5,206 research outputs found
Aspects of Urban Decline: Experiments with a Multilevel Economic- Demographic Model for the Dortmund Region
In this paper, selected results of a multilevel dynamic simulation model of the economic and demographic development in the urban region of Dortmund, FRG, are presented. The model simulates location decisions of industry, residential developers, and households, the resulting migration and commuting patterns, the land use development, and the impacts of public policies in the fields of industrial development, housing, and infrastructure.
In particular, the paper illustrates the capability of the model to capture not only urban growth processes, but also processes of urban decline. For this purpose, first the mechanisms which control spatial growth, decline, or redistribution of activities in the model are outlined. Second, it is demonstrated how the model reproduces the general pattern of past spatial development in the region. Third, results of simulations covering a wide range of potential over all economic and demographic development in the region are discussed
Teaching Physics Using Virtual Reality
We present an investigation of game-like simulations for physics teaching. We
report on the effectiveness of the interactive simulation "Real Time
Relativity" for learning special relativity. We argue that the simulation not
only enhances traditional learning, but also enables new types of learning that
challenge the traditional curriculum. The lessons drawn from this work are
being applied to the development of a simulation for enhancing the learning of
quantum mechanics
Twisted split-ring-resonator photonic metamaterial with huge optical activity
Coupled split-ring-resonator metamaterials have previously been shown to
exhibit large coupling effects, which are a prerequisite for obtaining large
effective optical activity. By a suitable lateral arrangement of these building
blocks, we completely eliminate linear birefringence and obtain pure optical
activity and connected circular optical dichroism. Experiments at around
100-THz frequency and corresponding modeling are in good agreement. Rotation
angles of about 30 degrees for 205nm sample thickness are derived.Comment: 6 pages, 4 figure
Single-cycle gap soliton in a subwavelength structure
We demonstrate that a single sub-cycle optical pulse can be generated when a
pulse with a few optical cycles penetrates through resonant two-level dense
media with a subwavelength structure. The single-cycle gap soliton phenomenon
in the full Maxwell-Bloch equations without the frame of slowly varying
envelope and rotating wave approximations is observed. Our study shows that the
subwavelength structure can be used to suppress the frequency shift caused by
intrapulse four-wave mixing in continuous media and supports the formation of
single-cycle gap solitons even in the case when the structure period breaks the
Bragg condition. This suggests a way toward shortening high-intensity laser
fields to few- and even single-cycle pulse durations.Comment: 4 pages, 6 figure
Carrier-envelope phase dependence in single-cycle laser pulse propagation with the inclusion of counter-rotating terms
We focus on the propagation properties of a single-cycle laser pulse through
a two-level medium by numerically solving the full-wave Maxwell-Bloch
equations. The counter-rotating terms in the spontaneous emission damping are
included such that the equations of motion are slightly different from the
conventional Bloch equations. The counter-rotating terms can considerably
suppress the broadening of the pulse envelope and the decrease of the group
velocity rooted from dispersion. Furthermore, for incident single-cycle pulses
with envelope area 4, the time-delay of the generated soliton pulse from
the main pulse depends crucially on the carrier-envelope phase of the incident
pulse. This can be utilized to determine the carrier-envelope phase of the
single-cycle laser pulse.Comment: 6 pages, 5 figure
Ultrashort light bullets described by the two-dimensional sine-Gordon equation
By using a reductive perturbation technique applied to a two-level model,
this study puts forward a generic two-dimensional sine-Gordon evolution
equation governing the propagation of femtosecond spatiotemporal optical
solitons in Kerr media beyond the slowly varying envelope approximation. Direct
numerical simulations show that, in contrast to the long-wave approximation, no
collapse occurs, and that robust (2+1)-dimensional ultrashort light bullets may
form from adequately chosen few-cycle input spatiotemporal wave forms. In
contrast to the case of quadratic nonlinearity, the light bullets oscillate in
both space and time and are therefore not steady-state lumps
Self-consistent calculation of metamaterials with gain
We present a computational scheme allowing for a self-consistent treatment of
a dispersive metallic photonic metamaterial coupled to a gain material
incorporated into the nanostructure. The gain is described by a generic
four-level system. A critical pumping rate exists for compensating the loss of
the metamaterial. Nonlinearities arise due to gain depletion beyond a certain
critical strength of a test field. Transmission, reflection, and absorption
data as well as the retrieved effective parameters are presented for a lattice
of resonant square cylinders embedded in layers of gain material and split ring
resonators with gain material embedded into the gaps.Comment: 5 pages, 6 figure
Estimating Multiattribute Spatial Choice Models
In this paper, an interactive computer program for estimating the parameters of spatial choice models with multiattribute utilities is presented. The models to be calibrated may be unconstrained, singly constrained, or doubly constrained random utility choice or entropy-maximizing interaction models. Utilities may be associated with choice alternatives (zones) or with the choices themselves (trips). The program maximizes the likelihood of the choice matrix (trip table) given observed choices (trips) using a combination of gradient search and Newton-Raphson iteration methods.
The paper contains a specification of the range of models that can be calibrated with the program and a description of its solution algorithm and organization, as well as an illustrative application and a listing of the source code
Coherent control of atomic excitation using off-resonant strong few-cycle pulses
We study the dynamics of a two-level system driven by an off-resonant
few-cycle pulse which has a phase jump at , in contrast to many
cycle pulses, under non rotating-wave approximation (NRWA). We give a closed
form analytical solution for the evolution of the probability amplitude for the upper level. Using the appropriate pulse parameters like
phase-jump , jump time , pulse width , frequency and
Rabi frequency the population transfer, after the pulse is gone,
can be optimized and for the pulse considered here, enhancement of
factor was obtained.Comment: 5 Pages, 7 Figure
- …