Ph.D Student | Harari Gal |
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Subject | Topological Insulator Lasers |

Department | Department of Physics |

Supervisor | ? 18? Mordechai Segev |

Full Thesis text |

Topological insulators are a phase of matter with an insulating bulk and conducting edges that first emerged in condensed matter physics. A topological insulator is characterized by a bulk bandgap where topological gapless unidirectional edge states reside. These edge states are robust to any perturbation that does not close the bandgap. In this way, deformations of the system like disorder, strain, or imperfections have little effect on the transport of such topological edge modes. The discovery of topological insulators, originally made for electrons in solids, has subsequently motivated the search of topological systems in optics and photonics with the aim to engineer robust optical devices. In photonics, the first topological insulators were proposed in gyromagnetic materials and demonstrated using strong magnetic fields in microwave frequencies. However, magneto-optics effects are very weak at optical frequencies. Consequently, a completely different approach was needed for making topological insulators in photonics. Topological protection is now known to be a ubiquitous phenomenon, occurring in many physical systems, ranging from photonics and cold atoms to mechanical systems.

Non-Hermitian topological systems is a more recent concept, with particular interest in optics, where loss and gain, which have no equivalence in electronic systems, play a vital rule. Topology in such systems is a topic of active research: the existence of topological phases, how to define the topological phases and their stability. It is now known that one-dimensional non-Hermitian systems can have topological edge states and stationary topological defect states. However, the inclusion of gain to 2D systems and its impact on laser action required a major leap.

Lasers are complex physical systems with very rich dynamics. They are fundamentally non-Hermitian, open and nonlinear systems. Laser cavities are typically designed to have a high Q-factor, to avoid disorder which strongly degrades the performance. A prominent effect of disorder is mode localization, which has profound implications in photonics. To a laser system this implies degraded overlap of the lasing mode with the gain profile, lower output coupling and multimode lasing, altogether resulting in an overall reduced efficiency and coherence.

This thesis presents the theoretical study and experimental demonstration of a topological insulator laser, a laser in which the topological edge states are made to lase. Thanks to their topological robustness the lasing is immune to disorder, defects and maintains single mode lasing even in the presence of disorder and defects. To grasp the non-trivial interplay between topology and non-Hermiticity a theoretical model is presented, unveiling the action of the topological insulator laser. It is shown how non-linearity helps to maintain topological protection even when the linear system is not immune to backscattering. The experimental results were obtained using InGaAs ring lasers arranged in an aperiodic array. The array is time-invariant, but thanks to the gain-saturation and open boundary conditions the system is rendered topologically robust.

My work established the high-efficiency lasing of a topological structure compared to the trivial (non-topological) structure and paves the way to active topological devices and the study of topological features of active non-linear media.