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Injected Spin Lasers

Final Report Summary - ISLAS (Injected spin lasers)

Fellow: Dr Antonio Hurtado

Project: Injected spin lasers (ISLAS)

The aim of this project was to investigate the effect of polarised optical injection into conventional and spin-polarised vertical-cavity surface-emitting lasers (VCSELs) where the polarisation of the emitted light emitted can be controlled through the injection of spin-polarised electrons. We have studied experimentally and in theory different phenomena arising when these devices are subject to different cases of polarised optical injection. Among others, these include injection locking, polarisation switching (PS) and bistability (PB) as well as a rich variety of nonlinear dynamics. In particular, nonlinear dynamics of optically-injected semiconductor lasers has been a fertile research topic for many years and the benefits are now beginning to be seen in stable injection-locked sources with very high bandwidths as well as in potential applications in encrypted communications systems based on chaos. Thus, this project, by combining the hitherto disparate fields of spin-polarised VCSELs and optical injected lasers offered a novel research approach of fundamental importance with many routes to potential applications.

We have first studied experimentally the effect of polarised optical injection in conventional VCSELs operating at the important telecom wavelength of 1 550 nm. Initially, we studied the properties of the polarisation switching (PS) and bistability (PB) that can be induced in these devices under orthogonally-polarised optical injection. Different shapes of PS and PB, including anticlockwise, clockwise and X-Shape, were observed for the first time in both the optical power and frequency domain. Additionally, the occurrence of very wide hysteresis cycles associated to PB has been reported for the first time. These works were also extended for the experimental analysis of PS and PB in a 1 550 nm-VCSEL under different polarised optical injection with arbitrary polarisation angle compared to that of the light emitted by the solitary device. The injection locking properties of these devices have also been experimentally analysed. Different shapes for the stable locking bandwidth were reported for a 1 550 nm-VCSEL when subject to parallel and to orthogonal polarised injection. Additionally, the different regions of nonlinear dynamics appearing outside the stable locking range have also been studied experimentally when the device was subject to parallel and to orthogonally-polarised optical injection. Experimental stability maps plotting the different regions of nonlinear dynamics in the plane of frequency detuning versus injection strength have been produced for both cases of polarised optical injection analysed. Different dynamics, including periodic (such as limit cycle and period doubling) as well as chaotic dynamics have been observed and reported for both cases of polarised optical injection. However, different shapes for the stability maps and very different qualitative behaviour were observed for both cases of polarised optical injection. Finally, the effect of the applied bias current on the regions of nonlinear dynamics has been analysed for the two cases of polarised injection considered.

Also, theoretical investigations have been carried out during the fellowship. An extension of the well-established spin flip model (SFM) has been developed. This extension is capable to reproduce theoretically the behaviour of a conventional as well as a spin-polarised VCSEL not only in its solitary state but also when it is subject to external optical injection. In fact, this novel extension of the SFM model is capable to reproduce the effect of any kind of polarised optical injection, including parallel, orthogonal or elliptically-polarised injection, in a simple, versatile and computing efficient way. The equations of the model have been recently reported and the model has already been used for the theoretical analysis of the injection locking properties and the nonlinear dynamics of a conventional 1 550 nm-VCSEL subject to parallel and orthogonal optical injection. Very good agreement has been found between theory and experiments. Additionally, the largest Liapunov exponent (LEE) method has been used in combination with this generated model for the production of numerically calculated stability maps which match very well the experimental findings. Finally, the analysis of the effect of polarised optical injection in spin-polarised VCSELs is the objective of undergoing research at present and results are expected to be reported and published after the end of the fellowship.

As a result of the work carried out during this project the fellow, Dr Antonio Hurtado, has authored or co-authored 14 papers in high-quality scientific journals; of these 9 have been already published, 2 are accepted for publication and 3 have been submitted and are either under review or pending final publication acceptance. Additionally, the results of this work have been presented in highly ranked scientific conference and workshops and published in their respective conference books of abstracts and proceedings.

In parallel, the fellow has been in charge of supervising the daily experimental work in the laboratory of three PhD students and has also been responsible of leading this team of researchers in the preparation and submission of high-quality conference and journal papers reporting both experimental and theoretical analysis of optically-injected VCSELs. In addition, the fellow has established a scientific collaboration with the group of nonlinear physics of the IFCA-UNICAN (Physics Institute of Cantabria - University of Cantabria) in Santander (Spain). This collaboration has proved to be very intense and successful in the study of optical injection in single- and multi-transverse mode 1 550 nm-VCSELs, and has led to several joint publications.

Moreover, the fellow has been responsible of supervising the work of a PhD student from that group during a three month research stay in the University of Essex (United Kingdom). The fellow has also established a collaboration with the optoelectronics computing group in the University of California at San Diego (USA) for the study of all-optical logic operation with vertical-cavity semiconductor optical amplifiers (VCSOAs). Moreover, during the period of the project the fellow has built an international reputation, has enhanced his professional relationships through different networking opportunities and has disseminated his research work in technical and scientific and conferences as well as through invited seminars in the IFCA-UNICAN (Santander, Spain) and in the Tyndall National Institute (Cork, Ireland).

To conclude, the work carried out in the frame of the IEF Marie Curie project ISLAS shows considerable promise and in time could yield novel knowledge and techniques for the development of optically-injected spin-polarised VCSELs with arbitrary but controlled emitted light polarisation. This breakthrough could in turn eventually lead to new applications and concepts in disparate fields such as spintronics, optoelectronics, optical communications and optical computing. Such developments could include among others spin based information processing and communication systems, chaos-based encrypted transmission systems, microwave frequency generation, spin-controlled VCSELs, etc. This is why we are currently continuing to undergo further research based in the technical challenges posed by this project and in the long term vision we expect spin-controlled optoelectronics devices such as spin-VCSELs to play a major role in future information technologies from an academic and research as well as from an industrial point of view.

List of keywords : vertical-cavity surface emitting lasers (VCSELs), optical injection, spin-polarised VCSELs, polarisation switching and bistability, injection locking, nonlinear dynamics.