1. WHERE ARE THE OPTICAL METAMATERIALS?

The unprecedented control of light in recent years through the use of optical metamaterials is derived from controlling the orientational or positional order of the nanostructure, rather than just the elemental shape or composition [1]. The nanostructures are comprised of subwavelength plasmonic elements, resonantly coupling light to matter, concentrating the light well below the diffraction limit, thus resulting in tremendous field enhancements and a plethora of exotic optical properties, such as negative refraction [2–5], epsilon-near-zero [6,7], amplification of evanescent fields [8,9], enhanced Raman scattering [10–12], and nanoscale chirality [13,14].

Great strides were made early on in the research field, thus demonstrating novel metamaterial properties at millimeter [2] and then micrometer wavelengths [15,16]. The success of these initial measurements have lead to disruptive questions and ensuing changes in the understanding of light–matter interactions. However, these measurements were predicated on the ability to create the desired structures. As the field has progressed to visible wavelengths, the need for reproducible subnanometer resolution elements has become paramount. Moreover, to develop these properties into usable materials, the nanoscopic elements need to be reliably replicated and assembled to create macroscopic devices. Assembly strategies must therefore be developed to not only control the nanoscopic order of the elements, giving rise to the unique optical properties, but enable high throughput, thus leading to macroscopic quantities for device applications [17] such as for transformational optics [18–23], antennas [24–26], lasers [27,28], transistors [29,30], sensors [31–37], and photovoltaics [38–40].

The field of optical metamaterials has matured into an evolutionary crossroad between intellectual endeavors and the pursuit of ubiquitous devices. Despite recent progress, much work still needs to be carried out to realize efficient assembly approaches to create these materials for device applications.

In this work, we present an overview of recent approaches used to develop optical metamaterials. Herein, the term macroscopic is defined as a material, which can easily be seen with the unaided eye, making it accessible for widespread applications. From a pragmatic viewpoint, optical metamaterials are classified into two categories:

Within each category, we highlight the advantages and challenges faced using different assembly approaches. Regarding the index of refraction—it is a bulk material property. In the context of this work, it is viewed as an effective index of refraction because most of the nanostructures presented are subwavelength in scale. Generally, the transmitted and reflected phases and amplitudes are better quantities to describe the properties of these materials. Given the immense volume of literature generated on this subject over the last decade and a half, it is not viable to include all fundamental contributions in this mini-review. We selected work, which, in our opinion, exemplifies the salient developments and, more importantly, are likely candidates for future progress.

2. METASURFACES

Optical metasurfaces are typically composed of one or a few subwavelength layers of plasmonic elements precisely oriented or positioned on a surface over large areas. Many devices could result from these surfaces such as flat lenses, which can arbitrarily shape wavefronts [41,42], the ability to engineer specific electric and magnetic field interactions [23,43,44], and chemical applications [45–47]. Below we survey recent progress.

A. Lithography

Electron beam lithography uses a beam of electrons to serial, point-by-point, write patterns into substrates and has been used extensively over the years to create optical metasurfaces, verifying and discovering many unique optical properties. Figure 1(a) is an optical metasurface created using electron beam lithography, establishing a negative index of refraction at 1.5 μm [48]. The metasurfaces consists of a $2\mathrm{mm}\times 2\mathrm{mm}$ array of paired gold nanorod elements ($220\mathrm{nm}\times 780\mathrm{nm}$). Sub-10 nanometer resolution metasurface elements were patterned into metallic films on silicon nitride transmission electron microscopy windows using a combination of electron beam lithography and mask lift-off, illustrating the advantage of combining different types of assembly approaches [Fig. 1(b)] [49].

 

Fig. 1. Scanning electron microscopy images of a $2\times 2\mathrm{mm}$ optical metasurfaces made using electron beam lithography. Reproduced with permission from Shalaev et al. [48]. (b) Transmission electron microscopy image of two plasmonic nanoprisms with nanometer interparticle spacing created using electron-beam lithography and lift-off techniques. Reproduced with permission from Duan et al. [49]. (c) Scanning electron microscope image of a $16\mathrm{\mu m}\times 16\mathrm{\mu m}$ split-ring resonator array fabricated using focused ion beam lithography with resonances in the near-infrared regime. Reproduced with permission from Enkrich et al. [50]. (d) Scanning electron image of a layered fishnet structure consisting of alternating layers of 30 silver and 50 nm magnesium fluoride fabricated using focused ion beam lithography. Reproduced with permission from Valentine et al. [51].

