192 research outputs found
Power-dependent shaping of vortex solitons in optical lattices with spatially modulated nonlinear refractive index
We address vortex solitons supported by optical lattices featuring modulation
of both the linear and nonlinear refractive indices. We find that when the
modulation is out-of-phase the competition between both effects results in
remarkable shape transformations of the solitons which profoundly affect their
properties and stability. Nonlinear refractive index modulation is found to
impose restrictions on the maximal power of off-site solitons, which are shown
to be stable only below a maximum nonlinearity modulation depth.Comment: 11 pages, 3 figures, to appear in Optics Letter
Observation of two-dimensional lattice interface solitons
We report on the experimental observation of two-dimensional solitons at the
interface between square and hexagonal waveguide arrays. In addition to the
different symmetry of the lattices, the influence of a varying refractive index
modulation depth is investigated. Such variation strongly affects the
properties of surface solitons residing at different sides of the interface.Comment: 14 pages, 5 figures, to appear in Optics Letter
Nonlinearity-induced broadening of resonances in dynamically modulated couplers
We report the observation of nonlinearity-induced broadening of resonances in
dynamically modulated directional couplers. When the refractive index of the
guiding channels in the coupler is harmonically modulated along the propagation
direction and out-of-phase in two channels, coupling can be completely
inhibited at resonant modulation frequencies. We observe that nonlinearity
broadens such resonances and that localization can be achieved even in detuned
systems at power levels well below those required in unmodulated couplers.Comment: 14 pages, 4 figures, to appear in Optics Letter
Anonymity and Rewards in Peer Rating Systems
When peers rate each other, they may choose to rate inaccurately in order to boost their own reputation or unfairly lower another’s. This could be successfully mitigated by having a reputation server incentivise accurate ratings with a reward. However, assigning rewards becomes a challenge when ratings are anonymous, since the reputation server cannot tell which peers to reward for rating accurately. To address this, we propose an anonymous peer rating system in which users can be rewarded for accurate ratings, and we formally define its model and security requirements. In our system ratings are rewarded in batches, so that users claiming their rewards only reveal they authored one in this batch of ratings. To ensure the anonymity set of rewarded users is not reduced, we also split the reputation server into two entities, the Rewarder, who knows which ratings are rewarded, and the Reputation Holder, who knows which users were rewarded. We give a provably secure construction satisfying all the security properties required. For our construction we use a modification of a Direct Anonymous Attestation scheme to ensure that peers can prove their own reputation when rating others, and that multiple feedback on the same subject can be detected. We then use Linkable Ring Signatures to enable peers to be rewarded for their accurate ratings, while still ensuring that ratings are anonymous. Our work results in a system which allows for accurate ratings to be rewarded, whilst still providing anonymity of ratings with respect to the central entities managing the system
Soliton modes, stability, and drift in optical lattices with spatially modulated nonlinearity
We put forward new properties of lattice solitons in materials and geometries
where both, the linear refractive index and the nonlinearity are spatially
modulated. We show that the interplay between linear and out-of-phase nonlinear
refractive index modulations results in new soliton properties, including
modifications of the soliton stability and transverse mobility, as well as
shape transformations that may be controlled, e.g., by varying the light
intensity.Comment: 12 pages, 4 figures, to appear in Optics Letter
Surface solitons at interfaces of arrays with spatially-modulated nonlinearity
We address the properties of two-dimensional surface solitons supported by
the interface of a waveguide array whose nonlinearity is periodically
modulated. When the nonlinearity strength reaches its minima at the points
where the linear refractive index attains its maxima, we found that nonlinear
surface waves exist and can be made stable only within a limited band of input
energy flows, and for lattice depths exceeding a lower threshold.Comment: 13 pages, 3 figures, to appear in Optics Letter
Rapid assessment of nonlinear optical propagation effects in dielectrics
Ultrafast laser processing applications need fast approaches to assess the nonlinear propagation of the laser beam in order to predict the optimal range of processing parameters in a wide variety of cases. We develop here a method based on the simple monitoring of the nonlinear beam shaping against numerical prediction. The numerical code solves the nonlinear Schrödinger equation with nonlinear absorption under simplified conditions by employing a state-of-the art computationally efficient approach. By comparing with experimental results we can rapidly estimate the nonlinear refractive index and nonlinear absorption coefficients of the material. The validity of this approach has been tested in a variety of experiments where nonlinearities play a key role, like spatial soliton shaping or fs-laser waveguide writing. The approach provides excellent results for propagated power densities for which free carrier generation effects can be neglected. Above such a threshold, the peculiarities of the nonlinear propagation of elliptical beams enable acquiring an instantaneous picture of the deposition of energy inside the material realistic enough to estimate the effective nonlinear refractive index and nonlinear absorption coefficients that can be used for predicting the spatial distribution of energy deposition inside the material and controlling the beam in the writing process
Template-based Fault Injection Analysis of Block Ciphers
We present the first template-based fault injection analysis of FPGA-based block cipher implementations. While template attacks have been a popular form of side-channel analysis in the cryptographic literature, the use of templates in the context of fault attacks has not yet been explored to the best of our knowledge. Our approach involves two phases. The first phase is a profiling phase where we build templates of the fault behavior of a cryptographic device for different secret key segments under different fault injection intensities. This is followed by a matching phase where we match the observed fault behavior of an identical but black-box device with the pre-built templates to retrieve the secret key. We present a generic treatment of our template-based fault attack approach for SPN block ciphers, and illustrate the same with case studies on a Xilinx Spartan-6 FPGA-based implementation of AES-128
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