Ultra-fast and non-Hermitian dynamics in spin-controlled anisotropic quantum dot micropillar lasers

• Applicants: Professor Dr.-Ing. Nils Christopher Gerhardt; Professor Dr. Stephan Reitzenstein
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548332436

The goal of this project is to develop a new class of ultra-fast lasers by combining high - quantum dot (QD)-based micropillar lasers with the ultra-fast polarization dynamics of spin-lasers. This unique combination allows us not only to address highly interesting fundamental physics questions but also to pave the way for a revolutionary new type of semiconductor lasers with ultra-fast modulation dynamics in the 250 GHz range and rich non-Hermitian properties controlled by anisotropy and carrier spin. Spin-lasers have recently emerged as a promising device technology to overcome the fundamental limitation of the intensity modulation bandwidth of conventional lasers by exploiting the ultra-fast and nonlinear dynamics of the coupling between carrier spin and photon spin in strongly anisotropic cavities. The spin dynamics are typically decoupled from the carrier-driven intensity dynamics and have been shown to reach frequencies beyond 200 GHz even at low carrier densities near laser threshold. In addition, it has recently been theoretically predicted that such spin-lasers are a new representative of the class of non-Hermitian photonic devices that are currently of great interest in fundamental research in the context of exceptional points and parity-time symmetry. QD microlasers represent another highly interesting class of lasers at the crossroads between classical and quantum physics. They feature high spontaneous emission coupling factors (ß) so that their threshold pump power can be reduced by orders of magnitude compared to conventional lasers. Combined with gain-coupling by the QD gain medium, this can lead to rich dynamic behavior in bimodal micropillar lasers. Interestingly, we recently demonstrated for the first time that polarization dynamics of bimodal micropillar lasers can in fact be controlled by spin injection.To achieve our goal and develop a new class of ultra-fast laser, we will adapt the architecture of micropillar lasers using elliptically shaped microcavities and coherent lateral coupling. Through the ellipticity of the micropillars, the mode splitting and thus the resonance frequency of the polarization oscillation can be elegantly controlled during fabrication. In addition, the evanescent mode coupling between two orthogonally shaped micropillars offers great potential to significantly increase the speed, efficiency, and robustness of the spin effects. We benefit from the longstanding and complementary expertise of both partners in micropillar laser nano-processing (TU Berlin) and on the development, study, and description of spin-controlled lasers (RUB). Our innovative spin-microlaser concept and our challenging research plan will allow us, for the first time, to fundamentally investigate the physics of single and laterally coupled spin-controlled microlasers and to realize spin-lasers with pronounced bimodal operation for ultra-fast, large-amplitude polarization dynamics, efficiently controlled by spin injection.

• DFG Programme: Research Grants