Clara Rivero-Baleine

Adaptive phase change metamaterials for infrared aperture control

This paper discusses the use of chalcogenide phase change materials to create tunable metamaterials as potential candidates for application to adaptive coded aperture control in the infrared. Phase change materials exhibit large and reversible changes in optical properties (Δn, Δk) when switched between the amorphous and crystalline phases. Thermally-induced phase transitions from the insulating amorphous to the conductive crystalline state can be controlled through external means, facilitating the design of reconfigurable metamaterial devices that operate with ultrafast response times. In this work, robust global stochastic optimization algorithms were combined with full-wave electromagnetic simulation tools to design periodic subwavelength chalcogenide nanostructured arrays to meet the specified device performance goals in each phase. The measured optical properties (n, k) of deposited chalcogenide thin films and nanofabrication constraints were incorporated into the optimization algorithm to guarantee that the designed nanostructures could be manufactured. By choosing the appropriate cost functions, adaptive metamaterials were designed to switch between transmissive and reflective, transmissive and absorptive, and reflective and absorptive states. These design demonstrations represent a significant step forward in the development of adaptive infrared metamaterials.

Evidence of spatially selective refractive index modification in 15GeSe 2 -45As 2 Se 3 -40PbSe glass ceramic through correlation of structure and optical property measurements for GRIN applications

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.

Ultralow Dispersion Multicomponent Thin‐Film Chalcogenide Glass for Broadband Gradient‐Index Optics

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.

Processing and properties of arsenic trisulfide chalcogenide glasses for direct laser writing of 3D microstructures

Arsenic trisulfide (As2S3) is a transparent material from ~620 nm to 11 μm with direct applications in sensors, photonic waveguides, and acousto-optics. As2S3 may be thermally deposited to form glassy films of molecular chalcogenide (ChG) clusters. It has been shown that linear and multi-photon exposure can be used to photo-pattern thermally deposited As2S3. Photo-exposure cross-links the film into a network solid. Treating the photo-patterned material with a polarsolvent removes the unexposed material leaving behind a structure that is a negative-tone replica of the photo-pattern. In this work, nano-structure arrays were photo-patterned in As2S3 films by multi-photon direct laser writing (DLW) and the resulting structure, morphology, and chemical composition were characterized and correlated with the conditions of the thermal deposition, patterned irradiation, and etch processing. Raman spectroscopy was used to characterize the chemical structure of the unexposed and photo-exposed material, and near infrared ellipsometry was used to measure the refractive index. Physical characterization including structure size and surface adhesion of nano-scale features is related to the processing conditions.

Evolution of the linear and nonlinear optical properties of femtosecond laser exposed fused silica

Three fused silica samples possessing different impurity levels and exposed to a near infrared femtosecond laser are investigated. The laser-induced defects are identified from absorption, luminescence, and Raman spectroscopy. Their linear and nonlinear optical properties are measured from Kramers-Kronig calculations and third-harmonic generation microscopy experiments. No conclusive correlation between the change in the optical properties, the initial impurity levels, and the photoinduced structures could be established based on the results obtained in this study. In addition, several hypotheses (densification and color center formation) have been rejected to explain why the linear and nonlinear optical properties of the photoinduced structures follow a contradicting evolution. This phenomenon is attributed to an experimental artifact on the measurement of the third-order susceptibility due to scattering of the photoinduced structures.

Fabrication and characterization of micro-structures created by direct laser writing in multi-layered chalcogenide glasses

Arsenic trisulfide (As2S3) is a chalcogenide (ChG) material with excellent infrared (IR) transparency (620 nm to 11 μm), low phonon energies, and large nonlinear refractive indices. These properties directly relate to commercial and industrial applications including sensors, photonic waveguides, and acousto-optics. Multi-photon exposure can be used to photopattern thermally deposited As2S3 ChG glassy films of molecular clusters. Immersing the photo-patterned cross-linked material into a polar-solvent removes the unexposed material leaving behind a structure that is a negative-tone replica of the photo-pattern. Nano-structure arrays that were photo-patterned in single-layered As2S3 films through multi-photon direct laser writing (DLW) resulted in the production of nano-beads as a consequence of a standing wave effect. To overcome this effect, an anti-reflective (AR) layer of arsenic triselenide (As2Se3) was thermally deposited between the silicon substrate and the As2S3 layer, creating a multi-layered film. The chemical composition of the unexposed and photo-exposed multi-layered film was examined through Raman spectroscopy. Nano-structure arrays were photopatterned in the multi-layered film and the resulting structure, morphology, and chemical composition were characterized, compared to results from the single-layered film, and correlated with the conditions of the thermal deposition, patterned irradiation, and etch processing.

