Tunable Metasurfaces Based on Mechanically Deformable Polymeric Substrates

Abstract

The emergence of metamaterials has presented an unprecedented platform to control the fundamental properties of light at the nanoscale. Conventional metamaterials, however, possess passive properties that cannot be modulated post-fabrication, limiting their application spectrum. Recent metasurface research has explored a plethora of active control mechanisms to modulate the optical properties of metasurfaces post-fabrication. A key active control mechanism of optical properties involves the use of mechanical deformation, aided by deformable polymeric substrates. The use of deformable polymeric substrates enables dynamic tuning of the optical properties of metasurfaces including metalenses, metaholograms, resonance, and structural colors, which are collectively relevant for biosensing and bioimaging. Deformable–stretchable metasurfaces further enable conformable and flexible optics for wearable applications. To extend deformable–stretchable metasurfaces to biocompatible metasurfaces, a fundamental and comprehensive primer is required. This review covers the underlying principles that govern the highlighted representative metasurface applications, encompassing stretchable metalenses, stretchable metaholograms, tunable structural colors, and tunable plasmonic resonances, while highlighting potential advancements for sensing, imaging, and wearable biomedical applications.

1. Introduction

Traditional optical components are bulky, expensive, and have challenges achieving high precision as a result of design complexities [1]. These limitations present a genuine concern, as the demand for compact and miniaturized optical devices in our daily lives is ever increasing. Recently, the conception of metasurfaces has offered a powerful and flexible platform for tailoring the fundamental characteristics of light at a subwavelength resolution [1,2,3]. Metasurfaces or planar structured interfaces are subwavelength-spaced structures with wavelength scale thickness capable of modulating the phase, amplitude, and polarization of incident light [4,5,6,7,8]. By exploiting the excellent subwavelength meta-engineering of metasurfaces, various unique meta-devices including planar metalenses, polarization elements, axicons, holograms, color filters, plasmonic metamaterials, electromagnetic brain-computer-metasurface (EBCM), and beam shapers have been demonstrated [2,6,9,10,11].

With recent efforts toward multifunctional metasurfaces, several pioneering works have also addressed the inherent drawbacks of conventional metasurfaces, including monochromatic aberrations [2,7,12], fixed wave response, and narrow operation bandwidth [13,14]. The improvement in efficiency and functionality can be traced to a better understanding of meta-atoms design, material composition, and the integration of hybrid materials [15]. Specifically, the versatility of metasurfaces is enabled by using meta-atoms of various geometric forms, dimensions, space orientations, integrated mechanisms, and wavefront manipulation schemes [16].

A critical survey of multifunctional metasurface research depicts a growing focus on active control of electromagnetic waves or tunable metasurfaces [15,17]. Tunable metasurfaces allow arbitrary control of optical properties post-fabrication. Tunable metasurfaces can be realized through the application of external stimuli including mechanical, electrical, and thermal stimuli [15,18]. When subjected to such external stimuli, the resonance frequency, resonance wavelength, and focal length of metasurfaces can be dynamically tuned to expand the spectrum of real-world applications. Among the external modulation techniques, mechanical stimuli have been extensively studied for dynamically tunable metasurfaces. Mechanical modulation of optical functionality is achieved through mechanical control of meta-atoms and dielectric resonators on deformable polymeric substrates.

The use of deformable substrates presents additional capabilities by allowing integration with non-planar surfaces to address the increasing demand for wearable, flexible, portable, and adaptive optics-electronics. Furthermore, the conformability and tunability of deformable biocompatible metasurfaces can greatly augment bio-imaging, biosensing, and biomedical wearables. Miniaturization and detection efficiency are important characteristics of early in vivo biomedical diagnostic devices that can be addressed with biocompatible metasurfaces. As a result, a fundamental and comprehensive primer is required to propel biocompatible metasurfaces.

In this review, we explore the progress made in tunable metasurfaces by focusing on mechanically deformable tunable metasurfaces. We first introduce the concept of flexible metasurfaces based on deformable polymeric substrates. We report on the applicable polymers by emphasizing the relevant properties that inform their use for tunable metasurfaces. We then consider the noteworthy metasurface applications based on deformable polymeric substrates. We specifically focus on metalenses, metaholograms, structural colors, and plasmonic resonances, which are collectively relevant for biosensing, bioimaging, and wearable devices. Finally, we conclude this review by providing our perspective on future developments for biomedical applications.

2. Flexible–Deformable Metasurfaces

The concept of flexible metasurfaces originates from the impregnation of diffractive elements or dielectric resonators on flexible solid surfaces [19]. Flexible substrates offer a promising platform for expanding the functionality of conventional metasurfaces for dynamic and real-time control of electromagnetic waves [20,21]. By using mechanical deformation, the fixed optical response of metasurfaces can be tuned to elicit tunable optical responses, including tunable focusing and imaging, tunable plasmonic resonances, and dynamic color switching, as shown in Figure 1 [22]. In principle, mechanical tuning is realized through the straining and bending of deformable–elastomeric substrates impregnated with meta-atoms. Straining and bending of deformable substrates induces a change in the periodicity or distance between adjacent atoms without altering the shape of meta-atoms [23]. This shift in unit cell periodicity ultimately translates into a change in the optical response of the metasurface.

Polymeric Substrates

Over the years, deformable polymeric substrates, including polydimethylsiloxane (PDMS) [24,25,26], polyamide (PI) [27], polyethylene terephthalate (PET) [28,29], polyethylene naphthalate (PEN) [30], poly(methyl methacrylate) (PMMA) [31], low-density polyethylene (LDPE) [32], silicone (SI) [33], and cyclic olefin copolymer (COC) [34], have enabled flexible metasurfaces [22]. To ensure effective integration with meta-atoms, the mechanical, optical, and physical properties of deformable substrates must be critically considered. Generally, the deformable substrate, which is a polymeric material, must have a low elastic modulus to allow easy and repeatable deformations via stretching or bending. In addition, a good adhesive property of the substrate is necessary to facilitate unrestricted integration of dielectric or diffractive materials. Optically, the preferred substrate must exhibit a low refractive index to minimize reflection loss while exhibiting a low absorption coefficient to enhance the transmission and propagation of electromagnetic waves [22]. Also, excellent optical transparency is favorable for broadband metamaterial applications. The aforementioned properties of the reported polymers are summarized in Figure 2 and

Table 1. Typical properties of polymeric–deformable substrates for mechanically tunable metasurfaces.

Polymeric Material	Refractive Index	Young’s Modulus E [MPa]	Dielectric Permittivity [THz]	Operating Temperature [°C]	Optical Transparency [400–800 nm, %]	Elastic Limit [%]	Reference
PDMS	1.4	0.36–0.87	2.3–2.8	−45–200	74–90%	200	[22,35,36]
Polyimide	1.50	1.5–3	3.24	440	–	2–15	[22,37,38]
PET	1.66	4–5.3	2.86	300	85–90.4%	–	[38,39,40]
LDPE	1.5	8.70–1370	2.2–2.4	456.2	–	79	[41,42,43,44]
COC	1.51–1.53	2600	2.33	–	94.5%	–	[34,40,45]

.

