147 research outputs found
Modulation instability in the weak normal dispersion region of passive fiber ring cavities
International audienceWe report the experimental observation of modulation instability in the weak normal dispersion region of a passive fiber ring cavity. We show that the fourth-order dispersion strongly modifies the dynamics of the cavity through the generation of new instability bands. Experimental results are in excellent agreement with theoretical predictions and numerical simulations
Topographic fibers: a platform for fundamental physical phenomena
International audienceWe review the fundamental physical phenomena associated with parametric resonance which can be successfully investigated in topographic fibers characterized by suitably engineered oscillating dispersion
Dynamics of Turing and Faraday instabilities in a longitudinally modulated fiber-ring cavity
International audienceWe experimentally investigate the roundtrip-to-roundtrip dynamics of the modulation instability spectrum in a passive fiber ring cavity presenting an inhomogeneous dispersion profile. By implementing a real-time spectroscopy technique we are able to record successive single-shot spectra, which display the evolution of the system toward a stationary state. We find that the two instability regimes (Turing and Faraday) that compete in this kind of inhomogeneous cavities not only differ by their characteristic frequency but also by their dynamical behaviour. The dynamic transition between those two regimes of instability is also presented
Momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals measured by angle resolved photoemission spectroscopy
We use angle resolved photoemission spectroscopy (ARPES) to study the
momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals.
We find that the Gamma hole pocket is fully gapped below the superconducting
transition temperature. The value of the superconducting gap is 15 +- 1.5 meV
and its anisotropy around the hole pocket is smaller than 20% of this value.
This is consistent with an isotropic or anisotropic s-wave symmetry of the
order parameter or exotic d-wave symmetry with nodes located off the Fermi
surface sheets. This is a significant departure from the situation in the
cuprates, pointing to possibility that the superconductivity in the iron
arsenic based system arises from a different mechanism.Comment: 4 pages, 3 figure
Universal symmetry-breaking dynamics for the Kerr interaction of counterpropagating light in dielectric ring resonators
Spontaneous symmetry breaking is an important concept in many areas of physics. A fundamentally simple symmetry-breaking mechanism in electrodynamics occurs between counterpropagating electromagnetic waves in ring resonators, mediated by the Kerr nonlinearity. The interaction of counterpropagating light in bidirectionally pumped microresonators finds application in the realization of optical nonreciprocity (for optical diodes), studies of PT-symmetric systems, and the generation of counterpropagating solitons. Here, we present comprehensive analytical and dynamical models for the nonlinear Kerr interaction of counterpropagating light in a dielectric ring resonator. In particular, we study discontinuous behavior in the onset of spontaneous symmetry breaking, indicating divergent sensitivity to small external perturbations. These results can be applied to realize, for example, highly sensitive near-field or rotation sensors. We then generalize to a time-dependent model, which predicts different types of dynamical behavior, including oscillatory regimes that could enable Kerr-nonlinearity-driven all-optical oscillators. The physics of our model can be applied to other systems featuring Kerr-type interaction between two distinct modes, such as for light of opposite circular polarization in nonlinear resonators, which are commonly described by coupled Lugiato-Lefever equations
Point defect distribution in high-mobility conductive SrTiO3 crystals
We have carried out positron annihilation spectroscopy to characterize the spatial distribution and the nature of vacancy defects in insulating as-received as well as in reduced SrTiO3 substrates exhibiting high-mobility conduction. The substrates were reduced either by ion etching the substrate surfaces or by doping with vacancies during thin film deposition at low pressure and high temperature. We show that Ti-vacancies are native defects homogeneously distributed in as-received substrates. In contrast, the dominant vacancy defects are the same both in ion-etched and substrates reduced during the film growth, and they consist of non-homogeneous distributions of cation-oxygen vacancy complexes. Their spatial extension is tuned from a few microns in ion-etched samples to the whole substrate in specimens reduced during film deposition. Our results shed light on the transport mechanisms of conductive SrTiO3 crystals and on strategies for defect-engineered oxide quantum wells, wires and dots
Two-dimensional superconductivity at a Mott-Insulator/Band-Insulator interface: LaTiO3/SrTiO3
Transition metal oxides display a great variety of quantum electronic
behaviours where correlations often play an important role. The achievement of
high quality epitaxial interfaces involving such materials gives a unique
opportunity to engineer artificial structures where new electronic orders take
place. One of the most striking result in this area is the recent observation
of a two-dimensional electron gas at the interface between a strongly
correlated Mott insulator LaTiO3 and a band insulator SrTiO3. The mechanism
responsible for such a behaviour is still under debate. In particular, the
influence of the nature of the insulator has to be clarified. Here we show that
despite the expected electronic correlations, LaTiO3/SrTiO3 heterostructures
undergo a superconducting transition at a critical temperature Tc=300 mK. We
have found that the superconducting electron gas is confined over a typical
thickness of 12 nm. We discuss the electronic properties of this system and
review the possible scenarios
Built-in and induced polarization across LaAlO/SrTiO heterojunctions
Ionic crystals terminated at oppositely charged polar surfaces are inherently
unstable and expected to undergo surface reconstructions to maintain
electrostatic stability. Essentially, an electric field that arises between
oppositely charged atomic planes gives rise to a built-in potential that
diverges with thickness. In ultra thin film form however the polar crystals are
expected to remain stable without necessitating surface reconstructions, yet
the built-in potential has eluded observation. Here we present evidence of a
built-in potential across polar \lao ~thin films grown on \sto ~substrates, a
system well known for the electron gas that forms at the interface. By
performing electron tunneling measurements between the electron gas and a
metallic gate on \lao ~we measure a built-in electric field across \lao ~of 93
meV/\AA. Additionally, capacitance measurements reveal the presence of an
induced dipole moment near the interface in \sto, illuminating a unique
property of \sto ~substrates. We forsee use of the ionic built-in potential as
an additional tuning parameter in both existing and novel device architectures,
especially as atomic control of oxide interfaces gains widespread momentum.Comment: 6 pages, 4 figures. Submitted to Nature physics on May 1st, 201
Mode-locking via dissipative Faraday instability
Emergence of coherent structures and patterns at the nonlinear stage of modulation instability of a uniform state is an inherent feature of many biological, physical and engineering systems. There are several well-studied classical modulation instabilities, such as Benjamin-Feir, Turing and Faraday instability, which play a critical role in the self-organization of energy and matter in non-equilibrium physical, chemical and biological systems. Here we experimentally demonstrate the dissipative Faraday instability induced by spatially periodic zig-zag modulation of a dissipative parameter of the system - spectrally dependent losses - achieving generation of temporal patterns and high-harmonic mode-locking in a fibre laser. We demonstrate features of this instability that distinguish it from both the Benjamin-Feir and the purely dispersive Faraday instability. Our results open the possibilities for new designs of mode-locked lasers and can be extended to other fields of physics and engineering
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