Metamaterials that derive their properties from the subwavelength structure of meta-atoms offer unprecedented flexibility for manipulating light-matter interaction. To promote the development of practical metadevices, considerable efforts have been made to achieve photonic metamaterials that exhibit an active response under external stimuli. The primary strategy is to incorporate active materials that possess variable refractive index into the design of metamaterials. Among the various active materials, phase-change materials have recently attracted particular attention due to their variation of material properties in a broad frequency band. On the other hand, as structures and geometric arrangement of building blocks (i.e., artificial atoms or meta-atoms) determine the properties of metamaterials, reconfigurable metamaterials based on mechanical tuning have also been intensively studied. Unlike most reported mechanically tunable metamaterials based on elastic and electromagnetic forces, directed self-assembly (DSA) of nanoparticles opens a new path for low-cost, large-area reconfigurable photonic systems. In this chapter, we present our findings on active photonics metadevices based on phase transition of vanadium dioxide (VO2) and reconfigurable nanowire assemblies. Our work shows the potential of the proposed metamaterial systems for applications ranging from electro-optical information process, storage, and display to energy-efficient smart windows.

We report the effect of topographical features on gold nanowire assemblies in a vertically applied AC electric field. Nanowires 300 nm in diameter ×2.5 μm long, and coated with ∼30 nm silica shell, were assembled in aqueous solution between top and bottom electrodes, where the bottom electrode was patterned with cylindrical dielectric posts. Assemblies were monitored in real time using optical microscopy. Dielectrophoretic and electrohydrodynamic forces were manipulated through frequency and voltage variation, organizing nanowires parallel to the field lines, i.e., standing perpendicular to the substrate surface. Field gradients around the posts were simulated and assembly behavior was experimentally evaluated as a function of patterned feature diameter and spacing. The electric field gradient was highest around these topographic features, which resulted in accumulation of vertically oriented nanowires around the post perimeters when dielectrophoresis dominated (high AC frequency) or between the posts when electrohydrodynamics dominated (low AC frequency). This general type of reconfigurable assembly, coupled with judicious choice of nanowire and post materials/dimensions, could ultimately enable new types of optical materials capable of switching between two functional states by changing the applied field conditions.

Ordered and chaotic superlattices have been identified in Nature that give rise to a variety of colours reflected by the skin of various organisms. In particular, organisms such as silvery fish possess superlattices that reflect a broad range of light from the visible to the UV. Such superlattices have previously been identified as 'chaotic', but we propose that apparent 'chaotic' natural structures, which have been previously modelled as completely random structures, should have an underlying fractal geometry. Fractal geometry, often described as the geometry of Nature, can be used to mimic structures found in Nature, but deterministic fractals produce structures that are too 'perfect' to appear natural. Introducing variability into fractals produces structures that appear more natural. We suggest that the 'chaotic' (purely random) superlattices identified in Nature are more accurately modelled by multi-generator fractals. Furthermore, we introduce fractal random Cantor bars as a candidate for generating both ordered and 'chaotic' superlattices, such as the ones found in silvery fish. A genetic algorithm is used to evolve optimal fractal random Cantor bars with multiple generators targeting several desired optical functions in the mid-infrared and the near-infrared. We present optimized superlattices demonstrating broadband reflection as well as single and multiple pass bands in the near-infrared regime.

We report an all-dielectric lossless optical mirror for the realization of controllable reflection phase based on an array of isolated dielectric nanoresonators. This dielectric mirror is comprised of a cross-shaped amorphous silicon nanoresonator array that has been designed to achieve a 99.8% reflectivity and zero reflection phase at the wavelength of 0.99 μm. The measured results from the fabricated sample match the theoretical predictions with 99.5% reflectivity and near-zero degree reflection phase at 1 μm, which is very close to the targeted wavelength. This concept and approach pave the way for synthesizing lossless artificial reflecting electromagnetic boundaries with arbitrary phase response and hold great promise in applications ranging from nanocavities to nanowaveguides and nanoantennas.

Ordered two-dimensional (2D) lattices were formed by assembling silica-coated solid and segmented Au nanowires between coplanar electrodes using alternating current (ac) electric fields. Dielectrophoretic forces from the ac field concentrated wires between the electrodes, with their long axis aligned parallel to the field lines. After reaching a sufficient particle density, field-induced dipolar interactions resulted in the assembly of dense 2D lattices that spanned the electrodes, a distance of at least ten wire lengths. The ends of neighboring Au wires or segments overlapped a fraction of their length to form lattice structures with a "running bond" brickwork-like pattern. The observed lattice structures were tunable in three distinct ways: (1) particle segmentation pattern, which fixed the lattice periodicity for a given field condition; (2) ac frequency, which varied lattice periodicity in real time; and (3) switching the field on/off, which converted between lattice and smectic particle organizations. Electric field simulations were performed to understand how the observed lattice periodicity depends on the assembly conditions and particle segmentation. Directed self-assembly of well-ordered 2D metallic nanowire lattices that can be designed by Au striping pattern and reconfigured by changes in field conditions could enable new types of switchable optical or electronic devices.

