Résumé
In the past decade, continuous interest and research efforts have been dedicated to the study of spin injection into Semiconductor based Light Emitting Device such as Spin-Light Emitting Diodes (Spin-LEDs) [1] and more recently Spin-LASERs such as Vertical Cavity Surface Emitting LASERs (VCSELs) [2]. Our study mainly focuses on the possibility to manipulate the polarization state of the electromagnetic field emitted by a Vertical External Cavity Surface Emitting Laser (VECSEL) thanks to spin-polarized carriers injected in the active medium of the structure. Our work on Spin-LASER is based on our expertise on III-V Spin-LEDs AlGaAs/GaAs/AlAs heterostructures [3-4-5] providing the emission at 800 nm of a circularly-polarized light (Spin polarization rate in the QWs greater than 30%) in the spontaneous recombination regime. The extension of this principle to coherent light should enable to considerably increase the spin polarization rate [6]. We have focused our effort on half-VCSEL structures implemented in external cavity configurations. This enables us to control a part of laser parameters as gain dichroism and birefringence which is useful to optimize spin injections effects on laser emission. The optically injected ½-VCSEL is constituted of a high reflection Bragg mirror (AlAs/GaAs), and an active medium made of 6 to 12 InGaAs QWs confined by GaAsP/GaAs barriers. As a first step to get electrically injected and spin controlled VECSELs, laser oscillations with a VECSEL incorporating an intra-cavity ferromagnetic spin-injector is needed. This step was challenging given the high absorption of classical ferromagnetic spin injector (typically 10% for 5 nm thickness) compared to optical gain in ½-VCSELs, limited to few %. To overcome this limitation, we grew by sputtering a (2.6nm MgO/1.8nm Co/3nm Pt) ferromagnetic Metal-Tunnel-Junction spin-injector in one node of the stationary electric field on the surface of the antiresonant ½-VCSEL structure. By optimizing the filtering and absorption effects of the spin-injector, we succeed to obtain laser oscillation within the new ½-VCSEL. This encouraging result is a first step towards the realization of an electrically pumped VECSEL with spin-polarized electrons. Another way to control electron spin-polarization state in ½-VCSELs consists in using circularly polarized optical pumping. Unfortunately, the polarization state of a laser depends on the competition between the gain dichroism, ΔG, and phase birefringence, γ, in the laser cavity. In order to evaluate the influence of such parameters on the emitted polarization state, we developed a dedicated vectorial model describing the laser eigenstates when taking into account laser parameters as gain dichroism (linear, circular) and phase parameters. Our first experiments showed that a ½-VCSEL implemented in a linear external cavity is in the regime where the linear birefringence is higher than the gain circular dichroism γ >> ΔG resulting in a linear polarization state at the laser output. Hence, to overcome the effects of linear birefringence, we implemented a specific laser cavity including circular phase anisotropy introduced by a Faraday Rotator. This optical component forces the laser to oscillate on circularly polarized eigenstates [7]. In this case, we demonstrate that the VECSEL output circular polarization orientation can be controlled with a high efficiency (100% left or right circular polarization) using the pump circular polarization orientation when switching from Left (σ-) to Right (σ+) Circular polarization. This witnesses an efficient spin-injection and spin-control into the semiconductor active medium of the laser.