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In focused ion beam lithography, a focused beam of ions is used to ablate and pattern substrates usually with tens of nanometer resolution and is considered faster for prototyping relative to electron beam lithography. Figure 1(c) show an example of an optical metasurfaces made using focused ion beam writing. The metasurfaces consist of a $16\mathrm{\mu m}\times 16\mathrm{\mu m}$ planar array of U-shaped gold split ring resonators, approximately 280 nm in scale, leading to operating wavelengths between 1–3 μm [50]. As an additional example a multilayered fishnet metamaterial with a refraction index below zero between 1.5–1.8 μm is shown in Fig. 1(d) and was patterned using focused ion beam writing [51].

Electron beam and focused ion beam lithography are typically limited to small areas, nanometer (or greater) feature resolution, low throughput, and require significant time and monetary investments, making them generally well suited for proof-of-principle demonstration [52].

B. Scaffolds

Taking advantage of the underlying symmetries of scaffolds may help to enable the fabrication of large-scale devices. A promising approach to create macroscopic metasurfaces is nanotransfer printing. The nanotransfer printing process is shown in Fig. 2(a) [53,54]. Photoresist was spun cast onto a silicon wafer coated with silicon nitride. Patterned molds made of poly(dimethylsiloxane) were stamped into the photoresist. The regions of silicon nitride not protected by the photoresist are removed with reactive ion etching. Next, the exposed silicon is plasma etched to a depth of 1–2 μm, creating a fishnet pattern in the silicon wafer. The excess silicon nitride and photoresist were remove from the silicon stamp with piranha solution. Alternating layers of silver, silicon dioxide, and magnesium difluoride were deposited onto the stamp, “inking” the stamp, using electron beam evaporation, thus mirroring the underlying fishnet pattern on the stamp. The multilayered metamaterial ink is finally transferred onto a target substrate.

 

Fig. 2. (a) Schematic of the nanotransfer printing process. (b) and (c) Scanning electron microscopy and photography images of the large-area metamaterial. Reproduced with permission from Chanda et al. [53] and Gao et al. [54].

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The resulting metasurfaces in Fig. 2(b) correspond to ${10}^5$ unit cells covering an area up to approximated $75{\mathrm{cm}}^2$. The throughput is nearly ${10}^8$ times faster than state-of-the-art focused ion beam lithography systems. The period of the structure is 0.85 μm and the widths of the fishnet ribs are 0.6 and 0.23 μm. The operating wavelength is between 1.2–2.6 μm with the real part of the refractive index reaching values of $-7$ at 2.4 μm. Figure 2(c) shows an optimized fishnet structure with feature sizes on the order of 100 nm, enabling the metasurfaces to operate at visible wavelengths [54]. The real component of the refractive index is calculated to be negative at a wavelength of 600 nm.

The high throughput and excellent uniformity over a macroscopic length scale makes nanotransfer printing an impressive approach to develop metasurfaces. The future refinement of the feature size and material composition will only strengthen this approach’s potential in developing metasurface devices. The above strategies also remain, to the best our knowledge, the only routes to create gradient-based metasurfaces for flat optics applications [43].

Other approaches have also been developed. Homeotropic-aligned silver nanowires in scaffolds were prepared by electrochemical anodization in porous alumina [55,56]. Aluminum was pretextured with a mold pressed into the surface, forming long-range, ordered indentations on the surface. The aluminum is then anodized under constant voltage and anodizing solution. The indentations serve as sites for hole generation in the initial stages of anodization. A highly ordered, honeycomb-like nanohole array was formed in the alumina and served as a scaffold [57]. Silver is then electrochemically deposited into the nanoholes, forming the homeotropic-aligned nanowires. The diameter of the nanowires was approximately 50 nm and several micrometers in length. The negative refraction of light at visible wavelength was demonstrated with these metasurfaces based on hyperbolic dispersion. Additionally hyperbolic metasurfaces have been demonstrated using binary layers of metals and dielectrics, such as silver and aluminum oxide [21] or other materials [58].

C. Phase Separation

Colloidal self-assembly offers the potential to produce high throughput optical metasurfaces with sub-nm resolution [59]. Organizing nanoparticles, through the minimization of free energy, to interfaces can yield large-area metasurfaces [60].