Reconfigurable near-IR metasurface based on Ge 2 Sb 2 Te 5 phase-change material

A reconfigurable metasurface made of Ge2Sb2Te5 phase-change material was experimentally demonstrated in the 1.55 μm wavelength range. A nanostructured Ge2Sb2Te5 film on fused silica substrate was optimized 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.

Ultra-thin, reconfigurable meta-optics using optical phase change materials (Conference Presentation)

The dramatic optical property change of optical phase change materials (O-PCMs) between their amorphous and crystalline states potentially allows the realization of reconfigurable photonic devices with enhanced optical functionalities and low power consumption, such as reconfigurable optical components, optical switches and routers, and photonic memories. Conventional O-PCMs exhibit considerable optical losses, limiting their optical performance as well as application space. In this talk, we present the development of a new group of O-PCMs and their implementations in novel meta-optic devices. Ge-Sb-Se-Te (GSST), obtained by partially substituting Te with Se in traditional GST alloys, feature unprecedented broadband optical transparency covering the telecommunication bands to the LWIR. A drastic refractive index change between the amorphous and crystalline states of GSST is realized and the transition is non-volatile and reversible. Optical metasurfaces consist of optically-thin, subwavelength meta-atom arrays which allow arbitrary manipulation of the wavefront of light. Capitalizing on the dramatically-enhanced optical performance of GSST, transparent and ultra-thin reconfigurable meta-optics in mid-infrared are demonstrated. In one example, GSST-based all-dielectric nano-antennae are used as the fundamental building blocks for meta-optic components. Tunable and switchable metasurface devices are developed, taking advantage of the materials phase changing properties.

Optical and crystal growth studies of ZnO-Bi2O3-B2O3 glass

Transparent ZnO–Bi2O3–B2O3 (ZBB) glasses were prepared using the melt quench technique. Various compositions of the glass containing stoichiometric ratios of Zn/Bi/B as well as some including As2O3 for redox control and LiNO3 for use as nucleation species, were studied. ZBB glass-ceramics containing nanocrystallites have a potential for use as low-cost UV-MWIR optical devices such as microlenses, waveguides, and photonic crystals. Our goal was to exploit crystal growth in the ZBB systems by heat treatment in order to obtain transparent glass-ceramics that contain homogenous volume crystallization. Thermal behavior was studied using differential scanning calorimeter (DSC) measurements. Physical and optical characterizations included Raman spectroscopy to identify molecular connectivity, energy-dispersive X-ray spectroscopy (EDX) for elemental analysis, VIS/NIR transmission and reflection spectroscopy for optical bandgap and IR transmissivity, X-ray diffraction (XRD) to determine crystal phase, and transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) to quantify size, number density, and identification of nanometer sized secondary phases. Heat treatments were used to nucleate and grow BiB3O6 and Bi2ZnB2O7 nanocrystals in ZBB. We explored new compositions within the ZBB system and heat treatment techniques to assess the transformation of the amorphous glass phase into the crystalline phase. In-situ XRD and TEM imaging was employed to correlate nucleation temperature, heat treatment temperature, and heat treatment duration with induced crystal phase. BiB3O6, Bi2ZnB2O7, and ZnO was found to grow on the surface of some compositions. Compositions and heat treatment procedures were developed to facilitate volume crystallization and reduce unwanted surface crystallization.

Design of broadband anti-reflective metasurfaces based on an effective medium approach

In this paper we show how to systematically design anti-reflective metasurfaces for the mid-infrared wavelength range. To do so, we have utilized a multilayer arrangement involving a judiciously nano-perforated surface, having air holes, arranged in a hexagonal fashion. By exploiting an effective medium approach, we optimized the dimensions of the surface features in our design. Here, we report a broadband reflectivity 3.5 − 5.5 μm that is below 10% over a broad range of incident angles 00 ≤ θ𝑖 ≤ 700 , irrespective of the incident polarization (TE, TM). Our experimental results are in excellent agreement with full-wave finite element simulations. This systematic approach can be used to design a wide variety of patterned metasurfaces, capable of controlling the phase of the incident optical field.