In this paper, we investigate an electric-field assisted assembly approach to create dense arrays of contact hole patterns with complex feature geometries. This hybrid strategy uses a spatially varying dielectrophoretic (DEP) force created by lithographically defined guiding features to assemble dense arrays of nanoparticles within the features, thereby replicating features within the starting pattern. For close-packed particle arrays, the half- and full-pitch of the contact hole array is defined by the starting nanoparticle core and shell diameter.

We present the design, fabrication, and characterization of broadband and wide-angle metasurface-based plasmonic waveplates. Such nanostructured waveplates are formed by an array of rectangular gold nanopatches on a dielectric spacer backed by a solid gold layer. By engineering the anisotropic resonant response of the nanopatch array and tailoring the light interference in the dielectric spacer, highly efficient polarization transformation can be achieved over a broad wavelength range with a wide field-of-view. As a proof-of-concept example, a half-wave plate at the near-infrared regime is demonstrated, showing a polarization conversion ratio and efficiency both higher than 92% within a wavelength range spanning 640 nm to 1290 nm.

A novel nature-inspired technique is introduced and employed to synthesize fractal random superlattices with custom mirror and multispectral filter properties in the visible spectrum. Fractal random Cantor bars are utilized as a representative model of superlattice structures found in nature. An important benefit of the design methodology is a reduction in the number of parameters required for optimization by a genetic algorithm (GA).

Quasi two-dimensional metasurfaces composed of subwavelength nanoresonator arrays can dramatically alter the properties of light in an ultra-thin planar geometry, enabling new optical functions such as anomalous reflection and refraction, polarization filtering, and wavefront modulation. However, previous metasurface-based nanostructures suffer from low efficiency, narrow bandwidth and/or limited field-of-view due to their operation near the plasmonic resonance. Here we demonstrate plasmonic metasurface-based nanostructures for high-efficiency, angle-insensitive polarization transformation over a broad octave-spanning bandwidth. The structures are realized by optimizing the anisotropic response of an array of strongly coupled nanorod resonators to tailor the interference of light at the subwavelength scale. Nanofabricated reflective half-wave and quarter-wave plates designed using this approach have measured polarization conversion ratios and reflection magnitudes greater than 92% over a broad wavelength range from 640 to 1290 nm and a wide field-of-view up to ± 40°. This work outlines a versatile strategy to create metasurface-based photonics with diverse optical functionalities.

Nanostructured optical coatings with tailored spectral absorption properties are of interest for a wide range of applications such as spectroscopy, emissivity control, and solar energy harvesting. Optical metamaterial absorbers have been demonstrated with a variety of customized single band, multiple band, polarization, and angular configurations. However, metamaterials that provide near unity absorptivity with super-octave bandwidth over a specified optical wavelength range have not yet been demonstrated experimentally. Here, we show a broadband, polarization-insensitive metamaterial with greater than 98% measured average absorptivity that is maintained over a wide ± 45° field-of-view for mid-infrared wavelengths between 1.77 and 4.81 μm. The nearly ideal absorption is realized by using a genetic algorithm to identify the geometry of a single-layer metal nanostructure array that excites multiple overlapping electric resonances with high optical loss across greater than an octave bandwidth. The response is optimized by substituting palladium for gold to increase the infrared metallic loss and by introducing a dielectric superstrate to suppress reflection over the entire band. This demonstration advances the state-of-the-art in high-performance broadband metamaterial absorbers that can be reliably fabricated using a single patterned layer of metal nanostructures.

In this paper, we demonstrate an ultra-thin, low-loss optical metamaterial filter with high transmission and near constant group delay across a broad pass-band from 3.0 to 3.5μ m. Deep-subwavelength air hole inclusions positioned at the corners of a conventional metallodiectric fishnet were used engineer the dispersive properties of the structure to have an impedance match to free space over the pass-band. The optical properties of the metamaterial filter were verified by experimentally fabricating and characterizing the optimized free-standing nano-notched fishnet. The measured experimental results agreed well with the simulated response, showing a high transmission band over the targeted wavelength band.

