590 research outputs found
The Localized Quantum Vacuum Field
A model for the localized quantum vacuum is proposed in which the zero-point
energy of the quantum electromagnetic field originates in energy- and
momentum-conserving transitions of material systems from their ground state to
an unstable state with negative energy. These transitions are accompanied by
emissions and re-absorptions of real photons, which generate a localized
quantum vacuum in the neighborhood of material systems. The model could help
resolve the cosmological paradox associated to the zero-point energy of
electromagnetic fields, while reclaiming quantum effects associated with
quantum vacuum such as the Casimir effect and the Lamb shift; it also offers a
new insight into the Zitterbewegung of material particles.Comment: 21 pages, 1 figur
Causality-based criteria for a negative refractive index must be used with care
Using the principle of causality as expressed in the Kramers-Kronig
relations, we derive a generalized criterion for a negative refractive index
that admits imperfect transparency at an observation frequency . It
also allows us to relate the global properties of the loss (i.e. its frequency
response) to its local behaviour at . However, causality-based criteria
rely the on the group velocity, not the Poynting vector. Since the two are not
equivalent, we provide some simple examples to compare the two criteria.Comment: slightly longer version of published PR
Improving the sensitivity of FM spectroscopy using nano-mechanical cantilevers
It is suggested that nano-mechanical cantilevers can be employed as high-Q
filters to circumvent laser noise limitations on the sensitivity of frequency
modulation spectroscopy. In this approach a cantilever is actuated by the
radiation pressure of the amplitude modulated light that emerges from an
absorber. Numerical estimates indicate that laser intensity noise will not
prevent a cantilever from operating in the thermal noise limit, where the high
Q's of cantilevers are most advantageous.Comment: 5 pages, 1 figur
Effects of electrostatic fields and Casimir force on cantilever vibrations
The effect of an external bias voltage and fluctuating electromagnetic fields
on both the fundamental frequency and damping of cantilever vibrations is
considered. An external voltage induces surface charges causing
cantilever-sample electrostatic attraction. A similar effect arises from
charged defects in dielectrics that cause spatial fluctuations of electrostatic
fields. The cantilever motion results in charge displacements giving rise to
Joule losses and damping. It is shown that the dissipation increases with
decreasing conductivity and thickness of the substrate, a result that is
potentially useful for sample diagnostics. Fluctuating electromagnetic fields
between the two surfaces also induce attractive (Casimir) forces. It is shown
that the shift in the cantilever fundamental frequency due to the Casimir force
is close to the shift observed in recent experiments of Stipe et al. Both the
electrostatic and Casimir forces have a strong effect on the cantilever
eigenfrequencies, and both effects depend on the geometry of the cantilever
tip. We consider cylindrical, spherical, and ellipsoidal tips moving parallel
to a flat sample surface. The dependence of the cantilever effective mass and
vibrational frequencies on the geometry of the tip is studied both numerically
and analytically
Electromagnetically induced transparency in an inverted Y-type four-level system
The interaction of a weak probe laser with an inverted-Y type four-level
atomic system driven by two additional coherent fields is investigated
theoretically. Under the influence of the coherent coupling fields, the
steady-state linear susceptibility of the probe laser shows that the system can
have single or double electromagnetically induced transparency windows
depending on the amplitude and the detuning of the coupling lasers. The
corresponding index of refraction associated with the group velocity of the
probe laser can be controlled at both transparency windows by the coupling
fields. The propagation of the probe field can be switched from superluminal
near the resonance to subluminal on resonance within the single transparency
window when two coupling lasers are on resonance. This provides a potential
application in quantum information processing. We propose an atomic
system for experimental observation
No anomalous scaling in electrostatic calibrations for Casimir force measurements
In a recent paper (Phys.Rev.A78, 020101(R) (2008)), Kim at al. have reported
a large anomaly in the scaling law of the electrostatic interaction between a
sphere and a plate, which was observed during the calibration of their Casimir
force set-up. Here we experimentally demonstrate that in proper electrostatic
calibrations the scaling law follows the behavior expected from elementary
electrostatic arguments, even when the electrostatic voltage that one must
apply to minimize the force (typically ascribed to contact potentials) depends
on the separation between the surfaces.Comment: Final versio
Electromagnetic Energy, Absorption, and Casimir Forces. Inhomogeneous Dielectric Media
A general, exact formula is derived for the expectation value of the
electromagnetic energy density of an inhomogeneous absorbing and dispersive
dielectric medium in thermal equilibrium, assuming that the medium is well
approximated as a continuum. From this formula we obtain the formal expression
for the Casimir force density. Unlike most previous approaches to Casimir
effects in which absorption is either ignored or admitted implicitly through
the required analytic properties of the permittivity, we include dissipation
explicitly via the coupling of each dipole oscillator of the medium to a
reservoir of harmonic oscillators. We obtain the energy density and the Casimir
force density as a consequence of the van der Waals interactions of the
oscillators and also from Poynting's theorem.Comment: 13 pages, no figures. Updated version with generalization to finite
temperature and added example
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