Chuang Qu

Polycrystalline metasurface perfect absorbers fabricated using microsphere photolithography

Microsphere photolithography (MPL) is a practical, cost-effective nanofabrication technique. It uses self-assembled microspheres in contact with the photoresist as microlenses. The microspheres focus incident light to a sub-diffraction limited array of photonic jets in the photoresist. This Letter explores the MPL technique to pattern metal-insulator-metal metasurfaces with near-perfect absorption at mid-wave infrared (MWIR) frequencies. Experimental results are compared to electromagnetic simulations of both the exposure process and the metasurface response. The microsphere self-assembly technique results in a polycrystalline metasurface; however, the metal-insulator-metal structure is shown to be defect tolerant. While the MPL approach imposes geometric constraints on the metasurface design, once understood, the technique can be used to create functional devices. In particular, the ability to tune the resonant wavelength with the exposure dose raises the potential of hierarchical structures.

Wire-Fed Additive Manufacturing of Transparent Glass Parts

This paper presents a new technique for additive manufacturing of transparent glass. In this process, transparent glass is wire-fed into a laser generated melt pool, which solidifies as the work piece is moved relative to a stationary laser beam. The key parameters are identified in terms of their effects on the morphology and transparency of printed walls. The relationship between these parameters is studied experimentally. It is demonstrated that the process parameters strongly affect the morphology and proper selection of the scan speed, feed rate and laser power can produce optimum results. A key advantage of this process relative to powder bed techniques is the ability to form optically transparent parts. The process parameters also determine the transmissivity of the final sample. The transmissivity is measured experimentally for builds with different process parameters.© 2015 ASME

Design and analysis of frequency-selective surface enabled microbolometers

Frequency Selective Surfaces (FSS) are periodic array of sub-wavelength antenna elements. They allow the absorptance and reflectance of a surface to be engineered with respect to wavelength, polarization and angle-of-incidence. This paper applies this technique to microbolometers for uncooled infrared sensing applications. Both narrowband and broadband near perfect absorbing surfaces are synthesized and applied engineer the response of microbolometers. The paper focuses on simple FSS geometries (hexagonal close packed disk arrays) that can be fabricated using conventional lithographic tools for use at thermal infrared wavelengths (feature sizes > 1 μm). The affects of geometry and material selection for this geometry is described in detail. In the microbolometer application, the FSS controls the absorption rather than a conventional Fabry-Perot cavity and this permits an improved thermal design. A coupled full wave electromagnetic/transient thermal model of the entire microbolometer is presented and analyzed using the finite element method. The absence of the cavity also permits more flexibility in the design of the support arms/contacts. This combined modeling permits prediction of the overall device sensitivity, time-constant and the specific detectivity.

Infrared metasurfaces created with off-normal incidence microsphere photolithography

Fabricating metasurfaces over large areas at low costs remains a critical challenge to their practical implementation. This paper reports on the use of microsphere photolithography (MPL) to create infrared metasurfaces by changing the angle-of-incidence of the illumination to steer the photonic jet. The displacement of the photonic jet is shown to scale with the diameter of the microsphere while the exposure dose scales with the square of the microsphere diameter. This process is robust in the presence of local defects in the microsphere lattice. The paper demonstrates patterning split ring resonators and tripole based metasurfaces using MPL, which are fabricated and characterized with FTIR. The combination of bottom-up and top-down approaches in off-normal incidence microsphere photolithography technique provides cost-effective, flexible, and high-throughput fabrication of infrared metasurfaces.

Droplet Growth Dynamics during Atmospheric Condensation on Nanopillar Surfaces

ABSTRACT The Gibbs free energy barrier for heterogeneous nucleation of a condensed droplet on a rough surface changes significantly with changes of humidity content in the condensing environment. The influence of environmental factors (ambient temperature and relative humidity) and substrate characteristics (topology, surface chemistry, and substrate temperature) on atmospheric condensation phenomenon is very important to elucidate the condensed droplet wetting state and condensate harvesting applications. Condensation from the humid air has been reported for plain silicon and fabricated nanopillar surfaces to facilitate condensate harvesting. Droplet growth and size distributions were recorded for 90 min. Spherical droplets condensed on the silicon surfaces and irregular-shaped droplets were observed on the nanopillar surfaces due to the pinning effect of the pillars. The effect of droplet pinning on coalescence events has been described based on the energy balance for the condensed droplets. A mathematical model reveals that certain dimensional combinations (pillar pitch, pillar diameter, and pillar height) of the nanopillar geometry are required to exhibit the pinning mechanism for condensed droplets. Regeneration of droplets was observed at void spaces generated from coalescence events. The growth of individual droplets was tracked over multiple time and length scales, starting from nucleation to get further insight into the direct growth and coalescence mechanisms. Abbreviation: ESEM: Environmental Scanning Electron Microscope; HCP: Hexagonal Closed-Packed; MPL: Microsphere Photolithography; RH: Relative Humidity

Fabrication of infrared broadband polarized emitting metasurfaces using microsphere photolithography

This paper describes the low-cost, scalable fabrication of 2D metasurface LWIR broadband polarized emitter/absorber. A Frequency Selective Surface (FSS) type design consisting of dipole antenna elements is designed for resonance in the 7.5-13 μm band. Frequency-domain Finite Element Method (FEM) is used to optimize the design with ellipsometrically measured properties. The design is synthesized to be broadband by creating a multiple cavities and by hybridizing the dipole modes with phonon resonances in a germanium/silica dielectric which separates metallic elements from a continuous ground plane. While IR metasurfaces can be readily realized using direct-write nanofabrication techniques such as E-Beam Lithography, or Focus-Ion Beam milling, or two-photon lithography, these technologies are cost-prohibitive for large areas. This paper explores the Microsphere Photolithography (MPL) technique to fabricate these devices. MPL uses arrays of self-assembled microspheres as optical elements, with each sphere focusing flood illumination to a sub-wavelength photonic jet in the photoresist. Because the illumination can be controlled over larger scales (several μm resolutions) using a conventional mask, the technique facilitates very low cost hierarchical patterning with sub-400 nm feature sizes. The paper demonstrates the fabrication of metasurfaces over 15 cm2 and are measured using FTIR and imaged with a thermal camera.

Substrate effects on the microsphere photolithography process (Conference Presentation)

Microsphere Photolithography (MPL) uses an array of self-assembled microspheres as optical elements. Flood illumination is focused to a photonic jet by each microsphere. Simulation and experiments show that photonic jet can be as small as λ/3, with collimation of more than a wavelength. This provides significant potential for pattern transfer of sub-micron patterns over large-areas and offers an inexpensive alternative to direct-write techniques such as e-beam lithography or two-photon absorption. This has applications such as SERS and SEIRA templates as well as metasurfaces to control radiation heat transfer. For these applications, the underlying substrate is important for the device performance and often presents a considerable index-contrast with the photoresist. The substrate significantly affects the behavior of the photonic jet and changes the necessary dose, minimum feature size, and morphology of the exposed area. This paper explores the effects of the substrate on the process. Numerical models using commercial (HFSS) frequency-domain Finite Element Method (FEM) is used to simulate the interaction of light with the microsphere/photoresist/substrate. The distribution of the electric field is used to predict the exposure curve for the process. In general, metals and high index materials cause significant standing waves in the photoresist which modifies the hole morphology and ultimate feature size. These predictions are compared to i-line illuminated experiments with SEM measured hole dimensions for aluminum, germanium, and glass substrates. The objective of the paper is to establish design rules for the process which can be incorporated into the device design.