Drop-cast evaporation approaches have been used to create locally ordered, large-area monolayer surfaces, wherein nanoparticle suspensions in volatile solvents are placed onto immiscible liquids and allowed to evaporate, confining the nanoparticles to the air–fluid interface. Locally hexagonally close-packed, centimeter-scale monolayer surfaces have been created by controlling the evaporation of toluene and the concentration of excess dodecanethiol ligand in a suspension of 6 nm diameter gold nanospheres [61]. The excess ligand promotes the nanospheres to the air–toluene interface of a droplet, drop-cast onto a substrate, forming monolayer surfaces. Further work expanded on this idea, phase transferring, self-assembling, and transporting macroscopic monolayer surfaces onto substrates using a single self-assembly process [62]. Binary metasurfaces composed of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and FePt nanoparticles with various symmetries have been constructed over large areas [Fig. 3(a)] [63]. These surfaces were created by drop-casting the binary nanoparticles in a hexane suspension onto the surface of diethylene glycol in a Teflon trough. The nanoparticle surfaces were then placed onto ${\mathrm{SiO}}_2\text{-}\mathrm{Si}$ substrates. Additional experiments have been carried out to characterize the plasmonic properties of surfaces using microspectrophotometry for various nanoparticle composition and order [64].

 

Fig. 3. (a) (left) Schematic of binary nanocrystal superlattice growth and transfer process. (right) Transmission scanning microscopy images and model of the binary superlattices. Reproduced with permission from Dong et al. [63]. (b) (left) Photographs of a Langmuir–Blodgett trough containing silver nanoparticles confined to the air–fluid interface and (right) corresponding transmission microscopy images as a function of surface pressure. Reproduced with permission from Tao et al. [65]. (c) Schematic of block copolymer scaffold assembly of gyroidal metamaterials. Reproduced with permission from Vignolini et al. [66].

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The packing of nanoparticles and ensuing optical properties of large-area monolayer colloidal surfaces can be tuned via surface pressure [Fig. 3(b)] [65]. Poly(vinyl pyrrolidone) stabilized silver nanocubes suspended in chloroform were placed at the air–water interface in a Teflon coated Langmuir–Blodgett trough. Upon evaporation of the chloroform, the hydrophobic nanocubes are confined to the air–water interface, forming a monolayer surface. As the surface area is decreased [Fig. 3(b)], the surface pressure increased, thus driving the monolayer surfaces from a gas to liquid crystal to solid phase. Images showing the evolution of the reflected spectrum from the monolayer surface in the Langmuir–Blodgett trough are shown in Fig. 3(b) (left column). In the dilute (gas) phase, the reflected spectrum is a yellow-green color; as the pressure is increased, the spectrum dramatically changes from orange in the liquid crystal phase to a silver-like color in the solid phase. The intensity of the reflected color also significantly increases with increasing surface pressure. The scanning electron microscopy images on the right side of Fig. 3(b) show the morphology of the nanocube monolayers changing with increasing surface pressure.

Microphase separation of block copolymers were utilized to create scaffolds to form macroscopic metasurfaces [66]. Isoprene-block-styrene-block-ethylene oxide block copolymers were self-assembled into a gyroid morphology [Fig. 3(c), top left]. The scaffolds were then exposed to ultraviolet light selectively removing the isoprene polymer from the scaffold once rinsed with ethanol. The resulting polymer network was then backfilled with gold using electrodeposition to a thickness of 200 nm. The remaining polymer was removed by plasma etching leaving a gold gyroid metasurfaces with feature sizes on the 10-nm length scale. A scanning electron microscopy image of the metasurfaces is shown in Fig. 3(c) (right). The metasurface has resonances in the visible part of the spectrum and also exhibited optical chirality.

Metal-coated elastomer metasurfaces were created on elastomeric substrates by self-assembling a monolayer of polystyrene spheres on a gold-coated glass substrate [67]. A thin layer of transparent elastomer was then drop-cast onto the sphere and cured. The elastomer polystyrene sphere layer was then removed from the glass substrate. The polystyrene spheres were then dissolved, leaving a nanocavity void on the surface of the elastomer. A 100 nm thick layer of gold was then sputter coated onto the surface of the elastomer. The reflectance peak shifted by approximately 2% with a 5% strain applied to the metal-elastomer metasurfaces.

