with a measured transmission efficiency up to 90% and diffraction efficiency over 99% at 1600 nm [111]. Our group has demonstrated a multi-color hologram utilizing Al nano-rods with different sizes to form a two-level phase modulation that is polarization- and wavelength-dependent. The hologram images are re-constructed at different angles for three primary colors [Fig. 11(c)] [112]. Wang et al. has proposed a metasurface consisting of three kinds of Si nano-blocks with subwavelength spacing and in-phase orientations in order to achieve full phase control in red, green, and blue [Fig. 11(d)] [113]. They have also demonstrated achromatic and highly dispersive meta-holograms by creating identical images at different wavelengths and distinct images at identical wavelength. To extend the practicability of meta-holograms, a reprogrammable 1-bit meta-hologram whose state can be switched between 0 and 1 by electrically controlling the loaded diodes is demonstrated by Li et al. [Fig. 11(e)] [114]. Malek et al. proposed a reconfigurable meta-hologram with Au nanorods on a stretchable polydimethylsiloxane substrate where the hologram image plane can be multiplexed by stretching [117].

 

Fig. 11 (a) SEM images of fabricated meta-holograms consisting of silicon nanopillars of various sizes. (b) Experimental holographic image obtained through an infrared camera from the sample illuminated with a collimated laser beam at a 1600 nm wavelength. (c) Schematic illustration of the designed multicolor meta-hologram under linearly polarized illumination. The meta-hologram structure is made of a pixel array consisting of aluminum (Al) nanorods that produce images R, G, and B in 405, 532, and 658 nm, respectively. The pixels are patterned on a 30-nm-thick SiO₂ spacer sputtered on an Al mirror. (d) Schematic of the highly dispersive meta-hologram which projects the red image of a flower, the green image of a peduncle and the blue image of a pot. The insert is the partial SEM image of the fabricated meta-device. The white scale bar corresponds to 1 μm. (e) Sketch of the dynamic holographic metasurface which projects the holographic images of the letters ‘P’, ‘K’, ‘U’ at the imaging plane. The middle panel shows the metasurface composed of an array of meta-atoms. Each meta-atom has a pin diode welded between the two metallic loops and independently controlled by a DC voltage through a via (see the unit cell in the upper left corner). (f) Left panel: phase distribution of the metasurface with fork gratings. Right panel: the SEM images of the fabricated plasmonic metasurfaces on an 80-nm-thick Al thin film by FIB. It includes spatially variant nano-slits with a size of ~50 nm by 210 nm. Scale bar is 3 µm. (g) Generated optical vortex beam from metasurface fork gratings with different incident beams. (h) The measured intensity profiles and corresponding interference patterns of the metasurface on the transmission side under the RCP illumination. Three real focal planes under the cross LCP transmission at z = 40, 100, and 160 µm are observed. (a) and (b) are reprinted with permission from [111]. Copyright 2016, Optical Society of America. (c) Reprinted with permission from [112]. Copyright 2015, American Chemical Society. (d) Reprinted with permission from [113]. Copyright 2016, American Chemical Society. (e) Reprinted with permission from [114]. Copyright 2017, Springer Nature. (f) and (g) are reprinted with permission from [115]. Copyright 2017, John Wiley and Sons. (h) Reprinted with permission from [116]. Copyright 2016, John Wiley and Sons.

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Optical phase singularities have recently received much interest due to their various promising applications in nonlinear optics [118], quantum information technology [119], optical tweezers and so on. Vortex beam is typically created by fork diffraction gratings and spiral phase plates. Chen et al. demonstrated a novel type of geometric metasurface fork grating that seamlessly combines the functionality of a metasurface phase plate for vortex-beam generation and that of a linear phase gradient metasurface for controlling the wave-propagation direction [Figs. 11(f) and 11(g)] [115]. Qiu’s group proposed an ultrathin nanostructured optical vortex generator with multiple focal planes along the longitudinal direction by combining the functionalities of spiral phase plates and focusing lens [Fig. 11(h)] [116]. The polarization state and the position of the focal plane can be controlled by manipulating the helicity of the incident light. They designed a series of Pancharatnam-Berry phase metasurfaces to produce the perfect vortex and vector beams, replacing the conventional vortex plate, Bessel converter and lens.

