Light-matter interaction and laser dynamics in nanophotonic structures

Thorsten Svend Rasmussen

Research output: Book/ReportPh.D. thesisResearch

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Abstract

This thesis deals with theoretical and numerical modelling of microscopic semiconductor lasers, with the perspective of applications in on-chip optical interconnects, signal processing and sensing at a microscopic scale. The primary focus is a particular kind of photonic crystal laser, known as the Fano laser. It is explained how this Fano laser is realised by replacing a conventional non-dispersive mirror by a highly dispersive and tunable one based on Fano interference between a continuum of modes and a single, discrete mode. This novel type of mirror leads to rich physics, with analysis revealing a number of exciting properties of Fano lasers. Computational models are developed to describe both the stationary and dynamical behaviour of Fano lasers. The rst method, described in chapter 2, consists of full 3D vectorial solutions of Maxwell's equations in the time domain using the nite-dierence time-domain method, including for the rst time also the eect of the active material, leading to lasing. These are primarily used for proof-of-concept example simulations due to their computational demands, but also show promise for more rigorous investigations in future work. The second method, developed in chapter 3, consists of calculating stationary solutions from a conventional oscillation condition, and deriving a dynamical model from coupling of a transmission-line description and temporal coupledmode theory, based on the stationary solutions. This leads to a exible ordinary dierential equation (ODE) model, adaptable to the applications of interest. It is also shown how this method can be transformed into an iterative travellingwave method, resolving the time-resolution limitation and providing a wide range of accuracy. The ODE model is used in chapter 4 to analyse the small-signal response of Fano lasers, revealing how the relaxation oscillation frequency depends on the Fano mirror quality factor, leading to behaviour not observed in conventional Fabry-Perot lasers. This leads to a highly-damped intensity modulation response, promising improved feedback stability and improved noise properties. The small-signal analysis also reveals an essentially unlimited frequency modulation bandwidth with generation of a pure frequency modulated signal by modulation of the nanocavity resonance frequency. The feedback stability of Fano lasers is analysed in chapter 5 by extending the ODE model as a generalisation of the conventional Lang-Kobayashi model, and it is shown numerically and analytically how Fano lasers suppress coherence collapse, with a critical feedback level orders of magnitude larger than comparable Fabry-Perot lasers. This is a crucial property for on-chip applications, where optical isolators are impractical. In order to contextualise this investigation, a more general study of how feedback properties depend on the device size is carried out. This reveals that the Lang-Kobayashi description is valid in most cases, and that the most important factor in the feedback stability of a device is the net gain at which it operates, independent of the device size. Due to the highly dispersive reectivity, Fano lasers are ideal candidates for switching schemes and pulse generation by tuning of the nanocavity resonance. This is the subject of study in chapter 6, which describes pulse generation using cavity dumping and active Q-switching, and investigates a specic example of applications of Fano lasers in neuromorphic photonic computing. Finally, chapter 7 deals with inclusion of slow-light eects in the Fano laser models, an eect which can be highly important in the photonic crystal platform. Based on a coupled-Bloch-mode approach, the Fano laser model is extended to include slow-light eects to rst order, and it is shown how this changes the stationary solutions. Chapter 8 concludes on the work and provides perspectives for future research and applications.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages149
Publication statusPublished - 2020

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