Phase separation is a useful mechanism to facilitate the development of large-area metasurfaces by confining monolayers of colloids to interfaces via energy minimization or the templating of scaffolds and remains a promising strategy for large-area, self-assembled metasurfaces.

D. External Fields

Using external fields to govern the orientational or positional order of plasmonic nanoparticles can lead to metamaterials with tunable optical properties.

Anisotropic metallic nanoparticles in suspensions are aligned when the induced energy arising from the polarizability of the nanoparticle from external electric fields is greater than the energy from thermal fluctuations [68]. Figure 4(a) shows a “pixel” of small aspect ratio ($<4$) polystyrene stabilized gold nanorods in suspension and aligned using a 60 Hz externally applied electric field ($\sim {10}^6[\mathrm{V}/\mathrm{m}]$) (bottom half of the image) [69, 70]. The nanorods align in the applied field direction (out of the page), suppressing the long axis absorption peak, giving rise to a red color when backlit with white light. The suspension without the electric field applied (top half of the image) is blue. By combining solvent evaporation and DC electric fields, micrometer-sized domains of homeotropic aligned cadmium sulfide nanorods have been developed [71]. The positional order of nanoparticles also can be controlled using optical frequency fields to trap and manipulate the position of plasmonic nanoparticles [72].

 

Fig. 4. (a) Switchable “pixel” of aligned gold nanorods in an organic suspension using external electric fields. The bottom part of the cuvette is red as a result of the nanorods aligning with the field (out of the page) and the top part is blue without the field applied. Reproduced with permission from Zheng et al. [69]. (b) Polarized optical microcopy images of lithographically patterned ferromagnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanorods between cross polarizers before (left) and after (right) shifting the transmission axis by 45°. Reproduced with permission from Wang et al. [73]. (c) Magnetic field aligned gold nanorods functionalized with liquid crystal ligands oriented planar (left) and hometropic (right). Reproduced with permission from Umadevi et al. [74]. (d) (left) Schematic representation of gold nanorods and lyotropic liquid crystal surfactants aligned using shear flow and magnetic fields. (right) Tunneling electron microscopy image of the aligned nanorods. Reproduced with permission from Liu et al. [77].

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Ferromagnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanorods encased in silica can be oriented using magnetic fields [73]. Figure 4(b) shows an image under crossed polarizers (of a panda bear) comprised of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanorods lithographically oriented on a substrate. The orientation of gold nanorods functionalized with liquid crystal ligands has also been demonstrated using magnetic fields, with field magnitudes on the order of 1 T, Fig. 4(c) [74,75]. Figure 4(d) shows schematically (left), through the combination of both shear flow and external magnetic fields, aligned gold nanorods stabilized in micelles of lyotropic surfactants [76,77]. The mixture was freeze fractured and imaged using transmission electron microscopy [Fig. 4(d) (right)]. Approaches to dynamically tune the optical response from plasmonic nanostructures have been demonstrated using thermally responsive liquid crystalline ligands to modulate the material phase [78]. Metasurface properties can also be tuned by changing the local dielectric properties by orienting liquid crystals surrounding the nanostructures [79].

The ability to dynamically tune the phase of the nanostructure with external fields may have great benefits to directing the assembly and optical responses from plasmonic nanostructures.

3. METAMOLECULES

Metamolecules are fundamental elements, giving rise to unique optical properties. These elements are on the order of tens of nanometers in size for visible light applications. The metamolecules are composed of subunits. These subunits are made from plasmonic materials, such as silver or gold, and typically are nanospheres or nanorods, although much more elaborate subunits can be created. The subunits are ordered either through position or orientation with respect to one another, forming the metamolecules and facilitating the optical response. Assembling such nanostructures at large scales is challenging. Here, we highlight some encouraging approaches.

A. Scaffolds

DNA mediated assembly uses complementary strands of DNA functionalized onto plasmonic nanoparticles to direct the spontaneous assembly of metamolecules [80]. Such structures are anticipated to have extraordinary properties such as enhanced electric fields and magnetic resonances at visible wavelengths [81]. DNA assembly was used to construct hetropentamers comprised of a central gold nanospheres with four larger gold nanoshells forming the nanostructure. Complementary DNA strands were used on the nanospheres and nanoshells to assemble in a solution. Local scattering measurements show rich optical property results from these nanostructures, including a magnetic response at near-infrared wavelengths [82].

