Review Article | Open Access
Sumbel Ijaz, Ahsan Sarwar Rana, Zubair Ahmad, Muhammad Zubair, Yehia Massoud, Muhammad Qasim Mehmood, "The Dawn of Metadevices: From Contemporary Designs to Exotic Applications", Advanced Devices & Instrumentation, vol. 2022, Article ID 9861078, 24 pages, 2022. https://doi.org/10.34133/2022/9861078
The Dawn of Metadevices: From Contemporary Designs to Exotic Applications
In recent years, metamaterials and metasurfaces have prospered in many fields of “science and technology,” covering the entire electromagnetic spectrum. Metasurface devices constituting of a set arrangement of meta-atoms translate into modern-day miniaturized means to achieve planar, ultrathin, multifunctional electromagnetic (EM) systems. Metasurfaces are ideal candidates to develop next-generation, lightweight, and fabrication-friendly optical components as they impart local and space-variant phase changes on incident EM waves, providing more comprehensive control over EM wavefronts. This attribute has been instrumental in realizing a variety of special beams for high-capacity data transmission and superresolution imaging. Furthermore, from the perspective of efficiency, the below-par performance of previously explored plasmonic-based metasurfaces can be enhanced by employing all-dielectric metasurfaces. All-dielectric metasurfaces with high refractive indices have high resonance quality factors, low cost, and CMOS fabrication compatibility. 2D materials-based metasurface design has succeeded in further reducing the device footprints for better integration in optoelectronic devices. The conventional, time- and computation-intensive EM solvers have largely been assisted by artificial intelligence techniques, resulting in quicker metasurface designing. This review focuses on the state-of-the-art meta-devices employed for wavefront manipulations of optical waves. The design variants and applications of metasurfaces constitute a prolific field for future research to meet existing challenges and make the devices more suitable for real-time applications.
A range of electromagnetic (EM) radiations spanning from radiofrequency to gamma rays (greater than about 1019 cycles per second), and many intermediate frequency regimes set up the EM spectrum. The radio, microwave, and infrared have wide-ranging applications in telecommunication, precisely for audio/video transmission and reception, broadcasting and navigation, satellite communication, radar signals, wireless communication systems, and thermal heating. However, the higher-frequency EM waves including ultraviolet, X-rays, and gamma rays are ideal for high-resolution bioimaging, sterilization, and radiotherapy. Optical spectrum extending from 400 THz to 750 THz is particularly suitable for lensing, holography, lasers, and optical communication since it has a major advantage of lying in the visible spectrum of EM waves (waves that can be detected with the help of our eyes) .
Mathematically, the time-varying electromagnetic (EM) wave propagating in free space follows the model given below : where is the polarization vector, is time, and represent real and imaginary parts of optical constant, and represents spatial structure; , , and provide the amplitude, phase, and frequency information, respectively, while stands for angular frequency. An in-depth examination of the expression leads to finding out the degrees of freedom of the EM wave elaborated in Figure 1(a), which are engineered for desired applications at nanoscale .
Only some of the EM wave characteristics undergo a variation when it propagates through a natural material as the size of the atoms is too small in comparison with its wavelength to make any significant changes. On the contrary, one can establish full control over EM wave’s properties such that they exhibit unconventional functionalities by modifying the spatial profile of the optical phase, as desired. This feature is almost impossible to realize using naturally present materials. However, by using 2D or 3D arrays of artificially engineered subwavelength nanoresonators known as “meta-atoms,” the phase of EM wave can be tailored as per requirement. When meta-atoms are arranged in 3D space with considerable thickness, they constitute a “metamaterial” whereas if their thickness is of subwavelength scale that arrangement is termed as “metasurface.” To state it another way, metasurfaces are described as an arrangement of constant amplitude dipoles with engineered phase profiles. This implies that the overall intensity of the field emitted from the metasurface is a superposition from many dipole antennas. Artificial EM metamaterials yield unconventional physical properties (as compared to naturally occurring materials), such as the negative dielectric constant, negative permeability, nonlinear effects, enhanced or suppressed backscattering, tunability, reconfigurability, enhanced plasmonic effects, engineered dielectric resonances, and strong chiral properties [4–6]. As a material is characterized by its electric and magnetic properties, therefore, the design of a metamaterial involves engineering of its electric permittivity (epsilon ‘’) and magnetic permeability (mu ‘’) as was indicated in 1968 . The parameters epsilon and mu are significant while considering a material and its interaction with an EM wave, since they determine the refractive index of a material, which, in turn, dictates the propagation, radiation, or scattering of the EM wave. The refractive index has real and imaginary components as ‘’ and ‘,’ which are related by Kramers–Kronig integral , and the complex refractive index is given by , determining whether the material would be a dielectric with or a passive one with . The refractive index depends on the constitutive parameters as .
1.1. Design of Metasurfaces
On a microscopic level, metamaterials can be deemed as artificially engineered heterogeneous entities that are designed by using natural materials in the form of special periodic/aperiodic arrangements. These arrangements employ fundamental building blocks known as meta-atoms or unit cells for the design. These meta-atoms tailor properties of the metamaterial in response to electromagnetic waves. Furthermore, these meta-atoms can also be found in the form of isotropic/anisotropic, single or supercell arrangements in a metasurface, as can be seen from Figures 1(b)–1(e). Since these periodic atoms employ natural materials in their design, the same cell configuration provides a multifaceted control of phase fronts. The electromagnetic multipoles stimulated inside meta-atoms behaving as scatterers  provide independent phase (and amplitude) manipulation capability at element level by engineering their geometry, such that EM wavefronts are precisely modified  for optoelectronic applications. An appropriate selection of spatial distribution, geometry, and orientation of subwavelength resonators and the material to be employed is essential to realize desired functionalities.
In terms of geometry, there are two fundamental profiles assumed by meta-atoms: rectangular and cylindrical. A meta-atom of rectangular nanobar shape introduces a phase shift of radians so that the state of polarization is switched from one to another effectively, thus acting as a half-wave plate (HWP). The HWPs can also be realized by combining two quarter-waveplates (QWPs), which introduce a phase shift of to the light beam passing through them. The cylindrical nanopillars are essentially the step-index waveguides as they have the field localized within them because of the difference of their refractive index with the surrounding air. The phase coverage in this case is achieved by varying the diameter obeying step-index waveguide theory [11, 12]. However, when meta-atoms are arranged so as to combine multiple anisotropic nanoantennas (HWPs), they form a supercell [13, 14] which helps exploiting functions that are not realized using single nanoresonators because of the provision of additional degrees of freedom . A supercell can be realized by arranging them laterally with equal or unequal dimensions. A design of four unit cells making a supercell is presented in  to induce chirality for high broadband asymmetric transmission for LCP incidence, whereas the one in Figure 1(e) is a hybrid, polarization-insensitive supercell, where both Pancharatnam-Berry (PB) and propagation phases are used to achieve full control of the phase of incident light of any polarization. Such a supercell has been shown to achieve circular- and giant spin-selective-asymmetric transmission .
Alongside geometry or shape of the meta-atom, the geometrical dimensions, i.e., lattice constant and lateral extent (length for rectangular profiles and diameter for cylindrical ones), pose optimization challenges in order to make the design feasible for fabrication. For example, the diameter of nanopillars cannot be decreased beyond a certain height-to-diameter ratio (aspect ratio). Similarly, a nanopillar cannot assume a diameter greater than the period of the unit cell itself, as constrained by the Nyquist criterion for fabrication of meta-atoms ()). Following such precautions during design and fabrication is the key to ensuring full-phase coverage of with the variation of diameter within allowable limits .
Metasurfaces can be classified according to the range of their operating frequencies such as microwave, terahertz, and optical  as they adopt sizes less than the operating frequency (they should have smaller dimensions than the corresponding operating wavelength). Another distinction in metasurface design can be made on the basis of their scattering properties, e.g., a metasurface can be reflective, transmissive, or absorptive . They can also be categorized considering the type of material they are made of, e.g., metallic/plasmonic , high-index dielectric , a combination of metal, and dielectric or a 2D material-based metasurface .
