1. Introduction

Metalenses are a type of flat lens technology that exploit metasurface optics to construct flat optical component that are lighter, thinner, and more compact than diffractive/refractive lenses [1–3]. Focusing and lensing components based on ultrathin metasurfaces are of great importance because of their significant potential practical applications in industrial vision system, biological imaging, and display. Metafocusing devices are the key building blocks for developing integrated optical components [4–6]. However, a severe issue, particularly for real-time imaging and display applications, is lack of reconfiguration which results from the natural passive response of each optical scatterer [3,7]. Despite the rapid progress, the post-fabricated fixed structures hinder the dynamic manipulation of light. Recently, keen interest has been bestowed upon reconfigurable and active devices [8–10]. The tuning mechanisms include electrical, chemical, mechanical, and thermal stimulation [11–16] by utilizing active materials [17–21], stimuli-responsive polymer gels [22], phase change material [23,24] and strain multiplexed stretchable substrates [25,26]. Mechanically switchable metasurfaces are widely being realized by micro/nano-electro-mechanical systems and stretchable substrates [25–28]. However, the extremely precise physical manipulation, low speed, and external strain required for radial stretching mechanism may not be quite feasible and compatible to integrate with practical applications. In addition, the toxicity and biodegradability of materials also set additional challenges which limit its applicability in bioimaging and biomedical applications [29]. Tunable metasurfaces based on non-toxic liquids environment such as water could offer a potential solution.

Hydrogel [30], a class of crosslinked functional hydrophilic polymeric networks, has been a research frontier in the last decade. A drastic change in the hydrophilicity/hydrophobicity and volume is observed in response to external stimuli. Being responsive to environmental stimulation such as solvent composition, temperature, pH, humidity, and electric fields; hydrogels are considered smart and intelligent materials [31,32]. The hydrogel can be classified according to its origin constitution, type of cross-linking and environmental responses [31]. Their good biocompatibility, and large deformability, impart them with many important applications such as biosensors, drug delivery systems, tissue engineering, contact lenses, and implantable devices [33–35]. Poly (N-isopropylacrylamide) (pNIPAAm) [36] is the most commonly used polymer skeleton to prepare thermosensitive hydrogel; it endowed the hydrogel with intelligent thermal sensitivity, hydrophilicity, non-toxicity, and biocompatibility. pNIPAAm can form soft hydrogels with a local critical solution temperature (LCST) of 32 °C below which they undergo a phase transition from a hydrophobic collapsed state to hydrophilic swollen state. The switching speed or the phase transition from the swollen state to collapse state occur with a characteristic response time ≈ 100 ms and are fully reversible up to several hundreds of cycles [34,37] . These properties are the main reasons for the wide range of applications, which span from industrial to biomedical sciences [30,33]. Due to its flexible swelling and shrinking both in lateral directions (free-standing hydrogel) and vertical directions (surface-attached hydrogel) in an aqueous solution [37,38], it has great potential for dynamic manipulation of wavefront engineering. Metallic nanostructures embedded on the responsive hydrogels film allow to tightly confine energy of light at their surface by the resonant excitation of surface plasmons and induce rapid local plasmonic heating and an associated volume change of the thermoresponsive polymer network. The variations in polymer volume fraction reversibly modulate refractive index and the distance between metallic nanostructures which pave the way to numerous important applications and could be used for ‘on-demand’ tuning of optical properties for adaptive optical devices [34]. Recently, the pNIPAAm hydrogel has been studied for active control of plasmonic resonance for biosensor application [34,37], actively tunable optical filter [39], surface-enhanced Raman spectroscopy [40], Wearable plasmonic-metasurface sensor [41], dynamic broadband beam steering [35,42] and meta holography [32,43].