The upper-left quadrant of Fig. 5(a) is a schematic of a DNA origami gold nanospheres ring assembly process [83]. Using DNA segments to build scaffolds engineered with specific binding sites to attach nanospheres are assembled into ring structures by inserting and deleting bases at selected sites. The resulting ring structure has a ring cross-sectional area of 10 nm and overall diameter of 62 nm. DNA handles were placed around the circumference of the rings acting as specific attachment points for DNA functionalized nanospheres. Various numbers and sizes of gold nanospheres with complementary DNA strands are attached to the DNA origami ring forming the nanostructures and imaged using transmission electron microscopy [Fig. 5(a), right]. The spectra from individual nanostructures were measured using dark-field spectroscopy and overlaid with simulation spectra retrieved from finite-element simulations, claiming the demonstration of a magnetic mode at visible wavelengths.

 

Fig. 5. (a) (upper left) Self-assembly schematic of the DNA nanospheres rings. (lower right) Transmission electron microscopy images of various rings produced using this approach. Reproduced with permission from Roller et al. [83]. (b) DNA mediated assembly schematic of 3D plasmonic nanoclusters. (lower left) Scanning electron microscopy image of a nanocluster. Reproduced with permission from Barrow et al. [84].

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The need for polarization independent, isotropic optical responses is important for many proposed applications using metamolecules. Figure 5(b) illustrates a DNA mediated assembly approach yielding 3D (out-of-plane) nanoclusters [84]. Planar nanoclusters of nanospheres functionalized with specific complementary DNA strands were assembled in a solution. The planar nanoclusters were then spun coated onto a substrate, which was coated with another thiolated DNA strand allowing for the attachment of an additional nanosphere. The 3D nanoclusters (tetramers, pentamers, and hexamers) were imaged using scanning electron microscopy, and a pentamer is shown in the bottom-left corner of Fig. 5(b). Scattering measurements on individual nanoclusters demonstrate polarization independence for the 3D nanoclusters, compared with the planar assemblies. The subnanometer interparticle gaps in the nanoclusters were calculated to enhance the magnitude of the incident electric field by 1000-fold. Nanostructures of face- or body-centered cubic micrometer-sized assemblies were assembled using DNA functionalized silver nanospheres [85]. Micro-spectrophotometry on the assemblies combined with simulations showed that the assemblies have epsilon-near-zero values at visible wavelengths.

The impressive advantages of a DNA mediated assembly of plasmonic nanostructures is the ability to program and direct the nanoscopic symmetry of the assemblies with nanometer resolution, giving rise to the unique metamaterial properties, as demonstrated by the striking examples illustrated above. Yet, for the foreseeable future, scaling these assemblies from individual nanostructure studies to macroscopic devices is constrained by availability and cost of the DNA as well as temperature and drying sensitivities. Even with these surmountable constraints, the approach is an exciting step toward the realization of optical metamaterial devices.

In contrast with DNA mediated assemblies, nanometer-sized scaffolds can be produced in bulk quantities and govern the local symmetry of the metamolecules yielding the desired optical properties.

Silver nanospheres functionalized with biotin-terminated polyethylene glycol ligands are randomly attached to streptavidin functionalized polystyrene spheres in suspension forming a metamolecule [Fig. 6(a)] [86]. The approach yields high throughputs with concentrations on the order of ${10}^{10}$ metamolecules per milliliter, as demonstrated in the photograph in Fig. 6(a) (lower left). Transmission electron microscopy images show the distribution of the nanospheres on the polystyrene spheres constituting the metamolecules [Fig. 6(a), right]. Scattering spectra from individual metamolecules give rise to a magnetic mode with an isotropic response near 700 nm. Alternatively, gold nanospheres can be grown in situ on polystyrene spheres, forming the metamolecules with electric and magnetic resonances at visible wavelengths [87]. A motivating result from these metamolecules is the observation of magnetic responses at visible wavelengths, given the isotropic organization of the plasmonic nanospheres on the scaffolds.

 

Fig. 6. (a) (top left) Schematic of the protein assembly of the plasmonic nanoclusters. (bottom left) Vials containing silver nanospheres coated with biotin (left), polystyrene nanospheres coated with streptavidin (center), and plasmonic nanocluster (right). (right side) Transmission electron microscopy images of the nanoclusters. Reproduced with permission from Sheikholeslami et al. [86]. (b) Schematic for the directed assembly of a 3D plasmonic nanocluster with icosahedral symmetry using genetically engineered viruses as scaffolds. (Left) Suspension of nanocluster backlight using white light. (Right) Representative nanocluster assemblies imaged with transmission electron microscopy. Reproduced with permission from Fontana et al. [88].

