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      Nonvolatile Reconfigurable Phase-Change Metadevices for Beam Steering in the Near Infrared

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          Phase-change materials for rewriteable data storage.

          Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. This review looks at the unique property combination that characterizes phase-change materials. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. The origin of the fast crystallization kinetics is also discussed.
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            Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces

            The concept of optical phase discontinuities is applied to the design and demonstration of aberration-free planar lenses and axicons, comprising a phased array of ultrathin subwavelength spaced optical antennas. The lenses and axicons consist of radial distributions of V-shaped nanoantennas that generate respectively spherical wavefronts and non-diffracting Bessel beams at telecom wavelengths. Simulations are also presented to show that our aberration-free designs are applicable to high numerical aperture lenses such as flat microscope objectives.
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              Resonant bonding in crystalline phase-change materials.

              The identification of materials suitable for non-volatile phase-change memory applications is driven by the need to find materials with tailored properties for different technological applications and the desire to understand the scientific basis for their unique properties. Here, we report the observation of a distinctive and characteristic feature of phase-change materials. Measurements of the dielectric function in the energy range from 0.025 to 3 eV reveal that the optical dielectric constant is 70-200% larger for the crystalline than the amorphous phases. This difference is attributed to a significant change in bonding between the two phases. The optical dielectric constant of the amorphous phases is that expected of a covalent semiconductor, whereas that of the crystalline phases is strongly enhanced by resonant bonding effects. The quantification of these is enabled by measurements of the electronic polarizability. As this bonding in the crystalline state is a unique fingerprint for phase-change materials, a simple scheme to identify and characterize potential phase-change materials emerges.
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                Author and article information

                Journal
                Advanced Functional Materials
                Adv. Funct. Mater.
                Wiley
                1616301X
                March 2018
                March 2018
                January 05 2018
                : 28
                : 10
                : 1704993
                Affiliations
                [1 ]College of Engineering; Mathematics and Physical Sciences; University of Exeter; Exeter EX4 4QF UK
                [2 ]Department of Electrical and Electronic Engineering; University of Bristol; Bristol BS8 1TH UK
                Article
                10.1002/adfm.201704993
                © 2018

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