טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentFreedman Barak
SubjectNonlinear Wave Interactions in Periodic and Quasiperiodic
Systems
DepartmentDepartment of Physics
Supervisor ? 18? Mordechai Segev
Full Thesis textFull thesis text - English Version


Abstract

My doctoral research focuses on linear and nonlinear wave dynamics in several optical systems, including shock waves, waveguides and photonic quasicrystals. In these systems, the dynamics are determined by the interplay between transport (dispersion /diffraction) effects and nonlinearity. The phenomena associated with these systems are numerous and include lattice solitons, waveguiding, defect dynamics, nonlinear energy transfer, spontaneous pattern formation and more. My research was carried out in the framework of nonlinear optics using photorefractive materials, in systems that are easy to create, control, manipulate and observe; However, the physics underlying these research topics is universal and applies to all nonlinear systems in nature, e.g., fluid dynamics, matter waves, atomic structures, etc.
 My Ph.D. research encompasses primarily the following topics

·        The first experimental observation of optical spatial shock-waves. The shock waves consist of two coupled kink and antikink beams that remain locked to each other throughout propagation. These coupled shock-wave pairs move undistorted at angles that fall outside their original angular sector of propagation, hence they exhibit superluminal propagation (in matter). These effects are driven by energy transfer between the tail regions of optical spatial shockwaves, resulting in a change in the propagation direction.

·        A new method of optical waveguiding, called "Grating Mediated Waveguide". The new method relies on Bragg diffractions from a 1D grating that gives rise to waveguiding in the direction normal to the grating wave vector and arises from cross-phase modulation. The waveguide structure consists of a shallow 1D grating that has a bell- or trough-shaped amplitude in the confinement direction.

·        I have used optical induction to create nonlinear photonic quasicrystals and show the first observation of wave dynamics in quasi-periodic optical lattices, including experiments on linear “discrete” diffraction from various lattice sites, lattice solitons, and experiments on nonlinearly interacting quasi-periodic lattices. These experiments show how an interacting photonic quasicrystal with a dislocation can heal itself through nonlinear interactions.

·        Phasons and phason strain are a special type of excitations unique to structures lacking periodicity, but with long range order (incommensurate structures). Quasicrystals are a special case of this type of structures. I studied defect dynamics arising from these excitations in interacting nonlinear photonic quasicrystals, and demonstrated the first quasi-periodic lattice whose structure incorporates phasons but no phonons at all. I demonstrated experimentally that phasons survive longer than phonons, and that nonlinear interactions in photonic quasicrystals can reduce the phason density within the structure.