For metal-based metasurfaces, plasmonic resonances resulting from the dynamic response of material’s electrons to the external electromagnetic field lead to strong light-matter interactions. Earlier, various transmission and reflection type subwavelength scatterer’s metasurfaces were demonstrated based on noble metals including gold  and silver . Atypical refractions were observed, which were either due to light scattered from the metallic antennas  or due to excitation of a particular mode originating from nanosieves and subwavelength waveguides. Plasmonic metasurfaces pose fabrication challenges and get contaminated when immersed in liquids . There exist ohmic losses in metals due to strong electron-electron and electron-phonon scattering, which limit the performance of metal-based plasmonic metasurfaces in the visible regime due to absorption. The energy of the impinged light is transformed to thermal energy in the material, thus making plasmonic-based metasurfaces less attractive for high-efficiency applications . The devices’ efficiency is even worsened when fabrication defects are factored in .
To attain high efficiency by mitigating nonradiative losses, all-dielectric metasurfaces have emerged as a promising alternative to plasmonics . All-dielectric metasurfaces exhibit both magnetic and electric dipoles leading to the creation of antiferromagnetic resonances  and higher-order Mie resonances. Higher-order Mie resonances refer to the generation of quadrupole and octupole configurations, along with electromagnetic dipole, for materials having , which leads to superior performance than plasmonics-based structures . This design of metasurfaces takes advantage of the low extinction coefficient at optical frequencies to attain maximum transmission by minimizing absorption loss . Metasurfaces based on silicon (Si)—one of the earliest known dielectrics—have extensively been designed because of the associated advantages of low cost and ease of fabrication. However, the performance of Si-based metasurfaces is better for infrared regime because of its 1.12 eV bandgap, while amorphous silicon (a-Si), because of having 1.7 eV bandgap, performs well for (near-infrared-visible frequencies) . An a-Si-based metasurface is reported in , achieving broadband operation and exhibiting an efficiency as high as 75% in the 1000-1400 nm range.
Other dielectric materials such as titanium oxide (TiO2)  and gallium nitride (GaN)  have also been explored for metasurfaces. These dielectrics acquire ultrahigh transmission efficiency for lensing and holography in visible regime. As an example in point, authors in  have demonstrated a high-performance metahologram for RGB wavelengths with >78% efficiency. However, the designs from these dielectrics require high aspect ratios (up to 10 or 15), to attain the desired phase making them infeasible for fabrication . For this reason, novel dielectric materials with high refractive indices and low extinction coefficients such as gallium phosphide (GaP) and hydrogenated amorphous silicon (a-Si:H) have been sought for use in visible regime. The addition of hydrogen in a-Si increases its mobility bandgap and decreases the extinction coefficient. Moreover, a-Si:H has added benefits of being a low-cost, highly efficient, and nanophotonic complementary metal-oxide-semiconductor- (CMOS-)- compatible material, thus making it an ideal candidate for designing devices requiring minimal loss for the optical regime.
In instances where asymmetric transmission plays a pivotal role in achieving targeted functionalities, such as those in quantum physics where controlling electrons’ spin momentum (polarization of the wave) is critical [36, 37], it is instrumental to be able to design a multifunctional metasurface where both phase and polarization state act as degrees of freedom. In , a monolayer all-dielectric metasurface is reported where asymmetric transmission of CP light is realized by wavefront engineering. The structural analysis of the transmission phenomenon has been carried out using the Jones calculus, where it attains a 76% asymmetric transmission parameter. So far, such a transmission  has mostly been possible for microwave or infrared domains. However, such structures have been employed to produce metaholograms for visible regime as well [30, 39].
The phase manipulation has helped to miniaturize devices to attain multiple functionalities, which are exceedingly needed in integrated optical systems. The versatile applications in the optical regime include flat optical elements , retroreflectors for visible backscattering communication , modulators, aberration-corrected flat meta-lenses , waveplates, mirrors, super imaging , polarization converters, polarimeters , optical vortex generators [44, 45], beam splitters, filters employing structural color to visualize detailed colors beyond sRGB , multiplexed metahologram devices for 3D projective displays, invisibility cloaking , and anticounterfeiting technologies [21–23].
1.2. Role of Modeling and Simulation
Traditionally, the design and optimization of metasurfaces are executed using time-intensive and well-established computational electromagnetic methods, including but not limited to finite element methods, finite difference time domain (FDTD) method, method of moments, and transmission line matrix methods. These methods rely on iterative calculations performed to zero in on the most efficient set of design parameters for the metasurface under consideration. The advent of high-speed computers popularized the use of computing software for this purpose, which, while using the same techniques as before, helped reduce the computational time and labor involved to a great extent. Using commercial software packages like Lumerical FDTD, computer simulation technology (CST-MWS), and Comsol Multiphysics, simulations are carried out for computing the target electromagnetic (EM) responses. However, they expend a drastic computational effort in selection of meta-atom design as literally hundreds of EM simulations are run to solve Maxwell equations on a case-by-case basis.
More recently, deep learning methods have gained attention in this domain primarily due to their ability to make the design optimization process faster. These methods mimic the way humans gain certain types of knowledge, have risen to the forefront in many fields of research where there is a significant amount of data to be processed, and make the overall process quicker, more accurate and bias-free. The greater accuracy associated with these methods stems from making a holistic analysis by including all possible variables into the design process. Additionally, while humans tend to lean towards certain outcomes for a particular design problem, a deep-learning routine is virtually free of any such tendencies and thus yields bias-free results. A neural network is used in  that is capable of predicting phase and characterizing 3D dielectric structures, thus making their design easier and faster (Figure 1(f)). In , a complex, all-dielectric metasurface is modeled using the information of its geometry and governing physical principles as the network’s input. The presented design is highly accurate as it achieves a very low average mean square error (MSE) (1.16×10-3) and is quicker than traditional simulation methods.
A hybrid architecture of convolutional neural network (CNN) and recurrent neural network (RNN) is employed in  to predict the optical response of a metasurface absorber and emitter with all important dimensional parameters and their shapes as inputs. The hybrid, 98% faster model results in an MSE for test datasets as and for absorber and emitter cases, respectively. Similarly, a design-independent generative adversarial network (GAN) has been applied in  to speed up the metasurface design process. The network is also capable of suggesting different possible metasurface designs for a bifocal metalens and a polarization-independent metalens.
The field of nanophotonics offers various sophisticated methods and technologies for fabricating and integrating these metasurfaces . These technologies include electron-beam lithography (EBL) , focused-ion beam (FIB) lithography , interference lithography, nanoimprint lithography , and femtosecond laser writing .
1.3. Working Principle of Metasurfaces
Unlike conventional optical elements that make use of phase accumulation for manipulating wavefronts, metasurfaces achieve this target in terms of phase, amplitude, and polarization using arrays of subwavelength optical resonators. EM waves can be modulated in terms of amplitude originating from ohmic losses and absorptive dissipation inherent to metals or plasmonic materials, in order to exploit absorption-based applications. For transmissive applications, phase-based metasurfaces are employed, which can be further divided into two types—a resonance type (design-dependent) involving propagation phase and geometrical type (usually associated with anisotropic structure) involving PB phase . The complete range of phase accumulation is a function of material, spatial distribution of nanoresonators, geometry, and orientation with reference to a nearby nanoantenna. PB phase metasurfaces are capable of showing greater transmission over a wide range of wavelengths, while their utilization is limited to circularly polarized incident light. For such a design, every HWP produces a phase shift of , where represents the angle at which the meta-atom is oriented, and the handedness of incident light dictates the sign. Upon incidence of light, the nanoresonators introduce an abrupt phase change in the optical path along the interface between two media  and mold the incident wave in the desired direction. The metasurface imparts local and space-variant phase change on incident EM wave and provides a planar opportunity for controlling amplitude, polarization , and phase of EM wavefronts such that it offers great potential for developing new optical components. It is also worth mentioning that phase-modulating metasurfaces that follow resonance-dependent mechanisms exhibit lower conversion efficiency (ratio between the power of cross-polarized light and that of incident light). In contrast, geometric metasurfaces rank higher when it comes to phase control by virtue of the spatially varying arrangements of nano-resonators [58, 59].
