632 research outputs found
Quelques reflexions sur le concept d'entropie issues d'un enseignement de thermodynamique
The authors establish the existence of a gap between physics' students conceptions concerning the notion of entropy and Clausius's, Boltzmann's, constructors of this concept. They analyze some basic features at the origin of these conceptions and some characteristics of the common sense interpretation of entropy. They conclude by some didactical propositions
Broadband Spectral Analysis of Non-Debye Dielectric Relaxation in Percolating Heterostructures
International audienc
HRM inside UK e-commerce firms : innovations in the ânewâ economy and continuities with the âoldâ
The e-commerce approach to people management (i.e. HRM) is popularly believed to be radically new and an innovative rewriting of the âoldâ rules of employment. Yet little is known about which HR practices are used by such companies, and what might explain these companiesâ policy selections in the realm of HR. Exploratory survey data based on a sample of 30 small-medium UK e-commerce firms reports use of employee involvement in decision-making, internal communication, financial participation and reward schemes, performance evaluation, training and provision for employment security. Insights from interviews with five senior managers from the sample augment the survey data with qualitative evidence on e-commerce firmsâ approach to HR. The findings suggest that this approach falls somewhere between radical ânewâ innovations and enduring continuities with âoldâ people management techniques, and that this has parallels with the experience of small-medium enterprises generally
Gain properties of dye-doped polymer thin films
Hybrid pumping appears as a promising compromise in order to reach the much
coveted goal of an electrically pumped organic laser. In such configuration the
organic material is optically pumped by an electrically pumped inorganic device
on chip. This engineering solution requires therefore an optimization of the
organic gain medium under optical pumping. Here, we report a detailed study of
the gain features of dye-doped polymer thin films. In particular we introduce
the gain efficiency , in order to facilitate comparison between different
materials and experimental conditions. The gain efficiency was measured with
various setups (pump-probe amplification, variable stripe length method, laser
thresholds) in order to study several factors which modify the actual gain of a
layer, namely the confinement factor, the pump polarization, the molecular
anisotropy, and the re-absorption. For instance, for a 600 nm thick 5 wt\% DCM
doped PMMA layer, the different experimental approaches give a consistent value
80 cm.MW. On the contrary, the usual model predicting the gain
from the characteristics of the material leads to an overestimation by two
orders of magnitude, which raises a serious problem in the design of actual
devices. In this context, we demonstrate the feasibility to infer the gain
efficiency from the laser threshold of well-calibrated devices. Besides,
temporal measurements at the picosecond scale were carried out to support the
analysis.Comment: 15 pages, 17 figure
Assessing the Polarization of a Quantum Field from Stokes Fluctuation
We propose an operational degree of polarization in terms of the variance of
the projected Stokes vector minimized over all the directions of the Poincar\'e
sphere. We examine the properties of this degree and show that some problems
associated with the standard definition are avoided. The new degree of
polarization is experimentally determined using two examples: a bright squeezed
state and a quadrature squeezed vacuum.Comment: 4 pages, 2 figures. Comments welcome
First-principles calculations for the adsorption of water molecules on the Cu(100) surface
First-principles density-functional theory and supercell models are employed
to calculate the adsorption of water molecules on the Cu(100) surface. In
agreement with the experimental observations, the calculations show that a H2O
molecule prefers to bond at a one-fold on-top (T1) surface site with a tilted
geometry. At low temperatures, rotational diffusion of the molecular axis of
the water molecules around the surface normal is predicted to occur at much
higher rates than lateral diffusion of the molecules. In addition, the
calculated binding energy of an adsorbed water molecule on the surfaces is
significantly smaller than the water sublimation energy, indicating a tendency
for the formation of water clusters on the Cu(100) surface.Comment: 5 pages, 3 figures, submitted to Phys. Rev.
Polarization of tightly focused laser beams
The polarization properties of monochromatic light beams are studied. In
contrast to the idealization of an electromagnetic plane wave, finite beams
which are everywhere linearly polarized in the same direction do not exist.
Neither do beams which are everywhere circularly polarized in a fixed plane. It
is also shown that transversely finite beams cannot be purely transverse in
both their electric and magnetic vectors, and that their electromagnetic energy
travels at less than c. The electric and magnetic fields in an electromagnetic
beam have different polarization properties in general, but there exists a
class of steady beams in which the electric and magnetic polarizations are the
same (and in which energy density and energy flux are independent of time).
Examples are given of exactly and approximately linearly polarized beams, and
of approximately circularly polarized beams.Comment: 9 pages, 6 figure
An insight into polarization states of solid-state organic lasers
The polarization states of lasers are crucial issues both for practical
applications and fundamental research. In general, they depend in a combined
manner on the properties of the gain material and on the structure of the
electromagnetic modes. In this paper, we address this issue in the case of
solid-state organic lasers, a technology which enables to vary independently
gain and mode properties. Different kinds of resonators are investigated:
in-plane micro-resonators with Fabry-Perot, square, pentagon, stadium, disk,
and kite shapes, and external vertical resonators. The degree of polarization P
is measured in each case. It is shown that although TE modes prevail generally
(P>0), kite-shaped micro-laser generates negative values for P, i.e. a flip of
the dominant polarization which becomes mostly TM polarized. We at last
investigated two degrees of freedom that are available to tailor the
polarization of organic lasers, in addition to the pump polarization and the
resonator geometry: upon using resonant energy transfer (RET) or upon pumping
the laser dye to an higher excited state. We then demonstrate that
significantly lower P factors can be obtained.Comment: 12 pages, 12 figure
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