M.Sc Thesis | |

M.Sc Student | Kaminski Noam |
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Subject | Linear and Non-linear Wave Guiding in Anomalous Dispersion Regime |

Department | Department of Electrical and Computer Engineering |

Supervisor | PROF. Meir Orenstein |

Full Thesis text |

In electromagnetics and photonics, anomalous dispersion regimes in which the wave number in the direction of propagation of a guided mode is reduced as the frequency is increased, form extremely interesting and peculiar phenomena. The later includes: 'backward waves' where the phase and the pulse energy propagate in anti-parallel directions, and 'negative group velocity' waves where the pulse envelope actually moves in the opposite direction of the pulse energy.

We first explore the continuous wave solutions in this abnormal dispersion regime. We focus on the understanding of the physical meanings of 'backward waves' and their difference from regular guided modes that have co-propagating power and phase. We also study some cases that can exhibit 'backward waves' in waveguides. We find that when the dispersion curve is abnormal, one must make sure that two conditions take place. The first condition states that the total power must flow away from the source and not towards it, while the second condition states that the power emitted from the source, in passive media, must not be amplified as it travels away from the source.

The work includes the intensive investigation of several types of related waveguides: plasmonic waveguide in the forbidden band-gap regime, single surface plasmonic waveguides and negative-positive anisotropic waveguide. The research deals with guided modes, evanescent modes and complex modes in general.

We continued with the research of electromagnetic pulses within such media. We extensively investigated the field of negative group velocity modes which exhibit seemingly unphysical behavior due to the fact that at certain times the pulse breaks into 3 peaks containing much higher energy than the one peak at former and later times, thus breaking the conservation of energy law. The paradox is resolved by introducing a new theorem of negative electromagnetic energy by and redefining the group velocity in non-transparent regimes.

To complete the investigation of pulse propagation in exotic media, we examined pulse propagation in a medium that exhibits nonlinear 'two photon gain', which negates the broadening effect of the dispersion to create a few optical cycles pulse. This idea was proved using an extensive numerical simulation (finite difference time domain) that simulated all the important material parameters that have an influence on the pulse shape and propagation. We showed that under plausible conditions, the 'two photon gain' can dominate the pulse behavior and a very efficient pulse compression is achievable.