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Genetically engineered viruses can be used as scaffolds to control not only the nanoscopic symmetry of the nanospheres constituting the metamolecules, as shown with DNA-mediated assemblies, but are capable of high throughput [88,89]. Figure 6(b) shows an image of a suspension of metamolecules containing ${10}^{11}$ nanoclusters per milliliter. The metamolecules are composed of an icosahedron cowpea mosaic virus where thiols (cysteine) were genetically engineered on the capsid of the virus at each of the 12 vertices. Gold nanospheres were covalently attached to the programmed thiols on the surface of the virus, thus utilizing the underlying symmetry of the virus to form 3D icosahedral plasmonic nanoclusters with resonance at visible wavelengths. The virus can readily be produced on gram-scales, providing a route to macroscopic quantities of metamolecules. The bulk absorbance was measured from suspensions and is in good agreement with finite-element simulations. Figure 6(b) shows transmission electron microscopy images representing the distribution of the nanocluster produced using this approach.

Nanometer-sized scaffolds offer the opportunity to control not only the interparticle interactions and symmetries but may be produced in large quantities, thus leading to bulk realizations of 3D isotropic metamolecules for interfacial, suspension, and aerosol applications.

B. Phase Separation

Similar to the metasurface assembly approach, phase separation can be applied to create metamolecules. Using flocculation, plasmonic clusters were formed by polystyrene-coated gold nanospheres, phase separated from dimethylformamide suspensions through the addition of water, causing the hydrophobic clusters to flocculate. The approach produced a wide variety of clusters with individual symmetries and responses [90].

4. PERSPECTIVES AND CONCLUSIONS

This mini-review highlighted several key assembly approaches used to address the constraints bottlenecking the realization of optical metamaterial devices. Two viable categories emerged for potential macroscopic optical metamaterials: metasurfaces and metamolecules. By carefully orienting or positioning plasmonic elements on substrates over macroscopic areas, an optical metasurfaces device may enable the specific tailoring of light–matter interactions. Metamolecules are basic elements producing unique optical responses and, when developed in macroscopic quantities, may usher in novel sensing applications at interfaces in suspensions and as aerosols.

The evolution of optical metamaterials from proof-of-principle experiments to practical devices requires control over multilength scales, bridging nanometer-sized elements with macroscopic throughput. Emphasis is placed on self-assembly methods where the nanostructures spontaneously assemble into macroscopic materials, offering a high throughput, low cost fabrication approach to produce optical metamaterial devices. However, self-assembly is merely an observation of how things put themselves together, not a machine in a laboratory that can be optimized by simply turning a dial. Directing or, more importantly, understanding the underlying interparticle forces governing self-assembly, e.g., van der Waals, electrostatic, entropic, will be key in moving the field forward.

Looking forward, we anticipate, through the combination of multidisciplinary assembly approaches, exploiting the advantages of each, hybrid-assembly strategies will be developed, potentially enabling macroscopic metamaterial devices. As an illustrative example, by chemically assembling nanoclusters and ordering them on substrates, patterned using electron beam lithography, bottom-up and top-down approaches can be merged with tremendous potential [14]. Similarly, self-assembled microsphere masks can be combined with lithography to create large area metasurfaces [91–93]. Additionally, by flocculating two different diameter spheres, each functionalized with complementary DNA strands, 3D icosahedral clusters are readily formed [94]. Recent work has also proposed that nanoclusters with tetrahedral symmetries can be formed simply by controlling the sizes of the two spheres in the electrostatic assembled reactions, without the need for a scaffold [95]. Although not a focus here and even with recent progress, the ability to theoretically forecast the self-assembly and optical responses from large collections of plasmonic elements remains critical to the development of optical metamaterial devices [96].

The aim here has been to convey the opportunities and hurdles facing the further refinement of optical metamaterial assemblies for device applications. An important conclusion that this work presented is the merging of different approaches, which may lead to fruitful hybrid assembly strategies. These hybrid assemblies will require a multidisciplinary effort to understand, predict, and govern the forces, thus leading to the nanostructure assemblies and ensuing optical properties.