12,748 research outputs found
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
Quantum Theory of a Resonant Photonic Crystal
We present a quantum model of two-level atoms localized in a 3D lattice,
based on the Hopfield theory of exciton polaritons. In addition to a
polaritonic gap at the exciton energy, a photonic bandgap opens up at the
Brillouin zone boundary. Upon tuning the lattice period or angle of incidence
to match the photonic gap with the exciton energy, one obtains a combined
polaritonic and photonic gap as a generalization of Rabi splitting. For typical
experimental parameters, the size of the combined gap is on the order of 25
cm^{-1}, up to 10^5 times the detuned gap size. The dispersion curve contains a
branch supporting slow-light modes with vanishing exciton probability density.Comment: 4 pages, 3 figure
La mise en oeuvre de la Liste unique des garanties.
L’élaboration, depuis le 1er janvier 2007, de la Liste unique des garanties éligibles aux opérations de refinancement de l’Eurosystème entraîne des adaptations dans les méthodes d’évaluation de la qualité des actifs remis en garantie.Eurosystème, Liste unique, garanties, collatéral, refinancement, qualité de signature, créances privées, actifs négociables.
Reflection-Free One-Way Edge Modes in a Gyromagnetic Photonic Crystal
We point out that electromagnetic one-way edge modes analogous to quantum
Hall edge states, originally predicted by Raghu and Haldane in 2D gyroelectric
photonic crystals possessing Dirac point-derived bandgaps, can appear in more
general settings. In particular, we show that the TM modes in a gyromagnetic
photonic crystal can be formally mapped to electronic wavefunctions in a
periodic electromagnetic field, so that the only requirement for the existence
of one-way edge modes is that the Chern number for all bands below a gap is
non-zero. In a square-lattice gyromagnetic Yttrium-Iron-Garnet photonic crystal
operating at microwave frequencies, which lacks Dirac points, time-reversal
breaking is strong enough that the effect should be easily observable. For
realistic material parameters, the edge modes occupy a 10% band gap. Numerical
simulations of a one-way waveguide incorporating this crystal show 100%
transmission across strong defects, such as perfect conductors several lattice
constants wide, larger than the width of the waveguide.Comment: 4 pages, 3 figures (Figs. 1 and 2 revised.
Artefacts in geometric phase analysis of compound materials
The geometric phase analysis (GPA) algorithm is known as a robust and
straightforward technique that can be used to measure lattice strains in high
resolution transmission electron microscope (TEM) images. It is also attractive
for analysis of aberration-corrected scanning TEM (ac-STEM) images that resolve
every atom column, since it uses Fourier transforms and does not require
real-space peak detection and assignment to appropriate sublattices. Here it is
demonstrated that in ac-STEM images of compound materials (i.e. with more than
one atom per unit cell) an additional phase is present in the Fourier
transform. If the structure changes from one area to another in the image (e.g.
across an interface), the change in this additional phase will appear as a
strain in conventional GPA, even if there is no lattice strain. Strategies to
avoid this pitfall are outlined.Comment: 9 pages, 7 figures, Preprint before review, submitted to
Ultramicroscopy 7 April 201
Resonant X-ray diffraction studies on the charge ordering in magnetite
Here we show that the low temperature phase of magnetite is associated with
an effective, although fractional, ordering of the charge. Evidence and a
quantitative evaluation of the atomic charges are achieved by using resonant
x-ray diffraction (RXD) experiments whose results are further analyzed with the
help of ab initio calculations of the scattering factors involved. By
confirming the results obtained from X-ray crystallography we have shown that
RXD is able to probe quantitatively the electronic structure in very complex
oxides, whose importance covers a wide domain of applications.Comment: 4 pages 4 figures, accepted for publication in PR
Distortion-corrected phase demodulation using phase-generated carrier with multitone mixing
We present a novel phase generated carrier (PGC) demodulation technique for homodyne interferometers which is robust to modulation depth variations and source intensity fluctuations. By digitally mixing the waveform with a multitone synthetic function (a linear combination of harmonics of the modulating signal), distortion can become negligible even in presence of large variations of the modulation depth. The technique only requires two mixers and can also provide the DC component of the phase in real time, without needing any previously recorded data or ellipse-fitting algorithms. We validate the technique with simulated waveforms and with experimental data from a wavelength metering experiment using an integrated unbalanced interferometer on-chip, showing that the technique corrects distortion without increasing the noise with respect to the standard PGC technique
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