80 research outputs found
Graphene Electrodynamics in the presence of the Extrinsic Spin Hall Effect
We extend the electrodynamics of two dimensional electron gases to account
for the extrinsic spin Hall effect (SHE). The theory is applied to doped
graphene decorated with a random distribution of absorbates that induce
spin-orbit coupling (SOC) by proximity. The formalism extends previous
semiclassical treatments of the SHE to the non-local dynamical regime. Within a
particle-number conserving approximation, we compute the conductivity,
dielectric function, and spin Hall angle in the small frequency and wave vector
limit. The spin Hall angle is found to decrease with frequency and wave number,
but it remains comparable to its zero-frequency value around the frequency
corresponding to the Drude peak. The plasmon dispersion and linewidth are also
obtained. The extrinsic SHE affects the plasmon dispersion in the long
wavelength limit, but not at large values of the wave number. This result
suggests an explanation for the rather similar plasmonic response measured in
exfoliated graphene, which does not exhibit the SHE, and graphene grown by
chemical vapor deposition, for which a large SHE has been recently reported.
Our theory also lays the foundation for future experimental searches of SOC
effects in the electrodynamic response of two-dimensional electron gases with
SOC disorder.Comment: 12 pages, 4 figure
Quantitative test of general theories of the intrinsic laser linewidth
We perform a first-principles calculation of the quantum-limited laser
linewidth, testing the predictions of recently developed theories of the laser
linewidth based on fluctuations about the known steady-state laser solutions
against traditional forms of the Schawlow-Townes linewidth. The numerical study
is based on finite-difference time-domain simulations of the semiclassical
Maxwell-Bloch lasing equations, augmented with Langevin force terms, and thus
includes the effects of dispersion, losses due to the open boundary of the
laser cavity, and non-linear coupling between the amplitude and phase
fluctuations ( factor). We find quantitative agreement between the
numerical results and the predictions of the noisy steady-state ab initio laser
theory (N-SALT), both in the variation of the linewidth with output power, as
well as the emergence of side-peaks due to relaxation oscillations.Comment: 24 pages, 10 figure
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
Breaking of PT-symmetry in bounded and unbounded scattering systems
PT-symmetric scattering systems with balanced gain and loss can undergo a
symmetry-breaking transition in which the eigenvalues of the non-unitary
scattering matrix change their phase shifts from real to complex values. We
relate the PT-symmetry breaking points of such an unbounded scattering system
to those of underlying bounded systems. In particular, we show how the
PT-thresholds in the scattering matrix of the unbounded system translate into
analogous transitions in the Robin boundary conditions of the corresponding
bounded systems. Based on this relation, we argue and then confirm that the
PT-transitions in the scattering matrix are, under very general conditions,
entirely insensitive to a variable coupling strength between the bounded region
and the unbounded asymptotic region, a result that can be tested experimentally
and visualized using the concept of Smith charts.Comment: 9 pages, 6 figures (final version, including newly added connection
to the concept of "Smith charts"
Nonlinear topological photonics
Rapidly growing demands for fast information processing have launched a race
for creating compact and highly efficient optical devices that can reliably
transmit signals without losses. Recently discovered topological phases of
light provide a novel ground for photonic devices robust against scattering
losses and disorder. Combining these topological photonic structures with
nonlinear effects will unlock advanced functionalities such as nonreciprocity
and active tunability. Here we introduce the emerging field of nonlinear
topological photonics and highlight recent developments in bridging the physics
of topological phases with nonlinear optics. This includes a design of novel
photonic platforms which combine topological phases of light with appreciable
nonlinear response, self-interaction effects leading to edge solitons in
topological photonic lattices, nonlinear topological circuits, active photonic
structures exhibiting lasing from topologically-protected modes, and harmonic
generation from edge states in topological arrays and metasurfaces. We also
chart future research directions discussing device applications such as mode
stabilization in lasers, parametric amplifiers protected against feedback, and
ultrafast optical switches employing topological waveguides.Comment: 21 pages, 12 figure
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