Owing to the polymeric structure, biocompatibility, and tunable mechanical properties; hydrogel-integrated tunable metasurfaces provide an unprecedented opportunity to overcome flexible reconfiguration challenges. In particular, free-standing thermoresponsive hydrogel holds a prominent position due to its drastic change in the hydrophilicity/hydrophobicity accompanied by large refractive index and volume phase transition in the lateral direction by a slight change in temperature around its LCST [38,44,45]. Below the LCST, polymer chains are hydrophilic, absorbs a considerable amount of water and exhibits a highly open-hydrated swollen structure. Once the critical temperature has been attained, the gel undergoes a phase transition with release of bound water, whereby the polymer chains become hydrophobic and in collapsed state [38,39]. The process is reversible, and dropping the solution temperature below the LCST, reverses the phase transition causing the free-standing hydrogel to become swollen again with water [38]. Because of this stimulation, the combination of plasmonic resonators and pNIPAM based hydrogel has enabled dynamic and reversible modulation of surface plasmons and wavefronts [42,43]. In this work, we demonstrate that by mounting the plasmonic resonators on freestanding hydrogel, the outgoing wavefront can be tuned via temperature stimulation due to position dependent phase discontinuity or more specifically, the curvature of the engineered wavefront; as a result it could be used for dynamic beam focusing and modulation functionality. Primarily, taking the advantage of the flexible and reversible switching behavior of hydrogel in aqueous solution, we design tunable metalens, which tune the focal position when it swells/collapses in lateral direction. As another application, we further designed an active metasurface with a fixed focal length and modulated focusing intensity under different states of hydrogel. We anticipate that further research will lead to the development of intelligent devices, on-chip sensing, and implantable wearable optical devices.

2. Structural design and principle analysis of tunable metalens

2.1 Structural design

The schematic of the proposed tunable metalens based on plasmonic resonators embedded on freestanding thermoresponsive hydrogel film as shown in Fig. 1(a). The system comprised of gold nanobrick resonators embedded and covalently crosslinked to the pNIPAAm-based free-standing hydrogel, dipped in water inside the glass container. The diameter of the whole metalens is $D = 14.76\; \mu m$. The nanobrick with height h = 35 nm are periodically positioned on the top of hydrogel. For maximum cross-polarization transmission efficiency, the plasmonic resonators are rotated at an angle of ${45^ \circ }$ along x-axis. The structural parameters of the unit cell and resonator are depicted in Fig. 1(b). The length, width, thickness, and lattice period of the nanobrick are denoted by L, w, t_h, and p, respectively. The refractive index and unit cell parameters of collapsed and swollen states are taken from Ref. [37–39]. Hydrogel films with different thickness have been investigated from micrometers to a few nanometers [44,45]. We take that freestanding hydrogel height is equal to the surface-attached hydrogel, i.e., 280 nm provided in Ref. [38]. The desired spatial phase profile for beam focusing is achieved from the dimension and position-dependent phase discontinuity of nanobrick, which is evaluated numerically by running a parametric sweep of the lateral dimensions between width w and length L of the nanobrick to extract the optimal parameters of the design. The dynamic focusing functionality of the proposed tunable hydrogel-loaded metalens is schematically illustrated in Fig. 1(c). The schematic depicts that the phase transition of hydrogel from the hydrophobic state (collapsed state) to hydrophilic state (swollen state) triggered by lowering the LCST temperature, characteristically modulate the position-dependent phase dysconnectivity; as a result, the focal distance of the proposed device can be tuned. Initially, the lattice period and refractive index of the hydrogel film in the collapsed state is $p({x,y} )= 360\; nm$, and ${n_h} = \,1.46$, respectively. While the refractive index of the surrounding water is ${n_w} = 1.334$. In the swollen state, the lattice periodicity transforms as $p({x,y} )\to p^{\prime}({x^{\prime}/{s_x},y^{\prime}/{s_y}} )= 525\; nm$, where ${s_x} = {s_y} = s \approx 1.459$ is the swollen ratio obtained by $s = \frac{{swollen\; state\; ({p^{\prime}} )}}{{collapsed\; stae\; (p )\; }}$. The refractive index accompanied with the volume phase transition change to ${n_h} = 1.34$. It is worth noting that changing the aqueous solution from water to ethanol will provide additional swelling. The refractive index of ethanol is ${n_e} = 1.36$. Because of the better solvent; the periodicity and refractive index of hydrogel will transform as $p({x,y} )\to {p^{\prime}}({x^{\prime}/{s_x},y^{\prime}/{s_y}} )= 600\; nm$, where ${s_x} = {s_y} = s \approx 1.666$ and ${n_h} = 1.37$. Additionally, the swelling and collapsing are reversible, and because of the crosslinked polymeric network the gold nanobrick will not perturb or detach from the original position. However, the swelling in ethanol is not reversible by temperature, but it can regain its original shape by changing the solvent with water [34,38].

