Development of the theoretical framework for semiconductor spin-lasers, injected with spin-polarized carriers, and evaluation of how they can outperform their conventional (spin-unpolarized) counterparts.

Intellectual Merit: Practical spintronic devices have shown a remarkable success for magnetic storage and sensing, but remain of limited use for advanced signal processing and digital logic. This proposal will explore alternative realizations of spin-based applications, stimulated by the experimental breakthroughs in spin-lasers: lasing threshold reduction and strong modulation of the emitted light, even at fixed injection intensity. The proposal will study how these principles can be utilized to: enable superior dynamical performance including enhanced bandwidth, chirp reduction, ultrafast switching, and provide guidance for future experimental efforts by exploring novel device concepts and geometries. By developing theoretical and modeling techniques, from rate and drift-diffusion equations to rigorous microscopic description, it will also be possible to advance the understanding of conventional lasers. This proposal can have a transformative effect, spin-lasers would enable secure communications, high-bandwidth interconnects, and reduced power-dissipation.

Broader Impacts: The majority of public school students from Buffalo have no exposure to physical sciences that subsequently deters them from considering careers in science and engineering. To address this concern, the PI will organize summer workshops (Lasers: Theory and Experiment) focusing on the participation of underrepresented groups. A textbook on spintronics will be completed, facilitated by the continued collaboration with J. Fabian (U. Regensburg), to address the lack of suitable resources in spintronics and offer a structured material that can be tailored in many different course offerings.

Project Report

Intellectual Merit: The success of practical spintronic devices operating at room temperature is largely limited to magnetoresistive effects implemented in spin-valves. While spin-valves have shown remarkable success for magnetic storage and sensing, they are of limited use for advanced signal processing and digital logic. Our work has focused on exploring spin-lasers and alternative operating principles of spintronics devices. There are three major differences between spin-lasers and their conventional (spin-unpolarized). (i) Injected carriers are spin polarized. (ii) The light emitted is circularly polarized due to the spin-polarized carriers. When an electron recombines with a hole in accordance with optical selection rules, the electron spin orientation determines the helicity (circular polarization) of the emitted photon, such that the total angular momentum is conserved. The output polarization can be controlled by adjusting either the injection polarization or the intensity. (iii) There are two lasing thresholds: each spin feeds one corresponding mode (polarization) and the imbalance of spin-up and spin-down carrier injection leads to two separate thresholds for majority and minority spin carriers. In the current period we have formulated a microscopic description of spin-lasers allowing us materials-specific prediction of their properties. By considering III-V gain quantum well-based lasers, we have formulated various figures of merits for the optical gain in spin-lasers that would ensure their desirable properties, such as effective spin-filtering, large threshold reduction, and high-frequency operation. Broader Impacts: Some spin-lasers are readily available since they can be based on commercial semiconductor lasers to which a source of circularly polarized light is added subsequently. Our findings can therefore reveal paths for an improved lasers operation where a versatile control of spin polarization could enable spin-enhanced information transfer and high-performance interconnects. Our research on spin-lasers has elucidated the influence of spin degrees of freedom on optical properties and devices, providing an effective way to strengthen links between the spintronics and optics communities.

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Suny at Buffalo
United States
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