Calcite is a birefringent material with optical anisotropy that becomes extreme in the infrared, allowing for the excitation of highly-directional, sub-diffractional hyperbolic modes. In this talk, I will discuss our recent work that focuses on understanding the optical behavior of hyperbolic modes supported within asymmetric nanostructures formed in calcite crystals with in-plane anisotropy, including our recent findings that demonstrate how the resonant frequency and directional power flow can be tuned by simply rotating gratings with respect to the crystal axes of calcite – without changing the shape of the gratings.
An infrared ultrafast laser (=1026 nm, ~160 fs) was used to synthesize nanodiamonds (ND) directly from an intense plasma in liquid ethanol (sans target) through the process of laser filamentation. The ethanol solution was characterized by its UV-VIS absorbance and optical emission spectra; and the nanodiamond particles by Raman spectroscopy and transmission electron microscopy (TEM) analysis. The absorbance spectra showed a single strong peak in the UV while the visible emission spectra displayed two lines above a background continuum. Raman spectroscopy showed the existence of D and G peaks that confirmed the presence of nanodiamonds. TEM analysis showed the nanodiamond particles had a narrow 2-5 nm size range with no aggregation and had visible lattice fringes of 2.06 Å corresponding to the diamond (111) phase. Electron energy loss spectroscopy (EELS) also confirmed the existence of nanodiamonds by revealing a 35 eV edge in the low loss region and a sharp onset of the carbon K-edge at 290 eV.
Theoretical work has identified a new type of hybrid nanoresonator akin to a loaded-gap antenna, wherein the gap between two collinearly aligned metal nanorods is filled with active dielectric material. The gap optical load has a profound impact on resonances supported by such a “nanogap” antenna, and thus provides opportunity for (i) active modulation of the antenna resonance and (ii) delivery of substantial energy to the gap material. To this end, we have (i) used a bottom-up technique to fabricate nanogap antennas (Au/CdS/Au); (ii) characterized the optical modes of individual antennas with polarization- and wavevector-controlled dark-field microscopy; (iii) mapped the spatial profiles of the dominant modes with electron energy loss spectroscopy and imaging; and (iv) utilized full-wave finite-difference time-domain simulations to reveal the nanoscopic origin of the radiating modes supported on such nanogap antennas.
In addition to conventional transverse and longitudinal resonances, these loaded nanogap antennas support a unique symmetry-forbidden gap-localized transverse mode arising from the splitting of degenerate transverse modes located on the two gap faces. This previously unobserved mode is strong (E2 enhanced ~20), tightly localized in the nanoscopic (~30 nm separation) gap region, and is shown to red-shift with decreased gap size and increased gap dielectric constant. In fact, the mode is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multi-functional components many seek.
Neutron irradiation of sapphire with 1 x 1022 neutrons(<EQ MeV)/m2 increases the c-axis compressive strength by a factor of 3 at 600 degree(s)C. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. 1-B and Cd shielding was employed during irradiation to filter our thermal neutrons (<EQ1 eV), thereby reducing residual radioactivity in the sapphire to background levels in a month. Yellow-brown irradiated sapphire is nearly decolorized to pale yellow by annealing at 600 degree(s)C with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200 degree(s)C for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600 degree(s)C by 0-20% in some experiments. However, the c- plane ring-on-ring flexure strength at 600 degree(s)C is doubled by irradiation. Elastic constants of irradiated sapphire are only slightly changed by irradiation. Infrared absorption and emission and thermal conductivity of sapphire are not affected by irradiation at the neutron fluence used in this study. Defects that might be correlated with strengthening were characterized by electron paramagnetic resonance spectroscopy. Color centers observed in the ultraviolet absorption spectrum were not clearly correlated with mechanical response. No radiation-induced changes could be detected by x-ray topography or x-ray diffraction.
We show that ion beam assisted deposition (IBAD) can result in films of metal or semiconductor nanoclusters in dielectric matrices with nonlinear optical (NLO) or photoluminescence (PL) properties. Gold nanocluster thin films consisting of metal clusters 5-30 nm in size embedded in a Nb2O5 or a SiO2 oxide matrix were deposited by ion beam assisted deposition by co-evaporation of Au and Nb or Si with O2+ ion bombardment. Semiconductor nanocluster films have been prepared by IBAD as well. Silicon-rich silica films were deposited by coevaporation of silica and silicon with and without simultaneous ion bombardment. PL attributable to defects in SiO2 was observed at about 550 nm. After annealing, the PL peak shifts to 750 nm and increases in intensity, indicating the formation of Si nanoclusters. Transmission electron microscopy (TEM) images of annealed IBAD films show a dense distribution of mostly spherically shaped crystalline Si nanoclusters, about 1-4 nm in diameter, in a featureless amorphous SiO2 matrix. TEM images of films prepared without ion assist showed many less crystalline Si clusters, that were more irregularly shaped, within 0.1 micrometers amorphous silica grains. Passivation of the films with hydrogen removes the defect PL peak and enhances the peak due to nanoclusters. We have also prepared Ge nanoclusters in silica films with and without IBAD.
Pulsed laser deposition has been used for the growth of high quality YBa2Cu3O7 and La0.67Sr0.33MnO3 thin films and multilayers for electronic device applications. In particular, YBa2Cu3O7 - (SrTiO3, CeO2) - La0.67Sr0.33MnO3 trilayer devices were fabricated to study the supercurrent suppression by the injection of a spin-polarized quasiparticle current. Our results show that the critical current for a YBa2Cu3O7 - 50 angstroms SrTiO3 - La0.67Sr0.33MnO3 device was found to decrease from 120 mA to 15 mA, for an injection current of 60 mA of spin polarized current yielding a negative current gain of approximately 1.8. The effect of film microstructure on the critical current suppression was investigated. Defects in the SrTiO3 and CeO2 layers were found to control the device properties. Once optimized, spin injection represents a new approach to fabricating superconducting transistors which could impact electronic systems for many important next generation.
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