One of the more noteworthy contributions in this context was made by the Capasso group, when they proposed a V-shaped structure with different arm lengths and opening angles for subwavelength thick structures to introduce rapid phase changes for incoming light for wavelength manipulation . Over the years, variously shaped plasmonic meta-atoms including nanogrooves, nanoslits, nanoholes, and split-ring resonators have also been explored to realize resonance metasurfaces. Phase gradient metasurfaces find applications in beam-steering, imaging, and holography .
In this review article, we will be presenting an overview of the metamaterials and metasurface-based devices that manipulate EM waves in the visible regime. The applications including absorption, metalensing, metaholography, polarization control, and the generation of different beams including airy, Bessel, and OAM will be discussed. Some other phenomena including reflection, chirality, sensing for refractive index, bioimaging, and biosensing will also be covered briefly.
Absorbers are the devices that transform electromagnetic waves into heat energy by absorbing them. Absorption was observed as an indirect phenomenon in 1902 by Wood, when anomalous reflection dips from the metallic gratings were observed . The absorptance is measured in terms of transmission and reflection of a material by using the relation stated in  where t is transmission and is reflection.
2.1. Metasurface/Metamaterial Absorbers
In the past, numerous plasmonic metamaterial and metasurface absorbers exhibiting plasmonic resonance have been profiled. But the scarcity of their reserves and high cost, poor thermal stability and inertness, uneconomical fabrication and pervasive nature hinder their use in applications involving solar thermophotovoltaics (STPVs), thermal emission, and hot electron generation. Aluminum (Al), which is both abundant and low cost, has also exhibited perfect absorption [49, 50]. However, its low melting point (660°C) and high chemical reactivity restrict its utilization for high-temperature applications . Nickel (Ni) , chromium (Cr) , titanium (Ti) , tungsten (W) , molybdenum (Mo), and tantalum (Ta) , as well as refractory materials like titanium nitride (TiN)  and zirconium nitride (ZrN) , have all been investigated for the design of both absorbers and emitters. Emitters essentially exhibit narrowband absorption response and are augmented with broadband absorbers in solar thermal photovoltaic (STPV) applications. High-index dielectrics including Si, TiO2 and Ge have also been reported for high absorptive performance [11, 69]. Multiband light absorption has been achieved by employing plasmonic-dielectric nanoantennas hybrid system or the all-dielectric metamaterials . Nevertheless, dielectric absorbers have limited bandwidth and thus cannot be used in high-power applications because of low thermal stability. Refractory materials have shown significant promise as absorber materials owing to their plasmonic behavior in optical regime coupled with higher thermal stabilities. They have excellent absorption and are CMOS-compatible, corrosion- and high-temperature-resistant, mechanically strong, and durable.
It is of primary interest to reduce the dimensions of graded structures/Fabry-Perot-type cavity-based absorber designs using metamaterials and metasurfaces to facilitate their integration with compact optoelectronic systems. But many of the designs require patterning on subwavelength scales amounting to complex fabrication processes. Multiple pairs of stacked metal/dielectric resonators are used frequently in the design of broadband absorbers because of the strong cavity resonance inside the film of dielectric material . Moreover, the authors in  claim that an assembly of resonators assuming different shapes in one period has the potential to exhibit resonance at different wavelengths simultaneously.
An awaited revolution in the absorber design was seen when Landy et al. proposed metal-dielectric-metal (MDM) triple-layer metamaterials as perfect EM wave absorbers in 2008 . Since then, various absorbers that assume the very topology for radiofrequency (RF), terahertz (THz) , mid-infrared (MIR) , near-infrared (NIR) , and visible frequencies [40, 41] have been designed with wider angular tolerance. Different types of EM resonances are to be credited for higher absorption of the incident light, such as Mie resonance , propagating surface plasmon resonance (PSPR) , localized surface plasmon resonance (LSPR) , and surface lattice resonance (SLR) , each of which can be single or multiple narrowband resonances.
Another characterization of absorber structure can be done from a macroscopic electromagnetic point of view, whereby the material’s effective parameters “” and “” dictate its impedance () , given as . The reflection becomes negligible for the wavelength at which the impedance is equal to that of free-space impedance while the transmission is already blocked by the ground plane, resulting in perfect absorption of incident waves because impedance is related to reflection and transmission as defined in 
With a view to investigate the relationship between structure geometry and achieve maximum absorption, a wide variety of structures, such as nanopillars , disks , nanopyramids , nanocones , nanoring grooved arrays , square rods , spheres, or randomly spread nanoparticles , have been explored. However, each of them poses a different set of design challenges, with the quest for an optimum configuration still underway.
Optical absorbers are employed in a wide arena of applications across the EM spectrum, such as photodetectors, photochemical and photovoltaic cells, optical filters, thermal imaging, stealth technology, thermal camouflaging and thermal light sources. On the other hand, wavelength-selective absorption finds applications in color filtering with enhanced color purity . All-dielectric  design configurations, as well as MDMs , have the potential for behaving as color filters.
Many efficient and broadband solar absorbers of different shapes that fully capture incident sunlight  have so far been proposed. In , a tungsten-based absorber with square- and cross-shaped designs having a total thickness of 250 nm is presented with near-unity absorption for the entire visible range (Figure 2(a)). In , a ZrN-based ultrabroadband absorber is presented for energy harvesting using STPV systems with an average absorption of >95% for 400-800 nm and 86% for 280-2200 nm (Figure 2(b)). In , a 250 nm thick chromium-based circular ring-shaped absorber is presented, both numerically and experimentally having for 500-710 nm and of 96.5% for 500-800 nm (Figure 2(c)). All of these structures are polarization- and angle-insensitive. A TiN-based wide-angle ultrabroadband absorber exhibiting a strong absorption of 87% for 250-2300 nm with ultrathin thickness (330 nm) is presented in  (Figures 2(d)–2(f)). The outstanding performance of this absorber is realized with the help of resonant modes while emittance of 29% is in the wavelength range of 5000-13000 nm. In , an optimized pair of tungsten-based absorber and emitter for an STPV system is analyzed (Figures 2(g) and 2(h)). Similarly, a tantalum-made cross-shaped absorber-emitter pair is shown in Figures 2(i) and 2(j), where the emitter is so sharply narrowband that it matches the bandgap (0.55 eV) of an indium-gallium-arsenide-antimonide (InGaAsSb) PV cell . The overall STPV efficiency has been reported as 41.8%.
3. Light Structuring (Metalensing, Holography, and OAM Generation)
Refraction of light in conventional optics obeys Snell’s law; however, for heterogeneous materials or metamaterials where there is an abrupt phase discontinuity at the interface, the light beam redirection is not only determined by a sudden change of refractive indices but also by the distribution of phase discontinuities . Generalized laws of refraction and reflection are demonstrated with the help of Figures 3(a) and 3(b), where it is evident that the interface leads to a noncoplanar anomalous beam generation, whose direction is dependent on the angle between the incident plane and antenna array. Abrupt phase discontinuity provides new degrees of freedom. V-shaped antennas have been deployed as phase elements with different opening angles, orientations and arm-lengths to modulate phase over a range of radians . C-shaped split-ring resonators , U-shaped split-ring resonators , and cross-shaped resonators  have also been explored to achieve phase discontinuities. The light reflection and refraction at the interface with a phase gradient is given by 
Here, represents the angle between propagation vector and its projection on the -plane whereas represents the projection angle of the wave vector on the -plane and -axis.
