phase solitons emerge from chirally charged chaotic areas

One of the main goals of the MOLOSSE project was to explore alternative approaches for the generation of dissipative solitons in relation to mode locking in semiconductor lasers and so we did. In this work, we show that a ring semiconductor laser submitted to coherent external forcing can host dissipative solitons whose existence is fundamentally based on the oscillatory nature of the laser system. Contrary to most optical solitons, the objects we describe host a chiral charge. The stability of only one of the two possible chiral charges results from the broken parity symmetry in a propagative system with non-instantaneous medium.

Dissipative optical solitons fall in most cases in two categories. In absence of external forcing (typically the case of mode-locked lasers) the phase of the optical soliton is free to evolve and the lasing threshold constitutes an example of a symmetry breaking bifurcation: one phase is selected, whatever it is, depending on initial conditions. In systems with external forcing (typically the case of so-called "cavity solitons") the phase of the solitons is locked to that of the forcing. Here we are in a conceptually different situation since the laser is an oscillatory medium. When submitted to forcing, it can lock or not to this forcing and also display many different dynamical regimes corresponding to the physics of the forced oscillator. In the specific case we study, the laser is uniformly locked to the external forcing except for the soliton itself, which exists essentially as a phase rotation embedded in a uniformly locked space. Due to this rotation the solitons carry a chiral charge whose sign is defined by the direction of the phase rotation, which we were able to measure thanks to the collaboration of our colleagues from Cork, Bryan Kelleher and Boguslaw Tykalewicz.

Thanks to our colleagues from Bari and Como (Lorenzo Columbo, Massimo Brambilla, Franco Prati) we have been able to reproduce our experimental findings with a detailed physical model but also to reduce this model to a very paradigmatic equation: a modified Ginzburg-Landau with resonant forcing. Interestingly, even with a term which breaks the parity symmetry of this equation, both chiral charges can exist unless the symmetry breaking term is brought to rather unrealistic values. From there, we infer that the non-instantaneous medium dynamics (certainely the most severe form of broken symmetry along the spatial dimension in propagative systems) is crucial for the stability of only one of the two possible chiral charges.

The paper as published in Physical Review Letters is available on our publications page: it is about Dissipative Phase Solitons in Semiconductor Lasers.