The fabrication process for flexible metasurfaces is typically informed by the desired features of the metasurface or the desired wavelength [46,47]. In a broader sense, the nanofabrication techniques can be classified into conventional lithography and direct-writing lithography, as shown in Figure 3. High resolution in the visible spectrum, which requires small features, can be achieved with conventional top-down nanofabrication techniques such as focused ion beam lithography (FIB) and electron beam lithography (EBL). In most cases, the operating temperature of the nanofabrication technique is far above the operating range of the polymeric substrate, necessitating the use of a sacrificial layer. Aside from the use of a sacrificial layer, an alternative fabrication technique to consider is soft lithography, which usually couples with any of the above standard lithography processes. In soft lithography, the inverse structure of the desired feature is patterned on a master substrate with standard lithography. After defining the pattern size, an appropriate polymer (usually PDMS) is cured on the structured master template. A subsequent peeling of the polymer leaves the features embedded within the polymer.

Nano-imprinting lithography (NIL) is a highly suitable fabrication technique for deformable substrates, and it is similar to soft lithography. In NIL, a pattern on a stamp is transferred to a silicon substrate by reactive etching processes. The silicon stamp is then deposited with the desired metal layers, which are later transferred to the target polymeric substrate, as shown Figure 3c. A more direct fabrication technique involves the use of nanostencils, as shown in Figure 3b. Nanostencil lithography offers a better alternative for direct and harmless writing of nanostructures on polymeric substrates in a single step by using a shadow mask.

3. Representative Metasurface Applications

3.1. Stretchable Metalens

Traditional optical systems based on refractive and diffractive elements exhibit limited optical characteristics due to inherent material limitations usually emerging from material dispersion [49]. To address such limitations in lenses, multiple diffractive and refractive elements are often cascaded to form doublet and triplet lenses [49,50]. Despite the improved focusing and imaging from the integration of multiple lenses, the resulting optical system is bulky, expensive, and difficult to realize due to the complexities associated with multiple-lens configurations. Metasurfaces can address such challenges by offering an ideal platform for realizing thin multifunctional flat lenses or metalenses that can exhibit wide-field imaging [51,52,53] and ultrahigh numerical apertures [54,55,56,57].

Metalenses consist of subwavelength meta-atoms that are carefully arranged to impart a phase shift or a phase delay to achieve a spherical wavefront [20,58,59]. To leverage metasurfaces for diffraction-limited metalens, the target phase profile at a point $\mathsf{\phi }\left(x,y\right)$ on a metalens must follow the equation [59,60]:

$$\mathsf{\phi }\left(x,y\right)=-\frac{2\pi }{\lambda }\left(\sqrt{x^2+y^2+f^2}-f\right)$$

(1)

where x and y are the spatial coordinates of the phase-shifting meta-atoms with respect to the center ray $\left(x=0,y=0\right)$, $\lambda$ is the wavelength in free space, and f is the focal length of the lens.

For focusing and imaging applications, all light paths must accumulate for constructive interference at the focal plane. This requirement is necessary to achieve complete phase coverage from 0 to 2π, necessary for full control of the optical wavefront [61]. As a result, metalenses can be realized by precisely positioning meta-atoms on substrates to generate phase discontinuities that accumulate to a complete 2π phase [20].

Despite full wavefront control at the nanoscale, chromatic aberration remains a lingering challenge arising from periodic lattice dispersion and light confinement in resonant or guided modes [7,62,63]. Recently, more effort has been directed toward achieving achromatic focusing and imaging, revealing dispersion engineering as a corrective approach for chromatic aberrations. Fundamentally, the total accumulated phase is a composite of phase shift imparted at a point on a metasurface ${\mathsf{\phi }}_{tot}\left(r,\lambda \right)$ and the phase accumulated via projection through free space ${\mathsf{\phi }}_p\left(r,\lambda \right)$. As a result, the dispersion of phase shift emerging at a point on the metasurface can be engineered to account for the wavelength dependence of the propagating phase as follows [61]:

$${\mathsf{\phi }}_m\left(r,\lambda \right)=-\frac{2\pi }{\lambda }l\left(r\right)$$

(2)

where $l\left(r\right)$ is the physical distance between the interface at position $r$ and the desired wavefront. Through this design approach, constructive interference can be preserved at different wavelengths to maintain a constant total accumulated phase which imparts constant focusing and imaging across all wavelengths [61,64].

In addition to dispersion engineering, spatial interleaving of metalenses with varying operating wavelengths has been utilized for the same focusing on multiple wavelengths [65,66]. In spatial interleaving, meta-atoms of each metalens are designed to exhibit local phase change such that light scattered by each group of meta-atoms constructively interfere in its respective focal spot at the designed wavelength.

Driven by the high demand for metalenses with broadband focusing and imaging, several wide-bandwidth achromatic metalenses have also been reported [67,68,69,70,71,72,73,74,75,76,77,78]. By carefully introducing the appropriate phase compensation for the phase, group delay, and group dispersion, chromatic aberration can be effectively corrected [7,79,80,81,82,83,84]. In theory, for the relative phase profile of a metalens, Equation (1) can be expressed as a Taylor series near a designed frequency ${\omega }_d$ as [7]:

$$\phi \left(r,\omega \right)=\phi \left(r,{\omega }_d\right)+\frac{\partial \phi \left(r,\omega \right)}{\partial \omega } |\begin{array}{c}\\ \omega ={\omega }_d\end{array}\left(\omega -{\omega }_d\right) +\frac{{\partial }^2\mathsf{\phi }\left(r,\omega \right)}{2\partial {\omega }^2} |\begin{array}{c}\\ \omega ={\omega }_d\end{array}{\left(\omega -{\omega }_d\right)}^2+\dots$$

(3)

where $\mathsf{\phi }$, $r$, and $\omega$ are the phase, radial coordinates, and angular frequency, respectively. The terms on the right-hand side represent the phase, relative group delay, and group delay dispersion, respectively. As seen in conventional metalenses, only the phase profile $\mathsf{\phi }\left(r,{\omega }_d\right)$ requirement is satisfied with neglect of metalens dispersion emerging from the high-order derivative terms. However, transmissive broadband achromatic metalenses can be designed by dispersion phase compensation such that (i) the phase term achieves spherical wavefront, (ii) the group delay term compensates for wave packets’ arrival time at the focus, and (iii) the group delay dispersion term ensures identical outgoing wave packets.

Tunable metalenses have received much attention as an alternative way to remediate the limitations of conventional metalenses. Tunable metalenses overcome the fixed optical properties of conventional metalenses by enabling the tuning of metalens focusing and imaging post-fabrication. The underlying principle of a tunable metalens lies within the fact that metalenses are made up of spatially arranged meta-atoms. As a result, meta-atoms can be dynamically tuned under external stimuli to generate the required phase shift for wavefront control [58]. The external tuning can be categorized in the form of light source control and electrical, thermal, and mechanical tuning [16,58]. In terms of light source tuning, the polarization of incident light can be varied to generate different responses such as dual field-of-view at the same focal plane and varifocal switching [85,86]. In electrically tunable metalens, a phase shift is generated by applying an external bias voltage across active materials including graphene [87,88] and liquid crystals [89,90,91,92]. A combination of electrical and thermal tuning has also been employed in thermos–optical systems to induce changes in inherent material properties such as a refractive index for tunable metalenses [93].

