The precise manipulation of light, a cornerstone of various applications such as optical sensing, imaging, and communication, is generally based on the alteration of the amplitude, phase, and polarization states of light, which ultimately induce a desired outcoming light such as beam steering or wavefront shaping [1–3]. Traditionally, these tasks have been accomplished by bulk optical components based on the refraction, reflection, absorption, and diffraction of light. Despite their optimized performance and prevalence, these traditional tools have fundamental limitations due to their bulkiness and heaviness, which make them less suited for modern electromagnetic and photonic systems, where integration and miniaturization are essential [4, 5].

The solution to these challenges has emerged in the form of metasurfaces, a single-layer or few-layer stacks of planar structures that can be readily fabricated using existing technologies such as lithography and nano-imprinting [6–10]. As the two-dimensional (2D) equivalents of bulk metamaterials, the meta-atoms of metasurfaces are designed to have their own functionality to introduce abrupt changes in optical properties [11–14]. The rapid advances in metasurfaces have facilitated an era of flat optics characterized by ultra-thin, lightweight, and planar devices capable of manipulating light with high precision and efficiency.

In this review, we would like to provide a summary and perspective by examining the recent advancements in metasurface technology with a focus on their extraordinary functionality. Beginning with an explanation of the frequently used design principles of meta-atoms such as electromagnetic phenomena including magnetic dipole and Pancharatnam–Berry (PB) phase, as well as their use in wavefront shaping and beam forming applications such as meta-lens and metasurface hologram techniques will be introduced. The functionality of metasurfaces can be numerously designed based on the feature of their meta-atom, or a subgroup of meta-atom functions such as super-pixels, which leads to various features such as phase gradient designer plates, retroreflectors, optical Janus metasurfaces, polarization conversion, nonlinear optical phenomena, single-photon manipulation, and pulse reshaping.

In section 2, we briefly review the basic principles used for designing meta-atoms. After that, we focus on various applications of metasurfaces by classifying their features and functionality. From simple amplitude/phase manipulation, various extraordinary features of metasurfaces such as multiplexing of optical wavefronts, data encryption, asymmetric transmission and Janus metasurfaces, and some quantum applications based on metasurface design will be reviewed.

The performance of the phase-gradient characteristics of a given metasurface is often verified by providing anomalous reflection and refraction, or designing a lens function phase profile to provide a meta-lens geometry. The addition of a gradient phase profile caused by metasurface into Snell’s law, often called a generalized Snell’s law that includes an out-of-the-plane reflection and refraction component, can be expressed as [15, 16],

Here, we assume the case that the xz plane is the plane of incidence, and the y-axis is perpendicular to the plane of incidence. n_i, n_t are refractive indexes for incident and refracted regions. Angle parameters θ_i, θ_r and θ_t indicate the in-plane angles of incident, reflected, and refracted light, respectively, at the metasurface, whereas φ_r and φ_t are the out-of-plane angles from the plane of incidence for reflected and refracted light, respectively. In addition, Φ indicates the phase gradient artificially made by the metasurface.

Using the generalized Snell’s law, it is considered that the reflected and refracted angle of incident light can be arbitrary tuned by the metasurface. An experimental demonstration of these phase gradient metasurfaces can be achieved by various types of meta-atoms, which will be covered in the next few sections.

2.1. Plasmonic Resonances of Metallic Nanostructure

Since the field of metasurfaces has been closely related to that of bulk metamaterials, the pioneering works of the metasurface design principle have frequently used plasmonic resonances and metallic nanostructures as their meta-atom. Wavefront shaping with metasurfaces has been shown as the first meta-atom offering full phase control of light from 0 to 2π, with a V-shaped antenna [15]. Carefully designed symmetric and anti-symmetric plasmonic resonance is hybridized to produce the strong phase modulation of cross-polarized scattered light from the meta-atoms. In a comprehensive parametric study of a given meta-atom geometry, a phase-gradient metasurface has been reported by changing the length and the orientation angle of a V-shaped nanoantenna, which works in the mid-infrared region as shown in Fig. 1(a).