Traditional optics—lenses, waveplates, filters, and the like—have existed for centuries and despite refinements; they have not largely been changed requiring machining and angling of the glass for realization of the desired operation. Lenses find applications in various fields of science including medicine, biology, optics, photonics, and security . They are employed in imaging, spectroscopy, endoscopy, full-color routing, robotic vision, and self-driving vehicles. The conventional lenses are bulky and unfit for being used in a compact system. More recently, metasurface optics has emerged offering compactness and a new range of functionalities not accessible with bulk optical elements. High-frequency simple and advanced fabrication techniques such as single-step lithography and vertical integration have favored ultrathin, lightweight, and multifunctional flat lenses, along with features of diffraction-limited focusing and high-resolution qualities. Such sophisticated, portable, and compact designs, owing to their potential for vertical integration and adaptability, are capable of replacing their conventional voluminous counterparts by meeting the challenges and providing opportunities for applications like cellphone camera modules, wearable displays, and bioimaging.
A flat lens is an assembly of optical antennas that are subwavelength-spaced scatterers engineered in a suitable size, shape, position, and orientation to scatter light with equal amplitude and cover phase over 0 to range . Many variants of metalens design including spin-selected, broadband, spin-insensitive, and polarization-controlled lenses have so far been presented. The efforts have continued to improve designs that are large in diameter, high in efficiency, and free from aberration to be used for sophisticated imaging. The intent to optimize the design parameters such as focusing efficiency, numerical aperture (NA), resolution, full-width half-maximum (FWHM), and Strehl ratio has resulted in a wide range of lens designs.
To have “high efficiency” lenses, a series of designs including multilayered metalens [69, 70] and all-dielectric metalenses [17, 71] have been proposed. A highly efficient, polarization-insensitive metalens composed of cylindrical nanopillars of a-Si:H () has exhibited an efficiency of 73.4% at the design wavelength of 633 nm. The authors in  have presented highly efficient, polarization-insensitive planar lenses on a glass substrate obeying the waveguiding effect for phase accumulation. The demonstrated lenses are TiO2 nanopillars-based, have a diameter of 300 microns, NAs of 0.85 and 0.6 with respective highest efficiencies of 60% and 90% for red (660 nm), blue (532 nm), and green (405 nm) wavelengths. A symmetric diffraction-limited spot of 0.64 has been recorded with high resolution, and the Strehl ratios for horizontal cuts are 0.8, 0.82, and 0.83 for green, blue, and red wavelengths, respectively. Figures 3(c) and 3(d) show scanning electron microscope (SEM) images of the fabricated elements with nanopillars, while in Figures 3(e) and 3(f), the measured focal spots of the metalens at wavelengths of 405 nm and corresponding horizontal cuts of the focal spot are shown with FWHM of 385 nm.
In conventional optics, material dispersion leads to multiple foci, resulting in chromatic aberration, which is mitigated using multiple lenses for compensation of the difference of phase. In contrast, a design in , with hyperboloidal phase, produces spherical wavefront without paraxial bound and supports “aberration-less” focusing even at high NA. In the same way, to realize an aberration-free design, planar lenses and axicons consisting of a phased assembly of subwavelength-spaced optical nanoresonators have been demonstrated in  (Figure 3(g)). A diffraction-limited achromatic focusing for 470-670 nm is demonstrated using nanostructures of a thickness of the order of wavelength in . Another wideband, highly efficient diffraction-limited achromatic metalens for wavelengths ranging from 1000 nm to 1700 nm is demonstrated with focusing efficiency of >64% in .
An important performance metric for metalenses is the size of the focus spot. The smaller the size, the better will be the resultant focus. A lens that is able to achieve the limiting focus spot size () is said to exhibit diffraction-limited focusing at that wavelength. A single metalens made of an array of GaN nanoantennas has been reported to achieve diffraction-limited focusing with a subwavelength resolution for the entire visible spectrum . A high-NA (0.97) metalens was demonstrated in , while has also been realized through immersion metalenses . As another design variant, a 40 nm thick multifoci metasurface lens has been experimentally demonstrated in , which has three focal planes with CP keeping polarization states intact at the output as that of incident light (Figures 3(h) and 3(i)). Some of the recent research has attempted to test the validity of diffraction-limiting concepts by exploring opportunities for subdiffraction-limited focusing, as highlighted in . Another important development in this domain has been the introduction of 2D materials, which have made flattening of lens design to atomically thin levels possible by virtue of their high refractive indices. In , a monolayer, ultrathin flat lens design method based on 2D materials using femtosecond laser writing is presented. This technique can induce local scattering inside the monolayer so as to aid in obtaining sufficient phase and amplitude modulation that has been a challenge for atomically thin 2D materials. The imaging with subwavelength resolution and diffraction limit for a range of magnifications and at different focal positions is achieved by the presented design (Figures 3(j)–3(p)).
Fresnel zone plates (FZPs) present yet another design variant for metalenses that function by diffraction-induced focusing of oblique incident light from half of the plate while blocking the other half using a binary mask, resulting in a gradual radial phase retardation. Nanoholes  (Figure 3(q)) and optical masks or nanoslits  at optical frequencies have been used for planar focusing. The phenomenon of superoscillation  is manifested when subdiffraction-limited  focusing occurs due to more frequent oscillations of band-limited functions as compared to the highest Fourier components. Longitudinal cross-sections of diffraction patterns and design of a superoscillatory lens have been shown in Figures 3(r)–3(t), which has a diameter of 40 microns , radially divided into 100 rings with a minimum annulus width of 200 nm. The lens’ depth of focus and focal length were 4 μm and 10 μm, respectively. A flat lens consisting of rows of metallic slits that achieves focusing by nanopatterned gold films is shown in Figure 3(u) . The simulated intensity of diffraction patterns for the design fabricated onto a 100 nm thick gold film layered on a glass substrate are shown within a propagation distance of 20 μm. For this design, the control of focusing behavior is a function of lens size and slit spacing. The applied design principle paves the way to engineer a wide range of optical components. In the past decade, extensive efforts have been made to achieve subdiffraction-limited resolution, where it was necessary either to place the lens in the near-proximity of the object or fabricate on it . In , a new microscope design for optical imaging with subdiffraction-limit is presented, surpassing the capabilities of conventional instruments and those of superlenses and hyperlenses. The proposed technique is so robust that it can be universally applied for an object placed thousands of nanometers away from the mask at any spectral wavelength, without depending on the lumens of the object itself and imposing no resolution limits.
The development of ultrathin metasurfaces has provided the means for greater control and better manipulation of angular momentum, amplitude, and phase, enabling the generation of special beams such as vortex, Bessel, and airy beams, which find applications in high-capacity data transmission, superresolution imaging, particle trapping, and the like.
An airy beam is a distinctive nondiffracting, self-accelerating waveform that traverses a curved trajectory in the forward direction . The generation of a finite-energy airy beam at 633 nm with 73% amplitude of cross-component by engineering the amplitude and phase of transmitted EM wave has been presented in . The amplitude profile of an airy beam can be defined by using where “” is the positive value that keeps the beam’s energy finite, “” is transversal scale, “” is the transverse coordinate, and “Ai” represents the airy function. Chen et al. have illustrated the shift in the direction of linearly polarized light by an all-dielectric, low-loss metasurface that imparts the desired phase and cross-polarized amplitude to incident light   (Figures 4(a)–4(c)).
