63 research outputs found
Observation of valley Landau-Zener-Bloch oscillations and pseudospin imbalance in photonic graphene
We demonstrate inter-valley Bloch oscillation (BO) and Landau-Zener tunneling
(LZT) in an optically-induced honeycomb lattice with a refractive index
gradient. Unlike previously observed BO in a gapped square lattice, we show
non-adiabatic beam dynamics that are highly sensitive to the direction of the
index gradient and the choice of the Dirac cones. In particular, a
symmetry-preserving potential leads to nearly perfect LZT and coherent BO
between the inequivalent valleys, whereas a symmetry-breaking potential
generates asymmetric scattering, imperfect LZT, and valley-sensitive generation
of vortices mediated by a pseudospin imbalance. This clearly indicates that,
near the Dirac points, the transverse gradient does not always act as a simple
scalar force as commonly assumed, and the LZT probability is strongly affected
by the sublattice symmetry as analyzed from an effective Landau-Zener
Hamiltonian. Our results illustrate the anisotropic response of an otherwise
isotropic Dirac platform to real-space potentials acting as strong driving
fields, which may be useful for manipulation of pseudospin and valley degrees
of freedom in graphene-like systems
Dark-bright gap solitons in coupled-mode one-dimensional saturable waveguide arrays
In the present work, we consider the dynamics of dark solitons as one mode of
a defocusing photorefractive lattice coupled with bright solitons as a second
mode of the lattice. Our investigation is motivated by an experiment which
illustrates that such coupled states can exist with both components in the
first gap of the linear band spectrum. This finding is further extended by the
examination of different possibilities from a theoretical perspective, such as
symbiotic ones where the bright component is supported by states of the dark
component in the first or second gap, or non-symbiotic ones where the bright
soliton is also a first-gap state coupled to a first or second gap state of the
dark component. While the obtained states are generally unstable, these
instabilities typically bear fairly small growth rates which enable their
observation for experimentally relevant propagation distances
Multiple flat bands and localized states in photonic super-Kagome lattices
We demonstrate multiple flat bands and compact localized states (CLSs) in a
photonic super-Kagome lattice (SKL) that exhibits coexistence of singular and
nonsingular flat bands within its unique band structure. Specifically, we find
that the upper two flat bands of an SKL are singular - characterized by
singularities due to band touching with their neighboring dispersive bands at
the Brillouin zone center. Conversely, the lower three degenerate flat bands
are nonsingular, and remain spectrally isolated from other dispersive bands.
The existence of such two distinct types of flat bands is experimentally
demonstrated by observing stable evolution of the CLSs with various geometrical
shapes in a laser-written SKL. We also discuss the classification of the flat
bands in momentum space, using band-touching singularities of the Bloch wave
functions. Furthermore, we validate this classification in real space based on
unit cell occupancy of the CLSs in a single SKL plaquette. These results may
provide insights for the study of flatband transport, dynamics, and nontrivial
topological phenomena in other relevant systems.Comment: 5 pages,4 figure
Valley vortex states and degeneracy lifting via photonic higher-band excitation
We demonstrate valley-dependent vortex generation in a photonic graphene.
Without breaking the inversion symmetry, excitation of two equivalent valleys
leads to formation of an optical vortex upon Bragg-reflection to the third
valley, with its chirality determined by the valley degree of freedom.
Vortex-antivortex pairs with valley-dependent topological charge flipping are
also observed and corroborated by numerical simulations. Furthermore, we
develop a three-band effective Hamiltonian model to describe the dynamics of
the coupled valleys, and find that the commonly used two-band model is not
sufficient to explain the observed vortex degeneracy lifting. Such
valley-polarized vortex states arise from high-band excitation without
inversion symmetry breaking or synthetic-field-induced gap opening. Our results
from a photonic setting may provide insight for the study of valley contrasting
and Berry-phase mediated topological phenomena in other systems
Photonic realization of a generic type of graphene edge states exhibiting topological flat band
Cutting a honeycomb lattice (HCL) can end up with three types of edges
(zigzag, bearded and armchair), as is well known in the study of graphene edge
states. Here we theoretically investigate and experimentally demonstrate a
class of graphene edges, namely, the twig-shaped edges, using a photonic
platform, thereby observing edge states distinctive from those observed before.