Figure 1. Meta-atoms based on plasmonic resonances. (a) Design principle and phase characteristics of V-shaped gold optical antennas offering full phase control of light from 0 to 2π. (b) Reflection-type metasurface that uses dipole antenna resonance on a metal mirror. (c) Babinet-inverted nanoantennas as a meta-atom to provide an ultra-thin, compact meta-lens. Reprinted with permission from N. Yu et al. Science 2011; 334; 333-337. Copyright © 2011, American Association for the Advancement of Science [15], S. Sun et al. Nano Lett. 2012; 12; 6223-6229. Copyright © 2012, American Chemical Society [19], and X. Ni et al. Light Sci. Appl. 2013; 2; e72. Copyright © 2013, X. Ni et al. [20].

Moreover, such phase-gradient characteristics of the plasmonic nanoantenna can be achieved with various geometries, including a C-shaped resonator [17, 18], mirror-imaged dipole antenna [19], and nanoapertures [20–22].

2.2. Pancharatnam-Berry Phase

Although plasmonic nanoantennas successfully demonstrate phase-gradient metasurfaces in the mid-infrared and near-infrared range, the resonant characteristics of nanoantennas often restrict the operation bandwidth within a certain range, and the high thermal loss of metallic geometry may reduce the overall performance, especially for transmission-type phase-gradient metasurfaces [23]. Yu et al. [23] conducted an experimental demonstration of beam deflection and achieved around 45% efficiency for incident light of varied polarizations within the visible spectrum (665–775 nm). They used a gradient metasurface made from a supercell containing transparent silicon nanodisks.

Despite the powerful light manipulation capabilities demonstrated by PB metasurfaces, their efficiency was noticeably low in early implementations. This arises from the inherent physics where an individual meta-atom might produce spin-conserved scatterings devoid of PB phases, thus hindering or negating the desired wave manipulation effects. Theoretical proofs indicated that the maximum operational efficiency of a single-layer PB metasurface is limited to 25% [24], a constraint later experimentally confirmed in the microwave spectrum [25].

One of the solutions to overcome the above-mentioned issues is applying a Pancharatnam–Berry phase to a metasurface [26, 27]. PB phase, often referred to as a geometric phase, is known as a strong and intuitive method to modulate the phase of circularly polarized light during the transmission of meta-atoms. When the metasurface is designed, the PB phase is often demonstrated by the rotation of meta-atoms as shown in Fig. 2(a). Using the Jones matrix form, the phase retardation caused by rotated meta-atoms can be expressed as [28, 29],

Figure 2. (a) Meta-atom that uses the Pancharatnam-Berry phase with a split-ring antenna array for beam steering. (b) Meta-hologram with helicity multiplexed images based on the Pancharatnam-Berry phase. (c) Examples of polarization-insensitive metasurfaces based on isotropic meta-atoms. Reprinted with permission from J. Zeng et al. Nano Lett. 2016; 16; 3101–3108. Copyright © 2016, American Chemical Society [29], D. Wen et al. Nat. Commun. 2015; 6; 8241. Copyright © 2015, D. Wen et al. [30], M. Khorasaninejad et al. Nano Lett. 2016; 16; 7229–7234. Copyright © 2016, American Chemical Society [34], and Z.-B. Fan et al. Phys. Rev. Appl. 2018; 10; 014005. Copyright © 2018, American Physical Society [35].

In this equation, t_u, t_v indicates the transmission coefficients of an anisotropic meta-atom along its principle axes, and φ indicates the angle between global axes (x, y) and local axes of the meta-atom (u, v). If we assume the circular polarization incidence, the Jones vector form of transmitted light, which has spin state σ, can be written as

which means that the cross-polarization phase component of transmitted light is delayed by twice the orientation angle of the meta-atom (exp(j2σφ)), whereas co-polarized light is not affected by the orientation angle.

According to Eq. (4), it is notable that it is possible to perfectly remove the co-polarization component of scattered light from the meta-atom by precisely designing the t_u and t_v to have a π-phase difference like a half-wave retarder. Moreover, since Eq. (4) is not directly affected by the wavelength, a metasurface using PB phase is quite suitable for designing broadband, highly efficient transmitted type phase gradient metasurfaces; Therefore, the principle has been widely applied to various meta-lenses and meta-holograms [30, 31] as shown in Fig. 2(b).

