12,219 research outputs found
Berry's phase and the anomalous velocity of Bloch wavepackets
The semiclassical equations of motion for a Bloch electron include an
anomalous velocity term analogous to a -space "Lorentz force", with the
Berry connection playing the role of a vector potential. By examining the
adiabatic evolution of Bloch states in a monotonically-increasing vector
potential, I show that the anomalous velocity can be explained as the
difference in the Berry's phase acquired by adjacent Bloch states within a
wavepacket.Comment: 2 pages, 1 figur
Some notions of decentralization and coordination in large-scale dynamic systems
Some notions of decentralization and coordination in the control of large-scale dynamic systems are discussed. Decentralization and coordination have always been important concepts in the study of large systems. Roughly speaking decentralization is the process of dividing a large problem into subproblems so that it can be handled more easily. Coordination is the manipulation of the subproblem so that the original problem is solved. The various types of decentralization and coordination that have been used to control dynamic systems are discussed. The emphasis was to distinguish between on-line and off-line operations to understand the results available by indicating the aspects of the problem which are decentralized
Coherent optical control of polarization with a critical metasurface
We describe the mechanism by which a metamaterial surface can act as an ideal
phase-controlled rotatable linear polarizer. With equal-power linearly
polarized beams incident on each side of the surface, varying the relative
phase rotates the polarization angles of the output beams, while keeping the
polarization exactly linear. The explanation is based on coupled-mode theory
and the idea of coherent perfect absorption into auxiliary polarization
channels. The polarization-rotating behavior occurs at a critical point of the
coupled-mode theory, which can be associated with the exceptional point of a
parity-time (PT) symmetric effective Hamiltonian
Dark-State Polaritons in Single- and Double- Media
We derive the properties of polaritons in single- and
double- media using a microscopic equation-of-motion technique. In
each case, the polaritonic dispersion relation and composition arise from a
matrix eigenvalue problem for arbitrary control field strengths. We show that
the double- medium can be used to up- or down-convert single photons
while preserving quantum coherence. The existence of a dark-state polariton
protects this single-photon four-wave mixing effect against incoherent decay of
the excited atomic states. The efficiency of this conversion is limited mainly
by the sample size and the lifetime of the metastable state.Comment: 7 pages, 6 figure
Pseudo-Hermitian Hamiltonians Generating Waveguide Mode Evolution
We study the properties of Hamiltonians defined as the generators of transfer
matrices in quasi- one-dimensional waveguides. For single- or multi-mode
waveguides obeying flux conservation and time-reversal invariance, the
Hamiltonians defined in this way are non-Hermitian, but satisfy symmetry
properties that have previously been identified in the literature as "pseudo
Hermiticity" and "anti-PT symmetry". We show how simple one-channel and
two-channel models exhibit transitions between real, imaginary, and complex
eigenvalue pairs.Comment: 7 pages, 2 figure
Optical Resonator Analog of a Two-Dimensional Topological Insulator
A lattice of optical ring resonators can exhibit a topological insulator
phase, with the role of spin played by the direction of propagation of light
within each ring. Unlike the system studied by Hafezi et al., topological
protection is achieved without fine-tuning the inter-resonator couplings, which
are given the same periodicity as the underlying lattice. The topological
insulator phase occurs for strong couplings, when the tight-binding method is
inapplicable. Using the transfer matrix method, we derive the bandstructure and
phase diagram, and demonstrate the existence of robust edge states. When gain
and loss are introduced, the system functions as a diode for coupled resonator
modes.Comment: 10 pages, 9 figure
PID control system analysis and design
With its three-term functionality offering treatment of both transient and steady-state responses,
proportional-integral-derivative (PID) control provides a generic and efficient solution to realworld
control problems. The wide application of PID control has stimulated and sustained
research and development to "get the best out of PID", and "the search is on to find
the next key technology or methodology for PID tuning".
This article presents remedies for problems involving the integral and derivative terms. PID design objectives,
methods, and future directions are discussed. Subsequently, a computerized, simulation-based approach
is presented, together with illustrative design results for first-order, higher order, and nonlinear plants. Finally,
we discuss differences between academic research and industrial practice, so as to motivate new research
directions in PID control
PID control system analysis, design, and technology
Designing and tuning a proportional-integral-derivative
(PID) controller appears to be conceptually intuitive, but can
be hard in practice, if multiple (and often conflicting) objectives
such as short transient and high stability are to be achieved.
Usually, initial designs obtained by all means need to be adjusted
repeatedly through computer simulations until the closed-loop
system performs or compromises as desired. This stimulates
the development of "intelligent" tools that can assist engineers
to achieve the best overall PID control for the entire operating
envelope. This development has further led to the incorporation
of some advanced tuning algorithms into PID hardware modules.
Corresponding to these developments, this paper presents a
modern overview of functionalities and tuning methods in patents,
software packages and commercial hardware modules. It is seen
that many PID variants have been developed in order to improve
transient performance, but standardising and modularising PID
control are desired, although challenging. The inclusion of system
identification and "intelligent" techniques in software based PID
systems helps automate the entire design and tuning process to
a useful degree. This should also assist future development of
"plug-and-play" PID controllers that are widely applicable and
can be set up easily and operate optimally for enhanced productivity,
improved quality and reduced maintenance requirements
Hidden Black: Coherent Enhancement of Absorption in Strongly-scattering Media
We show that a weakly absorbing, strongly scattering (white) medium can be
made very strongly absorbing at any frequency within its strong-scattering
bandwidth by optimizing the input electromagnetic field. For uniform
absorption, results from random matrix theory imply that the reflectivity of
the medium can be suppressed by a factor ~(l_a/lN^2), where N is the number of
incident channels and l,l_a are the elastic and absorption mean free paths
respectively. It is thus possible to increase absorption from a few percent to
> 99%. For a localized weak absorber buried in a non-absorbing scattering
medium, we find a large but bounded enhancement.Comment: 4 pages, 4 figure
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