Session HA: Condensed Matter and Nanoscience IV

Efficient Forward Second-Harmonic Generation from Archimedean Nanospirals

High electric field enhancements resulting from the plasmonic resonances in nanoparticle antennas create efficient harmonic generators. These nanoscale nonlinear sources also show significant enhancements in conversion efficiency as asymmetry is introduced into the plasmonic geometry on both the inter-particle and intra-particle level. The nanospiral shows a unique form of asymmetry because it is not the result of an altered symmetric system nor does it have any axis of symmetry. These experiments show that this asymmetry results in a highly efficient harmonic generator as well as an interesting two-dimensionally chiral optical element. This nonlinear signal demonstrated sensitivity to tuning both linear and circular polarization states of incident light as well as conversion between these states. We report SHG conversion efficiencies (7.8*10$^{\mathrm{-9}})$ that are amongst the highest reported in the field of plasmonics using a 15 fs pulse at a wavelength of 800nm. We also observe selective conversion between fundamental and emitted polarization states. These results are interesting for applications requiring optical control of high energy light from nanoscale sources.   

Active Modulation of Plamon-Induced Transparency in Gold:VO$_{2}$ Hybrid Nanostructures

Plasmon-induced transparency (PIT) is a classical analogue of electromagnetically induced transparency where a spectrally broad and a spectrally narrow plasmon resonance interact. This interaction can yield a sharp, narrow increase in transmission referred to as a transparency window. Here we demonstrate a novel method of reversibly and actively modulating the plasmon resonance of~multi-element plasmonic nanostructures which exhibits PIT by using the phase transition in vanadium dioxide. Vanadium dioxide (VO$_{2})$ modulates the near-field dielectric environment of metal nanostructures via its semiconductor-to-metal transition (SMT). The SMT shifts the plasmonic response of the metal nanostructure. We design these hybrid structures using three-dimensional, finite-difference time-domain (FDTD) simulations and fabricate them with a combination of high-resolution electron-beam lithography and VO$_{2}$ deposition. To measure the modulation, we use IR spectroscopy and observe the spectral change in the location of the PIT window as we heat the VO$_{2}$ to thermally induce the phase transition. As the SMT can also be induced optically, these hybrid structures could be useful nanoscale switches or sensors.   

Irradiation of commercial, high-T$_{\mathrm{c}}$ superconducting tape for potential fusion applications: electromagnetic transport properties

Samples of commercially available thin-film tapes based on the high-temperature superconductor YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{x}}$ (YBCO) have been ion-irradiated under conditions approximating that of neutron irradiation to investigate these materials for potential fusion magnet applications. Effects on supercurrent transport and vortex dynamics were studied. Three different levels of irradiation damage were applied to films that had been produced by three different processes that utilize the inclusion of nanostructures to produce different defects for pinning -- the immobilization of supercurrent vortices (magnetic flux quanta, to which applied current exerts a Lorentz force, causing dissipative motion; thus rendering the sample non-superconducting.) As such, the effect of irradiation on these pinning-effective defects is of interest. We examined normal state resistivity, residual resistivity, critical temperature, field dependence of critical current density $J_{c}$, the irreversibility line, and vortex creep rate -- for $H $parallel to the c axis. Preliminary results show improved radiation tolerance over other YBCO materials for comparative irradiation conditions, with no detrimental effects on $J_{c}$, and in one case have even enhanced it over a broad range of fields. Details as to the role of various nanostructures shall be discussed.   

The donor-acceptor relationship in Fe:GaN

Due to its electronic properties, semi-insulating gallium nitride (GaN) is a promising candidate for high power and high frequency devices. One of the biggest plagues in GaN growth is residual donors, which results in a more conductive material. Fe replaces Ga in the wurtzite crystal structure as Fe$^{3+}$. The energy level of Fe$^{2+/3+}$ is significantly lower than shallow donors. As a result, a Fe atom is able to accept a shallow donor electron. This process changes the charge state of Fe$^{3+}$ to Fe$^{2+}$ and the neutral donor to have a net positive charge. This charge exchange should continue until the amount of either Fe$^{3+}$ or the neutral donor is depleted. The samples in this study were hydride vapor phase epitaxy bulk grown GaN doped with Si, controlling the donor concentration, and Fe. The [Si]/[Fe] ratios ranged from 0.01 to 1.55. Electron paramagnetic resonance (EPR) spectroscopy is able to identify the Fe$^{3+}$ acceptor and neutral donor. Fe$^{2+}$ and positive charged donors are EPR inactive. The principal finding of this study was the simultaneous existence of both the neutral donor and Fe$^{3+}$ acceptor state when Si to Fe concentration ratios are in the range of 0.13 to 0.42. Current interpretations involve donor and acceptor states induced by Fe. This UAB work is funded by NSF.   