However, Zheng et al. [31] proposed a similar principle for designing high-efficiency PB phase metasurfaces. Using this principle, they successfully demonstrated a meta-hologram operating at 825 nm with an efficiency reaching nearly 80%. The reduction in efficiency can be attributed to the ohmic losses present in plasmonic metals, an unavoidable factor at optical frequencies.

Recently, a significant advancement was made in manipulating polarization and phase efficiency using elliptical nanopillars at a wavelength of 915 nm [32]. These nanopillars, which form the metasurface pixels, can create various symmetric and unitary Jones matrices and grant complete control over polarization and phase through the transmission matrix. When exposed to right circularly polarized (RCP) light, it converges to a spot, while left circularly polarized (LCP) light is directed into a doughnut-shaped region. Furthermore, this platform allows independent and comprehensive phase modulation for two perpendicular polarization states, suggesting potential applications in integrated conformal optical devices.

Their experiments highlighted the adaptability of the metasurface and showcased its abilities as a polarization beam splitter, wave plate, and lens, all achieving efficiencies surpassing 72%. This was accomplished by tailoring the ellipticity, size, and orientation of the nanoposts [32].

Although PB phase metasurfaces come with advantages such as a dispersion-free phase response and easy design/fabrication, they face limitations in spin-correlated functionalities. To overcome these limitations, Mueller et al. [33] proposed a method to design a single metasurface capable of accommodating two independent phase distributions for the input light of two orthogonal polarization states. By combining the propagation phase and PB geometric phase, this approach enables the separation of functionalities for the two orthogonal polarizations within the metadevices.

2.3. Polarization-independent Propagation Phase

Since the phase retardation caused by the PB phase only affected the cross-polarization component of incident circular polarization, most applications that use PB phase have polarization-sensitive characteristics. For polarization-insensitive applications, propagation phase or detour phase retardations are often or simultaneously applied to provide a larger degree of freedom in the metasurface [34, 35].

As shown in Fig. 2(c), the tuning of phase retardation for a polarization-insensitive metasurface uses isotropic meta-atoms such as cylinder or square nanopillars. Here, the size of the meta-atom may affect the variation of the effective refractive index of the guided mode through the meta-atoms. Therefore, these meta-atom types generally have a higher aspect ratio than meta-atoms based on plasmonic resonance or PB phase. Therefore, materials with a high refractive index, such as TiO₂, are often used to improve the effect of meta-atom size variation. Another example of polarization-independent phase retardation is the application of a detour phase, which often indicates the relative phase difference caused by spatial displacement of the meta-atom scatterer itself, which can be either longitudinal or transverse to the incident wave. However, the limitation of the detour phase is that the amount of phase delay is significantly altered by the operating wavelength determined by the ratio of spatial displacement and the wavelength. Thus it has a limitation to use for the broadband operation of the metasurface.

The phase delay mechanisms explained in Chapter 2 are not only applied alone but also adapted together to increase the degree of freedom to design the novel functionalities of meta-atoms. Such functionalities can be used for multiplexing phase information [36, 37], or more precise control of transmitted light that can simultaneously control the complex amplitude and polarization states [38, 39], displaying 2D/3D images together [40], and other applications. This Chapter will review the recent progress of metasurface applications that provide extraordinary functionalities with appropriate categorization.

3.1. Metasurfaces for Next-generation Display Applications

Based on the design principles of meta-atoms explained in the previous section, one of the most promising applications for metasurfaces operating in the visible range is replacing various optical display components for more compactness and better efficiency. Because of the usefulness of a metasurface in phase control, shaping and designing digital holograms has been frequently researched and is often selectively integrated with cavity structures for color [41, 42]. As shown in Fig. 3(a), a study by Hu et al. [43] shows the integration of a polarization-independent metasurface into a monolithic Fabry–Pérot (FP) cavity that acts as a RGB color filter to achieve low crosstalk and high efficiency. The 3D integration of metasurfaces by these two structures provides a function of microprints enabled by a monolithic color filter microarray as well as hologram generation. A microprint image is simultaneously observed when the device is illuminated by white light, while a full-color hologram image can be projected under full-color laser illumination [43].

