Experimental results and models for dielectrophoretic assembly of segmented metal-dielectric particles are reported. Multi-component particles were fabricated by templated electrodeposition, silica coating, and selective etching to yield Au and solvent-filled segments in desired patterns. Both single-component particles and segmented particles that contained alternating Au and etched regions in five different patterns were produced. Frequency-dependent dielectrophoretic assembly of each particle type was observed using optical microscopy. At each frequency, a given particle type exhibited either positive or negative dielectrophoresis, accumulating at regions of highest or lowest field gradient, respectively. Crossover frequencies between positive and negative dielectrophoresis differed with segmentation pattern. Near the crossover frequency, two of the five segmented particle types exhibited a 90 degrees rotation, reorienting such that their long axes were perpendicular to the direction of the applied field. A model treating the net particle-field interaction as a superposition of their metal and dielectric components was developed to describe the assembly behavior and was able to capture both the frequency-dependent position and orientation behaviors of the segmented particles. This superposition model was the simplest method that captured all experimental observations for realistic conductance values, outperforming models based on mixing rules for composite particles. Finally, we took advantage of our understanding of segmentation-dependent differences in dielectrophoretic response to separately control particle subpopulations in binary mixtures of multicomponent particles having different segmentation patterns. Understanding how the organization of distinct materials within multicomponent particles impacts their assembly behavior in an applied field is an important step towards particle-based reconfigurable optical devices such as responsive metamaterials.

The size and weight of conventional imaging systems is defined by costly non-planar lenses and the complex lens assemblies required to minimize optical aberrations. The ability to engineer gradient refractive index (GRIN) optics has the potential to overcome constraints of traditional homogeneous lenses by reducing the number of components in optical systems. Here, an innovative strategy to realize this goal based on monolithic GRIN media created in Ge-As-Se-Pb chalcogenide infrared nanocomposites is presented. A gradient heat treatment to spatially modulate the volume fraction of high refractive index Pb-rich nanocrystals within a glass matrix is utilized, providing a GRIN profile while maintaining an optical transparency. A first-ever correlation of material chemistry and microstructure, processing protocol, and optical property modification resulting in a prototype GRIN structure is presented. The integrated approach and mechanistic understanding illustrated by this versatile modification paradigm provides a platform for new optical functionalities in next-generation imaging applications.

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.

Optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. The University of Central Florida's Glass Processing and Characterization Laboratory, together with our collaborators, have been evaluating compositional design and processing protocols for both bulk and film strategies employing multicomponent chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. We review progress in forming ChG-based gradient refractive index (GRIN) materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and metalens structures realized through multiphoton lithography. We discussed current design and metrology tools that lend critical information to material design efforts to realize next-generation IR GRIN media for bulk or film applications. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

Infrared (IR) glass-ceramics (GCs) hold the potential to dramatically expand the range of optical material solutions available for use in bulk and planar optical systems in the IR. Current material solutions are limited to single- or polycrystalline materials and traditional IR-transparent optical glasses. GCs that can be processed with spatial control and extent of induced crystallization present the opportunity to realize an effective refractive index variation, enabling arbitrary gradient refractive index elements with tailored optical function. This work discusses the role of the parent glass composition and morphology on nanocrystal phase formation in a multicomponent chalcogenide glass. Through a two-step heat treatment protocol, a Ge-As-Pb-Se glass is converted to an optical nanocomposite where the type, volume fraction, and refractive index of the precipitated crystalline phase(s) define the resulting nanocomposite's optical properties. This modification results in a giant variation in infrared Abbe number, the magnitude of which can be tuned with control of crystal phase formation. The impact of these attributes on the GCs' refractive index, transmission, dispersion, and thermo-optic coefficient is discussed. A systematic protocol for engineering homogeneous or gradient changes in optical function is presented and validated through experimental demonstration employing this understanding.

