M.Sc Thesis

M.Sc StudentGurgov Hassid
SubjectDecoupling the Spatial and Temporal Dynamics in 3D
Nonlinear Wave Propagation
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


Propagation of a short laser pulse in bulk nonlinear medium naturally couples the pulse spatial and temporal dynamics. This coupling complicates and limits nonlinear interaction in various applications. For example, it prevents the experimental demonstration of 3D spatiotemporal solitons that were predicted twenty years ago, because spatiotemporal coupling gives rise to instabilities that compete with soliton formation.

In this work, a novel concept is developed for decoupling the spatiotemporal dynamics in 3D nonlinear propagation of a pulse-train beam. Evolution in the transverse directions is determined by a slow nonlinearity that responds to the time-averaged intensity of the pulse-train beam only, while a fast nonlinearity (which is much weaker than the slow nonlinearity) controls the dynamics in the temporal domain. We use this de-coupling mechanism for proposing and exploring spatio-temporal pulse train solitons and optical-control of spectral broadening.   

Spatio-temporal pulse train solitons, or trains of light bullets, consist of a sequence of short pulses that are collectively trapped in the transverse directions by a slow nonlinearity and each pulse is self-trapped in the longitudinal (temporal) direction by a fast nonlinearity. In spatiotemporal pulse-train solitons, the characteristic length of the slow nonlinearity corresponds to the diffraction length, while the length of the fast nonlinearity matches the dispersion length. Stability of the soliton is facilitated by the slow nonlinearity and requires that the diffraction length is much shorter than the dispersion length. Various aspects for the experimental observation of spatio-temporal pulse-train solitons are analyzed and windows of accessible parameter sets are suggested. Numerical simulations show stable propagation of the suggested solitons.

The second application of spatiotemporal decoupling investigated in this work, optical-control of self-phase modulation spectral broadening, is experimentally investigated by confining the pulsed beam into a waveguide which is induced by another beam, through a slow self-focusing nonlinearity. We also showed numerically, using accessible material and laser parameters, self-phase modulation spectral broadening of a pulsed beam that is guided and controlled by its average power, without using a second beam.