Figure 3. Metasurfaces for various display applications. (a) Integration of micro-cavity color filter with metasurface for low-crosstalk, full-color holography. (b) Dynamic meta-holography using space channel switching operation. (c) Replacement of fine metal mask into a metasurface mirror for micro-OLED. (d) Pixel-level color routing metasurface for an image sensor. (e) Using a meta-lens as a polarization-selective lens for augmented reality. (f) Achromatic meta-lens doublet for digital imaging. Reprinted with permission from Y. Hu et al. Light Sci. Appl. 2019; 8; 86. Copyright © 2019, Y. Hu et al. [43], H. Gao et al. Sci. Adc. 2020; 6; eaba8595. Copyright © 2020, H. Gao et al. [44], W.-J. Joo et al. Science 2020; 370; 459–463. Copyright © 2020, W.-J. Joo et al. [45], X. Zou et al. Nat. Commun. 2022; 13; 3288. Copyright © 2022, X. Zou et al. [46], G.-Y. Lee et al. Nat. Commun. 2018; 9; 4562. Copyright © 2018, G.-Y. Lee et al. [47], and W. Feng et al. Nano Lett. 2022; 22; 3969–3975. Copyright © 2022, American Chemical Society [49].

One of the advantages of using a metasurface for digital hologram application is its tiny pixel pitch, which enables hologram generation within a tiny region with a sufficiently high beam steering angle. The work shown in Fig. 3(b) successfully applied such an advantage by introducing a space channel meta-hologram, which used a similar approach to a seven-segment system in an electronic circuit [44]. The whole metasurface is divided into several spatial subsections and consists of silicon nitride nanopillars, and each subsection provides a segmented message as a hologram. Each spatial channel can be illuminated separately using a digital multimirror device (DMD), to support a dynamic hologram message with a high frame rate (9,523 frames per second).

Obviously, the light manipulation characteristics of metasurfaces are not restricted to hologram applications; They can also be applied to more practical display devices such as micro-OLEDs. As illustrated in Fig. 3(c), Joo et al. [45] incorporated a nanopillar-type metasurface into the bottom mirror of a micro-OLED device. The phase retardation from the metasurface is optimized to produce the appropriate resonance shift of the micro-OLED Fabry–Pérot cavity. This cavity was initially designed for a blue pixel, but was altered to the cavity condition of a red or green pixel due to the resonance shift. Using this method, it is possible to replace the fine-metal mask (FMM) for RGB subpixels, which presents practical challenges for achieving higher-resolution OLED displays.

Like these, metasurfaces can replace conventional optical elements with thinner, flatter ones, and even merge two functionalities of a conventional optical element into a single metasurface. The work introduced in Fig. 3(d) shows such an example, with a pixel-level color router that combines the functionalities of a microlens array and Bayer color filters. By using the inverse design method of genetic optimization, the proposed metasurface can also improve efficiency by about 84% in the visible range, which can be applied to span a white light into RGB colors and then focus each of them in different locations within a single metasurface [46]. Therefore, the proposed work can be applied to high-resolution CMOS image sensors up to a pixel size of 1 μm by 1 μm scale.

Another well-known application of metasurfaces is using meta-lens as a polarization selective see-through display element for augmented reality displays. As explained in Chapter 2, PB phase can only produce phase retardation for cross-polarization components, whereas co-polarization components are not affected. Therefore, a meta-lens eyepiece combined with a circular polarizer can simultaneously show the external environment and AR images, which has been demonstrated by nano-imprinting technology. A prototype meta-lens for AR application with a lens aperture of 20 mm and a high NA of 0.61 for achieving a wide FOV is depicted in Fig. 3(e) [47].

The meta-lens is one of the most well-known and fascinating candidates for metasurface-based application that can be practically used. Among numerous studies that designed and fabricated a meta-lens [48], the work introduced in Fig. 3(f) applied a doublet structure in a meta-lens to design an achromatic characteristic in the meta-lens. Unlike previously reported PB phase-based achromatic meta-lenses [49], the given geometry has polarization-independent characteristics for a broad range with a high NA of 0.8 in the visible range, which can lead to the application of high-quality, full-color hologram generation.

