arXiv subjects

Brian S. Dennis

Publications and source records attributed to Brian S. Dennis.

12 recordsLinked to original sources

arXiv2015

Diffraction limited focusing and routing of gap plasmons by a metal-dielectric-metal lens

Passive optical elements can play key roles in photonic applications such as plasmonic integrated circuits. Here we experimentally demonstrate passive gap-plasmon focusing and routing in two-dimensions. This is accomplished using a high numerical-aperture metal-dielectric-metal lens incorporated into a planar-waveguide device. Fabrication via metal sputtering, oxide deposition, electron- and focused-ion- beam lithography, and argon ion-milling is reported on in detail. Diffraction-limited focusing is optically characterized by sampling out-coupled light with a microscope. The measured focal distance and full-width-half-maximum spot size agree well with the calculated lens performance. The surface plasmon polariton propagation length is measured by sampling light from multiple out-coupler slits.

physics.optics↗

arXiv2014

Ultracompact Nano-Mechanical Plasmonic Phase Modulators

Dielectrics' refractive index limits photonics miniaturization. By coupling light to metal's free electrons, plasmonic devices achieve deeper localization, which scales with the device geometric size. However, when localization approaches the skin depth, energy shifts from the dielectric into the metal, hindering active modulation. Here we propose a nano-electromechanical phase modulation principle exploiting the extraordinarily strong dependence of the phase velocity of metal-insulator-metal(MIM) gap plasmons on dynamically variable gap size. We demonstrate a 23 μm long non-resonant modulator having 1.5 π rad range with 1.7 dB excess loss at 780 nm. Analysis shows an ultracompact 1 μm$^{2}$ footprint π rad phase modulator can be realized, more than an order of magnitude smaller than any previously shown. Remarkably, this size reduction is achieved without incurring extra loss, since the nanobeam-plasmon coupling strength increases at a similar rate as the loss. Such small, high density electrically controllable components may find applications in optical switch fabrics and reconfigurable flat plasmonic optics.

physics.optics↗

arXiv2011

Higher Energy Composite Fermion Levels in the Fractional Quantum Hall Effect

Even though composite fermions in the fractional quantum Hall liquid are well established, it is not yet known up to what energies they remain intact. We probe the high-energy spectrum of the 1/3 liquid directly by resonant inelastic light scattering, and report the observation of a large number of new collective modes. Supported by our theoretical calculations, we associate these with transitions across two or more composite fermions levels. The formation of quasiparticle levels up to high energies is direct evidence for the robustness of topological order in the fractional quantum Hall effect.

cond-mat.str-el↗

arXiv2007

Spin-texture and magneto-roton excitations at nu=1/3

Neutral spin texture ST excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin- flips. At low magnetic fields, the ST energy is below that of the magneto-roton minimum. With increasing in-plane magnetic field these modes energies cross at a critical ratio of the Zeeman and Coulomb energies of eta_c=0.020 +- 0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magneto-roton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.

cond-mat.str-el↗

arXiv2007

Control of optical emission in doped GaAs/AlGaAs nanofabricated quantum dots

Dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching exhibit highly homogeneous luminescence. Single quantum dots display spectral emission with peak energies and linewidths linked largely to the geometrical diameter of the dot and to the built-in electron population. Excitonic-like and biexcitonic-like emission intensities have activation energy of about 2 meV. These results highlight the potential of high quality nanofabricated QDs for applications in areas that require fine control of optical emission.

cond-mat.other↗

arXiv2007

Metamorphosis of a Quantum Hall Bilayer State into a Composite Fermion Metal

Composite fermion metal states emerge in quantum Hall bilayers at total Landau level filling factor $ν_T$=1 when the tunneling gap collapses by application of in-plane components of the external magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index.

cond-mat.mes-hall↗

arXiv2005

Observation of low-lying excitations of electrons in coupled quantum dots

Tunneling excitations of electrons in dry-etched modulation-doped AlGaAs/GaAs coupled quantum dots (QDs) are probed by resonant inelastic light scattering. A sequence of intra- and intershell excitations are found at energies determined by the interplay between the QD confinement energy $\hbar ω_0$ and the tunneling gap $Δ_{SAS}$, the splitting between the symmetric and anti-symmetric delocalized single particle molecular states. The narrow line-widths displayed by electronic excitations in these nanostructures indicate promising venues for the spectroscopic investigation of entanglement of electron states in these artificial molecules.

cond-mat.mes-hall↗

arXiv2005

Evidence of correlation in spin excitations of few-electron quantum dots

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configurationinteraction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.

cond-mat.mes-hall↗

arXiv2004

Observation of collapse of pseudospin order in bilayer quantum Hall ferromagnets

The Hartree-Fock paradigm of bilayer quantum Hall states with finite tunneling at filling factor $ν$=1 has full pseudospin ferromagnetic order with all the electrons in the lowest symmetric Landau level. Inelastic light scattering measurements of low energy spin excitations reveal major departures from the paradigm at relatively large tunneling gaps. The results indicate the emergence of a novel correlated quantum Hall state at $ν$=1 characterized by reduced pseudospin order. Marked anomalies occur in spin excitations when pseudospin polarization collapses by application of in-plane magnetic fields.

cond-mat.mes-hall↗

arXiv2003

Soft Magnetorotons and Broken-Symmetry States in Bilayer Quantum Hall Ferromagnets

The recent report on the observation of soft magnetorotons in the dispersion of charge-density excitations across the tunneling gap in coupled bilayers at total Landau level filling factor $ν_T=1$ is reviewed. The inelastic light scattering experiments take advantage of the breakdown of wave-vector conservation that occurs under resonant excitation. The results offer evidence that in the quantum Hall state there is a roton that softens and sharpens markedly when the phase boundary for transitions to highly-correlated compressible states is approached. These findings are interpreted with Hartree-Fock evaluations of the dynamic structure factor. The model includes the effect of disorder in the breakdown of wave-vector conservation and resonance enhancement profiles within a phenomenological approach. These results link the softening of magnetorotons to enhanced excitonic Coulomb interactions in the ferromagnetic bilayers.

cond-mat.mes-hall↗

arXiv2003

Spectroscopy of soft modes and quantum phase transitions in coupled electron bilayers

Strongly-correlated two-dimensional electrons in coupled semiconductor bilayers display remarkable broken symmetry many-body states under accessible and controllable experimental conditions. In the cases of continuous quantum phase transitions soft collective modes drive the transformations that link distinct ground states of the electron double layers. In this paper we consider results showing that resonant inelastic light scattering methods detect soft collective modes of the double layers and probe their evolution with temperature and magnetic field. The light scattering experiments offer venues of research of fundamental interactions and continuous quantum phase transitions in low-dimensional electron liquids.

cond-mat.mes-hall↗

arXiv2003

Observation of soft magnetorotons in bilayer quantum Hall ferromagnets

Inelastic light scattering measurements of low-lying collective excitations of electron double layers in the quantum Hall state at total filling nu_T=1 reveal a deep magnetoroton in the dispersion of charge-density excitations across the tunneling gap. The roton softens and sharpens markedly when the phase boundary for transitions to highly correlated compressible states is approached. The findings are interpreted with Hartree-Fock evaluations that link soft magnetorotons to enhanced excitonic Coulomb interactions and to quantum phase transitions in the ferromagnetic bilayers.

cond-mat.mes-hall↗