Melt size-dependent physical property variation is examined in a multicomponent GeSe2-As2Se3-PbSe chalcogenide glass developed for gradient refractive index applications. The impact of melting conditions on small (40 g) prototype laboratory-scale melts extended to commercially-relevant melt sizes (1.325 kg) have been studied and the role of thermal history variation on physical and optical property evolution in parent glass, the glass' crystallization behavior and postheat-treated glass ceramics, is quantified. As-melted glass morphology, optical homogeneity and heat treatment-induced microstructure following a fixed, two-step nucleation and growth protocol exhibit marked variation with melt size. These attributes are shown to impact crystallization behavior (growth rates, resulting crystalline phase formation) and induced effective refractive index change, n(eff), in the resulting optical nanocomposite. The magnitude of these changes is discussed based on thermal history related melt conditions.

New missions and technical systems require lightweight, high performance, wide-field of view (W-FOV) infrared (IR) imaging systems. Traditionally, multilayer antireflective (AR) coatings are utilized with optical components to facilitate these high performance demands. However, fundamental limits in these multilayer AR coatings currently prevent the extremely high broadband transmission (<95%) and W-FOV (<100°) requirements of next generation IR imaging systems from being realized. Furthermore, there is restricted availability of suitable thin film IR materials with high index contract used in these AR coatings, preventing tuning and broad application of the technology. By contrast, surface-engineered gradient reflective index (GRIN) films afford a substrate and application independent means of generating and tuning transmissive and W-FOV properties in optical components. Herein, we present efforts toward designing devices with highly AR properties from GRIN surfaces. GRIN surfaces are generated through lithographic patterning of optical surfaces and dry etching processes to generate dense arrays of air holes. The density of these air holes offer a mean to tune the index of refraction of the optical surface, providing highly AR properties in a tunable optical range. State-of-the-art laser writing technology enables us to achieve features of 500 nm and below with high throughput (<1.25 min write time per 1 cm2 patterned). Control of depth etch through standard etching processes (Al2O3 hard mask, deep etch using Bosch-process type or other dry etch) allows for fully tunable GRIN films.

Novel optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. UCF’s Glass Processing and Characterization Laboratory (GPCL) with our collaborators have been evaluating compositional design and processing protocols for both bulk and film strategies employing multi-component chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. This paper reviews progress in forming ChG-based GRIN materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and meta-lens structures realized through multiphoton lithography (MPL).

A novel photothermal process to spatially modulate the concentration of sub‐wavelength, high‐index nanocrystals in a multicomponent Ge‐As‐Pb‐Se chalcogenide glass thin film resulting in an optically functional infrared grating is demonstrated. The process results in the formation of an optical nanocomposite possessing ultralow dispersion over unprecedented bandwidth. The spatially tailored index and dispersion modification enables creation of arbitrary refractive index gradients. Sub‐bandgap laser exposure generates a Pb‐rich amorphous phase transforming on heat treatment to high‐index crystal phases. Spatially varying nanocrystal density is controlled by laser dose and is correlated to index change, yielding local index modification to ≈+0.1 in the mid‐infrared.

This work reports the processing and properties of a new chalcogenide glass film that can be photo-patterned by multiphoton lithography (MPL) with enhanced post-fabrication stability. Thermally evaporated germanium-doped arsenic selenide [Ge5(As2Se3)95] thin films were photo-patterned using the output of a mode-locked titanium:sapphire laser. The morphology, chemical structure, and optical properties of the material were studied before and after photo-patterning and compared for their long-term aging behavior and stability to previously investigated arsenic trisulfide (As2S3) films fabricated using similar MPL conditions. Relative to As2S3, thermally deposited Ge5(As2Se3)95 is found to offer higher photo-sensitivity and greater chemical stability after photo-patterning, as evidenced by lack of age-induced crystallization and reduced feature degradation over a four year aging period. These findings demonstrate the suitability of a new photo-patternable material for the creation of robust, long-lived functional infrared anti-reflective coatings and meta-optics.

