The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.

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Nuclear spin physics in quantum dots: an optical investigation

We have performed pump-probe differential transmission spectroscopy (DTS) measurements on In/sub 0.4/Ga/sub 0.6/As-GaAs-AlGaAs heterostructures, which show that at room temperature, injected electrons preferentially occupy the excited states in the dots and states in the barriers layers. The relaxation time of these carriers to the dot ground state is >100 ps. This leads to large gain compression in quantum-dot (QD) lasers and limits the attainable small-signal modulation bandwidth to /spl sim/ 5-7 GHz. The problem can be alleviated by tunneling "cold" electrons into the lasing states of the dots from an adjoining injector layer. The design, growth, and steady-state and small-signal modulation characteristics of tunnel injection In/sub 0.4/Ga/sub 0.6/As-GaAs QD lasers are described and discussed. The tunneling times, directly measured by three-pulse DTS measurements, are /spl sim/ 1.7 ps and independent of temperature. The measured small-signal modulation bandwidth for I/I/sub th/ /spl sim/ 7 is f/sub -3 dB/ = 23 GHz and the gain compression factor for this frequency response is /spl epsiv/ = 8.2 /spl times/ 10/sup -16/ cm/sup 3/. The differential gain obtained from the modulation data is dg/dn /spl cong/ 2.7 /spl times/ 10/sup -14/ cm/sup 2/ at room temperature. The value of the K-factor is 0.205 ns and the maximum intrinsic modulation bandwidth is 43.3 GHz. Analysis of the transient characteristics with appropriate carrier and photon rate equations yield modulation response characteristics identical to the measured ones. The Auger coefficients are in the range /spl sim/ 3.3 /spl times/ 10/sup -29/ cm/sup 6//s to 3.8 /spl times/ 10/sup -29/ cm/sup 6//s in the temperature range 15/spl deg/C<T<85/spl deg/C, determined from large-signal modulation measurements, and these values are smaller than those measured in separate confinement heterostructure QD lasers. The measured high-speed data are comparable to, or better than, equivalent quantum-well lasers for the first time.

Carrier dynamics and high-speed modulation properties of tunnel injection InGaAs-GaAs quantum-dot lasers

Semiconductor saturable absorber mirrors (SESAMs) using quantum dot (QD) absorbers exhibit a larger design freedom than standard quantum well absorbers. The additional parameter of the dot density in combination with the field enhancement allows for an independent control of saturation fluence and modulation depth. We present the first detailed study of the effect of QD growth parameters and post growth annealing on the macroscopic optical SESAM parameters, measuring both nonlinear reflectivity and recombination dynamics. We studied a set of self-assembled InAs QD-SESAMs optimized for an operation wavelength around 960 nm with varying dot density and growth temperature. We confirm that the modulation depth is controlled by the dot density. We present design guidelines for QD-SESAMs with low saturation fluence and fast recovery, which are for example important for modelocking of vertical external cavity surface emitting lasers (VECSELs).

Growth parameter optimization for fast quantum dot SESAMs.

We report on resonant photoluminescence (PL) of InGaN inclusions in a GaN matrix. The structures were grown on sapphire substrates using metal-organic chemical vapor deposition. Nonresonant pulsed excitation results in a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot nature of luminescence in InGaN-GaN samples. PL decay demonstrates strongly nonexponential behavior evidencing coexistence of quantum dots having similar ground-state transition energy, but very different electron-hole wave-function overlap.

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Quantum dot origin of luminescence in InGaN-GaN structures

Generating, manipulating and detecting electron spin polarization and coherence at room temperature is at the heart of future spintronics and spin-based quantum information technology. Spin filtering, which is a key issue for spintronic applications, has been demonstrated by using ferromagnetic metals, diluted magnetic semiconductors, quantum point contacts, quantum dots, carbon nanotubes, multiferroics and so on. This filtering effect was so far restricted to a limited efficiency and primarily at low temperatures or under a magnetic field. Here, we provide direct and unambiguous experimental proof that an electron-spin-polarized defect, such as a Ga(i) self-interstitial in dilute nitride GaNAs, can effectively deplete conduction electrons with an opposite spin orientation and can thus turn the non-magnetic semiconductor into an efficient spin filter operating at room temperature and zero magnetic field. This work shows the potential of such defect-engineered, switchable spin filters as an attractive alternative to generate, amplify and detect electron spin polarization at room temperature without a magnetic material or external magnetic fields.

/pdf/room-temperature-defect-engineered-spin-filter-based-on-a-5d41fnkqx8.pdf

Room-temperature defect-engineered spin filter based on a non-magnetic semiconductor.

