Negative permeability due to exchange spin-wave resonances in thin magnetic films with surface pinning

Copyright © 2010 The American Physical SocietyWe report a theory of the effective permeability of multilayered metamaterials containing thin ferromagnetic layers with magnetization pinned on either one or both surfaces. Because of the pinning and small film thickness, the lowest frequency magnetic resonances are due to nonuniform exchange spin waves with frequencies far above those expected for uniform ferromagnetic resonance in known magnetic materials. Yet, the coupling of the nonuniform spin-wave modes to the electromagnetic field is shown to be strong enough to lead, for magnetic parameters characteristic for conventional transition metal alloys, to negative values of the effective permeability at frequencies of several hundred gigahertzs. The permittivity of metals is already negative in this frequency range. Hence, this system represents a negative refractive index metamaterial at subterahertz frequencies. The ways by which to maximize the frequency and the strength of the negative magnetic response are analyzed

Exciton and electron-hole plasma formation dynamics in ZnO

Euan Hendry, M. Koeberg, and M. Bonn, Physical Review B, Vol. 76, article 045214 (2007). "Copyright © 2007 by the American Physical Society."We employ optical pump-THz probe measurements to study the formation of excitons and electron-hole plasmas following photogeneration of a hot electron-hole gas in the direct gap semiconductor zinc oxide. Below the Mott density, we directly observe the evolution of the hot electron-hole plasma into an insulating exciton gas in the 10 to 100 ps following photoexcitation. The temperature dependence of this process reveals that the rate determining step for exciton formation involves acoustic phonon emission. Above the Mott density, the density of the hot electron-hole plasma initially decreases very rapidly (~1.5 ps) through Auger annihilation until a stable plasma is formed close to the Mott density. In contrast to exciton formation, Auger annihilation is found to be independent of lattice temperature, occurring while the plasma is still hot

Photo-induced doping and strain in exfoliated graphene

Copyright © 2013 American Institute of PhysicsFile "01 flake before modification.jpg" is a microscope picture of a monolayer graphene flake before photomodification. Files "02 Raman map before modification 2D.txt" and "02 Raman map before modification G+D.txt" contain Raman data for the flake before modification. The columns in the files are [X coordinate; Y coordinate; Raman shift; intensity;]. The first file contains data for Raman shifts from 2470 to 2916 cm-1, the second file - data for shifts from 1306 to 1840 cm-1. File “03 PL image before modification.tif” is an Up-converted photoluminescence image of the flake before photomodification. File “04 PL image after modification.tif” is an Up-converted photoluminescence image of the flake after photomodification. File “05 flake after modification.jpg” is a microscope picture of a monolayer graphene flake after photomodification. Files “06 Raman map after modification 2D.txt” and “06 Raman map after modification G+D.txt” are Raman data for the flake after modification. File “07 AFM data.txt” is AFM data for a modified area of a monolayer graphene sample.The modification of single layer graphene due to intense, picoseconds near-infrared laser pulses is investigated. We monitor the stable changes introduced to graphene upon photoexcitation using Raman spectroscopy. We find that photoexcitation leads to both a local increase in hole doping and a reduction in compressive strain. Possible explanations for these effects, due to photo-induced oxygenation and photo-induced buckling of the graphene, are discussed.CEMPS Research Project Studentshi

Probing optical chirality in the near field of plasmonic nanostructures

PublishedWe directly probe the optical chirality in the near field of plasmonic nanostructures coated with a dye-doped polymer film by measuring the photoluminescence enhancement and analysing it with respect to its circular polarisation state

Controlling the generation of THz radiation from metallic films using periodic microstructure

We report on THz frequency generation via irradiation of microstructured semicontinuous silver films by femtosecond laser pulses. By patterning the film so as to produce an array of microstrips, we show that one can use periodic microstructure to control the way nanostructured metal films produce THz radiation when illuminated by femtosecond infrared laser pulses. A simple analytical model based on the field distribution arising from an array of THz dipole emitters is used to assess the experimental data, allowing us to explain some of the main features of the generated THz radiation patterns, including the strongly resonant features of the emission spectrum

Importance of diffraction in determining the dispersion of designer surface plasmons

Euan Hendry, Alastair P. Hibbins, and J. Roy Sambles, Physical Review B, Vol. 78, article 235426 (2008). Copyright © 2008 by the American Physical Society.By employing a modified modal matching approach, we obtain explicit analytical expressions relating frequency to in-plane wave vector for the surface electromagnetic mode confined at the interface between vacuum and a perfect conductor patterned with a two-dimensional square array of square holes. Our complete analytical formalism takes into account both multiple order waveguide modes and diffracted evanescent waves, hence overcoming the a priori assumptions intrinsic to previous descriptions of the dispersion of these surface waves. We validate our derived dispersion relation through comparison with that recently recorded at microwave frequencies using prism coupling. Finally, we show that diffracted evanescent waves play an important role in determining the dispersion, so that the electric field associated with “designer” surface modes is much more weakly confined to the interface than the field associated with surface plasmons on real metal surfaces

Subwavelength lateral confinement of microwave surface waves

Copyright © 2011 American Institute of PhysicsExperimental verification of the “domino plasmon,” recently proposed by Cano et al. [Opt. Express 18(2), 754, (2 010)] is presented. Using microwaves, it is demonstrated that this mode propagates along a periodic chain of metallic cuboids (“dominos”) and the dispersion of the mode is determined with results being compared to the predictions of analytical and numerical models. This mode is found to be surprisingly insensitive to the lateral width of the chain, even on a subwavelength scale. Having such tight confinement, “domino plasmons” show consider able promise for one-dimensional subwavelength guiding and focusing of electromagnetic fields

Reduction of carrier mobility in semiconductors caused by charge-charge interactions

Euan Hendry, M. Koeberg, J. Pijpers, and M. Bonn, Physical Review B, Vol. 75, article 233202 (2007). "Copyright © 2007 by the American Physical Society."We investigate the effect of charge-charge interactions on carrier mobility in titanium dioxide (TiO2) and silicon (Si) using terahertz spectroscopy. Charge scattering times and plasma frequencies are directly determined as a function of charge density. In Si, a linear increase in scattering rate for densities exceeding 1021 m−3 is attributed to electron-hole scattering. In contrast, in TiO2, charge-charge interactions are suppressed due to dielectric screening, highlighting the vastly different dielectric properties for these two materials