During the preparation of a polarization-based mode-locking experiment in a vertical cavity surface emitting laser, Mathias Marconi, who is preparing his PhD with Massimo Giudici, has noticed and analyzed quite in depth a quite unexpected polarization behavior in a VCSEL. The phenomenon (a kind of nonvolatile polarization memory) is so surprising that it seemed rather impossible at first but Mathias was convinced enough and he brought the proofs, congratulations to him.

It is quite well known that the polarization state of VCSELs may switch depending on parameters. The subject has been quite a hot topic for some years with many experimental data published and quite a lot of modelling as well, often based on the so-called Spin-Flip model. In many cases, polarization bistability has been reported and the optical control of the device's polarization state has also been achieved. Mathias's observations are however quite more puzzling. He has noticed that not only the polarization state of the device can depend on the initial condition (that would be quite expected), but also that the whole polarization-resolved L-I curve can depend on the history of the device. That means that, using some clever optical injection scheme, Mathias has been able to show that a device which is usually emitting on one polarization state (and shows no sign of current-induced polarization switching) can be converted in a permanent but reversible way in a device emitting on the other polarization state... The physical mechanism at play here is not known yet, but the data is here, and calls for an understanding...

The paper is on our publications page: it is about nonvolatile polarization control of a bistable VCSEL.