3.2. Multiplexing and Encryption Phase with Metasurface

As we explained in section 2, there are various methods to generate abrupt phase retardation, such as PB phase, detour phase, and guided modal phase, and these principles can be merged to provide a greater degree of freedom in metasurface design. Such a degree of freedom is used not only to control the transmitted/reflected light more precisely, but also for multiplexing phase information within a single metasurface. For example, one of the pioneering works for multiplexing metasurfaces was achieved by mixing the PB phase with a guided modal phase [50–52], which can produce two independent hologram images according to input polarization handedness [53–55]. Moreover, researchers are also developing a kind of metasurface that can express certain information either actively or passively. Such characteristics can be achieved by merging various metasurface design principles in a single metasurface. This section will briefly review some of these works.

Based on the polarization-selective multiplexing characteristics of metasurfaces, a metasurface integrated with liquid crystal can be one of the simplest but most practical uses of tunable optical phase generation [56, 57]. As shown in Fig. 4(a), a polarization-multiplexed achromatic dielectric meta-lens, which is designed with meta-atoms with anisotropic retarded phase properties, is integrated with twisted nematic liquid crystals to demonstrate tunable focal length characteristics [56].

Figure 4. Metasurfaces for multiplexing and encrypting optical information. (a) Tunable polarization-multiplexed meta-lens. (b) Optical encryption platform using a vectorial hologram with multiple polarization channels. (c) Multiplexing hologram information with cascaded metasurfaces. (d) Temperature-sensitive data encryption metasurface using thermally driven phase change ma€ial rods. (e) Multiplexing full-color RGB information by single metasurface through on-chip waveguide modes and PB phase. (f) Three-channel metasurfaces that simultaneously show nanoimprinting image and two holographic images. Reprinted with permission from X. Ou et al. Nano Lett. 2022; 22; 10049–10056. Copyright © 2022, American Chemical Society [56], J. Kim et al. ACS Nano 2022; 16; 3546–3553. Copyright © 2022, American Chemical Society [59], Q. Wei et al. Adv. Opt. Mater. 2022; 10; 2102166. Copyright © 2022, Wiley‐VCH GmbH [60], B. Lee et al. Adv. Funct. Mater. 2020; 31; 2007210. Copyright © 2020, B. Lee et al. Published by Wiley‐VCH GmbH [65], Z. Li et al. Laser Photonics Rev. 2022; 16; 2100638. Copyright © 2022, Wiley‐VCH GmbH [66], and S. Zhang et al. 2020; 14; 2000032. Copyright © 2020, WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim [67].

If the optical phase information is only multiplexed through a certain polarization channel, the maximum number of multiplexed data might be significantly restricted. To increase the number of multiplexed data, either optical angular momentum [58] or a vectorial hologram [59] can be used. The work proposed by vectorial hologram is shown in Fig. 4(b). To produce nine explicit pieces of information, the authors designed a superpixel consisting of nine types of subpixels, which is designed to reconstruct certain holographic images with a properly designed polarization state, and randomly placed for suppression of the grating effect. Each of the subpixels consists of two types of groups that produce opposite types of PB phase.

Recent advances in optimizing techniques, such as gradient optimizers, have led to the development of novel types of metasurfaces that can produce unique functionalities [60]. The work presented in Fig. 4(c) shows that the cascade alignment of two metasurfaces can give a new degree of freedom to design optical phase information. Based on the phase profile optimizer based on a gradient descent algorithm, the cascaded metasurface reconstructs four types of different images according to the orientation angle between two metasurfaces, as well as each metasurface providing its own hologram images by tracking and updating the gradient of the total mean square error calculated by the optimizer.

For active metasurfaces, phase change materials working in the visible or infrared range such as Ge₂Sb₂Te₅ or VO₂ have been applied in numerous works [61–64]. Among them, the works shown in Fig. 4(d) provide a unique route to temperature-sensitive data encryption in an active metasurface [65]. Two types of Ge₂Sb₂Te₅ nanorods, which have different phase-change temperatures because of different sizes, can be used to hide hologram image data that can only be observed in a very narrow region of a temperature condition. Although the proposed work only used two types of nanorods and a single type of phase change material, the given principle has more potential to express multi-level information by applying various sizes of meta-atoms or simultaneously adding other phase change materials.