Mechanical tuning is of particular interest as it allows dynamic control of metalens functionality while presenting an enabling platform for conformable and flexible optics. Mechanically tunable metalenses are realized by integrating meta-atoms on flexible or deformable materials. It is important to recognize that the phase imparted by individual meta-atoms depends on the shape, position, and orientation of the meta-atoms [20]. As a result, the tunability of metalenses based on deformable substrates is achieved by subjecting the deformable substrates to mechanical deformation in the form of straining or bending. The application of strain induces a change in the distance between adjacent meta-atoms (periodicity), which in turn translates into the change in the focal length of the metalens. Mechanically tunable metalenses have been realized by using stretchable–deformable polymers, predominantly PDMS [20], and other polymers such as PET [21], and dielectric elastomer actuators DEAs [94]. In general, the deformable polymer-substrates must exhibit high reversible stretching, low refractive index, good optical transparency, and a low absorption coefficient for metalenses of superior optical properties.

Agarwal et al. first demonstrated tunable metalenses by precisely arranging meta-atoms on a deformable substrate, as shown in Figure 4a [20]. By carefully orienting the meta-atoms on the deformable substrate, an arbitrary wavefront with opposite helicity and appropriate phase discontinuity was achieved for complete phase coverage. The metasurface was designed to exhibit an operating wavelength of 632.8 nm by template stripping. Subwavelength-scale gold arrays were deposited on a silicon substrate by electron beam lithography. The gold nanoarray was transferred to a PDMS film by casting and curing PMDS on the silicon substrate. Finally, PDMS was stripped from the silicon substrate with the gold nanoarray arrays adhered to the PDMS film. The tunability of the metasurface was evaluated under illumination with the right circular polarized light.

By stretching the metasurface from an unstretched ratio of 100% to a stretch ratio of 130%, the anomalous diffraction angle decreased from 14.9° to 11.4°. In terms of optical wavefront control, the focal length of the metasurface gradually increased from 150 µm to 250 µm upon stretching the metasurface by 30%, as shown in Figure 4(aiii). Interestingly, the focal length continuously decreased upon releasing the strain to the original unstretched position. The authors proposed a fundamental equation that describes the tunability of the metasurface. For a planar lens with an orientation function, the spherical wavefront for transmitted light must obey:

$$\phi \left(r\right)=\left(\pi /\lambda \right)\times \left(\sqrt{f^2+r^2+}-\left|f\right|\right),$$

(4)

where $r^2=x^2+y^2$. When the substrate is stretched by a stretch a ratio of s, the constant orientation function changes to:

$$\mathsf{\phi }\left(r\right)= \mathsf{\phi } \prime \left(\pi /\lambda \right)=\pi r^2/\left(2\lambda f^{\prime }\right),\mathrm{where}$$

(5)

$$f^{\prime }=s^2 f$$

(6)

It can be observed that when the metasurface is stretched by $s$, the focal length $f^{\prime }$ of the new lens increases by an $s^2$ factor of the original focal length $f$. Not long after the work demonstrated by Agarwal et al. [20], Kamali et al. conceived a metasurface with tunable microlens functionality of diffraction-limited focusing, as shown in Figure 4c [95]. Here, the authors exploited high-refractive hybrid meta-atoms uniformly arranged on a PDMS. The meta-atoms consist of a–silicon (a–Si) nanostructures deposited on a thin layer of aluminum with the entire nanostructure encapsulated in PMDS. The metasurface was fabricated by electron beam lithography, etching, and curing of PDMS after spin coating. In theory, the complete phase profile of the metasurface is given by:

$$\mathsf{\phi }\left(\rho ,\lambda \right) \approx \frac{\pi {\rho }^2}{\lambda f}$$

(7)

where $\rho$ represents the distance from an arbitrary point to the center of the lens and $\lambda$ represents the wavelength of the lens. When the metasurface is subjected to a stretching ratio of $\left(1+\epsilon \right)$, the focal length of the metasurface scales by a factor of ${\left(1+\epsilon \right)}^2$. The focal tuning capability of the metasurface was demonstrated experimentally by stretching the PMDS from 0% strain to 50% strain. The designed focal length of 600 µm changed to 1400 µm upon stretching, as shown in Figure 4(cii).

Figure 4. Mechanical tuning of metalenses. (ai) Illustration of a tunable metasurface based on a stretched deformable substrate (PDMS). (aii) Measured beam profile of the experimentally realized flat zoom lens based on (ai) with different stretch ratios, s. (top) Depicts a zoom lens with s = 100%; (center) depicts a zoom lens with s = 115%; and (bottom) depicts a zoom lens with s = 130%. (aiii) Depicts the measured (black dots) and calculated (red line) focal length of the flat zoom lens under different stretch ratios, s. (bi) Illustration of the metasurface unit cell. The silver–silica–silica (Ag–Si–Ag) composite meta-atom is encapsulated in PDMS. (bii) Measured beam profile of the experimentally realized metamirror depicting different focal lengths under stretch ratios of 0% (top), 10% (middle), and 20% (bottom). (biii) Depicts the measured and calculated focal length (right) and focusing efficiency (left) of the metamirror under different stretch ratios. (ci) Illustration of a metasurface based on silicon nanoposts encapsulated in PDMS. (cii) Depicts the measured beam profiles of the lens under radial strains (0–50%) in the axial plane (left) and focal plane (right). (ciii) Depicts the measured–calculated focal length of the lens as function of stretch ratios. Upon stretching, the focal length changes from 600 [µm] to 1400 [µm]. (a) Reprinted with permission from ref. [20]. Copyright (2016) American Chemical Society. (b) Reprinted under the Creative Commons Attribution 4.0 License (CC BY 4.0) from ref. [96]. (c) Reprinted with permission from ref. [95]. Copyright (2016) Wiley & Sons, Inc.

Figure 4. Mechanical tuning of metalenses. (ai) Illustration of a tunable metasurface based on a stretched deformable substrate (PDMS). (aii) Measured beam profile of the experimentally realized flat zoom lens based on (ai) with different stretch ratios, s. (top) Depicts a zoom lens with s = 100%; (center) depicts a zoom lens with s = 115%; and (bottom) depicts a zoom lens with s = 130%. (aiii) Depicts the measured (black dots) and calculated (red line) focal length of the flat zoom lens under different stretch ratios, s. (bi) Illustration of the metasurface unit cell. The silver–silica–silica (Ag–Si–Ag) composite meta-atom is encapsulated in PDMS. (bii) Measured beam profile of the experimentally realized metamirror depicting different focal lengths under stretch ratios of 0% (top), 10% (middle), and 20% (bottom). (biii) Depicts the measured and calculated focal length (right) and focusing efficiency (left) of the metamirror under different stretch ratios. (ci) Illustration of a metasurface based on silicon nanoposts encapsulated in PDMS. (cii) Depicts the measured beam profiles of the lens under radial strains (0–50%) in the axial plane (left) and focal plane (right). (ciii) Depicts the measured–calculated focal length of the lens as function of stretch ratios. Upon stretching, the focal length changes from 600 [µm] to 1400 [µm]. (a) Reprinted with permission from ref. [20]. Copyright (2016) American Chemical Society. (b) Reprinted under the Creative Commons Attribution 4.0 License (CC BY 4.0) from ref. [96]. (c) Reprinted with permission from ref. [95]. Copyright (2016) Wiley & Sons, Inc.