Bessel beams are Maxwell solutions of EM waves . They form self-reconstructing, nondiffracting, and infinite-energy beams having transverse intensity and are described by Bessel function of the first kind. Bessel beam traversing in the -direction is expressed in cylindrical coordinates as given by where “” is the amplitude, “” and “” are the corresponding lengthwise and axial wave vectors such that . In contrast to conventional axicons that are limited by an NA of 0.75 for generating 0th-order Bessel beams, meta-axicons have not shown such limitations benefiting from more degrees of freedom in terms of compactness, design flexibility and features for the same purpose. Both plasmonics-based and all-dielectric meta-axicons have been exploited to generate zero- and higher-order Bessel beams in . The study experimentally demonstrates that the all-dielectric, compact, and efficient meta-axicons platform, which is highly polarization-insensitive and has a high transmission efficiency of ~73.4%, is a winner in producing a multiorder Bessel beam for visible wavelength and with a propagation length of ~2600. In Figures 4(d)–4(f), the schematic description of two higher-order meta-axicons with different topological charges is shown . A polarization-insensitive metasurface based on fully symmetric cascaded unit cells has also confirmed the generation of nondiffractive Bessel beams both numerically and experimentally . They find plentiful applications in nanoparticle trapping , tractor beams , superresolution fluorescence imaging, light-sheet microscopy , optical bullets, and lithography and laser machining . Bessel beams are suitable for high-power applications including high-resolution radar imaging, wireless charging , and microwave drilling .
Fractional-dimensional space Bessel beams (where the beam has an arbitrary noninteger dimension less than or equal to three) can model complex structures and have meritoriously enjoyed researchers’ attention. A unique nondiffracting Bessel beam using fractional-dimensional space shown in Figures 4(g) and 4(h) is presented by Ehsan et al. in . The technique can potentially encompass the limiting cases of both the ordinary integer and fractional Bessel beams. It is also capable of preserving the nondiffractive nature of Bessel and other structured light beams, along with being self-healing in nature, which refers to the restoring of amplitude downstream of an obstacle. Such beams provide a cutting-edge for light-based manipulation and trapping in near-field applications . The wave equation is solved in fractional space where the Cartesian coordinates have fractional values such that , , and , where , , and are the spatial coordinates of fractional-dimensional space .
Light beams carry both linear  and angular momenta. The angular momentum can either be of spin (polarization-defined) or orbital (polarization-independent) types, as determined by the spatial distribution of light . Spin angular momentum assumes only two values per photon (), whereas the orbital angular momentum (OAM) can assume multiple values as . A beam of light specified by its spatial distribution, having an annular intensity profile that projects itself as a spiral wavefront possessing a beam-axial phase singularity, i.e., a zero-intensity region [89, 126] and possessing OAM, is defined as a vortex beam (VB). Azimuthal phase gradients are responsible for helical wavefronts that generate VBs . Beams with OAM have transformed many fields of modern-day photonics ranging from classical imaging to quantum communication systems. The wave having an OAM can provide an orthogonal degree of freedom to improve system capacity to hundreds of terabit per second (Tbps) for optical communication .
In conventional optics having larger footprints, many techniques including spiral phase plates (SPPs) , computer-generated holograms (CGHs) , fork grating , and cylindrical mode converters  have been deployed for OAM generation. In contrast, compact vortex beam generators have been developed by (i) downscaling conventional components, (ii) plasmonic vortex lenses , (iii) metasurfaces , (iv) integrated optical vortex emitters , and (v) subwavelength apertures and nanoslits. A compact, ultrathin, and miniaturized flexible plasmonic-based device capable of complex light manipulation to produce OAM has been experimentally demonstrated in  using 121,000 rotated helical nanosieves.
After realization of integer, as well as fractional-order OAM, rational-order OAM had been a challenge, which was taken up in  using a bilaterally symmetrical grating metadevice creating an optical beam with dynamically varying spiniform-encoded phase over rational range. When light transits through “” perpendicularly oriented, concentric SPPs, its OAM of enhances . As a proof-of-concept, it can be seen from Figures 4(i)–4(m) that as light passes through a series of transversely located SPPs, its OAM increases. The addition of light’s OAM is proportional to the inclusion of SPPs because there are more phase singularities in the wavefront, thereby leading to the generation of continuous OAM through a gradually varying aperture .
An ultracompact array of nanowaveguides that converts light to a vortex beam is experimentally demonstrated in , such that it enables all-fiber structured light applications. Similarly, a flat logarithmic-spiral zone plate (LSZP) capable of breaking in-plane symmetry and hence providing enormous structural degrees of freedom has been presented in . Optical vortices traverse crescent-shaped transverse intensity profiles with ~22% transmission efficiency and long focal depth in the broadband visible range, while the design in  traces light in a donut-like intensity. Analog OAM generation has also been realized using a single device, which benefits from linearly-varying transverse dislocation along symmetry axis . The advancements in OAM-based designs catalyze researchers’ interest to look for improved designs in terms of their signal-to-noise ratio (SNR). Therefore, in , a compact device with enhanced SNR and tunable multifocus VBs building multiple longitudinal vortices of different topological charges is presented.
The metalens design manifests a phase profile on the metasurface such that the waves that emerge out of the surface interfere constructively at the focal plane. A flat lens, in its simple configuration, follows a phase profile defined in Equation (8), whereas when merger of multiple phases takes place as given in Equation (10), the generation of “th-order” optical vortex beam, which is a superposition of Equations (8) and (9), is observed . To realize the phase gradient in a dielectric-only metasurface, each nanoantenna positioned at on the interface should ensure the required phase, given by the following equations: where “” represents the focal length, “” is the topological charge equal to the number of interspersed helices at the focal plane, “” and “” specify the location of nano-antennas in the metasurface, and design wavelength is given by “” .
Metaholography is another exciting field that counts on the manipulation of propagating EM waves. Holography is a real-world recording of an interference pattern resulting from diffraction in producing a light field that has complete information of the original scene. It is a lensless photography that carries the information of an object’s amplitude and phase. Using metasurfaces, computer-generated holography (CGH) algorithms are employed that produce the required amplitude and phase profiles for a hologram . CGHs require no object, which is an advantage over optical holography [144, 145]. If plasmonic materials are used in hologram designs, losses incur into the design ; hence, high refractive index nanostructures-based metasurfaces are employed . Ansari et al. have proposed a dielectric-only, transmission-type metasurface, which employs a-Si:H having low absorption. The nanostructures utilized in the design of “helicity multiplexed” hologram are made up of a high-index dielectric. The demonstrated metahologram exploits the polarization control of nano-HWPs for visible wavelengths, which are optimized in terms of their geometry and sizes for realizing overlapping magnetic and electric dipoles in accordance with Huygen’s principle .
In , polarization has been used as an extra degree of freedom and a visible domain metasurface with measured efficiency up to 61% possessing a much lower aspect ratio (≈ 4.7) has been presented, in comparison with the previously reported all-dielectric metasurfaces. The significant reduction in aspect ratio potentially eases out the fabrication complexities and cost requirements, making such CGHs an ideal candidate for their practical utility through large-scale production. The unit cell of the nanorod, along with its optical efficiency by varying orientation angle and wavelengths, is shown in Figures 5(a)–5(c). The proposed spin-controlled metahologram has performed better in terms of efficiency and image fidelities.
In , an ultrathin metasurface hologram with 30 nm thickness (~1/23rd of the operational wavelength) is experimentally demonstrated. The proposed design provides both amplitude and phase modulation and is capable of generating high-resolution low-noise images for visible wavelengths. In , another highly efficient, broadband, helicity multiplexed-type metahologram with a competitive image fidelity is presented. The hologram gives two distinct pieces of information as output for opposite helicity incident lights, which can be seen from Figures 5(d) and 5(e). In , a novel design with a 2D nanoarray of super-cells realizing a highly efficient, broadband, polarization-insensitive metahologram is presented. The hologram has been used to validate the polarization insensitivity of the proposed design numerically (Figure 5(f)), under illuminations from linear horizontal, linear vertical, left circular, and right circular polarization sources. The design has been characterized as a function of wavelength under unpolarized light incidence, where the efficiencies of 73%, 90% and 72% under the illumination of blue (488 nm), green (532 nm), and red (632 nm) lights, have been recorded respectively. The broadband nature of this design has been validated as its efficiency remains above 66% over the wavelength range (464 nm-650 nm) (Figure 5(g)). Similarly, a direction-multiplexed hologram has been demonstrated using a 2D array of all-dielectric super-cells to record different images in forward and backward directions. The proposed metahologram has been shown to provide a cost-effective solution with straightforward fabrication .
