Ultrafast Spin Lasers for Modulation Frequencies in the 100 GHz Range

• Applicants: Professor Dr.-Ing. Nils Christopher Gerhardt; Professor Dr.-Ing. Rainer Michalzik
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Experimental Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2013 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 250699912

In times of "High Definition Streaming", and "Cloud Computing", the social and economic development is closely linked to the solution of the bandwidth bottleneck. Short-range optical data transmission systems, e.g., for server clusters and data centers, are becoming increasingly important. Established approaches based on directly modulated semiconductor lasers such as vertical-cavity surface-emitting lasers (VCSELs) are key components for these networks. However, their modulation bandwidth is limited to values below 50 GHz, amongst others by the intrinsic speed of the coupled carrier-photon dynamics in these lasers. To meet the growing demands for short-range optical communication systems, novel, unconventional approaches to increase the modulation speed are urgently needed.A new, very promising approach is to explore spin and polarization modulation instead of conventional current (and thus intensity) modulation in spin-polarized VCSELs. The direct coupling between the spin state of the carriers and the spin state of the photons, i.e., the polarization of the emitted light, is here a crucial factor. The dynamics of the coupled spin system is decoupled from the intensity dynamics of the laser and its polarization state can be modulated much faster. In the last funding period, we were able to demonstrate that the system's resonance frequency can be increased to more than 200 GHz by deliberately introducing birefringence, theoretically enabling a single-channel data transfer rate of more than 240 Gbit/s. In addition, the modulation bandwidth in spin lasers, in contrast to conventional systems, is almost independent of the electrical operating point. This allows to develop highly energy-efficient transmission systems. These results impressively demonstrate the potential of the concept even without reaching its upper limits.Within the framework of this follow-up proposal, the successful device concept will be taken to the next level in close collaboration between the groups in Ulm and Bochum. The project targets to demonstrate the first spin-laser-based data transmission. The goal is to reach data rates of more than 100 Gbit/s with a special focus on the development of data transmission concepts with particularly low energy consumption. Furthermore, integrated and feasible device concepts are developed in which the necessary birefringence is specifically implemented by exploiting special surface gratings. The ultimate goal is to identify a concept that combines high birefringence with efficient spin injection for future practical devices. Thereby the project provides the key requirements for future electrically pumped high-performance spin lasers for a new generation of spintronics-based data transmission systems.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Martin Hofmann