Cheng et al. reported a reflection-type tunable metalens in the visible range, as shown in Figure 4b [96]. By taking advantage of the isolated gap surface plasmon (GSP) resonators with high reflective resonance, the authors realized a unique flat mirror encapsulated in PDMS. Analogous to Kamali et al. [95], Chen et al. [96] utilized a hybrid meta-atom consisting of a metal–dielectric–metal structure precisely arranged on PMDS film to enable full wave control. The metamirror was fabricated by standard lithography and lift-off processes. When the metamirror was illuminated with light, the focusing behavior was analogous to that of a positive lens. To determine the tunable focusing property, the metamirror was subjected to isotropic lateral stretching between 0% to 20% under white light illumination. Under these conditions, the focal length of the metamirror increased from the original value of 250 µm to 350 µm, as shown in Figure 4(biii). Relaxing the stretching gradually returned the focal length to its original value.

Ahmed et al. demonstrated a strain–multiplex metalens array with tunable light focusing and imaging capabilities [21]. Here, the authors demonstrated metal-coated hemispherical cavities which enabled phase discontinuity, required for full wave control upon stretching. The flexible diffractive metalenses (FMDLs) were fabricated on a glass substrate by laser interface lithography. The nanostructures were then transferred to Acrylate polymer/PET film using UV stamping and embossing processes. The light focusing and imaging properties of the FDMLs were evaluated under monochromatic and broadband illumination. For a stretch factor of 1.5, the far-field captured images of monochromatic light (red, green, and blue) showed comparable sizes due to the uniformity of the FDMLs. Under broadband illumination, an increase in stretch factor generated unfocused images.

3.2. Stretchable Metahologram

Holography is a wavefront reconstruction technique that exploits the fundamental properties of light, such as amplitude, phase, and polarization to record and reconstruct the interference patterns of an object. In traditional photography, only the intensity from the light field illuminating an object is recorded, resulting in an inaccurate reproduction of the object. Holography, however, preserves both the phase and amplitude of the original wave, allowing near-perfect reconstruction of images in three-dimensionality. A hologram is the interference pattern generated by the interaction of scattered light from an object and a coherent reference wave. In other words, a hologram encodes the entirety of a light field including the intensity and direction of all rays. This type of hologram is traditionally referred to as an amplitude hologram [97]. Due to low conversion efficiency in amplitude holograms, often resulting from incident power being scattered or reflected, the intensity of the interference pattern can be translated into phase variations for phase holograms [98]. Phase holograms provide a relatively high diffraction efficiency and can substantially increase the brightness of reconstructed images. Since metasurfaces allow tailored control of phase, amplitude, and polarization of light at the nanoscale, metasurfaces can be utilized to create holograms of varying functionalities referred to as metaholograms.

A noteworthy importance of metaholograms is their ability to extend the pixel pitch of holographic technologies, i.e., spatial light modulators (SLMs) to nanometer scale for better resolution, high field of view, and high-order diffraction [15]. Theoretically, the pixel–wavelength scale dependence of holographic images can be described by [99]:

$$\frac{1}{2i\lambda }\left(e^{i\eta }-e^{-i\eta } \right)=\frac{1}{p}$$

(8)

where $\lambda$ represents the wavelength of the incident light, $\eta$ is the half-maximum field of view FOV, and $p$ represents the pixel size. A wide viewing angle requires a smaller pixel size, which can be achieved with a metasurface. To design a subwavelength pixel metahologram, one must numerically compute the phase map required for the desired light propagation [15,98]. Additionally, meta-atoms must be carefully designed to achieve a physical realization of a complete phase shift from 0 to $2\pi$ [15]. Such phase control has been achieved by exploiting phase retrieval algorithms aided by computer-generated hologram designs [100,101,102,103,104]. Like all conventional metamaterials, metaholograms are limited to a singular functionality once fabricated. However, the demand for metaholograms that can store multiple images is eminent, leading to recent efforts towards tunable metaholograms. Like most conventional metamaterials, metaholograms can be tuned by external stimuli in the form of mechanical deformation, incident light source, and electrical biasing. An incident light source of varying polarization state and orbital angular momentum can be supplied to metasurfaces to generate multiplex metahologram images [105], helicity-controlled switchable images [106], color-tunable holograms [107], and switchable vectorial holograms [108]. By applying electrical biasing to electricity-sensitive materials, including conductive oxides and liquid crystals, electrically tunable metaholograms have been realized for light projection displays, mono-multicolor switchable metaholograms [109], and smart encryption [110]. An emerging concept of the tunable metahologram leverages reprogrammable or coded metasurfaces to produce arbitrary holographic images [98,111,112]. Digital coding of unit cells is exploited to enable dynamic control of metamaterial functionalities. Different functionalities can be achieved in real time by changing the coding sequence of the metasurface [98,113].

Mechanical tuning of metaholograms is another tuning mechanism of interest owing to the use of elastomeric–deformable substrates which enable flexible or conformal optics. The basic principle of mechanically tuned metaholograms is the deformation of polymeric substrates. By isotropically stretching the metasurface, the period of meta-atoms can change to reconfigure the optical wavefront.

A fundamental study on stretching-induced wavefront control of metaholograms was reported by Agarwal et al. [114], as shown in Figure 5a. Here, the authors exploited the Huygens–Fresnel transformation of a metasurface to explain the underlying principle of wavefront reconfiguration by stretching. According to the Huygens–Fresnel Principle, the electric field $E\left(x,y,z\right)$ at a distance $z$ from a metasurface must follow [114,115]:

$$E\left(x,y,z\right)={{\displaystyle \iint }}^{ }{E}_0\left(x_0,y_0\right)\left(\frac{-ik}{2\pi z}\right)e^{ikz}{e}^{ik\left[{\left(x-x_0\right)}^2+{\left(y-y\right)}^2\right]/2z}d{x}_{0}d{y}_0$$

(9)

After stretching the metasurface by a given stretch ratio $s$, the resulting electric field $E^{\prime }\left(x^{\prime },y^{\prime },z^{\prime }\right)$ can be represented as:

$$E^{\prime }\left(x^{\prime },y^{\prime },z^{\prime }\right)=E^{\prime }\left(sx,sy,s^{2}z\right)=E\left(x,y,z\right)e^{-ik\left(s^2-1\right)z}$$

(10)

Therefore, for a stretch factor of s, the electric field is a factor of $s^2$ farther away from the metasurface ($z$–direction) than its original unstretched position. The $x$ and $y$ directions of the field are enlarged by a factor of $s$compared to the original unstretched field distribution. Consequently, when a metasurface hologram is stretched by a factor of $s$, the size of the hologram image enlarges proportionally by the stretch factor $s$, as shown in Figure 5(aii). Likewise, the hologram image plane is displaced from the metasurface (z-direction) by a factor of $s^2$. This property was verified experimentally; a computer-generated hologram (CGH) was utilized to determine the phase distribution required for the reconstruction of single or multi-plane images. After extracting the physical dimension and orientation of resonators required for full wave control, the metasurface was fully realized on a deformable substrate, i.e., PDMS. Gold nanoparticles were deposited on a silicon wafer by electron beam lithography. PDMS was then cast on the wafer and stripped off after curing, leaving the gold nanostructures adhered to its surface. The metasurface was designed and used to observe two holographic images at 200 µm and 130 µm from the metasurface. Upon stretching the metasurface by a stretch ratio of 1.24, the image at 130 µm moves to 200 µm, demonstrating the switching functionality of the metasurface.