Conventionally, geometric holograms using spin-orbit-interactions (SOIs) exhibit only centrally symmetric images, thus limiting the capability of helicity multiplexing. As a possible solution, a spin-dependent, asymmetric metasurface is used to realize bidirectional propagation with high fidelity . In , an asymmetric, SOIs-dependent, bidirectional, helicity-multiplexed metahologram for the visible domain has been presented for applications spanning from asymmetric data copying in communications to duplex displays in smart mobile devices. PB and retardation phases are exploited in such a way that two exclusive data items can be multiplexed into a single all-dielectric metasurface. The resulting metahologram showed ITU and POSTECH logos in accordance with the illumination direction from each side (Figures 5(h) and 5(i)).
Image holography means the realization of an image above the surface of the structure, i.e., without using a receiving screen. In , an image hologram is presented producing an image above the surface of the structure under arbitrary illumination like natural light. An image hologram for the target image “NANO” is produced using Si geometric metasurface by incorporating both geometric phase and magnetic resonance. The holographic image produced by the phase-only element is calculated using Kirchhoff’s diffraction formula (Figure 5(j)). A high-fidelity reflection-based Fourier hologram has been designed for normally incident CP light that generates a wider image. The required phase mask is obtained using GS algorithm. The reflected light from silicon nanostructures that are quarter-wavelength thick projects a holographic image of a parrot (Figure 5(k)) in the far-field at the planes vertical to optical axis .
4. Applications of Metasurfaces
The compact and lightweight metasurface-based devices make next-generation system integration and device miniaturization possible. The characteristics of metamaterials to furnish unusual material properties and achieve the desired light-matter interaction have fueled a lot of interest in the optical research community, leading to different applications and uses.
4.1. Solar Thermophotovoltaic (STPV) Systems
Sun is a perpetual and enormously huge source of energy that can be tapped for clean and renewable energy by employing PV cells. The traditional photovoltaics have limited efficiency in accordance with the Shockley-Queisser limit , because of broadband nature of the solar spectrum. The incident photon is either of low energy than the bandgap of the cell such that it is not absorbed, or of higher energy than the bandgap, causing thermal energy losses, allowing an upper efficiency limit of 33% without concentrating sun’s spectrum and 41% with maximum achievable solar concentration  for single-junction PV cells. Solar thermophotovoltaics (STPVs) are an integration of a high-temperature, angle- and polarization-insensitive broadband absorber that absorbs the full spectrum of the sun and a spectral-selective emitter, which emits narrowband infrared radiation to PV cells, in order to couple solar radiation efficiently. They have been reported to exhibit a system efficiency as high as 85.4% . However, laboratory demonstrations have so far been successful in achieving efficiencies not more than 35% due to associated inevitable energy losses . Figure 6(c) shows a simplified diagram for an STPV system where the overall energy conversion process is decomposed into two parts.
Chiral materials are meant to exhibit asymmetric behavior as chirality and asymmetric transmission go hand in hand . Chirality indicates materials’ inability to superimpose on their image, posturing a different material to incident EM wave from two different directions. Chirality, on most of the occasions, is artificially introduced by using metamaterials as natural materials do not exhibit high chirality. Chiral metamaterial helices have exhibited circular dichroism (CD) (absorption of a particular polarization). A 3D structure that reflects a certain CP light and absorbs the opposite handedness is presented by Mai et al. . However, 3D chiral metamaterial helices employed for asymmetric transmission  cause increased difficulty in fabrication along with not being cost-effective, thereby needing 2D planar chiral metamaterials. Gansel et al.  proposed a highly efficient metamirror with CD enabling selective reflection with preserved handedness and complete absorption of other polarization. A multifunctional and fabrication-friendly design that exhibits full reflection for LCP without reversing its handedness and completely absorbing the other is presented in . The proposed light-matter interaction example paves the path for detection and manipulation of CP light, encryption, and chiral imaging . Wang et al. in  have elaborated metamirrors with CD for simultaneous spin-selective absorption and reflection using a multilayer structure. Since multilayer structures employed to achieve CD pose fabrication complexities, a unique single-layered all-dielectric metasurface for the visible regime is presented in . Similarly, Jing et al. demonstrated a single-layer structure for selective absorption with maximum reflectance efficiency of ~58% by using different orientations and dimensions of the unit cell .
A distinctive chiroptical effect of simultaneous spin conservation and spin isolation using a novel design approach based on di-meta-atomic (a pair of achiral nanofins) platform is illustrated for optical frequencies (Figure 6(a)). The mutual coupling has resulted in both constructive and destructive interferences, thereby inducing a giant linear chiroptical effect in the form of CD, along with spin conservation, leading to the asymmetric transmission of 70% and average CD exceeding 60%, respectively. The design achieves 98% absorption of RCP and 72% reflectance for copolarized parameters of LCP, hence maximizing the performance through a single-layer metasurface . There are possible applications of this phenomenon, such as chirality-dependent polarizers, sensors, spectroscopes, beam splitters, and polarization rotators and shapers . Multipole scattering takes place in diatomic geometry. The compact, on-chip realization of giant chiroptical effects had long been a challenge at optical frequencies . Naeem et al. in  have successfully achieved chiroptical effects by breaking the mirror and -fold () rotational symmetries. As much as 84% of cross-polarized light is reflected when RCP is illuminated and 80% is transmitted when LCP is impinged, thereby leading to 80% asymmetric transmission in reflection mode and 74% in transmission mode, verified using a hologram . CD-based reflection structures have a specific application in optical setups that avoid bulkiness. Metamirrors without selective absorption for holographic applications  and vector beam generation at optical frequencies  have already been presented (Figure 6(b)).
4.3. Polarization Conversion
Polarization control of EM waves is a key issue in communication and imaging systems. Chiral anisotropic structures are capable of converting polarization, which is enabled by electric and magnetic coupling. A compact polarization converter is put forward in  for a subwavelength regime. Moreover, a bilayer meta-atom exhibiting high-efficiency transmission provides perfect polarization conversion of both orthogonal transmissions with an efficiency of 85%. The conversion efficiency is defined by
The resulting efficiency values for transmission mode were reported as 59%, 58%, and 56% for , and 58%, 53.2%, and 48% for at three distances, respectively .
4.4. Optical Tweezing
Particle trapping is another interesting area that has been extensively explored for improvements employing benefits associated with metasurfaces. Since 1970s, researchers have been on the lookout for ways to achieve the trapping of micrometer-sized particles. The success came with the concept of optical force used in the design of “optical tweezers” in subnanometers and microscales. Optical force is calculated in accordance with Lorentz law as there are two forces acting on the nanoparticle, i.e., gradient force and scattering force. Gradient force acts along the vertical and horizontal directions and should be greater when a particle is trapped, as against scattering force acting along propagation axis (-axis). An optical tweezer that traps nanoparticles in the visible regime is presented numerically in , where the optical force was calculated on a 100 nm radius gold nanosphere. The particle was located at the focus of this metalens, resulting in a force of 30.8 pN, 24.7 pN, and 21.0 pN in the directions of three principal axes, respectively. The visible light trapping and the ease of manufacturing associated with these metadevices have made it possible to use them in many commercial optical gadgets requiring high-resolution imaging. Additionally, tweezers are employed to cool atoms and subatomic particles, and those in the micrometer range are used in microbiological research settings because of the transfer of momentum due to scattering of light .