 

Fig. 1. Schematic illustration of tunable metalens based on plasmonic resonators embedded on freestanding thermoresponsive hydrogel film. (a) Gold nanobrick array mounted on free-standing hydrogel and dipped in water inside the glass container. (b) The basic building block of the designed metalens comprised of SiO2 substrate, pNIPAAm based free-standing hydrogel, and gold nanobrick is on the top. The height of SiO2 substrate (t_si), hydrogel (t_h), and solvent (t_s) are considered as 1 µm, 280 nm, and 615 nm, respectively. u and v are the local coordinates of the meta-atom, x and y are the global coordinates, θ=45° is the angle between u and x. L and W are the spatially varying dimensions of the nanobrick. (c) 3D illustration of the proposed hydrogel-embedded tunable metalens.

Download Full Size | PDF

2.2 Working principle

The working principle of the hydrogel-embedded reconfigurable metalens is schematically illustrated in Fig. 2. Initially, the hydrogel is in the hydrated collapsed state (T > LCST), and part of the incident light transmitted to cross-polarized light with a spatially varying phase discontinuity. In this study, the freestanding hydrogel is chosen such that the hydrated hydrogel swells and collapses equally in lateral direction [38,43]. The variation in periodicity and refractive indexes by temperature stimulation of the solvent solution is shown in Fig. 2(a). The schematic depicts that changes in periodicity occur in the lateral sides “x- and y-directions”. The swelling and collapsing of the responsive hydrogel film reversibly modulate refractive index (due to the variations in polymer volume fraction) and the distance between metallic nanobricks (through volumetric change of the polymer networks). These parameters thus serve as facile handles to modulate optical properties [34,38]. By assuming that there is no (or weak) coupling effect among the adjacent unit cells, and their local phase discontinuity does not depend on the deformation of hydrogel. Thus, the uniform expansion of hydrogel changes the relative position of the embedded resonators; as a result, the wavefront of transmitted light can be adjusted locally depending on the relative position of these resonators, and the focal distance of the device can be tuned accordingly. Tunable metalens can be achieved by simply incorporating these reversible swelling and collapsing properties into the metasurface-embedded with hydrogel film as schematically depicted in Fig. 2(b). It reveals that when the linearly x-polarized input light hits on the proposed hydrogel-integrated metalens; in the collapsed state, the cross-polarized transmitted light will be focuses on the focal plane of “$z\; = \; f$”. While, in the swollen state, focal plane moves in the z-direction s² times of the original position. To realize a flat lens focusing a normally incident plane wave at a focal point f from the metalens plane, the phase modulation from the metalens must impart the spatial phase profile or orientation function $\phi (r )= \frac{{2\pi }}{\lambda }\left( {\sqrt {{f^2} + {r^2}} - f} \right)$ calculated by Fermat’s theorem [3–5]. For a lens with a small NA, under paraxial approximation, the targeted phase profile or orientation function $\phi (r )\; $ of the lens has the following form [25,46]

 

Fig. 2. Working principle of the tunable metalens based on plasmonic resonators embedded on freestanding thermoresponsive hydrogel film. (a) Schematic representation of the variation in periodicity and refractive index in the collapsed state (I), swollen state (II) in water, and swollen state in ethanol (III). Temperature values associated with different states are depicted in the figure. (b) The operational principle of the proposed dynamic lens based on metasurface-embedded with hydrogel film.

Download Full Size | PDF

The transmitted electric field, ${E_o}$, of the incident light on the collapsed metasurface can be written as ${E_\textrm{o}}({x,\,y} )= \,A{e^{ik\phi ({x,\; y} )}}$. When the metasurface is swollen by a factor of s, the new electric field of the swollen state transforms to E_o′, such that $E_\textrm{o}^\mathrm{^{\prime}}({x^{\prime},\,y^{\prime}} )= E_\textrm{o}^\mathrm{^{\prime}}({sx,\,sy} )= \,A{e^{ik\phi ({sx,sy} )}}$. To understand rigorously how electric field changes with s in the far-field $({r/\lambda \gg 1} ),$ we employ a Fresnel transformation to quantify the field distribution of light shaped by metasurface. Initially, for the collapsed state, the electric field profile $E({x,y;z} )$ at a distance z from a metasurface can be represented using Huygens-Fresnel Principle as [26,47,48].

