553 research outputs found
Demonstration of superluminal effects in an absorptionless, non-reflective system
We present an experimental and theoretical study of a simple, passive system
consisting of a birefringent, two-dimensional photonic crystal and a polarizer
in series, and show that superluminal dispersive effects can arise even though
no incident radiation is absorbed or reflected. We demonstrate that a vector
formulation of the Kramers-Kronig dispersion relations facilitates an
understanding of these counter-intuitive effects.Comment: 6 pages, 3 figures, accepted on Physical Review Letter
Faster-than-light effects and negative group delays in optics and electronics, and their applications
Recent manifestations of apparently faster-than-light effects confirmed our
predictions that the group velocity in transparent optical media can exceed c.
Special relativity is not violated by these phenomena. Moreover, in the
electronic domain, the causality principle does not forbid negative group
delays of analytic signals in electronic circuits, in which the peak of an
output pulse leaves the exit port of a circuit before the peak of the input
pulse enters the input port. Furthermore, pulse distortion for these
superluminal analytic signals can be negligible in both the optical and
electronic domains. Here we suggest an extension of these ideas to the
microelectronic domain. The underlying principle is that negative feedback can
be used to produce negative group delays. Such negative group delays can be
used to cancel out the positive group delays due to transistor latency (e.g.,
the finite RC rise time of MOSFETS caused by their intrinsic gate capacitance),
as well as the propagation delays due to the interconnects between transistors.
Using this principle, it is possible to speed up computer systems.Comment: 13 pages, 5 figures, 2001 Photonic West Plenary Tal
Fast light, slow light, and phase singularities: a connection to generalized weak values
We demonstrate that Aharonov-Albert-Vaidman (AAV) weak values have a direct
relationship with the response function of a system, and have a much wider
range of applicability in both the classical and quantum domains than
previously thought. Using this idea, we have built an optical system, based on
a birefringent photonic crystal, with an infinite number of weak values. In
this system, the propagation speed of a polarized light pulse displays both
superluminal and slow light behavior with a sharp transition between the two
regimes. We show that this system's response possesses two-dimensional,
vortex-antivortex phase singularities. Important consequences for optical
signal processing are discussed.Comment: 9 pages, 4 figures, accepted in Physical Review Letters (2003
Examining the relationship between student performance and video interactions
In this work, we attempted to predict student performance on a suite of
laboratory assessments using students' interactions with associated
instructional videos. The students' performance is measured by a graded
presentation for each of four laboratory presentations in an introductory
mechanics course. Each lab assessment was associated with between one and three
videos of instructional content. Using video clickstream data, we define
summary features (number of pauses, seeks) and contextual information (fraction
of time played, in-semester order). These features serve as inputs to a
logistic regression (LR) model that aims to predict student performance on the
laboratory assessments. Our findings show that LR models are unable to predict
student performance. Adding contextual information did not change the model
performance. We compare our findings to findings from other studies and explore
caveats to the null-result such as representation of the features, the
possibility of underfitting, and the complexity of the assessment.Comment: 4 pages, 1 figure, submitted to the PERC 2018 proceeding
Photonic crystal polarizers and polarizing beam splitters
We have experimentally demonstrated polarizers and polarizing beam splitters
based on microwave-scale two-dimensional photonic crystals. Using polarized
microwaves within certain frequency bands, we have observed a squared-sinusoid
(Malus) transmission law when using the photonic crystal as a polarizer. The
photonic crystal also functions as a polarizing beamsplitter; in this
configuration it can be engineered to split incident polarizations in either
order, making it more versatile than conventional, Brewster-angle
beamsplitters.Comment: 7 pages, 3 figures, published Journal Applied Physics 93, 9429 (2003
Microwave measurements of the photonic bandgap in a two-dimensional photonic crystal slab
We have measured the photonic bandgap in the transmission of microwaves
through a two-dimensional photonic crystal slab. The structure was constructed
by cementing acrylic rods in a hexagonal closed-packed array to form
rectangular stacks. We find a bandgap centered at approximately 11 GHz, whose
depth, width and center frequency vary with the number of layers in the slab,
angle of incidence and microwave polarization.Comment: 8 pages, 3 figures, submitted to Journal of Applied Physic
Innledning
Sosiologi i dag har i likhet med mange andre i år kastet seg over jubileumsÃÂtematikken. Hundreårsjubileet gir oss mulighet til å diskutere en rekke problemstillinger som favner fagområder som politikk, sosiologi, kulturteori, historie osv. Vårt fokus vil ligge på de historiske begivenheter som har ledet fram til, og dannet vår særegne nasjonale habitus. Vi kunne ha valgt å la jubileet danne utgangspunkt for en diskusjon og framstilling av det norske og en norsk identitet sett innenfra eller utenfra, men valgte bort denne vinklingen i denne omgang. Ikke fordi vi ikke finner denne typen problemstillinger interessante, men vi føler en viss metthet i forhold til den nærmest masochistiske trang vi nordmenn har til alltid å ha oss selv som tema. ââ¬ÂWhat do you think about Norway?ââ¬Â er som kjent en gjenganger når norske journalister møter utlendinger i og utenfor Norge
Non-Gaussian Statistics of Multiple Filamentation
We consider the statistics of light amplitude fluctuations for the
propagation of a laser beam subjected to multiple filamentation in an amplified
Kerr media, with both linear and nonlinear dissipation. Dissipation arrests the
catastrophic collapse of filaments, causing their disintegration into almost
linear waves. These waves form a nearly-Gaussian random field which seeds new
filaments. For small amplitudes the probability density function (PDF) of light
amplitude is close to Gaussian, while for large amplitudes the PDF has a long
power-like tail which corresponds to strong non-Gaussian fluctuations, i.e.
intermittency of strong optical turbulence. This tail is determined by the
universal form of near singular filaments and the PDF for the maximum
amplitudes of the filaments
Reconstructing weak values without weak measurements
I propose a scheme for reconstructing the weak value of an observable without
the need for weak measurements. The post-selection in weak measurements is
replaced by an initial projector measurement. The observable can be measured
using any form of interaction, including projective measurements. The
reconstruction is effected by measuring the change in the expectation value of
the observable due to the projector measurement. The weak value may take
nonclassical values if the projector measurement disturbs the expectation value
of the observable.Comment: 6 pages. Accepted in Phys. Lett.
- …