1,426 research outputs found
Exceptional points in topological edge spectrum of PT symmetric domain walls
We demonstrate that the non-Hermitian parity-time (PT) symmetric interfaces
formed between amplifying and lossy crystals support dissipationless edge
states. These PT edge states exhibit gapless spectra in the complex band
structure interconnecting complex-valued bulk bands as long as exceptional
points (EPs) of edge states exist. As a result, regimes exist where the edge
states can spectrally overlap with the bulk continuum without hybridization,
and leakage into the bulk states is suppressed due to the PT symmetry. Two
exemplary PT symmetric systems, based on valley and quantum hall topological
phases, are investigated, and the connection with the corresponding Hermitian
systems is established. We find that the edge states smoothly transit to the
valley edge states found in Hermitian systems if the magnitude of gain/loss
vanishes. The topological nature of the PT edge states can be established
within the non-Hermitian Haldane model, where the topological invariance is
found to be unaffected by gain or loss. Nonreciprocal PT edge states are
discovered at the interfaces between PT-Haldane phases, indicating the
interplay between the gain/loss and the magnetic flux. The proposed systems are
experimentally feasible to realize in photonics. This has been verified by our
rigorous full-wave simulations of edge states in PT-symmetric silicon-based
photonic graphene.Comment: 24 pages, 9 figures, 2 table
Far-field probing of leaky topological states in all-dielectric metasurfaces
© 2018 The Author(s). Topological phase transitions in condensed matter systems give rise to exotic states of matter such as topological insulators, superconductors, and superfluids. Photonic topological systems open a whole new realm of research and technological opportunities, exhibiting a number of important distinctions from their condensed matter counterparts. Photonic modes can leak into free space, which makes it possible to probe topological photonic phases by spectroscopic means via Fano resonances. Based on this idea, we develop a technique to retrieve the topological properties of all-dielectric metasurfaces from the measured far-field scattering characteristics. Collected angle-resolved spectra provide the momentum-dependent frequencies and lifetimes of the photonic modes that enable the retrieval of the effective Hamiltonian and extraction of the topological invariant. Our results demonstrate how the topological states of open non-Hermitian systems can be explored via far-field measurements, thus paving a way to the design of metasurfaces with unique scattering characteristics controlled via topological effects
Near-field imaging of spin-locked edge states in all-dielectric topological metasurfaces
A new class of phenomena stemming from topological states of quantum matter has recently found a variety of analogies in classical systems. Spin-locking and one-way propagation have been shown to drastically alter scattering of electromagnetic waves, thus offering an unprecedented robustness to defects and disorder. Despite these successes, bringing these new ideas to practical grounds meets a number of serious limitations. In photonics, when it is crucial to implement topological photonic devices on a chip, two major challenges are associated with electromagnetic dissipation into heat and out-of-plane radiation into free space. Both these mechanisms may destroy the topological state and seriously affect the device performance. Here, we demonstrate experimentally that the topological order for light can be implemented in all-dielectric on-chip prototype metasurfaces, which mitigate the effect of Ohmic losses by using exclusively structured dielectric materials, and we reveal that coupling of the system to the radiative continuum does not affect topological properties. We demonstrate the spin-Hall effect of light for spin-polarized topological edge states through near-field spectroscopy measurements.The work was supported by the National
Science Foundation (Grant Nos. CMMI-1537294 and EFRI1641069). The experimental part of the work was supported by
the Russian Science Foundation (Grant No. 16-19-10538) and
numerical calculations were partially supported by the
Russian Foundation for Basic Research (Grant No. 18-32-
20065). The work of AS, DAS, and YSK was partially supported
by the Australian Research Council. Research was partly
carried out at the Center for Functional Nanomaterials,
Brookhaven National Laboratory, which is supported by the
U.S. Department of Energy, Office of Basic Energy Sciences,
under Contract No. DE-SC0012704
Interpreting Attoclock Measurements of Tunnelling Times
Resolving in time the dynamics of light absorption by atoms and molecules,
and the electronic rearrangement this induces, is among the most challenging
goals of attosecond spectroscopy. The attoclock is an elegant approach to this
problem, which encodes ionization times in the strong-field regime. However,
the accurate reconstruction of these times from experimental data presents a
formidable theoretical challenge. Here, we solve this problem by combining
analytical theory with ab-initio numerical simulations. We apply our theory to
numerical attoclock experiments on the hydrogen atom to extract ionization time
delays and analyse their nature. Strong field ionization is often viewed as
optical tunnelling through the barrier created by the field and the core
potential. We show that, in the hydrogen atom, optical tunnelling is
instantaneous. By calibrating the attoclock using the hydrogen atom, our method
opens the way to identify possible delays associated with multielectron
dynamics during strong-field ionization.Comment: 33 pages, 10 figures, 3 appendixe
Engineering aspects of hydrothermal pretreatment: from batch to continuous operation, scale-up and pilot reactor under biorefinery concept
Different pretreatments strategies have been developed over the years mainly to enhance enzymatic cellulose degradation. In the new biorefinery era, a more holistic view on pretreatment is required to secure optimal use of the whole biomass. Hydrothermal pretreatment technology is regarded as very promising for lignocellulose biomass fractionation biorefinery and to be implemented at the industrial scale for biorefineries of second generation and circular bioeconomy, since it does not require no chemical inputs other than liquid water or steam and heat. This review focuses on the fundamentals of hydrothermal pretreatment, structure changes of biomass during this pretreatment, multiproduct strategies in terms of biorefinery, reactor technology and engineering aspects from batch to continuous operation. The treatise includes a case study of hydrothermal biomass pretreatment at pilot plant scale and integrated process design.The authors gratefully thank the Secretary of Public Education ofMexico – Mexican Science and Technology Council (SEP-CONACYT,Mexico) for the Basic Science Project -2015-01 (Ref. 254808), EnergySustainability Fund 2014-05 (CONACYT-SENER), Mexican Centre forInnovation in Bioenergy (Cemie-Bio), Cluster of Bioalcohols (Ref.249564) and the BMBF for the financial support (reference number:031B0660A).info:eu-repo/semantics/publishedVersio
Measurement and Interpretation of Fermion-Pair Production at LEP energies above the Z Resonance
This paper presents DELPHI measurements and interpretations of
cross-sections, forward-backward asymmetries, and angular distributions, for
the e+e- -> ffbar process for centre-of-mass energies above the Z resonance,
from sqrt(s) ~ 130 - 207 GeV at the LEP collider. The measurements are
consistent with the predictions of the Standard Model and are used to study a
variety of models including the S-Matrix ansatz for e+e- -> ffbar scattering
and several models which include physics beyond the Standard Model: the
exchange of Z' bosons, contact interactions between fermions, the exchange of
gravitons in large extra dimensions and the exchange of sneutrino in R-parity
violating supersymmetry.Comment: 79 pages, 16 figures, Accepted by Eur. Phys. J.
- …