arXiv subjects
Publications and source records attributed to S. M. Mahdavi.
We analyze the impact of loss in lattices of coupled optical waveguides and find that in such case, the hopping between adjacent waveguides is necessarily complex. This results not only in a transition of the light spreading from ballistic to diffusive, but also in a new kind of diffraction that is caused by loss dispersion. We prove our theoretical results with experimental observations.
We propose an exactly solvable waveguide lattice incorporating inhomogeneous coupling coefficient. This structure provides a classical analogue to the squeezed number and squeezed coherent intensity distribution in quantum optics where the propagation length plays the role of squeezed amplitude. The intensity pattern is obtained in a closed form for an arbitrary distribution of the initial beam profile. We have also investigated the phase transition to the spatial Bloch-like oscillations by adding a linear gradient to the propagation constant of each waveguides ($ α$). Our analytical results show that the Bloch-like oscillations appear above a critical value for the linear gradient of propagation constant ($ α> α_{c} $). The phase transition (in the propagation properties of the waveguide) is a result of competition between discrete and Bragg diffraction. Moreover, the light intensity decay algebraically along each waveguide at the critical point while it falls off exponentially below the critical point ($ α< α_{c} $).
A new technique has been devised for the analysis of extensive air shower data in observing the effect of the moon on this data. In this technique the number of EAS events with arrival directions falling in error circles centered about the moving moon is compared to the mean number of events falling in error circles with centers randomly chosen in the sky. For any assumed angular radius of the error circle the deficit in EAS event count in the direction of moon i.e., Nsky-Nmoon which is a moon-related effect is interpreted as the shadow of the moon. A simple theoretical model has been developed to relate Nsky to the angular radius of the error circle and has been applied to the counts from the moon's direction in order to extract the physical parameters of the shadow of the moon. The technique and the theoretical model has been used on 1.7 *10^5 EAS events recorded at Alborz observatory.
We introduce a non-perturbative two scale Kirchhoff theory, in the context of light scattering by a rough surface. This is a two scale theory which considers the roughness both in the wavelength scale (small scale) and in the scales much larger than the wavelength of the incident light (large scale). The theory can precisely explain the small peaks which appear at certain scattering angles. These peaks can not be explained by one scale theories. The theory was assessed by calculating the light scattering profiles using the Atomic Force Microscope (AFM) images, as well as surface profilometer scans of a rough surface, and comparing the results with experiments. The theory is in good agreement with the experimental results.
The roughness of glass surfaces after different stages of etching is investigated by reflection measurements with a spectrophotometer, light scattering, and atomic-force microscopy (in small scale), and Talysurf (in large scale). The results suggest, there are three regimes during etching, according to their optical reflectivity and roughness. The first and second regimes are studied by the Kirchhoff theory and the third one is studied by the geometric theory. Also, we compare the roughness obtained by the optical scattering to the AFM results.
We study the conditions for soliton-like wave propagation in the Photorefractive (PR) and electro-optic (i.e., Pockels) material, by using Nonlinear Schrodinger (NLS) equation. The complete NLS equation is solved analytically and numerically by transforming it into the phase space. Our results clearly show the existence of both the dark and bright solitary solutions for the PR medium. Interestingly, however, we find only one bright solitary solution in the Pockels case and there is no evidence of any dark solitary solution.