A reconfigurable metasurface made of Ge2Sb2Te5 phase-change material was experimentally demonstrated in the 1.55 mu m wavelength range. A nanostructured Ge2Sb2Te5 film on fused silica substrate was optimiz.ed to switch from highly transmissive (80%) to highly absorptive (76%) modes with a 7:1 contrast ratio in transmission independent of polarization, when thermally transformed from the amorphous to crystalline state. The metasurface was designed using a genetic algorithm optimizer linked with an efficient full-wave electromagnetic solver. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

An electric current triggered multifunctional vanadium dioxide (VO 2 ) integrated photonic metamaterial is presented. In our metamaterial design, the nanoengineered topologically continuous metallic structure simultaneously supports the optical and electrical functionalities. Moreover, acting as part of the resonating structure, the VO 2 thin film enables the tunable nature of the device. By presenting a series of proof-of-concept studies, we demonstrate the proposed hybrid metamaterial as a new platform for multifunction electro-optic control including reflectance switching, a rewritable memory process and manageable localized camouflage. The design methodology introduced here provides a universal approach to creating self-sufficient and highly versatile nanophotonic systems.

Nanosensor arrays have recently received significant attention due to their utility in a wide range of applications, including gas sensing, fuel cells, internet of things, and portable health monitoring systems. Less attention has been given to the production of sensor platforms in the μW range for ultra-low power applications. Here, we discuss how to scale the nanosensor energy demand by developing a process for integration of nanowire sensing arrays on a monolithic CMOS chip. This work demonstrates an off-chip nanowire fabrication method; subsequently nanowires link to a fused SiO2 substrate using electric-field assisted directed assembly. The nanowire resistances shown in this work have the highest resistance uniformity reported to date of 18%, which enables a practical roadmap towards the coupling of nanosensors to CMOS circuits and signal processing systems. The article also presents the utility of optimizing annealing conditions of the off-chip metal-oxides prior to CMOS integration to avoid limitations of thermal budget and process incompatibility. In the context of the platform demonstrated here, directed assembly is a powerful tool that can realize highly uniform, cross-reactive arrays of different types of metal-oxide nanosensors suited for gas discrimination and signal processing systems.

Boron-doped silicon nanowires (SiNWs) grown by the vapor-liquid-solid growth mechanism using silicon tetrachloride (SiCl4) as the silicon precursor and trimethylboron (TMB) as the boron source were studied to understand the axial and radial doping uniformity. TMB-doped SiNWs with diameters up to 400 nm and lengths > 7.5 μm were integrated into a global back-gated test structure with multiple electrodes for electrical characterization. From gate modulated measurements, the SiNWs were confirmed to be heavily doped p-type. Multiple four point resistivity measurements across a total length of 7.5 μm were taken on as-grown SiNWs. Resistivity, corrected for surface charge, was determined to be 0.01 +/− 0.002 Ω cm along the entire length of the as-grown boron doped SiNWs. This was also observed in the axial direction for etched SiNWs, with corrected resistivity of 0.01 +/− 0.003 Ω cm, therefore confirming the uniform p-type doping of SiNWs using TMB and SiCl4 as precursors.

The unique light-matter interaction in metamaterials, a type of artificial medium in which the geometrical features of subunits dominate their optical responses, have been utilized to achieve exotic material properties that are rare or nonexistent in natural materials. Furthermore, to extend their behaviors, active materials have been introduced into metamaterial systems to advance tunability, switchability and nonlinearity. Nevertheless, practical examples of versatile photonic metamaterials remain exceedingly rare for two main reasons. On the one hand, in sharp contrast to the broad material options available at lower frequencies, it is less common to find active media in the optical regime that can provide pronounced dielectric property changes under external stimuli, such as electric and magnetic fields. Vanadium dioxide (VO2), offering a large refractive index variation over a broad frequency range due to its near room temperature insulator-to-metal transition (IMT), has been favored in recent studies on tunable metamaterials. On the other hand, it turns out that regulating responses of hybrid metamaterials to external forces in an integrated manner is not a straightforward task. Recently, metamaterial-enabled devices (i.e., metadevices) with 'self-sufficient' or 'self-contained' electrical and optical properties have enabled complex functionalities. Here, we present a design methodology along with the associated experimental validation of a VO2 thin film integrated optical metamaterial absorber as a hybrid photonic platform for electrically driven multifunctional control, including reflectance switching, a rewritable memory process and manageable localized camouflage. The nanoengineered topologically continuous metal structure simultaneously supports the optical resonance and electrical functionality that actuates the phase transition in VO2 through the process of Joule heating. This work provides a universal approach to creating self-sufficient and highly-versatile nanophotonic systems.

