551 research outputs found
Microscopic description of exciton-polaritons in microcavities
We investigate the microscopic description of exciton-polaritons that
involves electrons, holes and photons within a two-dimensional microcavity. We
show that in order to recover the simplified exciton-photon model that is
typically used to describe polaritons, one must correctly define the
exciton-photon detuning and exciton-photon (Rabi) coupling in terms of the bare
microscopic parameters. For the case of unscreened Coulomb interactions, we
find that the exciton-photon detuning is strongly shifted from its bare value
in a manner akin to renormalization in quantum electrodynamics. Within the
renormalized theory, we exactly solve the problem of a single exciton-polariton
for the first time and obtain the full spectral response of the microcavity. In
particular, we find that the electron-hole wave function of the polariton can
be significantly modified by the very strong Rabi couplings achieved in current
experiments. Our microscopic approach furthermore allows us to properly
determine the effective interaction between identical polaritons, which goes
beyond previous theoretical work. Our findings are thus important for
understanding and characterizing exciton-polariton systems across the whole
range of polariton densities.Comment: 14 pages, 5 figure
Equatorial Waves in Rotating Bubble-Trapped Superfluids
As the Earth rotates, the Coriolis force causes several oceanic and
atmospheric waves to be trapped along the equator, including Kelvin, Yanai,
Rossby, and Poincar\'e modes. It has been demonstrated that the mathematical
origin of these waves is related to the nontrivial topology of the underlying
hydrodynamic equations. Inspired by recent observations of Bose-Einstein
condensation (BEC) in bubble-shaped traps in microgravity ultracold quantum gas
experiments, we show that equatorial modes are supported by a rapidly rotating
condensate in a spherical geometry. Based on a zero-temperature coarse-grained
hydrodynamic framework, we reformulate the coupled oscillations of the
superfluid and the Abrikosov vortex lattice resulting from rotation by a
Schr\"odinger-like eigenvalue problem. The obtained non-Hermitian Hamiltonian
is topologically nontrivial. Furthermore, we solve the hydrodynamic equations
for a spherical geometry and find that the rotating superfluid hosts Kelvin,
Yanai, and Poincar\'e equatorial modes, but not the Rossby mode. Our
predictions can be tested with state-of-the-art bubble-shaped trapped BEC
experiments.Comment: 11 pages, 5 figure
Multi-Scale Attention for Audio Question Answering
Audio question answering (AQA), acting as a widely used proxy task to explore
scene understanding, has got more attention. The AQA is challenging for it
requires comprehensive temporal reasoning from different scales' events of an
audio scene. However, existing methods mostly extend the structures of visual
question answering task to audio ones in a simple pattern but may not perform
well when perceiving a fine-grained audio scene. To this end, we present a
Multi-scale Window Attention Fusion Model (MWAFM) consisting of an asynchronous
hybrid attention module and a multi-scale window attention module. The former
is designed to aggregate unimodal and cross-modal temporal contexts, while the
latter captures sound events of varying lengths and their temporal dependencies
for a more comprehensive understanding. Extensive experiments are conducted to
demonstrate that the proposed MWAFM can effectively explore temporal
information to facilitate AQA in the fine-grained scene.Code:
https://github.com/GeWu-Lab/MWAFMComment: Accepted by InterSpeech 202
Miniature probe for allâ optical double gradientâ index lenses photoacoustic microscopy
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146638/1/jbio201800147.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146638/2/jbio201800147_am.pd
Nonlinear dynamics of exciton-polariton Bose-Einstein condensate
Exciton-polariton Bose-Einstein condensates (BECs) are newly
emerged quantum systems that are capable of showing macroscopic
quantum phenomena with intrinsic open-dissipative nature. The
spatial distribution of the polariton density, without any
external potential, can be controlled by the geometric shape of
the pumping laser, enabling the investigation of polariton
dynamics with topologically non-trivial configurations.
Meanwhile, exciton-polaritons have spin degrees of freedom
inherited from excitons and photons, making it a candidate for
the realization of quantum logic gates.
In this thesis, we will investigate theoretically the nonlinear
dynamics of exciton-polariton BECs involving both polaritons'
spatial degrees of freedom and spin degrees of freedom, and
interactions between them. This thesis is organised as follows:
In Chapter 1, we will present an overall review of
exiton-polariton systems and important properties of polariton
BECs and then introduce the dynamical equations with various
interactions that will serve as the main theoretical tool for
subsequent chapters. Several polariton pumping and trapping
techniques appearing in later chapters will also be introduced.
In Chapter 2, we will investigate the superfluidity properties of
a single-component polariton condensate under an incoherent
annular pumping configuration. By studying the stability
properties of polariton persistent currents, we find that the
persistent currents can exhibit dynamical instability and
energetic-like instability according to different parameter
region. A stability phase diagram will be given and its relation
with the Landau's criterion will be discussed. In Chapter 3, we
will investigate the spin dynamics of a two-component polariton
condensate under a homogeneous pumping configuration. Owing to
the Josephson coupling, there exist multiple steady state
solutions that allow of controlled spin state switching. A
desynchronized region where there exists no stable steady
solution is found. In the desynchronized region, a desynchronized
state beating periodically over time can exist, which will serve
as a building block of spin waves presented in the next chapter.
In Chapter 4, by combining results from the previous two chapters
we will investigate generally the nonlinear dynamics of polariton
condensates under an annular pumping configuration. The
spin-orbit interaction provided by the Josephson coupling
supports azimuthon states that have simultaneous modulations in
both amplitude and phase. The azimuthon states, when viewed in a
different polarization basis, form rotating spin waves that can
be referred to as the optical ferris wheel. In Chapter 5, results
from previous chapters will be extended to micocavities that
support the anisotropic TE-TM splitting interaction. Rotating
singularities (small-scale vortices) are found as a result. Their
properties and experimental observation techniques will be
discussed.
Chapter 2-5 provide a theoretical framework for the nonlinear
dynamics of polariton condensates. They rely mostly on optical
trapping techniques and are ready to be tested in experiments. In
Chapter 6, polaritons trapped by an engineered periodic mesa
potential will be discussed.
We will investigate the band structure of polaritons under the
influence of the periodic potential together with discussions on
the phase-modulated interference pattern which corresponds to the
polariton Talbot patterns observed in experiments
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