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Vol. 54, No. 31 / November 1 2015 / Applied Optics
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Toward high throughput optical metamaterial assemblies JAKE FONTANA*
AND
BANAHALLI R. RATNA
Naval Research Laboratory, 4555 Overlook Ave S.W., Washington DC 20375, USA *Corresponding author:
[email protected] Received 7 May 2015; revised 1 July 2015; accepted 7 July 2015; posted 7 July 2015 (Doc. ID 239770); published 3 August 2015
Optical metamaterials have unique engineered optical properties. These properties arise from the careful organization of plasmonic elements. Transitioning these properties from laboratory experiments to functional materials may lead to disruptive technologies for controlling light. A significant issue impeding the realization of optical metamaterial devices is the need for robust and efficient assembly strategies to govern the order of the nanometersized elements while enabling macroscopic throughput. This mini-review critically highlights recent approaches and challenges in creating these artificial materials. As the ability to assemble optical metamaterials improves, new unforeseen opportunities may arise for revolutionary optical devices. © 2015 Optical Society of America OCIS codes: (160.3918) Metamaterials; (250.5403) Plasmonics; (220.0220) Optical design and fabrication; (220.3740) Lithography; (220.4241) Nanostructure fabrication. http://dx.doi.org/10.1364/AO.54.000F61
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 1559-128X/15/310F61-09$15/0$15.00 © 2015 Optical Society of America
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: 1. Metasurfaces, which are macroscopic in area and typically much thinner than the wavelength of light because thicker structures are difficult to efficiently assemble and intrinsically lossy. 2. Metamolecules, which are individual elements with inherent metamaterial properties and can be macroscopic in quantity. 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
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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 minireview. 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 mm × 2 mm array of paired gold nanorod elements (220 nm × 780 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]. In focused ion beam lithography, a focused beam of ions is used to ablate and pattern substrates usually with tens of
Fig. 1. Scanning electron microscopy images of a 2 × 2 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 μm × 16 μm 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].
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 μm × 16 μm 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
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].
Research Article 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. The resulting metasurfaces in Fig. 2(b) correspond to 105 unit cells covering an area up to approximated 75 cm2 . The throughput is nearly 108 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. Homeotropicaligned 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].
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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 Fe3 O4 and FePt nanoparticles with various symmetries have been constructed over large areas [Fig. 3(a)] [63]. These surfaces were created by dropcasting the binary nanoparticles in a hexane suspension onto the surface of diethylene glycol in a Teflon trough. The nanoparticle surfaces were then placed onto SiO2 -Si substrates.
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,
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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Additional experiments have been carried out to characterize the plasmonic properties of surfaces using microspectrophotometry for various nanoparticle composition and order [64]. 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.
Research Article 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 (