4.5. Refractive Index Sensing
The refractive index is a constitutive parameter of a material that characterizes its EM response. Conventionally, differernt methods including photonic crystals , ring resonator sensors, and optical fiber systems  were used for measuring refractive index. Metamaterials have also shown promise in the field of refractive index (RI) sensing. A low-loss high-performance RI sensor composed of TiO2 with three different shapes (cylindrical, square, and elliptical) is presented in . The sensor achieves sharp resonance dips indicative of high sensitivity, i.e., high -factor for four liquids (ethanol, carbon tetrachloride, pentanol, and water) having refractive indices between 1.333 and 1.453. RI sensors find industrial applications in food processing, chemical and environmental monitoring, biological sensing, biomedical, and quality control . A metasurface design where nanoresonators (supporting electric dipoles) are combined with a ring resonator (supporting magnetic dipole) has the potential to increase the sensitivity of local index change . Chiral metasurfaces, having broken mirror symmetry and exhibiting different optical behaviors for left or right CP light, have also aided the field of optical sensing  (Figures 6(d)–6(i)).
Transmissive metasurfaces are reflective when they are backed by an added ground plane but the efficiency remains low because of the losses associated with the ground plane. All-dielectric metasurfaces support not only transmission-based but also are useful for reflection-based applications, where plasmonic metasurfaces have performed well conventionally. Near-perfect all-dielectric reflectors with 99.7% efficiency surpass metallic resonators with 80% reflection. A polycrystalline silicon- (poly-Si-) based dielectric resonator for maximum cross-polarized reflection and phase coverage is presented in . Moreover, metareflectarrays (reflective metasurfaces) find applications in the domain of polarization selectivity. They are also capable of polarization conversion of incident EM waves into their opposite handedness upon reflection .
4.7. Microscopic Metavehicle
Another very recent and interesting application of nanostructured dielectric metasurfaces is in imparting an arbitrary phase gradient on an incident wavefront , where optomechanical effects are utilized to construct optical metavehicles. Metavehicles are microscopic particles having the potential to traverse significant distances using low-power plane-wave illumination. Directed propulsion is achieved by elongating nanoantennas in a propulsion direction while they are steered through the variation of polarization of the incident light and their complex patterned movement is realized. An added advantage is their ability to self-correct their motion while acting as locomotives for microscopic cargo and unicellular organisms. They have been shown to transport other microscopic objects by lateral pushing .
4.8. Active/Tunable Metasurfaces
“Real-time”, active, flat metasurfaces that are capable of exhibiting changes in their optical properties upon triggers have also been explored in order to exploit tunability. In , liquid crystals- (LCs-) integrated metaholograms for nanosized dynamic displays are illustrated, which respond differently to various external stimuli. LCs are optically nonlinear, birefringent, and anisotropic in nature, thereby facilitating polarization control on a recurrent basis. The stimuli are sent to the device in electric, thermal, or mechanical form . Phase change materials (PCMs) including germanium-antimony-tellurium (GST) alloys  and vanadium dioxide (VO2) , mechanical actuations by altering geometric parameters or exploiting microelectromechanical systems (MEMS), and chemical reactions have helped such novel realizations. The inclination-dependent LC properties such as dielectricity, viscosity, elasticity, and optical birefringence  have provided leverage for controlling optical characteristics. Such responsive LC materials have been employed in various fields including displays (electric field), electric papers (pressure), and biological and chemical sensors (bacteria, molecular binding). The simple geometry of LC-integrated metasurface systems, where LC film is confined between two glass substrates over which preprogrammed metasurfaces are attached, provides an arbitrary control of polarization state of outgoing light. Figures 6(j)–6(m) show how the external stimuli are sensed in the cell so that encoded polarization is produced. The simulation results for optimizing the cell are presented in Figure 6(k), where using a contour map, the selection of optimized length (), and breath () is achieved for high cross-transmission efficiency. The achieved simulated results of reconstructed hologram for LCP and RCP incidences are presented in Figures 6(l) and 6(m), respectively. The device performance has been evaluated experimentally as well, with an overall metasurface height of 380 nm.
In continuation of the ongoing art of utilizing light refraction, Lininger et al. have pioneered a design of a meta-lens that is also reconfigurable . They have done so by coupling infiltrate LCs between microscopic pillars to achieve bending control of light. Previously, lenses were either combined or physically moved to have multiple bending possibilities. The LC control has enabled dynamically reconfigurable structured light. Significant modification is observed in focusing after infiltration of different liquid crystals, owing to the change of RI in structure’s vicinity. The metalens infiltration is done in progression from no infiltration to partial infiltration and then complete infiltration, progressing circumferentially throughout the structure, and the results are shown in Figures 6(n)–6(q) for uninfiltrated, partially infiltrated, and fully infiltrated scenarios.
In the domains of public health, environmental monitoring, and military sector, the detection of biological and chemical substances is specifically important. Substance detection and visual reporting implementation on miniaturized scale aid these practical utilities. Therefore, a compact sensor platform that rapidly responds to gases using LCs without any further complex instrumentation to integrate LCs and holographic metasurfaces is proposed in , with the purpose to readily sense the presence of volatile gas and exhibit an instant visual holographic alarm. The holographic metasurface gas sensor platform provides a safety signal indicating the presence/absence of a dangerous gas. The two pieces of information are embedded in light polarization states, i.e., RCP corresponds to a “safe signal” and LCP corresponds to a “dangerous signal.”
In order to further harness the potential of such exquisite benefits of metasurfaces, tunability is indispensable as it conserves space for ideal metadevices exhibiting various electromagnetic responses by integrating diverse functionalities. The authors in  have illustrated the electrooptic effect to attain tunability for resonance wavelength and focal length using a visible metadevice. The designed metasurface has diffractive optical elements of barium titanate (BaTiO3) on indium titanium oxide- (ITO-) coated substrate serving as an electrode. The electrical tuning causes the RI of the material to vary. Three different choices of applying voltage have been explored, i.e., application of (i) same potential to each of the elements, (ii) same potential to elements in either a row or a column, and (iii) different potentials to each of the elements. The tunability is validated by characterizing the parameters including point spread function (PSF), FWHM, and imaging. The resonance shift to different wavelengths (488 nm, 532 nm, and 633 nm) in the visible regime has been achieved under biased voltages of -8.9 V, -16.7 V, and -33.6 V, respectively.
4.9. Metasurfaces for Biomedical Applications
Metasurfaces, while having induced deep interest in the consumer optoelectronics sector, have inspired and empowered novel bioimaging and biosensing devices as well. The extensive space requirements coupled with below-par performance of conventional optics hinder their deployment in clinical environments for accurate diagnoses. For studying internal organs at the millimeter scale and even beyond, high-resolution imaging can benefit from the inclusion of meta-lenses, such as in OCT (Figure 6(r)). It finds useful presence in ophthalmological examination, head and neck cancers, heart-related diseases, and medical diagnosis of various dermatological conditions. OCT provides morphological information of the sample by detecting the changes in RI of diverse biological media. Magnetic resonance imaging (MRI) is another medical imaging technique relying on strong magnetic fields and radiowaves for neuroimaging. MRI scans are time-consuming procedures, thus making the patient uncomfortable. Metasurfaces have been explored to reduce scan times and enhance MRI quality. They can simultaneously control and redistribute the radiofrequency electromagnetic field, improving SNR greatly and sufficiently reducing the scanning time  (Figure 6(a)). Likewise, DNA typing has also been realized using metasurfaces. A tortuous array of elliptical metasurfaces that detects binding of molecules has been designed and presented in .
Besides imaging for clinical purposes, a swift, robust, and dependable means for sensing to be employed in studies related to bacteria and viruses is also the need of the hour. Conventional methods in this arena are both tedious and time-consuming, while the methods including polymerase chain reaction (PCR) and mass spectrophotometry, despite being quicker, are exceptionally complex and costly to be used for swift detection on a frequent basis. Biosensors formulated using plasmonic- and/or dielectric-based metasurfaces present a rapid way to detect various microorganisms and cancer cells. Gold nanorod antennas have been used on a graphene layer to function as a metasurface biosensor whose performance is evaluated by measuring the detection of antibody immunoglobulin G (IgG), which binds with a biomolecule named protein A/G  (Figure 6(t)). Metasurface biosensors are employed in cancer biomarker detection (Figure 6(u)) for breast, liver, and oral cancers. An optofluidic, silicon nanoposts-based metasurface biosensor has been developed for breast cancer biomarkers detection to detect breast cancer at an early stage .