A detailed study of multiphoton lithography (MPL) in arsenic trisulfide (As2S3) films and the effects on nanoscale morphology, chemical networking, and the appearance of the resulting features by the chemical composition, deposition rate, etch processing, and inclusion of an antireflection (AR) layer of As2Se3 between the substrate and the As2S3 layer is reported. MPL was used to photo-pattern nanostructured arrays in single- and multilayer films. The variation in chemical composition for laser-exposed, UV-exposed, and unexposed films is correlated with the etch response, nanostructure formation, and deposition conditions. Reflection of the focused beam at the substrate back into the film produces standing wave interference that modulates the exposure with distance from the substrate and produces nanobead structures. The interference and the modulation can be controlled by the addition of an AR layer of As2Se3 deposited between the substrate and the As2S3 film. Relative to structures produced in a single- layer As2S3 film having no AR layer, photo-patterning in the multilayer As2S3-on-As2Se3 film yields pillar-shaped structures that are closer to the targeted shape and are narrower (120 versus 320 nm), more uniform, and better adhering to the substrate. Processing methods are demonstrated for fabricating large-area arrays with diffractive optical function. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)

Thermally-induced nucleation and growth of secondary crystalline phases in a parent glass matrix results in the formation of a glass ceramic. Localized, spatial control of the number density and size of the crystal phases formed can yield ‘effective’ properties defined approximately by the local volume fraction of each phase present. With spatial control of crystal phase formation, the resulting optical nanocomposite exhibits gradients in physical properties including gradient refractive index (GRIN) profiles. Micro-structural changes quantified via Raman spectroscopy and X-ray diffraction have been correlated to calculated and measured refractive index modification verifying formation of an effective refractive index, neff, with the formation of nanocrystal phases created through thermal heat treatment in a multi-component chalcogenide glass. These findings have been used to define experimental laser irradiation conditions required to induce the conversion from glass to glass ceramic, verified using simulations to model the thermal profiles needed to substantiate the gradient in nanocrystal formation. Pre-nucleated glass underwent spatially varying nanocrystal growth using bandgap laser heating, where the laser beam’s thermal profile yielded a gradient in both resulting crystal phase formation and refractive index. The changes in the nanocomposite’s micro-Raman signature have been quantified and correlated to crystal phases formed, the material’s index change and the resulting GRIN profile. A flat, three-dimensional (3D) GRIN nanocomposite focusing element created through use of this approach, is demonstrated.

One-dimensional Au nanoparticle arrays encapsulated within freestanding SiO2 nanowires are fabricated by thermal oxidation of Au-coated Si nanowires with controlled diameter and surface modulation. The nanoparticle diameter is determined by the Si nanowire diameter and Au film thickness, while the interparticle spacing is independently controlled by the Si nanowire modulation. The optical absorption of randomly oriented Au nanoparticle arrays exhibits a strong plasmonic response at 550 nm. Scanning transmission electron microscopy (STEM)-electron energy loss spectrum (EELS) of nanoparticle arrays confirmed the same plasmonic response and demonstrated uniform optical properties of the Au nanoparticles. The plasmonic response in the STEM-EELS maps is primarily confined around the vicinity of the nanoparticles. On the other hand, examination of the same nanowires by energy-filtered transmission electron microscopy also revealed significant enhancement in the plasmonic excitation in the regions in between the nanoparticles. This versatile route to synthesize one-dimensional Au nanoparticle arrays with independently tailorable nanoparticle diameter and interparticle spacing opens up opportunities to exploit enhanced design flexibility and cost-effectiveness for future plasmonic devices.