Similarly, for a given swollen ratio s, the electric filed transforms to $\,E^{\prime}({x^{\prime},\,y^{\prime};z^{\prime}} )$ such that

This equation manifest that, for a given ratio of s the electric field will becomes s² times of the original collapsed metasurface electric field.

2.3 Design procedure

The design process starts with the calculation of the target-phase profile $\phi ({x,y\; } )$ at a design wavelength of λ=700 nm obtained through Eq. (1). The preset designed focal length is set as 15 µm in the proposed design. The spatially distributed spherical phase profile of the whole lens, calculated through Eq. (1) at the designed wavelength λ=700 nm, f = 15 μm, s = 1, and period p = 360 nm is shown in Fig. 3(a). The desired targeted phase profile along the x-axis (at y = 0), calculated through Eq. (1), with different swollen ratio as depicted in Fig. 3(b). Each resonators satisfying the desired phase profiles need to be arranged accordingly. The next step is to build up a library of nanobricks through finite-difference time-domain (FDTD) solver to find out an optimal structure geometry that can reproduce the targeted phases profile. A separate library under different states is established by running a parametric sweep of the lateral dimensions between width w and length L of the nanobrick to extract the optimal parameters of the structure. For the sake of simplicity and convenience, the collapsed state library for s = 1 is presented in Fig. 3(c). The last step of the proposed design is searching of targeted phase profile and identification of a nanobricks structural geometry from the established libraries that matched the targeted phase profile (Fig. 3(b)) under different states. The desired resonators are found by minimizing the transmission error $\Delta T = |{{e^{i\phi }} - |t |{e^{i\angle t}}} |$, where t is the complex transmission coefficient, and minimizing the difference between the desired unity-amplitude phase profile ${e^{i\phi }}$ and phase profile of the resonator $\mathrm{\Delta }\phi = {e^{i\phi }} - {e^{i\angle t}}$. Thus, in our algorithm, the transmission error is reduced by first identifying a set of resonators with the appropriate phase profile, and then choosing the resonator with the largest transmission amplitude. The optimized parameters (length and width of the nanobricks) of the whole metalens whose transmitted phase are match to the targeted phase are identified and drawn in Fig. 3(d-e). The linear spatial phase profile (in equal steps of π/3) covering complete phase of 2π with nearly constant transmission amplitude chosen from the library with different states of hydrogel are shown in Fig. 3(f) and their parametric values are listed in Table 1.

 

Fig. 3. Design procedure of the tunable metasurface embedded with hydrogel film. (a) The desired spherical phase mask of the metalens calculated through Eq. (1) at the designed wavelength λ=700 nm. (b) The desired phase profile along x-axis at y = 0, calculated under different swollen ratio. (c) Phase library simulated at s = 1 and p = 360 nm, under the normal incident of light λ=700 nm. The unit cells are scanned between the width w and length L of the nanobricks. (d) The desired length L and (e) width W of nanobricks identified from library that matched to the targeted phase profile through searching algorithms. (f) The full phase coverage of 2π with a nearly constant transmission amplitude extracted from (d) and(e).

Download Full Size | PDF

Table 1. Parametric values of optimized nanobrick resonators.