Multiplexing of holograms can also be conducted by illumination directions and method, and one recent representative exampleis shown in Fig. 4(e) [66]. In this work, the meta-atoms are a simple rectangular rod type that produce PB phase, which is designed for producing hologram information (for red light) directly illuminated from the bottom of the metasurface. However, two more phase information (for green and blue lights) are integrated into the metasurface by applying a detour phase along the horizontal and vertical directions, and the input signals are provided through the waveguide mode in the metasurface plate. Since these waveguide modes are often used in recent AR applications, the given metasurface has a great potential for multiplexing full-colored images without using a multiple number of waveguides. Another example of data multiplexing with a metasurface is shown in Fig. 4(f). It can simultaneously show three channels of multiplexed images consisting of one 2D micro-image and two far-field holographic images, respectively [67].

3.3. Asymmetric Transmission and Janus Metasurfaces

The optical Janus effect, inspired by a Roman god with two different faces, refers to the optical phenomenon displaying different information according to the observation side. The phenomenon has been classified as asymmetric reflection and transmission, and the asymmetric reflection has been simply demonstrated without applying meta-atoms because the reflection coefficients can be different without chiral geometry. Therefore, a simple nanoparticle deposition on an asymmetric material interface can lead to the optical Janus reflection [68], and by applying a liquid-permeable random nano-island F-P etalon, such a Janus reflection has been applied to optical camouflage applications as shown in Fig. 5(a) [69]. In this work, it has been shown that the optical camouflage condition, which means that the overall reflection spectra of a liquid-sensitive region is matched to a nearby reference region, can be accompanied by the optical Janus effect. On the other hand, asymmetric transmission often requires more complicated meta-atom geometry such as cascade stacking of metasurfaces or chiral meta-atoms [70, 71]. Recently, a study by Chen et al. [71] provided a directional Janus metasurface that can display two different holographic images from a metasurface plate composed of cascaded subwavelength-scale impedance sheets as shown in Fig. 5(b). Using the rotatory alignment of meta-atoms, the authors analytically derived the unidirectional features on their metasurfaces, and by distributing two types of meta-atoms that provide phase gradient characteristics for forward (or backward) direction but totally block their transmission for backward (or forward) direction, it was possible to independently design the phase gradient characteristics of a given metasurface for forward and backward side illuminations of incident light.

Figure 5. Metasurfaces for asymmetric optical characteristics, i.e. Janus metasurface. (a) Asymmetric optical camouflage using FP cavity consisting of random distributed metallic nano-islands. (b) Directional Janus metasurface using cascaded subwavelength anisotropic impedance sheets. (c) Polarization-selective Janus metasurface composed of cascaded layers of metallic nanorods and nanowires. (d) Thermally active Janus metasurface for directional holography in the terahertz region. (e) A single-layered Janus metasurface based on precise control of PB phase. (f) Polarization-independent diffractive Janus metasurface for asymmetric diffraction. Reprinted with permission from T. Kim et al. Light Sci. Appl. 2020; 9; 175. Copyright © 2020, T. Kim et al. [69], K. Chen et al. Adv. Mater. 2019; 32; 1906352. Copyright © 2019, WILEY‐VCH Verlag GmbH [71], R. Ji et al. Nanomaterials 2021; 11; 1034. Copyright © 2021, MDPI [72], B. Chen et al. Light Sci. Appl. 2023; 12; 136. Copyright © 2023, B. Chen et al. [73], X. Liang et al. Opt. Express 2021; 29; 19964–19974. Copyright © 2021, Optical Society of America [74], and H.-D. Jeong and S.-Y. Lee, Optik 2023; 274; 170499. Copyright © 2023, H.-D. Jeong and S.-Y. Lee [76].

Instead of using unidirectional light transmission and spatial multiplexing to realize an optical Janus transmission, the work shown in Fig. 5(c) applied polarization multiplexing for an optical Janus metasurface [72]. The cascaded layers of metallic nanorods, dielectric spacers, and metallic nanowire were successfully designed to provide unidirectional phase gradient characteristics only available for certain linear polarization, whereas its orthogonal polarization is not affected by the phase gradient profile designed by the nanorods.