Oblique angle transmission spectroscopic measurements on InGaN/GaN dot-in-a-wire heterostructures

Nearly defect-free GaN nanowires embedded with intrinsic InGaN quantum dots presents the opportunity to control the surface and longitudinal optical phonons modes which are dependent on the nanowire fill-factor, doping densities of the nanowires and the presence of InGaN dots. These effects are observed using the oblique angle transmission infrared spectroscopy. The tunable optical properties of these nanostructures show potential for phonon assisted photodetectors whose spectral responses can be tailored by varying a combination of these three parameters. The optical anisotropy along the growth ($c$-) axis of the GaN nanowire introduces the polarization agility seen in the transmission spectra, which can lead to polarization agile photodetectors.   

Numerical Investigation of Particle Shape Effects on the Spatial Distribution of Near-field Electromagnetic Flux

It is well known that the radiative transport between particles, particularly the case of nano-sized particles, can exceed the black body limit when the separation between the particles less than the thermal wavelength $\lambda \sim hc/(k_BT)$. Recent work has investigated the total and spatial distribution of the electromagnetic energy flux into closely separated bodies using the boundary element method. This computational method permits the insertion of test surfaces into the two-body system to readily assess the directionality of the electromagnetic flux at additional points in space. By studying the effect of particle shape and orientation of the flux through such a test surface, it will be possible to establish geometric configurations to directionally channel the electromagnetic flux through a system of particles. In this presentation, the spatial distribution of the electromagnetic flux between a sphere and a spheroid is investigated as a function of the eccentricity of the spheroid.   

A study of electrophoretic deposition (EPD) of carbon nanotubes on insulator substrates

Since the discovery of carbon nanotubes (Ijima, 1991) and isolation of graphene (Geim and Novoselov, 2003), there has been enormous interest toward the incorporation of these novel nanomaterials in electronic device applications. To this end, a wide array of economical coating strategies have been proposed, e.g. Langmuir Blodgett (LB) method, drop casting, spray and dip coating, ink jet printing, to name a few. Recently, electrophoretic deposition (EPD) has gained substantial attention as a versatile technique for the deposition of uniform coatings of micro/nanoparticles on predominantly conducting substrates or films atop insulating surfaces and substrates with open pores. In this study, we have successfully performed, for the first time, EPD of acid-refluxed CNTs on glass and plastic (polyimide) substrates. The deposition was accomplished on an initial, semi-transparent layer of drop-casted CNTs on hydrophilic, APTES treated glass substrates. To the best of our knowledge, thus far there exists no report of demonstration of EPD of CNTs on insulator substrates by utilizing the conducting properties of the CNTs. This exciting development is believed to significantly contribute to EPD coated CNT thin film transistor technology where uniform CNT coatings on dielectric films of Al$_{2}$O$_{3}$, SiO$_{2}$ are desired as the active semiconducting layer.   

Some Comments on the Mathematical Analysis of the Mechanical Properties of Graphene

Graphene sheets have mechanical properties which may be modeled using the classical theory of plates [1]. To obtain the associated mechanical parameters, data is fitted to the solutions of these mathematical models. While the fundamental equations are PDE's, an averaging over the space variables result in an ODE for which time is the independent variable. This ODE is the Duffing equation which describes1-dim, nonlinear oscillations. While the coefficients of this equation are constant, their explicit values depend on the functional form assumed for the fundamental mode of the grapheme sheet and other issues such as the particular functions used for the damping/dissipative terms. This work focuses on the coefficients of the derived Duffy equation and examines how their values are modified by selection of different functional forms for the fundamental mode.\\[4pt] [1] A. Eichler et. Al., ``Nonlinear damping in mechanical resonators made from carbon nanotubes and grapheme,'' NATURE Nanotechnology, Vol. 6 (June 2011), 339-342.