Wavelength dispersion of optical materials is known to affect the performance of optical components and systems. To eliminate chromatic aberrations, the concept of achromatic metalens that imparts a wavelength-dependent phase contribution is proposed to compensate the dispersive accumulated phase through light propagation. Aieta et al. realized achromatic metalens using an aperiodic arrangement of coupled rectangular dielectric resonators, which operates at telecomm wavelengths [Fig. 12(a)] [120]. This design preserves the same focal length when the lens is illuminated with near-infrared light beams at three discrete wavelengths of 1300, 1550, and 1800 nm. Similarly, Faraon’s group presented a method for designing multi-wavelength metasurfaces using unit cells with meta-atoms of different resonant modes which can be used to implement a lens with two focal distances at two wavelengths, or a lens converging at one wavelength and diverging at the other [Fig. 12(b)] [121]. Avayu et al. introduced dense vertical stacking of independent metasurfaces, where each layer is made from a different material, and is optimally designed for a different spectral band [Fig. 12(c)] [122]. Khorasaninejad et al. proposed an achromatic metalens operating over a continuous 60-nm bandwidth in the visible [Fig. 12(d)], although the bandwidth is not enough for practical applications at the moment [123]. Our group proposed a design principle to realize achromatic metasurface that can eliminate the chromatic aberration over a continuous wavelength region from 1200 to 1680 nm for circularly-polarized incidence in a reflection scheme [Fig. 12(e)] [68]. Integrated unit elements on metasurface with smooth and linear phase dispersion are used to compensate the phase requirement of different wavelengths.

 

Fig. 12 (a) The simulated performance of the multi-wavelength achromatic metalens with the focal length of 7.5 mm and the diameter of 600 μm for various wavelengths. Broadband incident light is employed to illuminate the backside of the lens. (b) Upper panel: schematic illustration of scattering element composed of two kinds of a-Si nanoposts. Lower panel: the top view and 30-degree-tilt view of the SEM images for the metalens, respectively. (c) The artist’s view and schematic illustration of the three-layer structure metalens are shown in the left panel and the right panel, respectively. An interlayer distance of 200 nm is designed to prevent the near-field crosstalk between the nano-sized antennas in the different layers. The diameters and the separations of nano-disks are 125 and 185 nm in the Au-based layer, 85 and 195 nm in the Ag-based layer, and 120 and 150 nm in the Al-based layer, respectively. (d) The simulated and experimental results of the focal length versus wavelength for the achromatic metalens. Also shown in the figure is the experimental results for the geometric-phase-based metalens. It shows 1.5% variation in focal length between 490 to 550 nm for the achromatic metalens, which is close to the simulation result of 1.2% variation. The insert is a top-view SEM image of the fabricated sample. (e) Schematic of the achromatic metalens where the focal point becomes a single spot with the optimized phase compensation. (f) Schematic illustration of normal dispersion in refractive prisms and conventional lenses in the left panel. The other right panels are schematics of metalenses with negative, zero, positive and hyper-dispersive in dispersion-controlled metasurfaces. (a) Reprinted with permission from [120]. Copyright 2015, The American Association for the Advancement of Science. (b) Reprinted with permission from [121]. Copyright 2016, Optical Society of America. (c) Reprinted with permission from [122]. Copyright 2017, Springer Nature. (d) Reprinted with permission from [123]. Copyright 2017, American Chemical Society. (e) Reprinted with permission from [68]. Copyright 2017, Springer Nature. (f) are reprinted with permission from [124]. Copyright 2017, Optical Society of America.

