10,452 research outputs found
Robust self-trapping of vortex beams in a saturable optical medium
We report the first observation of robust self-trapping of vortex beams
propagating in a uniform condensed medium featuring local saturable
self-focusing nonlinearity. Optical vortices with topological charge m=1, that
remain self-trapped over ~ 5 Rayleigh lengths, are excited in carbon disulfide
using a helical light beam at 532 nm and intensities from 8 to 10 GW/cm^2. At
larger intensities, the vortex beams lose their stability, spontaneously
breaking into two fragments. Numerical simulations based on the nonlinear
Schr\"odinger equation including the three-photon absorption and nonpolynomial
saturation of the refractive nonlinearity demonstrate close agreement with the
experimental findings.Comment: 27 pages, 7 figures,to be published in Phys. Rev. A (2016
Effect of particle polydispersity on the irreversible adsorption of fine particles on patterned substrates
We performed extensive Monte Carlo simulations of the irreversible adsorption
of polydispersed disks inside the cells of a patterned substrate. The model
captures relevant features of the irreversible adsorption of spherical
colloidal particles on patterned substrates. The pattern consists of (equal)
square cells, where adsorption can take place, centered at the vertices of a
square lattice. Two independent, dimensionless parameters are required to
control the geometry of the pattern, namely, the cell size and cell-cell
distance, measured in terms of the average particle diameter. However, to
describe the phase diagram, two additional dimensionless parameters, the
minimum and maximum particle radii are also required. We find that the
transition between any two adjacent regions of the phase diagram solely depends
on the largest and smallest particle sizes, but not on the shape of the
distribution function of the radii. We consider size dispersions up-to 20% of
the average radius using a physically motivated truncated Gaussian-size
distribution, and focus on the regime where adsorbing particles do not interact
with those previously adsorbed on neighboring cells to characterize the jammed
state structure. The study generalizes previous exact relations on monodisperse
particles to account for size dispersion. Due to the presence of the pattern,
the coverage shows a non-monotonic dependence on the cell size. The pattern
also affects the radius of adsorbed particles, where one observes preferential
adsorption of smaller radii particularly at high polydispersity.Comment: 9 pages, 5 figure
Recent advances and open challenges in percolation
Percolation is the paradigm for random connectivity and has been one of the
most applied statistical models. With simple geometrical rules a transition is
obtained which is related to magnetic models. This transition is, in all
dimensions, one of the most robust continuous transitions known. We present a
very brief overview of more than 60 years of work in this area and discuss
several open questions for a variety of models, including classical, explosive,
invasion, bootstrap, and correlated percolation
- …