100 research outputs found
Optical isolation with nonlinear topological photonics
It is shown that the concept of topological phase transitions can be used to
design nonlinear photonic structures exhibiting power thresholds and
discontinuities in their transmittance. This provides a novel route to devising
nonlinear optical isolators. We study three representative designs: (i) a
waveguide array implementing a nonlinear 1D Su-Schrieffer-Heeger (SSH) model,
(ii) a waveguide array implementing a nonlinear 2D Haldane model, and (iii) a
2D lattice of coupled-ring waveguides. In the first two cases, we find a
correspondence between the topological transition of the underlying linear
lattice and the power threshold of the transmittance, and show that the
transmission behavior is attributable to the emergence of a self-induced
topological soliton. In the third case, we show that the topological transition
produces a discontinuity in the transmittance curve, which can be exploited to
achieve sharp jumps in the power-dependent isolation ratio.Comment: 11 pages, 7 figure
Flat Bands Under Correlated Perturbations
Flat band networks are characterized by coexistence of dispersive and flat
bands. Flat bands (FB) are generated by compact localized eigenstates (CLS)
with local network symmetries, based on destructive interference. Correlated
disorder and quasiperiodic potentials hybridize CLS without additional
renormalization, yet with surprising consequencies: (i) states are expelled
from the FB energy , (ii) the localization length of eigenstates
vanishes as , (iii) the density of states
diverges logarithmically (particle-hole symmetry) and algebraically (no
particle-hole symmetry), (iv) mobility edge curves show algebraic singularities
at . Our analytical results are based on perturbative expansions of the
CLS, and supported by numerical data in one and two lattice dimensions
Edge Modes, Degeneracies, and Topological Numbers in Non-Hermitian Systems
We analyze chiral topological edge modes in a non-Hermitian variant of the 2D
Dirac equation. Such modes appear at interfaces between media with different
"masses" and/or signs of the "non-Hermitian charge". The existence of these
edge modes is intimately related to exceptional points of the bulk
Hamiltonians, i.e., degeneracies in the bulk spectra of the media. We find that
the topological edge modes can be divided into three families
("Hermitian-like", "non-Hermitian", and "mixed"), these are characterized by
two winding numbers, describing two distinct kinds of half-integer charges
carried by the exceptional points. We show that all the above types of
topological edge modes can be realized in honeycomb lattices of ring resonators
with asymmetric or gain/loss couplings.Comment: 6 pages, 3 figures, and Supplementary Materials, to appear in Phys.
Rev. Let
Photonic Anomalous Quantum Hall Effect
We experimentally realize a photonic analogue of the anomalous quantum Hall
insulator using a two-dimensional (2D) array of coupled ring resonators.
Similar to the Haldane model, our 2D array is translation invariant, has zero
net gauge flux threading the lattice, and exploits next-nearest neighbor
couplings to achieve a topologically non-trivial bandgap. Using direct imaging
and on-chip transmission measurements, we show that the bandgap hosts
topologically robust edge states. We demonstrate a topological phase transition
to a conventional insulator by frequency detuning the ring resonators and
thereby breaking the inversion symmetry of the lattice. Furthermore, the
clockwise or the counter-clockwise circulation of photons in the ring
resonators constitutes a pseudospin degree of freedom. We show that the two
pseudospins acquire opposite hopping phases and their respective edge states
propagate in opposite directions. These results are promising for the
development of robust reconfigurable integrated nanophotonic devices for
applications in classical and quantum information processing
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
Reconfigurable topological phases in next-nearest-neighbor coupled resonator lattices
We present a reconfigurable topological photonic system consisting of a 2D
lattice of coupled ring resonators, with two sublattices of site rings coupled
by link rings, which can be accurately described by a tight-binding model.
Unlike previous coupled-ring topological models, the design is translationally
invariant, similar to the Haldane model, and the nontrivial topology is a
result of next-nearest couplings with non-zero staggered phases. The system
exhibits a topological phase transition between trivial and spin Chern
insulator phases when the sublattices are frequency detuned. Such topological
phase transitions can be easily induced by thermal or electro-optic modulators,
or nonlinear cross phase modulation. We use this lattice to design
reconfigurable topological waveguides, with potential applications in on-chip
photon routing and switching.Comment: 5+5 pages, 3+5 figures, published versio
Anomalous single-mode lasing induced by nonlinearity and the non-Hermitian skin effect
Single-mode operation is a desirable but elusive property for lasers
operating at high pump powers. Typically, single-mode lasing is attainable
close to threshold, but increasing the pump power gives rise to multiple lasing
peaks due to inter-modal gain competition. We propose a laser with the opposite
behavior: multi-mode lasing occurs at low output powers, but pumping beyond a
certain value produces a single lasing mode, with all other candidate modes
experiencing negative effective gain. This behavior arises in a lattice of
coupled optical resonators with non-fine-tuned asymmetric couplings, and is
caused by an interaction between nonlinear gain saturation and the
non-Hermitian skin effect. The single-mode lasing is observed in both frequency
domain and time domain simulations. It is robust against on-site disorder, and
scales up to large lattice sizes. This finding might be useful for implementing
high-power laser arrays
Compact Discrete Breathers on Flat Band Networks
Linear wave equations on flat band networks host compact localized
eigenstates (CLS). Nonlinear wave equations on translationally invariant flat
band networks can host compact discrete breathers - time periodic and spatially
compact localized solutions. Such solutions can appear as one-parameter
families of continued linear compact eigenstates, or as discrete sets on
families of non-compact discrete breathers, or even on purely dispersive
networks with fine-tuned nonlinear dispersion. In all cases, their existence
relies on destructive interference. We use CLS amplitude distribution
properties and orthogonality conditions to derive existence criteria and
stability properties for compact discrete breathers as continued CLS.Comment: 12 pages, 6 figure
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