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Contrary to the effort to eliminate wavelength dispersion, the use of dispersion of metasurface can also provide opportunities for applications that capitalize the effect. Arbabi et al. experimentally demonstrated dielectric gratings and focusing mirrors exhibiting positive, negative, zero, and enhanced dispersions [Fig. 12(f)] [124]. Super-dispersive off-axis metalenses for high resolution spectroscopy have also been demonstrated [125].

3.2 Polarization control

As a fundamental property of electromagnetic waves, polarization has been widely used in a wide range of applications. Conventional optical components usually rely on the birefringence of crystals to manipulate the polarization states of light. Orthogonally polarized waves propagating through the crystal accumulate different phases between two principal axes. These optical components suffer from a number of drawbacks such as being bulky and heavy with narrow operating bandwidth and limited material choices. Metasurface-based wave plates being light and compact, on the other hand, promise to be capable of broadband operation with a range of materials to choose from. Such wave plates can modulate the phase delay between two orthogonal polarizations by splitting the incident light. We shall review the recent development of metasurface-based wave plates for applications ranging from low-phase modulation in quarter-wave plates to high-phase modulation in polarimetry.

Although metasurfaces consisting of nano-antennas made of noble metal were initially used to demonstrate quarter-wave plates, dielectric metasurfaces have been introduced to improve device efficiency in visible and near-infrared range. Recently transparent conducting oxides (TCOs) developed as part of the CMOS-compatible process are being investigated as alternative materials for dielectric metasurfaces. Kim et al. fabricated a TCO metasurface working as a quarter-wave plate in the reflection mode as shown in Figs. 13(a) and 13(b) [126]. The metasurface can be operated over the broad bandwidth in the near-infrared regime (λ = 1.75 to 2.5 μm). The reflected beam exhibits a high degree of circular polarization state close to unity [Fig. 13(c)].

 

Fig. 13 (a) Schematic of the TCO-metasurface serving as the quarter-wave plate in the reflection mode and unit cells of plasmonic resonators with associated geometrical parameters. The material of gallium-doped zinc oxide (Ga:ZnO) is chosen to be TCO. P_x and P_y are the periodicity in x- and y-directions, respectively. (b) Top-view SEM image of the fabricated Ga:ZnO metasurface. (c) The measured polar diagrams of polarization state for the reflected beam at wavelengths of 1.6, 1.9, and 2.0 μm. The wavelength for reflected beam of the bare glass is 1.9 μm. Here both of P_x and P_y, as indicated in the schematic diagram of the unit cell, are designed to be 750 nm. (d) Schematic illustration of the Z-shaped chiral metasurface as the CD waveplate. (e) Top-view SEM image of the fabricated Z-shaped left-handed chiral metasurface. The theoretical and experimental polar diagrams for polarization statesof (f) the linearly-polarized transmission spectra with the RCP incident light and (g) the elliptically-polarized reflection spectra (close to circular polarization) with the LCP incident light at the wavelength of 1.56 μm. (h) Schematic of a monolithically integrated metasurface. The monolithic device is composed of QCLs forming two arms with integrated dielectric waveguides on which antennas are arranged. (i) Optical microscope (OM) image of the fabricated antenna structure arranged as a second-order grating with the white scalar bar of 100 μm. Each antenna has dimensions of 21 μm in length, 3 μm in width, and 0.4 μm in height. Measured polarization state of the active metasurface with pumped current in the left arm (j) equal to 3.49 A or (k) varied from 3.39 to 3.54 A, while keeping the current in the right arm faxed at 3.67 A. The output polarization state can be tuned from linear to near circular with the superposition of polarizations of the emitted light from two arms. (a)-(c) are reprinted with permission from [126]. Copyright 2016, American Chemical Society. (d)-(g) are reprinted with permission from [127]. Copyright 2017, Springer Nature. (h)-(k) are reprinted with permission from [128]. Copyright 2017, American Chemical Society.

