Similar to nitrogen-vacancy centers in diamond and impurity atoms in silicon, interstitial gallium deep paramagnetic centers in GaAsN have been proven to have useful characteristics for the development of spintronic devices. Among other interesting properties, under circularly polarized light, gallium centers act as spin filters that dynamically polarize free and bound electrons reaching record spin polarizations (close to 100%). Furthermore, the recent observation of the amplification of the spin filtering effect under a Faraday configuration magnetic field has suggested that the hyperfine interaction that couples bound electrons and nuclei permits the optical manipulation of its nuclear spin polarization. Even though the mechanisms behind the nuclear spin polarization in gallium centers are fairly well understood, the origin of nuclear spin relaxation and the formation of an Overhauser-like magnetic field remain elusive. In this work we develop a model based on the master equation approach to describe the evolution of electronic and nuclear spin polarizations of gallium centers interacting with free electrons and holes. Our results are in good agreement with existing experimental observations. In particular, we are able to reproduce the amplification of the spin filtering effect under a circularly polarized excitation in a Faraday configuration magnetic field. In regard to the nuclear spin relaxation, the roles of nuclear dipolar and quadrupolar interactions are discussed. Our findings show that, besides the hyperfine interaction, the spin relaxation mechanisms are key to understand the amplification of the spin filtering effect and the appearance of the Overhauser-like magnetic field. To gain a deeper insight in the interplay of the hyperfine interaction and the relaxation mechanisms, we have also performed calculations in the pulsed excitation regime. Based on our model's results we propose an experimental protocol based on time resolved spectroscopy. It consists of a pump-probe photoluminescence scheme that would allow the detection and the tracing of the electron-nucleus flip-flops through time resolved PL measurements.

We demonstrate the detection of coherent electron-nuclear spin oscillations related to the hyperfine interaction and revealed by the band-to-band photoluminescence (PL) in zero external magnetic field. On the base of a pump-probe PL experiment we measure, directly in the temporal domain, the hyperfine constant of an electron coupled to a gallium defect in GaAsN by tracing the dynamical behavior of the conduction electron spin-dependent recombination to the defect site. The hyperfine constants and the relative abundance of the nuclei isotopes involved can be determined without the need of electron spin resonance technique and in the absence of any magnetic field. Information on the nuclear and electron spin relaxation damping parameters can also be estimated from the oscillations damping and the long delay behavior.

A technique to measure low frequency noise in illuminated photodiodes is presented, and some 1/f noise results are given for InGaAs devices. The ability of photodiodes to convert laser noise into RF noise is also discussed, together with other types of 1/f noise arising directly from the optical fiber, and particularly from scattering phenomena inside the fiber.

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Time-resolved optical orientation experiments have been performed in dilute bismidestructures. Bulk layers with bismuth fractions in the range 1-3.8% and quantum wells with bismuth fractions in the range 2.4-7% were investigated. A clear decrease of the electron spin relaxation time is evidenced in both cases when the bismuth content increases. These results can be well interpreted by the increased efficiency of the spin relaxation mechanisms due to the bismuth induced larger spin-orbit interaction in these alloys.The incorporation of small concentrations of bismuth (Bi) into GaAs yields a significant reduction of the band gap energy 0F 1 , 2 . As a consequence the dilute bismide alloys, GaAs 1−x Bi x , are interesting for potential optical telecommunication or photovoltaic applications 3,4 . As Bismuth is a heavy atom, dilute bismides are also characterized by a much larger spin-orbit interaction compared to GaAs 5 . Indeed, an increase of the valence band spin-orbit (SO) split-off energy Δ was clearly evidenced in GaAsBi with values reaching ~800 meV and above (i.e. twice the GaAs value) for a bismuth composition of about 10% 6 .This remarkable property has triggered massive efforts to improve alloy growth, as the condition Δ > could lead to a significant reduction of Auger or Inter Valence Band Absorption loss mechanisms in the NIR telecommunication range as suggested by Sweeney's group et al 7 , 8 . This system has also been proposed as a good candidate for spintronic applications by Fluegel et al. as the enhanced SO coupling in this material allows for a composition-dependent SO engineering and hence possible electron spin tuneability and manipulation 5 .For these reasons, the role of the SO interaction on the electron Landé g-factor in bulk GaAsBi was studied both theoretically and experimentally 9 , 10 . It was shown that the introduction of a Bi fraction of the order of 0.03 led to an increase of the g-factor by a factor of 3. Similar observations were made on the exciton g-factors in quantum wells in the low temperature range 11 .Besides, the elctrons spin relaxation time is a key parameter for spin manipulations as the spin memory should last longer than the manipulation time in order to make use of the electron spin as the information vector. Tong et al. predicted theoretically that the electron spin relaxation time in GaAsBi decreases drastically when the Bi content increases 12 . The experimental investigation of the electron spin relaxation rate in a bulk sample with a bismuth fraction of 2.2% confirmed these predictions 9 . The result was in good agreement with the characterization of the product . (where is the electron Landé factor) determined by Pursley et al. via Hanle effect measurements 13 . However, a measurement of the bismuth dependence of the electron spin relaxation time is still lacking although it is essential to assess the potential of dilute bismides for spintronic applications. In this work we have measured the electron spin dynamics by time-and polarization-resolved photoluminescen...

The electron spin dynamics is studied by time-resolved optical orientation experiments in strained InGaAs/GaAs quantum wells (QWs) grown on (111) or (001) substrates. For a given well width, the electron spin relaxation time in (111) InGaAs QWs decreases by an order of magnitude when the indium fraction in the well varies only from 4% to 12%. In contrast, the electron spin relaxation time depends weakly on the indium fraction in similar InGaAs quantum wells grown on (001) substrates. The strong variation of the electron spin relaxation time in (111) strained quantum well can be well interpreted by the Dyakonov-Perel spin relaxation mechanism where the conduction band spin-orbit splitting is dominated by the structural inversion asymmetry (Rashba term) induced by the piezoelectric field. In (001) QWs, due to the absence of piezoelectric field, the electron spin relaxation time is solely controlled by the Dresselhaus term. These results demonstrate the possibility to engineer the electron spin relaxation time in (111)-oriented quantum wells by the piezoelectric field induced by the built-in strain.

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