Ever smaller and better laser applications

(27-01-2022) In his doctorate, Nicolas Poulvellarie conducted fundamental research that will allow lasers and microscopes to be optimised even further in the future.

Non-linear optics or photonics deals with phenomena that occur at such high light intensities that the propagating medium no longer behaves linearly; it can be compared to speakers that produce a distorted sound when the amplifier is turned up too loud.

Non-linear photonics has attracted increasing attention in recent decades. This is due to the invention of the laser. Lasers are being developed ever smaller so that they can be integrated into computers, for example; this is called integrated photonics.

"Second-harmonic generation is also a subject that attracts a lot of attention in photonics and has many applications, such as lasers and microscopes," Nicolas says.

Second-harmonic generation ( SHG , also called frequency doubling ) is a non-linear optical process in which two photons of the same frequency interact with a non-linear material (e.g. quartz), are "combined" and generate a new photon with twice the energy of the initial photons. This technique makes it possible, for example, to make things visible in medical microscopes that would otherwise remain hidden.

"In my PhD I investigated this interaction between photons and non-linear III-V materials. The research aimed at gaining a better understanding of second- harmonic generation in III-V integrated structures. Thanks to my research, lasers and microscopes can be further optimised in the future," Nicolas concludes.

Read a more detailed summary or the entire PhD

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PhD Title: Second-Harmonic Generation in III-V Integrated Structures

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Contact: Nicolas Poulvellarie, Bart Kuyken

Nicolas Poulvellarie studied in France where he obtained his engineering degree at the "École nationale supérieure des sciences appliquées et de technologie de Lannion".

He started to work on integrated structures at ULB during his master thesis. At this time, the study was about third order nonlinearities in silicon waveguides. To be more specific, it was the study of the optical equivalent of event horizons. This work resulted in a publiciation in Physical Review Applied entitled "Highly Nondegenerate Two-Photon Absorption in Silicon Wire Waveguides" for which Nicolas is first author.

He then starts a joint PhD at ULB and Ghent University. The PhD is about second order nonlinearities in III-V integrated structures. The research done led to several publications. Two as a first author and one as a co-author. The first one to be published is entitled "Second-harmonic generation enabled by longitudinal electric-field components in photonic wire waveguides" and is an experimental paper linked to a theoretical one named "Influence of longitudinal mode components on second harmonic generation in III-V-on-insulator nanowires" for which he is co-author. The last paper being published is entitled "Efficient type II second harmonic generation in an indium gallium phosphide on insulator wire waveguide aligned with a crystallographic axis" and is linking theory and experimental results.

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Editor: Jeroen Ongenae - Final editing: Ilse Vercruysse