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Circular dichroism (CD) is a phenomenon that takes place when the left-circularly polarized (LCP) and the right-circularly polarized (RCP) components of the incident light experience different transmittance. Hu et al. developed an all-dielectric metasurface composed of Z-shaped chiral structures of silicon film on a SiO₂ substrate to realize the CD waveplate in the transmission mode [Figs. 13(d) and 13(e)] [127]. Experimental results show CD up to 97% and a high extinction ratio of 345:1. Polar diagrams for the polarization state of the metasurface-based CD waveplate clearly show that the RCP incident component is being transmitted while LCP reflected [Figs. 13(f) and 13(g)]. To demonstrate continuous linear-to-circular dynamic control of the polarization state, Liang et al. developed a monolithically integrated metasurface structure where metallic antennas are integrated onto a SPP waveguide [Figs. 13(h) and 13(i)] [128]. The SPP waveguide couples radiation from a semiconductor-based THz quantum cascade laser (QCL) into SPP waves which are then scattered and collimated into the direction that is perpendicular to the waveguide surface. The emitted laser beam with desired polarization can then be generated with the careful design of the metasurface. When the emitted intensities of the two laser beams with π/2 phase difference are manipulated independently, the polarization state of the combined beam can evolve continuously from linear to circular [Figs. 13(j) and 13(k)].

Metasurface-based half-wave plates appear to be more difficult to produce because the phase difference up to π needs to be sustained between the two orthogonal polarizations with equal amplitudes. Liu et al. proposed a generic method to design the half-wave plate that is wavelength independent based on the Pancharatnam-Berry geometric phase and the mirror effect of anisotropic optical antennas [Figs. 14(a) and 14(b)] [129]. The bisection angle of the antenna pairs determines the optics axis where the incident and image polarizations have mirror symmetry. Experimental results in Fig. 14(c) show high degree of linear polarization in the transmitted image in the near-infrared range.

 

Fig. 14 (a) Schematic diagram of the metasurface functioning as the half-wave plate. (b) SEM images of two samples with the optical axis angle of 0° and 45°. The metasurface consists of Au nanorods on a glass substrate. (c) Analytical calculations (the curves) and experimental results (the symbols) of state-of-polarization analysis for the two samples. (d) Optical microscopic photograph of a fabricated sample. Inserted is the schematic of the metasurface. (e) and (f) are simulated and measured cross-polarized transmission spectra, respectively, of the metasurface with different geometric parameters. (g) Schematic of an electrically controllable metasurface operating in the reflection mode. (b) The reflection phase of the active metasurface in relation to various biasing conditions at a specific wavelength of 5.94 μm. Inset shows the diagram describing the complex reflection coefficient for three conditions of bias with the increase in frequency. (a)-(c) are reprinted with permission from [129]. Copyright 2017, American Chemical Society. (d)-(f) are reprinted with permission from [130]. Copyright 2016, AIP Publishing LLC. (g) and (h) are reprinted with permission from [131]. Copyright 2017, American Chemical Society.

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Wu et al. fabricated a bi-layer ultrathin anisotropic metasurface as a half-wave plate to convert the circularly polarized light almost completely to its cross polarization [Fig. 14(d)] [130]. The metasurface is composed of an F4B substrate on which either side is deposited with periodic 180°-twisted double-cut split ring resonators. Simulation and experiment demonstrate that the transmission efficiency of cross-polarized light is greater than 94% at the resonant frequency in microwave regime [Figs. 14(e) and 14(f)]. The resonance frequency can be effectively adjusted by changing the geometric parameters of the metasurface. Park et al. further presented an actively controllable metasurface that operated in a reflection scheme [Fig. 14(g)] [131]. Figure 14(h) demonstrates that the largest swing in the reflection phase reached over 180° with the electrical tuning of the gap-plasmon resonators featuring the metal-insulator-metal (MIM) structure near critical coupling. However, such high swing in the reflection phase is accompanied with substantial loss.