Since the Janus metasurface often requires complicated, cascaded, or chiral meta-atom geometry, it was not simple to apply active functionality to the Janus metasurface. However, recent work reported by Chen et al. [73] successfully demonstrated the active Janus metasurface working in the terahertz region as illustrated in Fig. 5(d). The tuning mechanism for active functionality uses VO₂ integrated meta-atoms for a thermally active impedance sheet. In conjunction with other cascaded metallic layers, the authors proposed four independent phase information in their work, which are tuned by illumination direction and thermal conditions, respectively.

Like these, metasurfaces with Janus functionalities have greatly interested to many researchers. Some works have focused on multiplexing more information with more complex designs of meta-atoms for further degrees of freedom, whereas some others have focused on providing Janus functionality with simpler structures [74]. The work shown in Fig. 5(e) designed a Janus metasurface without using cascaded metasurface sheets. It is based on PB phase, therefore only working for circularly polarized light. By applying a modified iterative algorithm to optimize the bidirectional hologram caused by opposite phase distribution caused by PB phase, they successfully demonstrated a Janus hologram with a single-layer metasurface plate. However, the limitation of the proposed work is that the metasurface should work with a certain circular polarization, and additional polarization filters should be placed to remove the co-polarized light from the PB phase-based metasurface.

In addition, the work shown in Fig. 5(f) also demonstrated an asymmetric metasurface with a single-layer structure using a mixed-cavity structure composed of single and double-step nanoapertures [75, 76]. In this work, the difference in effective periods observed by the forward and backward sides is a key feature for optical Janus characteristics. Here, the periodic arrangement of square-shaped meta-atoms enables polarization-independent Janus functionality in its diffraction property. However, the proposed work was only limited to generating a diffracted beam and did not show more complicated phase-gradient information such as a meta-lens or meta-hologram.

3.4. Metasurfaces for Photons and Pulse Manipulations

Recent great advances in metasurface technologies have expanded the use of metasurfaces from conventional electromagnetic wave wavefront engineering to single-photon manipulation and pulse reshaping [77–80]. Figure 6 shows some recent works that applying metasurfaces in such photon-based or pulse-reshaping applications.

Figure 6. Metasurfaces for single-photon manipulations and pulse reshaping. (a) Generation of spin angular momentum coded single photon with metasurface. (b) Generation of photon pairs by spontaneous parametric down-conversion in lithium niobate quantum optical metasurfaces. (c) Metasurface for quantum optical state reconstruction. (d) Observation of photoluminescence dynamics in plasmonic metasurface coupled with quantum dots. (e) Using epsilon-near-zero metasurface for photon acceleration. (f) Compact optical pulse shaping device consisting of two metasurfaces within parallel silver mirrors. Reprinted with permission from S. I. Bozhevolnyi et al. Adv. Mater. 2020; 32; 1907832. Copyright © 2020, S. I. Bozhevolnyi et al. [85], T. Santiago-Cruz et al. Nano Lett. 2021; 21; 4423–4429. Copyright © 2021, T. Santiago-Cruz et al. [86], K. Wang et al. Science 2018; 361; 1104–1108. Copyright © 2018, K. Wang et al. [87], M. Iwanaga et al. ACS Photonics 2018; 5; 897–906. Copyright © 2018, American Chemical Society [88], C. Liu et al. ACS Photonics 2021; 8; 716–720. Copyright © 2021, American Chemical Society [90], and R. Geromel et al. Nano Lett. 2023; 23; 3196–3201. Copyright © 2023, American Chemical Society [91].

In the field of plasmonics, spiral-shaped gratings as shown in Fig. 6(a) are often considered a useful geometry for the conversion of the optical spin state, which is assisted by spin angular momentum interactions during surface plasmon polariton (SPP) coupling [81–84]. The work shown in Fig. 6(a) demonstrates the generation of a highly directional single-photon with a certain spin angular momentum state, based on SPP mode coupling from a spiral-shaped metasurface composed of concentric periodic width-varying dielectric nanoridges [85]. The tightly focused pump beam illustrated as green cones in Fig. 6(a) may produce a strong longitudinal electric field, which can activate the quantum emitter located at the center of the metasurface, and is finally guided to only produce a certain spin state of photons by SPP mode coupling into surrounding nanoridges.