Infrared (IR) absorbers have been previously studied for use in narrow-band, multi-band, and broadband applications. Here, we investigate through synthesis and experiment a broadband absorber structure based on an electromagnetic band-gap type of metasurface. A genetic algorithm is successfully employed to optimize a structure with a single patterned Pd screen to exhibit high absorptivity over more than an octave bandwidth across the mid-IR regime.

Metamaterials have the potential to create optical devices with new and diverse functionalities based on novel wave phenomena. Most practical optical systems require that the device properties be tightly controlled over a broad wavelength range. However, optical metamaterials are inherently dispersive, which limits operational bandwidths and leads to high absorption losses. Here, we show that deep-subwavelength inclusions can controllably tailor the dispersive properties of an established metamaterial structure thereby producing a broadband low-loss optical device with a desired response. We experimentally verify this by optimizing an array of nano-notch inclusions, which perturb the mode patterns and strength of the primary and secondary fishnet nanostructure resonances and give an optically thin mid-wave-infrared filter with a broad transmissive pass-band and near-constant group delay. This work outlines a powerful new strategy for realizing a wide range of broadband optical devices that exploit the unique properties of metamaterials.

In this work, we present the design, numerical experiments, and analysis of a plasmonic metamaterial thin film based on subwavelength nano-notch loaded modified fishnet structures. The resulting device offers a simultaneous bandpass filtering functionality with a broad enhanced optical transmission window and a gapless negative-zero-positive index transition to enable polarization-independent passive beam-steering. This unique characteristic is made possible by the introduced subwavelength nano-notches, which provide fine tuning and hybridization of the external and internal surface plasmon polariton modes. This allows tailoring of the dispersive properties of the plasmonic metamaterial for broadband operation. Specifically, a multilayer nanostructured modified fishnet with feature sizes accessible by modern nanofabrication techniques is presented, exhibiting a broad passband at the mid-infrared wavelengths from 3.0 to 3.7 µm and stopbands elsewhere in the 2.5 ~4.5 µm window. The transmittance normalized to area is around 3 dB within the broad 20% bandwidth of the passband. Additionally, the effective index undergoes a smooth transition from negative unity through zero to positive unity with low loss within the passband. The physical mechanism and the angular dispersion of the metamaterial are analyzed in detail. Finally, full-wave simulations of a prism formed from this metamaterial are performed to demonstrate that the proposed structure achieves simultaneous polarization-insensitive passive beam-steering and filtering functionalities.

In this paper, we show that the optical metamaterial dispersion can be controllably tailored to produce an ultrathin, broadband filter with a flat transmission band and near-constant in-band group delay in the 3 ~ 3.5 μm midwave infrared window. It is achieved by strategically incorporating deep-subwavelength nano-notches into a fishnet structure. The deep-subwavelength inclusions, which have critical dimensions of less than 1/15 ± 1/100 of the shortest design wavelength, can perturb the mode patterns and strength of the unmodified fishnet nanostructure resonances, thereby shaping the dispersion in the effective medium properties to meet the specific design metrics. The designed free-standing optical metamaterial filter was fabricated and characterized, showing strong agreement between measured and simulated results. This powerful design approach will dramatically expand the opportunities to create new and practical broadband metamaterial-enabled optical devices and components.

We demonstrate a flexible thin film zero refractive index optical metamaterial with matched impedance to free space and low absorption loss at 1.55 mu m. The metallo-dielectric multilayer structure with fishnet geometry was optimized by a genetic algorithm. The fabrication process and characterization approach are described. The experiment results agree well with the theoretical predictions, showing an effective index of n(eff) = 0.072 + 0.51i and an impedance of Z(eff)/Z(0) = 1.009 - 0.021i.

In this paper, we experimentally demonstrate an artificial perfect magnetic mirror constructed from an array of cross-shaped dielectric resonators in the optical range. The TM01δ mode of the cross-shaped dielectric resonator was exploited to provide a near-unity reflectivity and a near-zero reflection phase, mimicing a theoretical perfect magnetic conductor. A sample of the artificial mirror was fabricated using standard e-beam lithography and was further characterized, showing a strong agreement with simulated predictions.

Electric-field-assisted deterministic assembly is used to position individual nanowires in high density arrays on silicon integrated circuits. Forces induced on the solution-suspended nanowires by a spatially varying, nonuniform electric field determines the position of each wire with respect to lithographic features on the circuit. Two electrode structures used to create the spatial variations in field strength are compared experimentally and theoretically. This technique is applied to fabricate bioprobe-coated nanowire resonator device arrays for chip-based sensing applications.