The ability to extract useful information from light polarization has motivated the invention of polarimetry. This optical component is developed to determine the polarization state and Stokes parameters of an arbitrary light source. Our group has numerically and experimentally displayed a reflective metasurface polarization generator (MPG) to simultaneously produce six polarization states from a single light source with linear polarization [Figs. 15(a) and 15(b)] [132,134]. The MPG composed of gap-plasmon resonators is capable of broadband operation in the visible regime since Al is chosen as the plasmonic metal and the concept of design is based on the Pancharatnam-Berry theory. The efficiency variation at different locations of the proposed MPG remains small. High broadband extinction ratios for all generated polarization states in the visible regime have been achieved as shown in Fig. 15(c).

 

Fig. 15 (a) Schematic of the gap-plasmon metasurface for the generation of six polarization states with the incident light of linear polarization. (b) Diagram showing numerical and experimental results of scattering intensity in the left and right panels, respectively, with the 600-nm wavelength of the incident light. The middle panel shows the corresponding SEM images with a white scale bar of 1 μm. (c) The measured polarization extinction ratio for six generated polarization states. The results validate the metasurface preserving a broadband operation in the visible regime. (d) Schematics explaining the unit pixel and off-axis multichannel generation of the OAM metasurface. Each nanorod has dimensions of 220-nm long, 80-nm wide, and 30-nm thickness. (e) Schematic diagram illustratingthe superposition of two kinds of OAM states upon the illumination of linearly-polarized incident light, which can be decomposed into RCP and LCP incident light. Each kind of circularly polarized incident light can generate off-axis reflection light with four OAM states of ℓ = 1 to ℓ = 4. (f) and (g) are SEM image, numerical and experimental results for the fabricated metasurface with two kinds of superpositions of the OAM states. One is for states of ℓ = 1 and ℓ = −1, and the other is for states of ℓ = 3 and ℓ = −3. The white double-headed arrows refer to the angle of linearly-polarized incident light and the axis for the transmission of the polarizer. (a)-(c) are reprinted with permission from [132]. Copyright 2017, American Chemical Society. (d)-(g) are reprinted with permission from [133]. Copyright 2017, John Wiley and Sons.

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The orbital angular momentum (OAM) of light is the component of angular momentum of a light beam that is dependent on the field spatial distribution but not on polarization. It has drawn much attention due to its versatile applications arising from superposition of OAM states, such as vector beam generation, ultrasensitive angular measurement, spin object detection, quantum entanglements, and so on. Yue et al. has employed the Pancharatnam-Berry geometric phase to experimentally validate a single broadband gap-plasmon metasurface capable of generating superposition of multiple OAM states [Fig. 15(d)] [133]. The polarization of incident light can be simply tuned to create arbitrary superposition of OAM states in four channels as shown in Figs. 15(e)-15(g). This method has diverse applications in both classical and quantum physics.

3.3 Active metasurfaces

A major challenge to develop a multi-functional optical component is to decouple the strong correlation between its physical geometry and optical functionality. Putting together several optical elements of different functions is one way to solve the problem. But this solution inevitably encounters other undesirable challenges such as complex fabrication processes, overly sophisticated surface morphology, and bulky devices. Even when they are successfully made, multi-functional metasurfaces created this way lack the ability to actively manipulate their optical responses. There is obviously a need to develop actively tunable metasurfaces to replace the static multi-functional optical components. In this section, we shall survey tunable metasurfaces that are controlled with either mechanics, electricity, temperature or light.

Gutruf et al. demonstrated a mechanically tunable all-dielectric resonator metasurface at visible frequencies as shown in Fig. 16(a) [135]. This metasurface is composed of an array of dielectric resonators embedded in a flexible substrate made of PDMS that can be mechanically bended and stretched. The optical response of the metasurface exhibits remarkable resonance shifts under different degrees of uniaxial strain. As shown in Figs. 16(b) and 16(c), Ee et al. demonstrated a mechanically reconfigurable metasurface capable of continuous manipulation of light wavefront by changing the lattice constant of an embedded Au nanorod array through stretching the substrate [136]. These efforts have paved the way for the development of reconfigurable broadband optical devices.

