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Thèse Année : 2013

Electron and nuclear spin dynamics in GaAs microcavities

Dynamique de spin des electrons et des noyaux dans les microcavit es GaAs

Rakshyakar Giri
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Résumé

The rotation of the plane of polarization of light upon transmission through a magnetized medium is known as Faraday rotation (FR). In non-magnetic semiconductors FR can be produced by optically orienting the spin of electrons. The main objectives of this thesis is (i) to demonstrate large FR due to optically oriented electrons using an n-doped bulk GaAs microcavity, and (ii) to show that the FR can also be used to measure the nuclear spin dynamics without disturbing it. By using optical orientation of electron gas in n-doped bulk GaAs confined in a microcavity (MC), FR up to 19$^\circ$ in the absence of magnetic field is obtained. This strong rotation is achieved because the light makes multiple round trips inside the MC. Fast optical switching of FR in sub-microsecond time scale is demonstrated by sampling the FR in a one shot experiment under pulsed excitation. A concept of FR cross-section as a proportionality coefficient between FR angle, electron spin density and optical path is introduced. This FR cross-section which defines the efficiency of spin polarized electrons in producing FR is estimated quantitatively and compared with the experimental results. Non-destructive measurement of nuclear magnetization in n-GaAs via cavity enhanced FR of an off-resonant light beam is also demonstrated. In contrast with the existing optical methods, this detection scheme does not require the presence of non-equilibrium electrons. Applying this detection scheme to the metallic n-GaAs sample, nuclear FR is found to vary non-monotonously after pump beam is switched off. It consists of two components: one with short decay time ($\sim$10 s) and another with longer decay time and opposite sign ($\sim$200 s). These two contributions to nuclear FR are attributed to two groups of nuclei: (i) nuclear spins situated within the localization radius of donor-bound electrons, which are characterized by fast dynamics, and (ii) all other nuclear spins in the sample characterized by much slower relaxation rate. The results suggest that, even in degenerate semiconductors nuclear spin relaxation is limited by the presence of localized electron states and spin diffusion, rather than by Korringa mechanism. Nuclear FR in the insulating sample, in contrast with the metallic sample, is found to vary monotonously, but again consists of two components. The fast component is even faster than that of the metallic sample ($\sim$1 s), and the slow component decays in the same time scale as that of the metallic sample. Main microscopic mechanisms responsible for nuclear FR is found to be conduction band spin splitting induced by Overhauser field. It dominates nuclear FR in both metallic (conduction band states partly occupied) and insulating (Fermi level below the bottom of the conduction band) samples. FR resulting from the spin unbalanced occupation of donor bound electron states is only observed in metallic sample.
Nous avons obtenu des angles de rotation Faraday (RF) allant jusqu' a 19 par orientation optique d'un gaz d' electrons dans GaAs de type n inclus dans une microcavit e (Q=19000), sans champ magn etique. Cette forte rotation est obtenue en raison des multiples allers-retours de la lumi ere dans la cavit e. Nous avons egalement d emontr e la commutation optique rapide de la RF a l' echelle sub-microseconde en echantillonnant le signal de RF sous excitation impulsionnelle mono-coup. De la d epolarisation de la RF en champ magn etique transverse, nous avons d eduit un temps de relaxation de spin de 160 ns. Le concept de section e cace de RF, coe cient de proportionnalit e entre l'angle RF, la densit e de spin electronique, et le chemin parcouru, a et e introduit. La section e cace de RF, qui d e nit l e cacit e du gaz d' electrons a produire une RF, a et e estim ee quantitativement, et compar ee avec la th eorie. Nous avons egalement d emontr e la possibilit e de mesurer de mani ere non destructive l aimantation nucl eaire dans GaAs-n, via la RF ampli ee par la cavit e. Contrairement aux m ethodes existantes, cette d etection ne n ecessite pas la pr esence d' electrons hors equilibre. Par cette technique nous avons etudi e la dynamique de spin nucl eaire dans GaAs-n avec di erents dopages. Contrairement a ce qu'on pourrait attendre, le d eclin de la RF nucl eaire est complexe et consiste en deux composantes ayant des temps de relaxation tr es di erents. Deux e ets a l origine de la RF nucl eaire sont identi es: le splitting de spin de la bande de conduction, et la polarisation en spin des electrons localis es, tous deux induits par le champ Overhauser. Le premier e et domine la RF nucl eaire dans les deux echantillons etudi es, tandis que la RF induite par les electrons localis es n'a et e observ ee que dans l' echantillon m etallique.
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Dates et versions

tel-00839477 , version 1 (28-06-2013)

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  • HAL Id : tel-00839477 , version 1

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Rakshyakar Giri. Electron and nuclear spin dynamics in GaAs microcavities. Other [cond-mat.other]. Université Montpellier II - Sciences et Techniques du Languedoc, 2013. English. ⟨NNT : ⟩. ⟨tel-00839477⟩
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