How an oxide shell affects the ultraviolet plasmonic behavior of Ga, Mg, and Al nanostructures.
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The ultraviolet (UV) range presents new challenges for plasmonics, with interesting applications ranging from engineering to biology. In previous research, gallium, aluminum, and magnesium were found to be very promising UV plasmonic metals. However, a native oxide shell surrounds nanostructures of these metals that affects their plasmonic response. Here, through a nanoparticle-oxide core-shell model, we present a detailed electromagnetic analysis of how oxidation alters the UV-plasmonic response of spherical or hemisphere-on-substrate nanostructures made of those metals by analyzing the spectral evolution of two parameters: the absorption efficiency (far-field analysis) and the enhancement of the local intensity averaged over the nanoparticle surface (near-field analysis).
Dr. Everitt is the Army's senior technologist (ST) for optical sciences, a senior executive currently working for the Army Research Laboratory in Houston, TX. Through his adjunct appointment in the Duke Physics Department, he leads an active experimental research group in molecular physics, novel terahertz imaging, nanophotonics, and ultrafast spectroscopy of wide bandage semiconductors with colleagues on campus and through an international network of collaborators. Four principal research areas are being pursued: 1) Molecular Physics. The longest research effort involves the use of molecular rotational spectroscopy and time-resolved techniques to investigate molecular collision dynamics. These studies will help us develop more efficient terahertz sources, detect and identify clouds of trace gases, and understand nonequilibrium atmospheres and interstellar media. In collaboration with Prof. Frank De Lucia, formerly of Duke Physics, Dr. Everitt was the first to map out the complete rotational and vibrational energy transfer map of methyl fluoride, leading to the demonstration of a compact, tunable terahertz laser for use in ground-based spectroscopy and astronomical observation. Their double resonance technique has now been adapted as a new means for remotely identifying the constituents of a trace gas cloud at distances up to 1 km. 2) Terahertz Imaging. This newest activity uses powerful, cw, tunable millimeter- and submillimeter-wave sources to adapt various coherent imaging techniques to the terahertz spectral region. Interferometry, digital holography, tomography, synthetic aperture RADAR, ISAR, ellipsometry, and polarimetry are all explored to develop practical tools for non-destructive measurements of visually opaque materials. The lab contains a unique combination of tunable sources, Schottky diode detectors, heterodyne receivers, and bolometers, plus a one-of-a-kind THz beam characterization and imaging instrument. The lab also explores ways of optimizing and accelerating these slow imaging methodologies, including methods for mapping strain in opaque composite materials with on-campus collaborators Profs. Nan Jokerst, Willie Padilla, and David Smith. 3) Ultraviolet Nanoplasmonics. Using metal nanoparticles to concentrate electromagnetic fields locally is an area of active research, most of which concentrates on using metal nanoparticles active in the visible and ultraviolet spectral regions. There are significant advantages of extending plasmonics into the ultraviolet, including enhanced Raman cross sections, accelerated photo-degradation of toxins, and accelerated excitonic recombination. In partnership with Profs. Jie Liu (Duke Chemistry), April Brown (Duke ECE), Naomi Halas (Rice Univ.), Fernando Moreno (Univ. Cantabria), and others, we have been identifying and exploring new nanostructured metals including rhodium, gallium, and aluminum for ultraviolet plasmonics. We have recently demonstrated ultraviolet surface enhanced Raman spectra and tailored photocatalytic behavior of important chemical reactions. 4) Ultrafast Spectroscopy. This effort concentrates on the ultrafast spectroscopic characterization of wide bandgap semiconductor heterostructures and nanostructures. We use independently tunable pump and probe wavelengths that span the ultraviolet-visible-infrared regions from 200 nm to 12 microns with pulses shorter than 150 fs. The objective is to manipulate and control carrier, exciton, and phonon transport and relaxation pathways in metal oxide and III-N semiconductors, sometimes doped with rare-earth atoms, using quantum efficiency, cw and time-resolved photoluminescence and differential transmission measurements. Areas of recent interest include characterization of efficient phosphorescence in sulfur-doped ZnO with Prof. Jie Liu, carrier dynamics in III-N epilayers and multiple quantum wells with Prof. April Brown, and characterization of radiative and nonradiative recombination of rare earth dopants in wide bandgap semiconductor hosts.
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