PhD defence by Thorsten S. Rasmussen

Title:  Light-matter interaction and laser dynamics in nanophotonic structures

Principal supervisor: Professor Jesper Mørk, DTU Fotonik, Denmark
Co-supervisor: Yi Yu, DTU Fotonik, Denmark

Evaluation Board
Associate Professor Martijn Wubs, DTU Fotonik, Denmark
Professor Andrea Fiore, Eindhoven Univ. of Technology, the Netherlands
Priv.-Doz. Dr. Uwe Bandelow, Humboldt University of Berlin, Germany

Master of the Ceremony
Senior Researcher Phillip Trøst Kristensen, DTU Fotonik

This thesis deals with theoretical and numerical modelling of microscopic semi- conductor lasers, with the perspective of applications in on-chip optical intercon- nects, 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 ex- plained how this Fano laser is realised by replacing a conventional non-dispersive mirror by a highly dispersive and tunable one based on Fano interference be- tween 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 prop- erties of Fano lasers.

Computational models are developed to describe both the stationary and dy- namical behaviour of Fano lasers. The first method, described in chapter 2, consists of full 3D vectorial solutions of Maxwell's equations in the time domain using the finite-difference time-domain method, including for the first time also the effect 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 coupled- mode theory, based on the stationary solutions. This leads to a flexible ordinary differential equation (ODE) model, adaptable to the applications of interest.  It    is also shown how this method can be transformed into an iterative travelling- wave 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 re- sponse, promising improved feedback stability and improved noise properties. The small-signal analysis also reveals an essentially unlimited frequency mod- ulation 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 col- lapse, 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 reflectivity, 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 specific example of applications of Fano lasers in neuromorphic photonic computing.
Finally, chapter 7 deals with inclusion of slow-light effects in the Fano laser mod- els, an effect 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 effects to first 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.

Register to join. Please write to Susanne Kolodziejczyk to join the defence.



tir 27 okt 20
13:30 - 16:30


DTU Fotonik



Via Zoom. Write to Susanne Kolodziejczyk to join.

DEADLINE: 26 October.