Guo et al. conceived a metahologram with dual color tunability [118]. By carefully designing unit cells or meta-atoms according to the Pancharatnam–Berry (PB) phase [119], a complete phase coverage can be obtained for full wave control. Moreover, a stretch of metasurfaces induces a change in the periodicity of unit cells, which translates into a change in optical transmission. By exploiting these two theories, the authors designed titanium nanopoles on a PDMS substrate. When the metasurface was stretched from 0% to 30%, a dual color change between red and green light is observed.

Although mechanically tunable metaholograms have progressed at a slow pace, recent works in this field have been particularly focused on flexible metaholograms. In 2019, Burch et al. developed a flexible reflection metasurface hologram with a millimeter operating wavelength range, as shown in Figure 5c [117]. To realize the metasurface, a three-layered c–ring gold nanostructure was employed as meta-atoms on PMMA film. The Gerchberg-Saxton algorithm (GSA) was utilized to recover the required phase distribution of the designed metasurface. The generated phase distribution was then mapped onto the distribution of the meta-atoms to acquire the meta-atom dimensions. The hologram metasurface was then fabricated by electron beam evaporation on rigid substrate and transferred to the PMMA film by lift-off processes. The hologram metasurface created holographic images upon irradiation at wavelength of 3.19 mm. Zhao et al. demonstrated a flexible large-area metahologram on defect-free flexible PMMA film, as shown in Figure 5b [116]. To fabricate the metasurface, the authors developed a novel adhesive-free double-faced nanotransfer lithography process. Gold nanostructures were deposited on a donor PET film by electron-beam evaporation and transferred to the PMMA film without any adhesive layers. This technique is particularly useful in fabricating large area meta-devices, which has been difficult to realize via conventional nanofabrication methods. To demonstrate the hologram functionality, gold ring hologram nanostructures were designed via the computer-generated holography technique and fabricated on the PMMA film. This process was then replicated for different metals to demonstrate the versatility of the technique. The fabricated hologram features were then observed under light-emitting diodes (LEDs) illumination with a varying angular spectrum, as shown in Figure 5(biii).

3.3. Plasmonic Resonance

Resonance is a basic light–matter interaction that is ubiquitous in plasmonic materials. When light is incident to a metal, two fundamental interactions are observed as a result of electron cloud excitation: propagating excitations known as surface plasmon polaritons (SPP) or non-propagating excitations known as surface plasmon resonance (SPR). Surface plasmon polaritons refer to the surface propagating electromagnetic waves that are bound with the motion of free electrons at the surface of a metal. On the contrary, surface plasmon resonance occurs when electromagnetic waves couple with the collective oscillation of free electrons inside a metal at a resonant frequency. The resonant frequency generally depends on the size, shape, composition, geometric arrangement, and optical surroundings of the metal [120,121]. When the size of the metallic material is on the order of nanometers, coherent oscillation at the resonant frequency is confined to a small region, resulting in strong field enhancement. The resulting resonance is referred to as localized surface plasmon resonance (LSPR). Since SPR frequency is material-dependent, one can greatly enhance SPR frequency using tailored material design.

The recent advances in metasurfaces have paved the way for the adoption of subwavelength periodic structures that can greatly enhance localized surface plasmon resonance. Meta-atoms can be periodically arranged on substrates such that light scattered by each meta-atom arrives in phase with the plasmon resonance induced in the neighboring atom by the incident light. This coupling increases the quality factor of the overall resonance [120]. The use of metasurfaces also presents additional advantages such as reduced losses owing to the reduced dimensions of meta-atoms [121]. To realize plasmonic metasurfaces, meta-atoms are periodically arranged on substrates, which in most cases are dielectric materials such as quartz and silicon [122]. These generic substrates have limited tunability which restricts the application of plasmonic metamaterials. Plasmonic metamaterials of this form are regarded as passive metamaterials and operate over a narrow bandwidth at the time of fabrication [123]. However, active tuning of plasmonic metasurface properties is necessary to satisfy novel application requirements.

As already established, fine-tuning of resonant frequency can be achieved by controlling the geometric parameters of meta-atoms as well as their surrounding media [122]. Frequency tunability has been demonstrated with optical, electronic, ferroelectric, and thermal tuning mechanisms [124]. An alternative tuning approach is to exploit the elasticity of compliant or elastomeric substrates. By mechanically deforming elastomeric substrates, the periodicity of embedded meta-atoms can be controlled to induce a spectral shift in resonance frequency or wavelength for novel biosensing applications. The use of elastomeric substrates also introduces additional functionality by enabling conformal and flexible optics. Here, we define meta-atoms as nanostructures that encompass the breadth of nanoparticles, split ring resonators, micro-ring resonators, and planar metallic nanostructures.

Pryce et al. developed a metasurface that constituted planar split ring resonators arrayed on PDMS. In this study, arrays of gold split ring resonators were patterned on a silicon substrate using e-beam lithography and then transferred to PDMS using soft lithography techniques [123]. By considering the slit ring resonators as electrical LC resonators with a resonant frequency governed by ${\omega }_{0 }\sim {\left(LC\right)}^{-1/2}$, the resonant frequency could be tuned by changing the distance between coupling resonators upon mechanical deformation. For symmetric resonators, when the metasurface was stretched uniaxially in the y-direction by a strain of 10%, a blue shift of resonant wavelength (30 nm) was observed. Better tunability in resonant width was obtained for asymmetric resonators. A maximum elastic strain of 10% generated a blue shift with twice the resonant wavelength of the symmetric split ring resonators. Pryce et al. advanced the previous work [123] to achieve resonant tuning in the infrared region [125]. The metasurface was fabricated by following the protocol in the previous work described above. In this approach, the prospect of coupled resonators on PDMS as surface-enhanced infrared absorption (SIERA) substrate was investigated. The coupled resonators were designed to exhibit a predetermined resonant frequency of 5.78 µm. After applying a reversible strain of 25%, the coupling distance between the resonators decreased with a subsequent resonant wavelength shift to 6.27 µm. This wavelength corresponds to the vibrational mode of p-mercaptoaniline (pMA), demonstrating the potential application of the metamaterial for sensing.

Aksu et al. reported a simple fabrication of nanostructures on various flexible substrates by using nanostencil lithography, as shown in Figure 6a [32]. Using a variety of polymers offers new possibilities to extend plasmonic metamaterials to substrates with high durability and stretchability. With all the polymers including PDMS, LDPE, and parylene, gold nanoparticles were patterned by electron beam lithography and etching processes in conjunction with a nanostencil. PDMS was particularly suited for the nanostencil fabrication approach, as it also facilitates the fabrication of high-quality nanostructures with sharp edges. Fine control of resonance wavelengths was achieved by stretching the various polymers. For PDMS with a designed periodicity of 1.51 µm, stretching by 21.3% increased the periodicity to 1.83 µm. When the strain was released, the substrate returned to its original state. In terms of LDPE, a strain of 16% shifted the designed periodicity (1.51 µm) to 1.575 µm with a corresponding redshift (160 nm) of resonant wavelength.