View Table

2.4 Simulation results and discussion

To interpret our theoretical results and analyze the tuning functionality of our proposed design, we perform numerical simulations using FDTD solver by Lumerical Solution. In the numerical simulation, the optical conductivity and periodicity of the collapsed and swollen state of hydrogel is taken from Refs. [37–39]. To mitigate the calculation burden, a moderate-sized metalens with a small numerical aperture (NA) was used for the demonstration of tunability. However, the design is scalable for the larger NA and larger focal length [49]. To realize strong focusing, we chose $f = 15\; \mu m$, diameter $D\; = \; 14.76\; \mu m$, so that NA ≈ 0.49 is expected, which should be adequate to enable the diffraction limited operation of the designed metalens. The structure shown in Fig. 4(a), is the total number of nanobrick resonators, equipped with an array of 41 × 41 unit cells. The metasurface was excited from the bottom by normally incidence x-polarized plane EM wave. Periodic boundary conditions were employed in the $x\; \& \; y - $ direction, and PML boundary conditions on the top and bottom ($z - $ direction). A monitor that records the transmitted wave intensity transmitted from the metasurface, correspondingly, calculates the phase and transmittance of light, was placed at a distance of 0.2 μm above from the metasurface. To investigate the tuning of focal length, the recorded simulated data from the monitor for the swollen ratio s = 1, s = 1.459, and s = 1.666 were then reconstructed in the far-field longitudinal intensity profiles. The far-field longitudinal light profiles of the transmitted focusing waves under the collapsed and swollen state in water and ethanol are presented in Fig. 4(b). The corresponding cross-sectional line profiles of the transmitted beam along the focal plane ($z$-axis) and their extracted focal length as a function of the swollen ratios are analyzed and presented in Fig. 4(d-e). The result shows that the focal length of the metalens in the collapsed state with s = 1 is ∼14.2 μm, which shifted to ∼32.3 μm and ∼41.6 μm for the swollen state in water and ethanol, as s increases from 1.459 to 1.666, respectively. The simulated values are closely match with the theoretically predicted values (black line) of $f^{\prime} = {s^2}f$ and confirms the focusing performance of the device. As expected, the NA increases with the swollen states of metalens, which leads to shorten the full width at the half maximum (FWHM) of focal spots [46]. The possible decrease in FWHM values with respect to NA are plotted in Fig. 4(g). The simulated values (orange squares) match well with their theoretically predicted value of FWHM = 0.51λ/(NA) which is approximately λ/(2 NA), where NA ≈D/2f. The results indicate that the device is diffraction limited under the swollen states of hydrogel. Furthermore, we also examined the broadband response of the designed metalens in the whole spectrum of visible range, and it is found from the Fig. 4(h) that the focusing and tuning response of the designed metalens is wavelength dependent and the focus deviates greatly with wavelength. It is worth noting that, the designed device features a thermoresponsive tunable metalens in the visible spectrum, which can be utilized for sensing purposes as well.

 

Fig. 4. Simulation results. (a) Top view of the hydrogel-integrated tunable metalens. (b) The reconstructed far-field longitudinal intensity profiles under the collapsed and swollen state in water and ethanol at the design wavelength of λ=700 nm. (c) Intensity beam profiles along the focal plane on the x-z plane. (d) Normalized electric field distribution along the optical axis (z-axis) under different swollen states. (e) Simulated focal length values (orange square) extracted from (e) and theoretically predicted (s²f) focal length (black line) as a function of swollen states of the hydrogel. (f) FWHM curves extracted from (c). (g) Simulated FWHM values (orange square) and theoretically predicted (λ/2 NA), diffraction limited spot sizes as a function of NA. (h) The broadband focusing response of the hydrogel-integrated metalens.

Download Full Size | PDF

3. Intensity tunable metalens

Hydrogel is quite versatile in functionality that it can be used as intensity-tunable metalens which is another interesting application of hydrogel-integrated reconfigurable metasurfaces. Dynamic control of electromagnetic strength is of great importance in sensing and biomedical applications, and it can be realized via metasurfaces by tailoring the amplitude and phase of the EM wave simultaneously [50]. It is worth noting that the amplitude of anomalous refracted waves in metasurfaces is typically fixed by their dimensions and structural geometry. Thus, by choosing optimized dimensions of plasmonic nanobrick, one can modify the light-matter interaction, engineer the amplitude, and impart an abrupt phase shift to the incident wave. Here, the versatility of plasmonic resonators embedded on hydrogel film that can dynamically tailor the transmission efficiency and confined it into the same focal spot under different states of hydrogel is presented. To demonstrate the intensity tunable functionality of the hydrogel-integrated metalens, we utilize gold nanobrick as the basic building block of the design, which comprised of glass substrate, and gold resonator with thickness/height h = 35 nm embedded and covalently crosslinked on the top of pNIPAAm based free-standing hydrogel as shown in Fig. 1(b).

