טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentOren Lahav
SubjectNonlinear Optics with Pulse-Train Beams
DepartmentDepartment of Physics
Supervisor Professor Cohen Oren
Full Thesis textFull thesis text - English Version


Abstract

My research includes two directions in nonlinear propagation of ultrashort optical pulses. In one topic, I demonstrated experimentally the first three-dimensional spatiotemporal soliton (3D-STS): a three dimensional localized wavepacket that propagates in a homogeneous medium while maintaining its shape in all three dimensions, i.e. two transverse (spatial) and one longitudinal (temporal) directions. In the second topic, we developed a method for induction of long-lived atmospheric optical waveguides by ultrashort laser pulses.

Three-dimensional spatiotemporal solitons:

Optical 3D-STS were proposed almost 30 years ago, however, their experimental observation is still considered a significant challenge in nonlinear optics. Their generation requires robust balance between linear diffraction, dispersion and nonlinearity. Finding a system which supports stable propagation of high dimensional solitons is not trivial. One of the main difficulties is the space-time coupling which makes the propagation of an intense ultrashort pulse in bulk medium complicated. To overcome this, I used a decoupling mechanism which was proposed by our group few years ago and is based on employing a medium with two types of nonlinearities with different response times. Using this method, I experimentally demonstrated a new type of spatiotemporal solitons: spatiotemporal pulse-train solitons. These are sequences of ultrashort pulses that are collectively trapped in both transverse directions by a slow nonlinearity and each pulse is self-trapped in the time domain by a fast nonlinearity. In addition, we used this method to demonstrate also enhancement of self-phase modulation spectral broadening inside a two dimensional spatial soliton. This work opens the possibility for experimental investigations of various solitons phenomena in three-dimensions.

Induction of atmospheric waveguides by laser pulses: 

A continuous-wave (CW) laser beam that propagates in the atmosphere spreads as a consequence of diffraction and scattering by small particles (pollution). On the other hand, intense ultra-short optical pulses can propagate as stable filaments for several kilometers. During propagation, the filamenting pulse initiates complex nonlinear dynamics in the densities of free electrons, ions, air density and in the level of molecular alignment. This chain of events, which is only partially understood, can lead to optical effects, e.g. waveguides, in the wake of laser filaments. However, until recently it was generally believed that all optical effects die out after several nanoseconds, which is the characteristic lifetime of the generated plasma. We discovered long-lived (microsecond-scale) air density dynamics in the wake of atmospheric laser filaments. We demonstrated a waveguiding effect that is formed by the acoustic waves which are generated by the filaments. These induced long-lived waveguides can be useful for numerous applications of laser filemantation in the atmosphere, from power transmission through these channels to backward propagation of coherent and incoherent radiation for remote sensing.