Chen et al. numerically proposed a mechanically stretchable metasurface that allows the modulation of optical properties in the mid-infrared region [128]. In the study, square gold nano-patches were periodically arranged on a PDMS substrate. When the designed metasurface was stretched by a moderate bi-axial strain of 0.15, a sharp switch of reflectance and absorbance was observed in the mid-infrared region from 7 µm and 20 µm. The potential integration of silicon photonics with flexible substrates was demonstrated by Chen et al. [129]. In this study, silicon photonic circuits were patterned by electron beam lithography and dry etching. The circuit was then transferred to a PDMS substrate by transfer and lift-off processes. To evaluate the tunability of the device, a compressive uniaxial strain of 9% was applied ratioing in a slight shift of resonance wavelengths (0.2 nm) compared to the resonance extinction ration, which increased five-fold.

Gutruf et al. reported a tunable metasurface that consists of all-dielectric nanostructures deposited on a deformable substrate, as shown in Figure 6b [126]. To realize the metasurface, a uniform array of cylindrical TiO₂ resonators was patterned on PDMS by electron beam lithography and transfer techniques. The optical characteristics of the metasurface were investigated without stretching and upon stretching the device uniaxially in the x–direction. The unstretched metasurface showed distinct electric dipolar resonance at 591 nm under white light illumination. By stretching the metasurface by 6% under y–polarized excitation, a distinct red shift of the resonance by 30 nm was observed, as shown in Figure 6(bii). Comparable to simulation results, an increase in strain from 0% to 6% resulted in transmission enhancement by 70% at 591 nm. The observed resonance originated from electric dipole resonances, which agrees well with the simulation results.

Liu et al. demonstrated a metasurface that exploits the gap distance between arrayed nanoparticles for continuous and reversible resonance tuning [130]. A square array of nano gold disks was fabricated on PDMS by interference lithography, gold sputtering, and lift-off process. By stretching the metasurface by 100% strain, the distance between adjacent gold nanodisk reduced remarkably from 140 nm to 10 nm. The transmission spectra of the metasurface were measured under polarized light perpendicular to the gap distance parallel to the varying applied strain from 0% to 100%. A distinct dipole resonance red shift to longer wavelengths was observed. Liu et al. reported a mechanically tunable metasurface that allows dynamic control of plasmonic resonance in the visible and near-infrared ranges [131]. Unlike conventional works that utilize single dielectric resonators or metallic nanostructures, the authors exploited out-of-plane layered gold nanoribbon arrays on a deformable substrate. The out-of-plane nanoribbon layers were fabricated on a deformable substrate, i.e., PDMS, using electron beam lithography coupled with a transfer process. To evaluate resonance tuning under strain, the metasurface was designed with different periods (587 nm, 802 nm) and layer separation heights (21 nm, 53 nm, 23 nm, 55 nm). By stretching the metasurface by a strain of 0% to 15%, a red shift in the visible and near-infrared range was observed. Overall, the resonance wavelength shifted to a longer wavelength upon increasing strain, suggesting a linear relationship between the period of the metasurface. This linear relationship is expressed as ${\lambda }_{res}=P$, where $P$ represents the period of the metasurface and ${\lambda }_{res}$ represents the resonance wavelength.

Yang et al. proposed a metasurface based on aluminum nanoparticle arrays embedded in a deformable substrate for broadband resonance control in the visible spectrum [132]. Aluminum nanoparticles were fabricated on glass using photolithography, etching, electron-beam deposition, and lift-off process. To begin with, Al nanoparticles were designed to support narrow and well-defined dipolar and quadripolar resonance modes. When the metasurface was stretched—a strain of 0% to 50%—in the long axis with polarized illumination in the same direction, the dipolar resonance red shifted with wavelength tunability across the spectral range of 600 nm to 700 nm. By stretching in the short axis with light polarization in the same direction, the quadripolar resonance exhibited spectral tunability from 500 nm to 600 nm.

Yu et al. reported an interesting study on periodic metasurfaces by describing the arrangement of meta-atoms from the perspective of crystallography, i.e., Bravais lattices [133]. From the crystallographic symmetry standpoint, the authors demonstrated the concept of a strain-enabled phase transition of periodic metasurfaces. In the study, Au nanodisks were fabricated using electron beam lithography to assume an initial hexagonal lattice or phase. This lattice arrangement was then transferred to a PDMS substrate. The phase transition from the hexagonal lattice to different Bravais lattices as well as the spectral evolution was tracked upon straining the initial lattice along its axis of symmetry. Upon stretching the initial lattice along the rotational axis $\theta =90°$ with strain between 0% to 30%, the resonance peak red-shifted under horizontal polarization. The peak shift for straining along the lower rotational axis $\theta =0°$ is less compared to straining along $\theta =90°$. The red shift of resonance peaks under vertical polarization is observed to be analogous to the red shift in 1D gratings of higher periodicities.

Wand et al. conceived a nanolasing platform by exploiting hybrid quadrupole plasmons [134]. The nanolasing platform comprises an array of gold nanoparticles embedded in PDMS. Gold nanoparticles were fabricated on PDMS using photolithography, etching, electron-beam deposition, and lift-off process. The metasurface exhibited sharp hybrid quadrupole lattice (HQL) resonance and strong near-field interactions. When the applied strain was moderately increased from 0% to 5%, the HQL resonance shifted by 45 nm. Chen et al. reported a structurally tunable metasurface by exploiting plasmonic lattices arrayed in between symmetric micro rods on PDMS, as shown in Figure 6c [127]. The metasurface platform was fabricated using photolithography, etching, electron-beam deposition, and lift–off process. The metasurface comprises gold plasmonic gratings sandwiched between tapered gold micro rods on PDMS. When the metasurface is stretched in the y–direction under polarization light in the same direction, the resonance wavelength of the metasurface shifts from 744 nm to 836 nm with a strain variation from 1.6% to 3.5%, as shown in Figure 6(cii). A linear relationship between the strain and resonance wavelengths is attributed to the increase in period upon straining. The experimental resonance wavelengths generated an excellent mechanosensitivity of 48 ± 5 nm shift per 1% uniaxial external strain. Yoo et al. demonstrated a metasurface that allows dynamic tuning of resonance via mechanical stretching [135]. The metasurface platform comprises asymmetric gold pyramid arrays embedded on a PDMS substrate. The asymmetric gold arrays were fabricated by standard photolithography and transferred to PDMS by template stripping aided by a cylindrical roller. By stretching the PDMS upon white light illumination, the angle between the pyramid face and the incident light changes with a consequential change in surface plasmons polaritons coupling. When the metasurface was illuminated with polarized white light, the resonance peak wavelength gradually shifted from 545 nm to 682 nm upon straining between 0% and 9% in the same direction of light polarization.

