response to writing beam

It has been done for a month now: Margherita Turconi has defended her PhD, congratulations!

The jury included several experts on the topics of excitability and optical localized structures: Massimo Giudici (Institut Non Linéaire de Nice), Alejandro Giacomotti (Laboratoire de Photonique et Nanostructures), Philippe Grelu (Institut Carnot de Bourgogne, president), Damià Gomila (Institute for Cross-Disciplinary Physics and Complex Systems) and Franco Prati (Università degli studi dell'Insubria) assisted to Margherita's defence and took part to the discussion which followed. Several insteresting points were raised, underlining the quality of Margherita's work and presentation. Although Margherita (rightfully) decided not to discuss all the results she obtained during her thesis, the presentation included an already very dense set of results.

After an introduction of the two topics which are central to her work (localized structures and excitability), she went on presenting some the research to which she contributed during the last three years.

Demonstration of the control of excitable pulses in a laser with injected signal:

Although the excitable character of this system has been expected for a very long time (since 1998) and several experimental observations have been interpreted in terms of noise-triggered excitable pulses, there were to date no demonstration of the control of excitable pulses, ie measurements of an actual excitability threshold. That is now done.

Numerical analysis of excitable localized states in a laser with saturable absorber:

A laser with saturable absorber (without spatial degrees of freedom, ie with very low Fresnel number) is known to behave like an excitable system for adequately chosen parameter values. It is also known (if on the contrary its Fresnel number is large enough) to host localized structures (and even localized vortices. What was not clear yet was about the coexistence of these two properties: during her defence, Margherita showed numerically that for some parameters, excitable localized structures exist in a model of laser with saturable absorber.

Can excitable localized structures be observed experimentally?

Probably due to the time scales in the experiment (which do not match at all the ones of the model due to the thirty centimeters long optical cavity we have been using) the excitability observed numerically was not observed experimentally, in spite of the extensive study of the response of the system to external perturbations. However, periodic and noise-triggered nucleation of localized states were observed experimentally. Due to the properties of the bifurcation leading to the oscillatory regime (analyzed experimentally), we have been able to argue that gradients intrinsic to the system can lead to excitable localized structures. An analogous argument has recently been made on theoretical basis by the IFISC group (Dissipative soliton excitability induced by spatial inhomogeneities and drift).

Congratulations to Margherita for her defence and for the good work done. The slides are available here: Excitability and laser localized structures in semiconductor microcavities.