Engineering topics

Dolan, James A.

Publications and source records attributed to Dolan, James A..

3 records

DOE OSTI2021

Polarization-Insensitive Medium-Switchable Holographic Metasurfaces

The adoption of metasurfaces has led to revolutionary advances in holography due to improved compactness, integrability, and performance. Switchable meta-holograms projecting different replay field images in a controllable manner are highly desirable. Still, existing technologies generally rely on the use of polarized light and additional optics to facilitate switching. Consequently, the potential benefits afforded through the use of metasurfaces are limited both by the system complexity and a fixed relationship between the optical input and output. In this manuscript, we demonstrate polarization-insensitive metasurfaces encoding arbitrary and independent holograms, which can be switched between by changing the refractive index of the infiltration medium while maintaining identical illumination conditions. Finally, by sidestepping the requirements for high-performance light sources, switching optics, or delicate alignment, this approach points toward ultracompact and cost-effective switchable meta-holograms for various practical applications, such as holographic image projection, eye-perceptible sensors, optical information storage, processing, and security.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Nucleation and growth of blue phase liquid crystals on chemically-patterned surfaces: a surface anchoring assisted blue phase correlation length

In condensed matter, the correlation length is an essential parameter which describes the distance over which a material maintains its structural properties. In liquid crystals, for the orientational order parameter this characteristic length is of the order of nanometers, while in solid crystals it can extend to macroscopic length scales. Here, we report the measurement of the correlation length, or persistence length, of the crystallographic orientation of blue phases (BPs)—chiral liquid crystals with long-range 3D-crystalline structures and submicron-sized lattice-parameters. These materials exhibit phase transformations that have been identified as the liquid analog of crystal-crystal martensitic transformations. In this work, we use liquid epitaxial growth to achieve spontaneous BP-crystal nucleation and subsequent growth. Specifically, we design patterned substrates made of a binary array of regions with different liquid crystal anchoring, which facilitate a uniform nucleation and growth of BP-crystals with (100)-lattice orientation and a simple cubic symmetry. Furthermore, our results indicate that this simple cubic BP, the so-called blue phase II (BPII), forms first on the patterned surface, thereby propagating the growth of domains in directions parallel or perpendicular to the patterned regions. These results are used to understand the emergence of a surface anchoring assisted BPII-correlation length, taken as the distance over which the BP preserves its lattice orientation, as a function of time and pattern characteristics. We found that BPII single crystals can be achieved on patterned regions whose lateral dimensions are equal to or larger than 10 μm, consistent with our measurements of the BPII-correlation length. This newly acquired understanding of the role of surfaces on the formation of BPs is then used to design processes that permit formation of macroscopically large mono-domain, single-crystal BPs by relying on significantly reduced patterned areas (only part of the area is patterned), a feature that could benefit the applications of this intriguing class of materials.

correlation length↗

DOE OSTI2021

Broadband Liquid Crystal Tunable Metasurfaces in the Visible: Liquid Crystal Inhomogeneities Across the Metasurface Parameter Space

Optical metasurfaces—planar nanostructured devices that can arbitrarily tailor the wavefront of light—may be reconfigured by changing their dielectric environment. The application of external stimuli to liquid crystals is a particularly promising means of tuning the optical properties of embedded metasurfaces because of liquid crystals' large and broadband optical anisotropy. However, the detailed behavior of liquid crystals immediately adjacent to the nanostructured meta-atoms elements is often overlooked, despite the optics of the device depending sensitively on this behavior (e.g., the spectral position of the meta-atom resonances). This is of increasing concern as the wavelength of operation further approaches the short-wavelength end of the visible spectrum and, therefore, the length scale of the inhomogeneities in the liquid crystal director field. In this manuscript, we undertake a fully comprehensive study, across the metasurface geometrical parameter space, of broadband (450-700 nm) all-dielectric liquid crystal tunable metasurfaces operating in the visible. Here, through combined experimental characterization, liquid crystal modeling, and optical simulations, we reveal and quantify the improved accuracy with which the optical properties of the liquid crystal tunable metasurfaces may be described, and identify the underlying physical mechanism: the three-dimensional spatial overlap of the liquid crystal director field and metasurface optical near fields in the vicinity of the meta-atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