3.4. Structural Color Filters

Another area of burgeoning interest in metasurface research is structural colors. In general, interference, diffraction, or scattering between light and resonators on metasurfaces can generate structural colors with superior color vibrancy, spatial resolution, and durability [136]. Here, the observed structural color is dictated by the physical geometry of resonators. Structural colors have been successfully implemented in applications such as color filtering, imaging, printing, colorimetric sensing, and anti-counterfeiting [137,138]. In conventional structural color applications, resonators are fabricated on rigid substrates [138,139,140] with spectral information which cannot be modified post-fabrication. This form of structural color is referred to as static color. However, active tunable structural colors can be obtained by using soft mechanically deformable substrates. These substrates allow dynamic and reversible control of the lateral distance between resonators with a consequential change in the optical response (color) of the metasurface. The use of deformable–elastomeric substrates also enables conformal optics to address the ever-increasing demand for flexible, wearable, and adaptive optics.

In 2017, Song et al. demonstrated a dynamic tunable structural color based on a tensile elastomeric substrate, i.e., PDMS, as shown in Figure 7a [141]. PDMS was applicable in this work due to its viscoelasticity, which allows reversible stretching under external force, and chemical stability. To realize a structural color, aluminum nanoparticles were fabricated on a PDMS substrate using interference photolithography. By laterally stretching one side of the PDMS from an unstretched position of 0% strain to a stretched length of 31.6% strain, the resonant wavelength is observed to shift from ~530 nm to ~620 nm. This corresponds to a structural color shift from green to fuchsia, as shown in Figure 7(bii). The resonant wavelength is further observed to increase linearly with the strain amount. To probe the underlying mechanism of the resonant shift, FDTD simulations were employed. The shift in resonance and corresponding structural color change emanates from the change in the period of aluminum nanoparticles. In another study [137], Tseng et al. realized a full-color tunable plasmonic device based on a two-dimensional array of rectangular aluminum nanostructures, as shown in Figure 7b. This work extended the structural color control reported by Song et al. to achieve full-spectrum control. Here, aluminum nanostructures, patterned on a silicon substrate by electron beam lithography, were transferred to a deformable–elastomeric substrate, i.e., PDMS. The device was engineered to portray green light in its relaxed state. By stretching the device along the horizontal axis and vertical axis, the green color shifted to red and blue, respectively. The scattering spectra of the device were observed under dark-field spectroscopy with a broadband white light supplied by a laser. In the relaxed state, the device exhibited a cyan color which shifted to green (10% strain), yellow (13% strain), and red (23% strain and 32% strain), upon stretching in the horizontal axis. When the device was stretched in the vertical axis by 31% strain, the color of the device shifted to blue and then purple. The combined stretching effect in both the vertical and horizontal axis allowed color tuning of the device across the visible spectrum with a moderate elastic strain of not more than 35%. The device further demonstrated active switching between three adjacent patterns of the letters O, W, and L with corresponding periods of (x, y) = (429 nm,348 nm), (400 nm, 400 nm), and (348 nm, 429 nm), respectively. As shown in Figure 7(biii), when the device was stretched horizontally by 15%, only patterned L could be observed. Likewise, stretching the device vertically by 26% revealed only pattern O.

Although the works mentioned above rapidly advanced structural color control on deformable substrates, the stretchable approach is limited by its inherent anisotropic properties, i.e., vertical stretching typically causes compression in the horizontal axis producing reversible effects for light with polarizations. As a result, Zhang et al. investigated a polarization-insensitive all-dielectric metasurface that allows distinct color control across the entire visible spectrum [136]. To realize the metasurface, squared-shaped TiO₂ nanoblocks were uniformly patterned on a multilayer substrate using e-beam lithography. The TiO₂ nanoblocks were transferred to a PDMS substrate using lift-off and transfer processes. To demonstrate the mechanical color tunability, the reflection spectra of the metasurface with x-polarization and y-polarization light were examined in a relaxed state. By straining the metasurface in the x-direction from 0% to 40%, the structural color from both polarizations simultaneously switched from blue to green and red, denoting complete coverage over the entire visible spectrum.

Kim et al. demonstrated a mechanochromic transmissive optical filter based on stretchable and flexible polymer embedded with silicon nanostructures, as shown in Figure 7c [142]. The silicon nanostructures were fabricated using laser lithography and transferred to a PDMS substrate via transfer processes. By stretching the metasurface uniaxially under white light illumination, the transmitted color shifted from yellow to magenta, as shown in Figure 7(cii). The spectral change was observed to be a function of the distance between nanowire arrays, which varies upon stretching. To evaluate the device as an optical filter, the metasurface was sandwiched between blue, green, and red lasers while subjected to a stretching ratio of 0%, 20%, 40%, and 60%. In the bare state, red light (635 nm) and green light (532 nm) showed a wide diffraction angle compared to blue light (450 nm). After applying a stretch ratio below 40%, the red light seamlessly proceeded without any diffraction. However, an increase in the strain of over 40% resulted in the diffraction of red light. To further extend the application range of structural colors, Agata et al. conceived a metasurface sticker that can augment the photoluminescence of different optical surfaces [143]. A hexagonal array of aluminum nanoparticles was fabricated on a dielectric substrate using nanoimprinting. Al nanoparticles were then embedded in PDMS via transfer processes. By stretching the sticker by 40% in the x–direction, the hexagonal symmetry transforms into a square symmetry. To demonstrate the emission control as a function of this transformation, the sticker was placed on a PMMA luminescent layer. The induced rearrangement from stretching enabled a stronger emission of the luminescent layer compared to the emission of the luminescent layer alone. To thoroughly capture the reported works, a detailed summary encompassing the applicable polymeric substrates, the active materials or nanostructures, and the performance metrics, which includes the degree of stretching and the tunable functionality is presented in

Table 2. Summary of representative metasurface applications and performance metrics.