Our main findings are: (i) the twig edge is a generic type of HCL edges
complementary to the armchair edge, formed by choosing the right primitive cell
rather than simple lattice cutting or Klein edge modification; (ii) the twig
edge states form a complete flat band across the Brillouin zone with
zero-energy degeneracy, characterized by nontrivial topological winding of the
lattice Hamiltonian; (iii) the twig edge states can be elongated or compactly
localized along the boundary, manifesting both flat band and topological
features. Such new edge states are realized in a laser-written photonic
graphene and well corroborated by numerical simulations. Our results may
broaden the understanding of graphene edge states, bringing about new
possibilities for wave localization in artificial Dirac-like materials.Comment: 13 pages, 4 figure
Persistent homology analysis of a generalized Aubry-Andr\'{e}-Harper model
Observing critical phases in lattice models is challenging due to the need to
analyze the finite time or size scaling of observables. We study how the
computational topology technique of persistent homology can be used to
characterize phases of a generalized Aubry-Andr\'{e}-Harper model. The
persistent entropy and mean squared lifetime of features obtained using
persistent homology behave similarly to conventional measures (Shannon entropy
and inverse participation ratio) and can distinguish localized, extended, and
crticial phases. However, we find that the persistent entropy also clearly
distinguishes ordered from disordered regimes of the model. The persistent
homology approach can be applied to both the energy eigenstates and the
wavepacket propagation dynamics.Comment: Published version. 8 pages, 9 figure
Unconventional Flatband Line States in Photonic Lieb Lattices
Flatband systems typically host "compact localized states"(CLS) due to
destructive interference and macroscopic degeneracy of Bloch wave functions
associated with a dispersionless energy band. Using a photonic Lieb
lattice(LL), we show that conventional localized flatband states are inherently
incomplete, with the missing modes manifested as extended line states which
form non-contractible loops winding around the entire lattice. Experimentally,
we develop a continuous-wave laser writing technique to establish a
finite-sized photonic LL with specially-tailored boundaries, thereby directly
observe the unusually extended flatband line states.Such unconventional line
states cannot be expressed as a linear combination of the previously observed
CLS but rather arise from the nontrivial real-space topology.The robustness of
the line states to imperfect excitation conditions is discussed, and their
potential applications are illustrated
Flatband Line States in Photonic Super-Honeycomb Lattices
We establish experimentally a photonic super-honeycomb lattice (sHCL) by use
of a cw-laser writing technique, and thereby demonstrate two distinct flatband
line states that manifest as noncontractible-loop-states in an infinite
flatband lattice. These localized states (straight and zigzag lines) observed
in the sHCL with tailored boundaries cannot be obtained by superposition of
conventional compact localized states because they represent a new topological
entity in flatband systems. In fact, the zigzag-line states, unique to the
sHCL, are in contradistinction with those previously observed in the Kagome and
Lieb lattices. Their momentum-space spectrum emerges in the high-order
Brillouin zone where the flat band touches the dispersive bands, revealing the
characteristic of topologically protected bandcrossing. Our experimental
results are corroborated by numerical simulations based on the coupled mode
theory. This work may provide insight to Dirac like 2D materials beyond
graphene
Topologically protected vortex transport via chiral-symmetric disclination
Vortex phenomena are ubiquitous in nature, from vortices of quantum particles
and living cells [1-7], to whirlpools, tornados, and spiral galaxies. Yet,
effective control of vortex transport from one place to another at any scale
has thus far remained a challenging goal. Here, by use of topological
disclination [8,9], we demonstrate a scheme to confine and guide vortices of
arbitrary high-order charges10,11. Such guidance demands a double topological
protection: a nontrivial winding in momentum space due to chiral symmetry
[12,13] and a nontrivial winding in real space arising from collective complex
coupling between vortex modes. We unveil a vorticity-coordinated rotational
symmetry, which sets up a universal relation between the topological charge of
a guided vortex and the order of rotational symmetry of the disclination
structure. As an example, we construct a C3-symmetry photonic lattice with a
single-core disclination, thereby achieving robust transport of an optical
vortex with preserved orbital angular momentum (OAM) that corresponds solely to
one excited vortex mode pinned at zero energy. Our work reveals a fundamental
interplay of vorticity, disclination and higher-order topological phases14-16,
applicable broadly to different fields, promising in particular for OAM-based
photonic applications that require vortex guides, fibers [17,18] and lasers
[19].Comment: 11 pages, 4 figure
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