We investigate the carrier dynamics in self-assembled InAs/GaAs quantum dots under strictly resonant excitation of the ground state. The spectral selectivity of the resonant excitation allows us to study the physical properties of a class of dots characterized by an energy distribution comparable to the excitation laser spectrum. We detect no Stokes shift of the photoluminescence (PL) line. The PL decay time yields a straightforward determination of the radiative recombination time.

Time-resolved photoluminescence in self-assembled InAs/GaAs quantum dots under strictly resonant excitation

Optically detected magnetic resonance measurements are carried out to study formation of Ga interstitial-related defects in Ga(In)NAs alloys. The defects, which are among dominant nonradiative recombination centers that control carrier lifetime in Ga(In)NAs, are unambiguously proven to be common grown-in defects in these alloys independent of the employed growth methods. The defects formation is suggested to become thermodynamically favorable because of the presence of nitrogen, possibly due to local strain compensation.

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Dominant recombination centers in Ga(In)NAs alloys: Ga interstitials

Room-temperature spin-dependent recombination in a series of GaAs1−x
Nx solid solutions (x = 2.1, 2.7, 3.4%) has been observed as manifested by a more than threefold decrease in intensity of the edge photoluminescence upon switching from circular to linear polarization of the exciting light or upon the application of a transverse magnetic field (∼300 G). The interband absorption of the circularly polarized light is accompanied by the spin polarization of conduction electrons, which reaches 35% with an increase in the pumping level. The observed effects are explained in terms of the dynamic polarization of deep paramagnetic centers and the spin-dependent trapping of conduction electrons on these centers. The electron spin relaxation time, as estimated from the dependence of the edge photoluminescence depolarization in the transverse magnetic field (the Hanle effect) on the pumping intensity, is on the order of 1 ns. According to the adopted theory, the electron spin relaxation time in the presence of spin-dependent recombination is determined by a slow spin relaxation of localized electrons. The sign (positive) of the g factor of localized electrons has been experimentally determined from the direction of the magnetic-field-induced rotation of their average spin observed in the three GaAsN crystals studied.

Spin-dependent recombination in GaAsN solid solutions

The paper studies the circularly polarized photoluminescence (PL) from dilute GaAsN alloys with nitrogen content of 1%–3.4%, grown on GaAs substrates. The room-temperature PL is found to consist of two bands whose splitting grows with increasing nitrogen content. The analysis of the PL circular polarization has shown that the PL bands originate from the splitting of light- and heavy-hole subbands, induced by an elastic strain in GaAsN layer. The dependence of the energy gap of unstrained GaAsN on the nitrogen content has been calculated using the measured light- and heavy-hole splittings.

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Determination of strain-induced valence-band splitting in GaAsN thin films from circularly polarized photoluminescence

The effect of a longitudinal magnetic field on the optical spin orientation and spin-dependent recombination in dilute nitrides GaAsN has been studied for the first time. We have found that intensity $I$ and circular polarization degree $\ensuremath{\rho}$ of the edge photoluminescence, excited in GaAsN alloys by circularly polarized light at room temperature, grow substantially in the longitudinal magnetic field $B$ of the order of 1 kG. This increase depends on pump power, and, in the region of weak or moderate powers, the $I$ and $\ensuremath{\rho}$ can reach a twofold value. We included the magnetic-field suppression of spin relaxation of the deep paramagnetic centers in the two-charge-state model, which considers the spin-dependent recombination of spin-polarized free electrons on the centers. The modified model describes qualitatively the rise of $\ensuremath{\rho}$ and $I$ in a magnetic field under different pump intensities. Experimental dependences $\ensuremath{\rho}(B)$ and $I(B)$ are shifted with respect to zero of the magnetic field by a value of $\ensuremath{\sim}$200 G, while the direction of the shift reverses with change of the sign of circular polarization of pumping. As a possible cause of the discovered shift we consider the Overhauser field, arising due to the hyperfine interaction of an electron bound on a center with nuclei of the crystal lattice in the vicinity of the center.

V. K. Kalevich

Papers

Time-resolved photoluminescence in self-assembled InAs/GaAs quantum dots under strictly resonant excitation

Dominant recombination centers in Ga(In)NAs alloys: Ga interstitials

Spin-dependent recombination in GaAsN solid solutions

Determination of strain-induced valence-band splitting in GaAsN thin films from circularly polarized photoluminescence

Amplification of spin-filtering effect by magnetic field in GaAsN alloys