טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentCarmon Tal
SubjectNonlinear Optics and Solitons
DepartmentDepartment of Physics
Supervisor ? 18? Mordechai Segev


Abstract

Beams of light tend to broaden as they propagate in linear medium. However, in a nonlinear medium (in which the index of refraction is proportional to the intensity) self-focusing can compensate for diffraction. In this case, the beam can create an index change similar to the one of an optical fiber and to guide itself in this self-induced fiber. These beams keep their shape and dimensions invariant along propagation. We called this type of beams spatial solitons. The beam that creates the fiber can be composed of more than one mode (e.g. TEM00 + TEM01+...), in this case we called this type of soliton: composite soliton.

           

Here, I report on a new kind of composite solitons that lives in a 3-dimensional bulk; one dimension for propagation and the two transverse dimensions in which the beam is self-trapped. More specifically, I present the first observation of a dipole soliton: a multimode soliton composed of a single-hump component and a double-hump (dipole-type) component that jointly self-trap in two transverse dimensions and propagate in the third dimension. This discovery opened the door for new directions in soliton science. The progress from planar systems (e.g. slab waveguides) to 3D media enabled us to demonstrate phenomena that have no counterpart in 2D, such as a new type of vector soliton for which the energy structure rotates while propagating.  These rotating propeller solitons consist of a dipole that rotates throughout propagation and makes a double helix structure in space, jointly trapped with a rotating bell-shaped component. We have studied these propeller solitons experimentally, analytically, and numerically and set forth the experimental and the theoretical foundation of the first ever composite solitons carrying angular momentum. The combination of rotation and propagation in 3D enabled us to go forward and investigate new phenomena such as collisions in space between rotating vector solitons.


In the second part of this research, I report on spontaneous pattern formation in incoherent cavities. Organized structures of trains of solitons can sometimes emerge spontaneously from a homogeneous state with random noise. We call this phenomenon spontaneous pattern formation. This type of structures can arise in systems with different degrees of coherency (from a fully coherent laser beam to white light from a regular incandescent bulb).


 In our recent series of papers, we have demonstrated pattern formation in incoherent cavities. We first demonstrated pattern formation in temporally incoherent cavities and showed that the bandwidth of the spatial frequency spectrum of the pattern narrows as the feedback increases. This opened the way to our subsequent work on cavities in which the circulating radiation is fully incoherent (both temporally and spatially). I demonstrated experimentally that such fully incoherent cavities exhibit two distinct thresholds: the first modulation instability threshold when self-focusing overcomes incoherent diffraction and the second cavity threshold when gain overcome cavity losses. These two different thresholds are experimentally distinguishable through the fact that the first threshold (the modulation instability threshold) is independent of the feedback, whereas the second threshold (the cavity threshold) is feedback-dependent.