Substrate for Metasurface	Representative Application	Specific Functionality	Active Material	Stretchability	Tunable Property	Year
Polydimethylsiloxane (PDMS)	Metalens	Flat zoom lens [20]	Gold (Au) nanorod	30% strain	Focal length 150–250 µm, Operating wavelength 632.8 nm	2016
		Zoom imaging metalens [144]	Graphene oxide film	10% strain	Focal length 212–376 µm, 450–650 nm	2021
		Microlens [95]	Silicon nano-post on thin aluminum oxide	0–50% strain	Focal length 600–1400 µm, Operating wavelength 915 nm	2016
		Reflecting metalens [96]	Silver (Ag)—Silica (SiO2)—Silver (Ag) resonators	0–20% uniaxial strain	Focal length 250–350 µm Operating wavelength 670 nm	2019
		Metalens [128]	Alumina nanopillars	–	average tuning range 109. 2 µm	2021
		Flat zoom lens [145]	Crystalline silicon nanoposts	137% strain	Focal length 320–440 nm, Operating wavelength 680 nm	2018
		Beam splitter, reflective mirror [146]	Silicon nanoblock-thin alumina	0–70% uniaxial strain	500–580 nm, 532 nm	2022
Polydimethylsiloxane (PDMS)	Metalens	Conformal optics [147]	Silicon nano-post on thin aluminum oxide	–	Operating wavelength 915 nm	2016
	Metahologram	Image switching [114]	Gold nanorod	0–24% strain	Focal length 150–232 µm	2017
		Color metahologram [118]	TiO2 nanopoles	–	–	2020
	Resonance modulation	Mechano-sensitive plasmonic resonator [127]	Tapered gold microrods-gold gratings	1.6–3.5% uniaxial strain	Resonance wavelength 744 nm–836 nm	2018
		Resonance tuning [130]	Gold nanogaps	100% uniaxial strain	Resonance wavelength red shift	2017
		Flexible photonics [129]	Gold micro-ring resonators	9% uniaxial strain	Resonance wavelength 0.2 nm shift	2012
		Tunable Surface-Enhanced Raman Spectroscopy [148]	Silica-gold nanoparticles	0–30% uniaxial strain	581–625 nm	2013
		Resonance frequency tuning [125]	Gold split ring resonators	25% uniaxial strain	Resonance frequency 5.78–6.27 µm shift	2011
		Dynamic polarization control, birefringence tuning [133]	Gold nanodisks	0–30% axial strain	Resonance wavelength red shift	2022
		Nanolasing [134]	Gold nano particles	0–5% strain	Resonance wavelength 35 nm shift	2018
Polydimethylsiloxane (PDMS)	Resonance modulation	Surface-Enhanced Raman Spectroscopy [149]	Gold nanoprism, Silver nanoprism, Gold-silver nanoprism	20% uniaxial tension	910–983 nm, 1011–971 nm, 937–1001 nm	2018
		Resonant frequency tuning [123]	Planar coupled split ring resonator (AuSRR)	10% uniaxial strain	Resonance wavelength, 30 nm shift	2010
		Strain, chemical, biological sensing [150]	Gold split rings	25% strain	0.5–3.0 THz, Terahertz	2018
		Electromagnetic device [151]	Silver nanowires	100% uniaxial stretching	Operating frequency (8–12 GHz)	2019
		Resonance tuning [135]	Gold nanoholes and pyramid	0–9% uniaxial strain	Resonance wavelength 545–682 nm	2015
		Dielectric resonator [126]	Cylindrical TiO2	6% uniaxial strain	Resonance wavelength 591–625 nm, optical transmission >70%	2016
		Resonance tuning [32]	Gold nanoparticles	21.3% uniaxial strain	Resonance wavelength 1.51–1.83 µm	2011
		Dynamic tunable plasmonics [132]	Aluminum nanoparticles	0–50% uniaxial strain	Resonance wavelength 500–700 nm	2016
Polydimethylsiloxane (PDMS)	Resonance modulation	Resonance wavelength tuning [131]	Layered gold nanoribbon	Uniaxial strain 0–9%, 0–13.7%s	Resonance wavelength 660–685 nm, 887–921 nm, Visible to Near-Infrared range	2018
		Dynamic infrared tuning [128]	Gold nanopatches	15% biaxial strain	Absorbance/reflectance 7–20 µm shift, Mid-infrared region	2021
	Color filter	Full color tuning, image-switching [137]	Aluminum nanostructure	Horizontal strain; 0–32%, Vertical strain, 0–31%	Red shift 495–645 nm, Blue shift 495–440 nm	2020
		Color tuning [136]	Square TiO2 nano-blocks	40% uniaxial strain	Color tuning range 450–650 nm	2020
		Optical emission control [143]	Aluminum nanoparticles	40% uniaxial strain	Emission intensity red shift	2021
		Near infrared color filter [152]	Amorphous silicon nanodisk		Transmittance peak 600–1000 nm	2019
		Dynamic color tuning [141]	Aluminum nanoparticles	36% strain	Resonance wavelength 530–620 nm	2017
		Color filter [153]	Silicon nanoblocks	–	Color tuning range purple–red	2020
Polydimethylsiloxane (PDMS)	Color filter	Circular dichroism [154]	Gold nanorod trimer	0–20% uniaxial strain	55% ircular dichroism	2022
Polyethylene Terephthalate (PET)	Metalens	Focusing metalens array [21]	Silver-nickel nanostructures	0–50% strain	Imaging wavelength range 450 nm, 532 nm, 635 nm	2021
	Resonance Modulation	Switchable optics [155]	Gold nanotrenches	Vertical bending	Terahertz frequency	2021
		Active optical devices [23]	Gold zero-nanometer gaps	Repeatable bending	Transmission efficiency 78% Operating wavelength microwave regime	2021
Low Density Polyethylene (LDPE)	Resonance modulation	Resonance tuning [32]	Gold nanorods	5% uniaxial strain, 16% uniaxial strain	Resonance shift 160 nm, 3000–3230 nm	2011
Silicone	Resonance modulation	Resonance tuning [33]	Gold-titanium-silica nanodisk	107% uniaxial strain	resonance tuning 770–1310 nm	2015
Polyimide (PI)	Laser	Random laser [156]	Zinc oxide nanorods	40% bending strain	decreased spectral intensity	2019
	Resonance modulation	Solar absorber [157]	Thin gold-tungsten layers	×800 bending	Optical absorption 97%	2020
		Terahertz modulation [158]	Polyimide-aluminum-polyimide film	28% biaxial strain	Resonance frequency shift, 3.4%	2020

.

4. Conclusions and Future Outlook

In this review, we have presented a comprehensive outlook on the use of mechanical deformation for tunable metasurfaces. Metasurfaces allow the design of spatial inhomogeneity over optically thin surfaces for precise control of the phase, polarization, and amplitude of light. By utilizing metasurfaces aided by deformable substrates, a variety of applications including stretchable metalenses, stretchable holograms, plasmonic resonances, and dynamic color tuning have been realized. Specifically, we described tunable metalenses with focusing and imaging capabilities that can be tuned across the entire visible spectrum via mechanical stretching. For metaholograms, we described the fundamental principles of mechanically tunable metaholograms with image-switching capabilities via mechanical stretching. We have also provided the current progress of dynamic color tuning via mechanical stretching, which is very promising for modern display technologies. Plasmonic resonance is relevant for various broadband applications including sensing in the visible and terahertz regime. As a result, we have presented a comprehensive overview of tunable plasmonic resonances enabled by mechanical stretching.

While metasurfaces have greatly revolutionized flat optics, we foresee the adoption of metasurfaces based on deformable–biocompatible substrates for various in vivo bio-imaging applications. Collectively, metalenses, metaholograms, structural colors, and plasmonic resonances are potential gamechangers for novel biosensing, bio-imaging, and wearable biomedical applications. Metalenses, in particular, are promising for in vivo bio-imaging without compromising performance. Specifically, the increasing demand for miniaturized-biocompatible in vivo diagnostic and imaging devices can be addressed with biocompatible metalenses. A noteworthy biopolymer that can facilitate this application is silk, which possesses excellent biocompatibility, optical properties, and mechanical robustness. Plasmonic metasurfaces with tunable resonances are relevant for broadband sensing applications. Aside from their standalone usage, flexible plasmonic metasurfaces can be integrated into wearable devices to enhance the sensing capabilities of biomedical wearables. Wearable skin, such as strain–gauge sensors that heavily relies on mechanical stress, can be tremendously augmented with flexible plasmonic metasurfaces. Likewise, structural colors have been demonstrated as efficient color-based strain sensors that can be employed in soft robotics, active color filters, and displays. More importantly, the development of structural color-based sensors based on deformable–biocompatible substrates will usher in color-based strain sensors that can ultimately supplement biomedical wearables. Consequently, we envision that the extension of flat optics to biocompatible metasurfaces will advance bio-implantable optics for better diagnosis and treatment of various diseases.