 

Fig. 16 (a) SEM image of the metasurface with TiO₂ resonators embedded in PDMS, where blue indicates PDMS and green TiO₂. (b) SEM image of the Si handling wafer for the fabrication of the tunable metasurface after PDMS curing and stripping. Scale bar: 400 nm. Experimental demonstration of the tunable metasurface mounted on four linear translation stages with (c) photographs of an unstretched (left) and a stretched (right) PDMS film. Scale bar: 10 mm. (d) Schematic of the ON state of the LC cell electrically controlled via in-plane potentials. Upper panel: LC ordering has been switched to planar (green) except for a very thin layer at the bottom (blue) with residual twist due to strong surface anchoring. Bottom panel: a hybrid LC cell with nanostructured metasurface. LC switching from twisted to planar state is complete both in the bulk and in the plane of the metasurface. Black arrow indicates the direction of rubbing that sets LC alignment at the top cover. SEM images of (e) the fabricated zig-zag metasurface and (f) a small fragment of the metasurface taken at 52°to the structure’s normal. Dashed box indicates elementary unit cell of zig-zag pattern. (g) The upper-left panel: SEM image of the fabricated prototype of electronically-controlled beam-steering for operation at 100 GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Schematic diagram shows the control of resonance frequency and the phase shift of a transmitted electromagnetic wave through the applied current to the heating electrode of each metasurface unit-cell (red arrow). (h) Schematic illustration of the electrical switching of infrared light for a Fano-resonant metasurface integrated with graphene. (i) SEM image of a metasurface fabricated on top of graphene. Scale bar: 3 μm. (a) Reprinted with permission from [135]. Copyright 2016, American Chemical Society. (b) and (c) are reprinted with permission from [136]. Copyright 2016, American Chemical Society. (d)-(f) are reprinted with permission from [137]. Copyright 2015, John Wiley and Sons. (g) Reprinted with permission from [138]. Copyright 2016, Springer Nature. (h) and (i) are reprinted with permission from [139]. Copyright 2015, American Chemical Society.

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Being able to electrically control the functionality of a metasurface is always desirable for fast switching and ease of operation. Buchnev et al. proposed a scheme with which an active near-infrared metasurface loaded with nematic liquid crystal (LC) can be electrically controlled to shift its resonance in near infrared [Figs. 16(d)-16(f)] [137]. As a well-established and affordable technology, LCs in the nematic phase offer interesting material properties such as broadband optical non-linearity and birefringence. However, the strong anchoring force from the interaction of nematic LC molecules with the surface limits the efficiency of the LC-loaded devices. In order to circumvent this issue, a suspended metasurface was fabricated, successfully eliminating the strong surface anchoring effect of LC molecules by reducing the area of the supporting substrate as shown in purple [Fig. 16(d)]. This work opens up metasurface-based applications in light modulator, switches, or active control in beam steering and polarization.

Hashemi et al. demonstrated active beam steering with an electronically-controlled metasurface made of vanadium dioxide (VO₂) [Fig. 16(g)] [138] which is a phase-change material that behaves like a semiconductor at room temperature and enters an abrupt insulator-to-metal transition that is triggered by either thermal, electrical, optical or mechanical stimulation. Steering is achieved by controlling the electric current applied to individual unit-cells and collectively they shape up the wavefront of the transmitted electromagnetic wave and deflect it to specified directions. Wang et al. also used VO₂ to realize metasurface-based switchable quarter-wave plate in terahertz [140]. Such metasurface is composed of complementary electric split-resonators embedded in VO₂. Moreover, Dabidian et al. experimentally demonstrated electrical control of the reflectivity in mid-infrared using back-gated single layer graphene (SLG) as shown in Figs. 16(h) and 16(i) [139]. SLG is a promising 2D material for light modulation because electrostatic or chemical doping can alter its optical conductivity. Efficiency in modulating mid-infrared reflectivity can be improved by integrating plasmonic metasurfaces with SLG. An order of magnitude improvement has been demonstrated with this approach.