5. Future Directions: Design of Metasurfaces Based on 2D Materials
In order to miniaturize an optical structure to the atomic extent, 2D and van der Waals materials have emerged in the world of nanophotonics. Their capability of manipulating light flow due to their strong inelastic interactions with light invokes researchers’ interest in exploiting the unprecedented miniaturization of optoelectronic components. Ultrathin flat optics is potent in controlling light at the subwavelength scale in comparison with the existing conventional refractive optics. High RIs of 2D materials support achieving the atomically thin dimensions. However, attaining diffraction-limited focusing and superresolution imaging through these materials are challenging because of their thickness-limited spatial resolution. A universal method that transforms the 2D monolayer into ultraflat lenses is described in . Femtosecond laser direct writing technique generates local scattering in monolayer, providing an alternative of phase and amplitude modulation that otherwise had been a challenge for atomically thin 2D materials. The proposed technique achieves highly efficient 3D focusing with diffraction-limited imaging with nanometer resolution. Additionally, variable magnification for different focal positions under diffraction-limit has been realized. The design has remarkably eased out downscaling of optical devices composed of 2D materials by introducing a novel approach for fabricating ultrathin devices .
A single-layered molybdenum disulfide (MoS2) having a giant optical path length (OPL) helps engineer the phase fronts of optical beams. The authors in  have exploited this feature in materializing the idea of sheer compactness using a few layers of MoS2 with a total thickness of less than 6.3 nm. The electrotunability of RI for layered MoS2 enables its utilization in microlenses having programmable focal lengths and an ultrahigh accuracy phase shifters, which are impossible to realize by bulk solids. In , the design of rewritable optical devices including lenses, prisms, and metalenses has been discussed. Upon light-matter interaction, rewritability is realized in these materials due to polaritons generation, which accumulate light and enable matter detection. Their shapes and sizes are tweaked to achieve reconfigurability . The presented lenses assume flat 2D shapes as compared to their 3D counterparts: semicircle instead of a semisphere, triangle instead of a prism, and a simple line instead of an optical fiber .
Miniaturization poses an acute challenge of adjusting numerous unit elements in a minuscule device with nonregular arrangements. In , a hybrid approach is developed analytically such that the random sieves control the amplitude, phase, and polarization of visible light for high accuracy. The model is experimentally demonstrated with ultrahigh capacity of nanosized holes (>34000 in number) to manifest subdiffraction-limited focusing. Moreover, a highly efficient diffraction-limited hologram is presented, which, in addition, is free from twin images present in conventional holography. Likewise, a multilayered, MoS2-based, atomically thin photon-sieve for high-quality optical devices supporting strong excitonic resonances at the band-nesting region of MoS2 is demonstrated in . The design employs optical wavefront engineering in order to accumulate phase in OPL and confines the electromagnetic resonances of nanomodes, resulting in high transmission. Additionally, it facilitates high-fidelity nanofabrication. The supercritical 2D lens is capable of breaking diffraction limit and exhibits aberration-less high-resolution imaging in air. The authors have also reported a 10 times improvement in efficiency in relation to 2D nanosieve hologram obtained using their design. Photonic platforms made out of TMDCs open up new opportunities for future 2D metaoptics and nanophotonics .
The utilization of 2D materials is not only limited to transmissive applications but also has been extended to absorptive devices. In , a tunable absorber with absorption in the visible region based on a MoS2 monolayer is presented. The design of the absorber is such that MoS2 is sandwiched between Ag and SiO2. It offers absorption in optoelectronics applications and provides tunability with geometric and optical parameters.
Amplitude, phase, and polarization are three fundamental characteristics of light waves, which open avenues to explore them as degrees of freedom for various application manifestations. Information can be stored in either of them; thus, wavefront manipulation becomes realizable. Conventional optical elements having the capability to manipulate light including lenses, wave plates, polarizers, beam splitters, and reflectors are voluminous and thus get ruled out of being used in integrated photonic devices. To overcome such issues, metasurfaces have been the focus of attention in the fields of optics and photonics, primarily due to tunable intrinsic properties of permittivity and permeability. Earlier, most of the metasurface designs were plasmonic material-based raising serious challenges in terms of fabrication, cost, and high resistive losses. The ability of metasurfaces to manipulate the EM wavefront is more efficiently exploited using all-dielectric materials to mitigate the issues associated with metal-based metasurfaces for transmissive applications.
With the passage of time, various novel phenomena and design techniques for ultracompact metasurfaces have emerged and matured, offering a myriad of applications. Metasurfaces are found in almost every field of science including medicine, biology, optics and photonics, and security. Many sophisticated techniques including electron beam lithography, focused ion beam, interference lithography, nanoimprint lithography, and femtosecond laser writing exist to fabricate metasurfaces at nanoscale. Optical absorbers find utilization in applications covering the full range of EM spectrum, including but not limited to photodetectors, stealth technology, photovoltaic and photochemical cells, optical filters, thermal light sources, thermal imaging, and thermal camouflaging, while single-wavelength or narrowband absorbers find applications in color filtering with enhanced color purity. A variety of metalenses—spin-selected, broadband, spin-insensitive, and polarization-controlled—are employed in high-resolution medical imaging, defect detection, spectroscopy, endoscopy, full-color routing, robotic vision, self-driving vehicles, cellphone camera modules, and wearable displays. There exist possibilities to revolutionize various areas of present-day photonics, such as imaging and classical and quantum communication systems using light beams with OAM. The field of nanophotonics can benefit from chirooptical effects in terms of molecular sensing, biological and biochemical separation, advanced imaging, and precision spectroscopy. Optical tweezers are employed to cool atoms and subatomic particles, and those in the micrometer range are used in microbiological research. RI sensors developed using metasurfaces find industrial applications in chemical and environmental monitoring; biomedical, biological, and biochemical sensing; quality control; food processing; and the like. Light beams—airy, Bessel, and OAM—are beneficial in machining by laser, lithography, trapping of nanoparticles, and tractor beam microscopy. High-power applications like wireless charging, ultraresolution radar imaging, microwave drilling, and the creation of through-the-wall radar make use of Bessel beams. Active and tunable metasurfaces have also been proposed having the tendency to dynamically modulate the EM wave properties. Many methods such as electrical, mechanical, and thermal have been applied to metasurfaces for exploring tunability. The electrooptic effect has aided electrically tunable metadevices to exhibit multifunctional characteristics by varying the electrical input, thus making them useful for tunable integrated systems. Earlier, meta-atom design was solely dependent on computational electromagnetic methods like finite element methods, finite-difference time-domain method, method of moments, and transmission line matrix methods for studying their characteristics. Nowadays, efficient deep learning architectures are complementing conventional methods to speed up accurate design procedures. To further miniaturize the dimensions of structures, two-dimensional van der Waals materials are also being explored, with the result that atomically thin optical lenses measuring less than 6.3 nm in thickness have been fabricated, which consist of just a few films of MoS2. In the world of flexible ultrathin and ultracompact devices, metasurfaces surely have a major role to play in shaping the future of light-matter interaction and its manipulation. It can be safely claimed that the research in this direction will progress further by digging deeper into relevant physical concepts and phenomena, in a bid to materialize more useful real-life applications.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
S. I, A. S. R, and Z. A has collectively written the manuscript, while M. Z, Y. M. and M. Q. M overall supervised the writing and polished it through various revisions.
The authors would like to acknowledge the research funding to the KAUST Innovative Technologies Labratories ( ITL) from King Abdullah University for Science and Technology (KAUST).
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