The design process begins with searching of the targeted phase profile with tunable transmittance intensity in the collapsed and swollen libraries. The optimal parameters of the design are identified and picked through searching algorithm such that each resonator of the metalens impart the same phase profile of Fig. 3(a), with tunable transmittance intensity under the collapsed and swollen states. Schematic of the designed intensity controllable metalens (Fig. 5(a)) indicates that the design ensures the transmitted electric field from different radial coordinates must interfere constructively at the same focal point f, under the swollen and collapsed states and thus it focuses to the same focal point. The desired nanobrick resonators were equipped with an array of 41 × 41 unit cell and diameter D = 14.76 μm. The operated wavelength is λ=700 nm and the preset designed focal length is set at f = 15 μm. The optimal parameter of plasmonic resonator; the length L and width W of nanobricks, identified from the library that matched to the same phase profile and ensure dynamically tunable transmission efficiency with same focusing point are drawn in Fig. 5(c) and 5(d), respectively. The focusing performance of the designed hydrogel-integrated metalens with tailored intensity are shown in Fig. 5(b). For the collapsed state, the cross polarized component of EM beam is tightly focused into a focal plane of 18.2 μm (Black curve). When it comes to the swollen state, the transmitted light is confined into the same focal spot of 18.2 μm as that in the collapsed state, while the intensity is tuned to 60% at s = 1.459 and 30% at s = 1.666 in the swollen state of hydrogel in water (Blue curve) and ethanol (Pink curve), respectively. Figure 5(c) shows the decomposed tunable intensity distributions of the transmitted light in x–z plane. It is clearly observed that under all states of hydrogel, the transmitted light with tunable intensity profile is well focused at z = 18.2μm. These results manifest a new avenue for dynamic beam shaping and adaptive optics on a metasurface platform through simple temperature stimulation.

 

Fig. 5. Insity tunable metalens (a) Schematic of focusing contrast of the designed intensity controllable metalens. (b) Normalized electric field distribution along the optical axis (z-axis) with different swollen ratio. (c) The desired length and (d) width of nanobricks identified from the library that matched to the target phase profile under the collapsed as well as in the swollen states. (e) Reconstructed far-field longitudinal intensity profiles under the collapsed (I) and swollen state in water (II) and swollen state in ethanol (III).

Download Full Size | PDF

4. Routes to realize the proposed designs experimentally

pNIPAAm-based hydrogel has been widely studied for sensing applications, and many of them were experimentally realized by fabricating the hydrogel thin film on the dielectric substrate, which is similar to our proposed metasurface. Thus, our proposed designs can be validated experimentally by following the processes reported in Refs. [34,37,38], which have been experimentally realized, having gold meta-atoms on the glass substrate with the same lattice periodicity, operating wavelength, and height of meta-atoms for biosensing applications. The fabrication process starts with the state-of-the-art electron beam lithography process to structure the proposed design on the photoresist, followed by the deposition of gold by e-beam evaporation [51]. After the lift-off process, the fabricated structure is needed to covalently crosslink and attach to the pNIPAAM polymer network using N-methylenebis(acrylamide) as a crosslinker via UV light irradiation. To form the free-standing hydrogel embedded with nanostructures, the glass substrate is needed to detach through swelling in ethanol [38]. Furthermore, optical waveguide spectroscopy could be used to characterize the thickness, refractive index, and swollen ratio of the hydrogel film. It is worth mentioning that the reversable swelling and collapsing may probably associated with the buckling of hydrogel or nanostructures which can be study by morphological characterization using SEM and atomic force microscopy to study the topography of the proposed design [37–39].

5. Conclusion

In conclusion, we have demonstrated a reconfigurable metalens based on plasmonic resonators embedded on freestanding thermoresponsive hydrogel film capable of dynamically tuning focal position of the transmitted light. The beam focusing and tuning are realized by dimension and position-dependent phase discontinuity of plasmonic resonators. The uniform phase transition of hydrogel from the hydrophobic state to hydrophilic state triggered by lowering the LCST temperature, characteristically modulate the position-dependent phase discontinuity; as a result, the focal distance of the proposed device can be tuned. Initially, the hydrogel is in the collapsed state, and the focus is observed at ∼14.2 μm. With the phase transition, the focal position is shifted to ∼32.3 μm and ∼41.6 μm for the swollen state in water and ethanol, respectively. In addition to the tunable focal length metalens, we further designed an active metasurface with a fixed focal length of ∼18.2 μm and modulated focusing intensity at different states of hydrogel. It is worth noting that such a design may suffer from low transmission efficiency and aberrations as a result of constraints such as limited pattern size, fabrication imperfections on a soft flexible substrate and losses. Development of high efficiency and polarization insensitive metasurfaces with dielectric (Si, TiO2), as opposed to plasmonic resonators, could be a substantial advancement. Additionally, further advances in metalens technology can be incorporated to correct for monochromatic and chromatic aberrations.