Instead of electrical control, Sautter et al. developed an all-dielectric metasurface with a nematic LC cell that can be tuned with heat to dynamically adjust electric and magnetic resonances in near infrared [Figs. 17(a) and 17(b)] [141]. As shown in Fig. 17(c), with the increase of the nematic LC temperature, a dramatic spectral shift in the electric resonance is appeared from the embedded metasurface while the spectral position of the magnetic resonance remains unchanged below the phase transition temperature of about 58 °C.

 

Fig. 17 (a) Schematic of the silicon nanodisk metasurface integrated into an LC cell. The cell can be heated by a resistor mounted on the backside of the silicon handle wafer. (b) SEM image of the silicon nanodisk metasurface. (c) Experimentally measured transmittance spectra of the metasurface for linearly polarized light and a systematic variation of the temperature. The resonance positions of the electric resonance are plotted as red dots; the resonance positions of the magnetic resonance are marked as cyan squares. The phase transition is indicated by the white dashed line. (d) Reconfigurable metasurfaces optically written in the phase-change film imaged at λ = 633 nm. Left panel: Fresnel zone-plate pattern. Middle panel: binary super-oscillatory lens pattern. Right panel: the fabricated eight-level greyscale hologram designed to generate a V-shaped five-spot pattern. Inset: computer-generated greyscale hologram with 121 × 121 pixels. Scale bar: 10 μm. (e) Oblique incidence SEM image of the all-dielectric phase-change reconfigurable metasurface with a 750-nm period grating fabricated by FIB milling in a 300-nm-thick amorphous GST film on silica. (a)-(c) are reprinted with permission from [141]. Copyright 2015, American Chemical Society. (d) Reprinted with permission from [142]. Copyright 2015, Springer Nature. (e) Reprinted with permission from [143]. Copyright 2016, AIP Publishing LLC.

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To demonstrate active metasurfaces controlled with light, Wang et al. exploited light-induced phase transition of germanium antimony telluride (GST) alloy to accomplish a variety of metasurface-based devices such as Fresnel zone plate, lens and hologram [Fig. 17(d)] [142]. The GST alloy is another important phase-change material with advantages of low optical loss in near-infrared, non-volatility, high stability, and quick response. The material can be melted or quickly quenched to an amorphous state with a short high-density laser pulse and transformed back into a metastable cubic crystalline state at an annealing temperature between the glass transition and the melting point. The use of GST rod as a basic building block for all-dielectric metasurfaces was numerically simulated [144]. Furthermore, the GST nano-grating metasurface was experimentally demonstrated for high-quality near-infrared transmission and reflection resonances as shown in Fig. 17(e) [143].

4. Conclusions

In less than a decade since the inception of metasurface, a wide range of fabrication technologies have been developed and applied to demonstrate some of the amazing features that conventional optical elements cannot produce. Still, the pursuit of more advanced fabrication methods and perfection of existing techniques are ongoing in order to meet the demands for low cost, high throughput, large area, good reproducibility, and high resolution. In this paper, numerous fabrication techniques of the optical metasurfaces with static and tunable functions have been surveyed. The maturity of these fabrication techniques along with the development of novel materials is the key to the future of optical metasurfaces. Wavefront shaping and polarization control using metasurfaces have seen impressive progress in recent years. Active metasurfaces with tunable functionality continue to emerge. A number of applications of the optical metasurfaces are not included in this article, such as nonlinear metasurfaces [145–151], parity-time symmetry metasurfaces [152–154], and so on. Effort in the community is picking up at an accelerated pace for more advanced optical metasurfaces that will revolutionize the optics industry in the not-so-distant future and change the way people think about what the ultrathin optics can do.