Spontaneous parametric down-conversion (SPDC), known as a nonlinear phenomenon that converts one photon of higher energy into a pair of photons of lower energy, is often considered an important process for the generation of entangled photon pairs [86]. The work shown in Fig. 6(b) used a nonlinear metasurface to strongly enhance SPDC. The metasurface is made of nonlinear material such as lithium niobate, to produce a photon pair with an enhancement of two orders of magnitude compared to the unpatterned film material of the same thickness. The metasurface structure used in this work is composed of an arrangement of nanoresonators in the shape of truncated pyramids, which can significantly enhance the electric field near the metasurface.

Various metasurfaces have been developed not only for the generation and enhancement of entangled photons, but also have been applied to the detection of interfered photon states. The work shown in Fig. 6(c) demonstrates the interference of a multiphoton state and its reconstruction with a metasurface. It has been shown that an all-dielectric metasurface can be used to measure the correlation states of a multiphoton density matrix without direct polarization measurement detection by splitting the N-photon state encoded in polarization into a spatial split of M output slots designed by the metasurface [87].

In addition to photon manipulations, metasurfaces have also been used to enhance photoluminescence (PL), which is the phenomenon of light emission from matter stimulated by external light energy. As shown in Fig. 6(d), a configuration to couple plasmonic metasurfaces with a layer of quantum dots (QDs) has been demonstrated to enhance PL intensity [88]. Here, an Ag nano-mesh structure of 330 nm periodicity, optimized by rigorous coupled-wave analysis of a scattering-matrix algorithm, was fabricated on InAs QDs. High absorption of light from the metasurface may lead to the fluorescence enhancement.

Another extraordinary flat optical functionality observed in the metasurface is photon acceleration from a time-varying epsilon-near-zero metasurface, which is depicted in Fig. 6(e). This photon acceleration, first experimentally observed in the optical domain using gaseous plasma [89], is a kind of nonlinear light phenomenon that indicates the self-driven frequency blueshift of a beam due to the time-varying effect induced by the same beam. To achieve such characteristics, the medium itself needs to have time-varying characteristics, which is not simply achieved in ordinary dielectrics, but can be achieved with the epsilon-near-zero metamaterial made by an indium tin oxide (ITO) antenna array. The work shown in Fig. 6(e) observes the blueshift of the excitation pulse, where the amount of blueshift is increased according to the incident intensity [90].

Finally, the work shown in Fig. 6(f) demonstrates the application of metasurfaces to reshape a pulse [91]. The proposed structure consists of two metasurfaces placed laterally along a single side of a silver mirror cavity. When incident light illuminates the first metasurface, the first metasurface is designed to have simultaneous focusing and steering properties. Here, the focal point properties are designed differently according to the incident spectra, so it is focused at different points on the second metasurface. The second metasurface is designed to have spectral encoding with a reverse phase gradient regarding the first metasurface, similar to a retroreflector [92] with spectral encoding. By applying the proposed structure, it has been shown that precise spectral tuning of an incident pulse can be obtained by appropriately designing the phase shift profile of the second metasurface.

In conclusion, the advancement of metasurface technology has substantially revolutionized optical manipulation, overcoming the limitations of traditional bulk optical components and enabling unprecedented functionalities. With the effective design of meta-atoms, we have witnessed a range of applications such as wavefront shaping, beam forming, phase gradient design, retroreflection, single-photon generation and manipulation, etc. The development of metasurfaces also provides a paradigm shift toward flat optics characterized by planar, lightweight, and ultra-thin devices that offer high precision and a degree of design freedom. As we further explore the electromagnetic and quantum phenomena driven in metasurfaces, we anticipate unveiling novel optical functionalities in designing flat optics. This burgeoning field of metasurfaces promises exciting opportunities for future photonic technologies and brings us closer to the era of fully integrated and miniaturized optical systems.

National Research Foundation of Korea funded by the Korean government Ministry of Science and ICT (No. 2022R1F1A1062278); The Technology Innovation Program (P20019400) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data sharing is not applicable to this article because no new data were created or analyzed in this study.