Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\Gamma$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations

Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\Gamma$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations

Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic-field-induced circular polarization of photoluminescence, $P_c$, is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behaviour of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $\tau_{se} = 33~\mu$s, and hole, $\tau_{sh} = 3~\mu$s, bound in the exciton.Comment: 17 pages, 16 figure

Dynamic Polarization of Electron Spins in Indirect Band Gap (In,Al)As/AlAs Quantum Dots in a Weak Magnetic Field: Experiment and Theory

A novel spin orientation mechanism - dynamic electron spin polarization - has been recently suggested in Phys. Rev. Lett. 125, 156801 (2020)PRLTAO0031-900710.1103/PhysRevLett.125.156801. It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field, and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting. © 2021 American Physical Society.We thank M. M. Glazov and M. O. Nestoklon for fruitful discussions. The experimental part of this research has been supported by the Deutsche Forschungsgemeinschaft (Grant No. 409810106) and by the Russian Foundation for Basic Research (Grant Nos. 19-52-12001 and 19-02-00098). M.B. acknowledges the support by the Deutsche Forschungsgemeinschaft (ICRC TRR 160, project A01). The theoretical studies by D.S.S. were supported by the RF President Grant No. MK-5158.2021.1.2, the Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS,” and the Russian Foundation for Basic Research Grant Nos. 19-52-12054 and 20-32-70048. The theoretical studies by A.V.S. were supported by the Russian Foundation for Basic Research Grant No. 19-02-00184

Optimization-Based Control Design Techniques and Tools

International audienceStructured output feedback controller synthesis is an exciting new concept in modern control design, which bridges between theory and practice in so far as it allows for the first time to apply sophisticated mathematical design paradigms like H∞-or H2-control within control architectures preferred by practitioners. The new approach to structured H∞-control, developed during the past decade, is rooted in a change of paradigm in the synthesis algorithms. Structured design may no longer be based on solving algebraic Riccati equations or matrix inequalities. Instead, optimization-based design techniques are required. In this essay we indicate why structured controller synthesis is central in modern control engineering. We explain why non-smooth optimization techniques are needed to compute structured control laws, and we point to software tools which enable practitioners to use these new tools in high technology applications. I. MOTIVATIONS In the modern high technology field control engineers usually face a large variety of concurring design specifications such as noise or gain attenuation in prescribed frequency bands, damping, decoupling, constraints on settling-or rise-time, and much else. In addition, as plant models are generally only approximations of the true system dynamics, control laws have to be robust with respect to uncertainty in physical parameters or with regard to un-modeled high frequency phenomena. Not surprisingly, such a plethora of constraints presents a major challenge for controller tuning, due not only to the ever growing number of such constraints, but also because of their very different provenience. The dramatic increase in plant complexity is exacerbated by the desire that regulators should be as simple as possible, easy to understand and to tune by practitioners, convenient to hardware implement, and generally available at low cost. Such practical constraints explain the limited use of black-box controllers, and they are the driving force for the implementation of structured control architectures, as well as for the tendency to replace hand tuning methods by rigorous algorithmic optimization tools. II. STRUCTURED CONTROLLERS Before addressing specific optimization techniques, we introduce some basic terminology for control design problems with structured controllers. Given a plant P in state-space Pierre Apkarian is with ONERA