721 research outputs found
Semiconductor Surface Studies
Contains research summary and reports on three research projects.Joint Services Electronics Program (Contract DAAG29-83-K-0003
Semiconductor Surface Studies
Contains research objectives and summary of research on one research project.Joint Services Electronics Program (Contract DAAB07-76-C-1400
Weyl points and line nodes in gapless gyroid photonic crystals
Weyl points and line nodes are three-dimensional linear point- and
line-degeneracies between two bands. In contrast to Dirac points, which are
their two-dimensional analogues, Weyl points are stable in the momentum space
and the associated surface states are predicted to be topologically
non-trivial. However, Weyl points are yet to be discovered in nature. Here, we
report photonic crystals, based on the double-gyroid structures, exhibiting
frequency-isolated Weyl points with intricate phase diagrams. The surface
states associated with the non-zero Chern numbers are demonstrated. Line nodes
are also found in similar geometries; the associated surface states are shown
to be flat bands. Our results are readily experimentally realizable at both
microwave and optical frequencies.Comment: 6 figures and 8 pages including the supplementary informatio
Non-Abelian Generalizations of the Hofstadter model: Spin-orbit-coupled Butterfly Pairs
The Hofstadter model, well-known for its fractal butterfly spectrum,
describes two-dimensional electrons under a perpendicular magnetic field, which
gives rise to the integer quantum hall effect. Inspired by the real-space
building blocks of non-Abelian gauge fields from a recent experiment [Science,
365, 1021 (2019)], we introduce and theoretically study two non-Abelian
generalizations of the Hofstadter model. Each model describes two pairs of
Hofstadter butterflies that are spin-orbit coupled. In contrast to the original
Hofstadter model that can be equivalently studied in the Landau and symmetric
gauges, the corresponding non-Abelian generalizations exhibit distinct spectra
due to the non-commutativity of the gauge fields. We derive the genuine
(necessary and sufficient) non-Abelian condition for the two models from the
commutativity of their arbitrary loop operators. At zero energy, the models are
gapless and host Weyl and Dirac points protected by internal and crystalline
symmetries. Double (8-fold), triple (12-fold), and quadrupole (16-fold) Dirac
points also emerge, especially under equal hopping phases of the non-Abelian
potentials. At other fillings, the gapped phases of the models give rise to
topological insulators. We conclude by discussing possible
schemes for the experimental realizations of the models in photonic platforms
Transparent and ‘opaque’ conducting electrodes for ultra-thin highly-efficient near-field thermophotovoltaic cells
Transparent conducting electrodes play a fundamental role in far-field PhotoVoltaic systems, but have never been thoroughly investigated for near-field applications. Here we show, in the context of near-field planar ultra-thin ThermoPhotoVoltaic cells using surface-plasmon-polariton thermal emitters, that the resonant nature of the nanophotonic system significantly alters the design criteria for the necessary conducting front electrode. The traditional ratio of optical-to-DC conductivities is alone not an adequate figure of merit, instead the desired impedance matching between the emitter and absorber modes along with their coupling to the free-carrier resonance of the front electrode are key for optimal device design and performance. Moreover, we demonstrate that conducting electrodes 'opaque' to incoming far-field radiation can, in fact, be used in the near field with decent performance by taking advantage of evanescent photon tunneling from the emitter to the absorber. Finally, we identify and compare appropriate tunable-by-doping materials for front electrodes in near-field ThermoPhotoVoltaics, specifically molybdenum-doped indium oxide, dysprosium-doped cadmium oxide, graphene and diffused semiconductors, but also for 'opaque' electrodes, tin-doped indium oxide and silver nano-films. Predicted estimated performances include output power density ~10 W/cm 2 with > 45% efficiency at 2100 °K emitter temperature and 60 Ω electrode square resistance, thus increasing the promise for high-performance practical devices.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies ( Contract W911NF-13-D-0001
Reflection-Free One-Way Edge Modes in a Gyromagnetic Photonic Crystal
We point out that electromagnetic one-way edge modes analogous to quantum
Hall edge states, originally predicted by Raghu and Haldane in 2D gyroelectric
photonic crystals possessing Dirac point-derived bandgaps, can appear in more
general settings. In particular, we show that the TM modes in a gyromagnetic
photonic crystal can be formally mapped to electronic wavefunctions in a
periodic electromagnetic field, so that the only requirement for the existence
of one-way edge modes is that the Chern number for all bands below a gap is
non-zero. In a square-lattice gyromagnetic Yttrium-Iron-Garnet photonic crystal
operating at microwave frequencies, which lacks Dirac points, time-reversal
breaking is strong enough that the effect should be easily observable. For
realistic material parameters, the edge modes occupy a 10% band gap. Numerical
simulations of a one-way waveguide incorporating this crystal show 100%
transmission across strong defects, such as perfect conductors several lattice
constants wide, larger than the width of the waveguide.Comment: 4 pages, 3 figures (Figs. 1 and 2 revised.
‘Squeezing’ near-field thermal emission for ultra-efficient high-power thermophotovoltaic conversion
We numerically demonstrate near-field planar ThermoPhotoVoltaic systems with very high efficiency and output power, at large vacuum gaps. Example performances include: at 1200 °K emitter temperature, output power density 2 W/cm[superscript 2] with ~47% efficiency at 300 nm vacuum gap; at 2100 °K, 24 W/cm[superscript 2] with ~57% efficiency at 200 nm gap; and, at 3000 °K, 115 W/cm[superscript 2] with ~61% efficiency at 140 nm gap. Key to this striking performance is a novel photonic design forcing the emitter and cell single modes to cros resonantly couple and impedance-match just above the semiconductor bandgap, creating there a ‘squeezed’ narrowband near-field emission spectrum. Specifically, we employ surface-plasmon-polariton thermal emitters and silver-backed semiconductor-thin-film photovoltaic cells. The emitter planar plasmonic nature allows for high-power and stable high-temperature operation. Our simulations include modeling of free-carrier absorption in both cell electrodes and temperature dependence of the emitter properties. At high temperatures, the efficiency enhancement via resonant mode cross-coupling and matching can be extended to even higher power, by appropriately patterning the silver back electrode to enforce also an absorber effective surface-plasmon-polariton mode. Our proposed designs can therefore lead the way for mass-producible and low-cost ThermoPhotoVoltaic micro-generators and solar cells.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-13-D-0001
Generalized Gilat-Raubenheimer method for density-of-states calculation in photonic crystals
Efficient numeric algorithm is the key for accurate evaluation of density of
states (DOS) in band theory. Gilat-Raubenheimer (GR) method proposed in 1966 is
an efficient linear extrapolation method which was limited in specific
lattices. Here, using an affine transformation, we provide a new generalization
of the original GR method to any Bravais lattices and show that it is superior
to the tetrahedron method and the adaptive Gaussian broadening method. Finally,
we apply our generalized GR (GGR) method to compute DOS of various gyroid
photonic crystals of topological degeneracies.Comment: 7 pages, 2 figures; typos added, appendix B added. Programs are
available at: https://github.com/boyuanliuoptics/DOS-calculatio
Time-reversal in dynamically-tuned zero-gap periodic systems
We show that short pulses propagating in zero-gap periodic systems can be
reversed with 100% efficiency by using weak non-adiabatic tuning of the wave
velocity at time-scales that can be much slower than the period. Unlike
previous schemes, we demonstrate reversal of {\em broadband} (few cycle) pulses
with simple structures. Our scheme may thus open the way to time-reversal in a
variety of systems for which it was not accessible before.Comment: Accepted for publication in Phys. Rev. Letter
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