The spin dependence of the lifetime of electrons excited in ferromagnetic cobalt is measured directly in a femtosecond real-time experiment. Using time-and spin-resolved two photon photoemission, we show that the lifetime of majority-spin electrons at 1 eV above the Fermi energy is twice as long as that of minority-spin electrons. The results demonstrate the feasibility of studying spin-dependent electron relaxation in ferromagnetic solids directly in the time domain and provide a basis for understanding the dynamics of electron transport in ferromagnetic solids and thin films. [S0031-9007(97)04853-9]

%'e show that excitons form with a time constant~~20 ps following the creation of electron-hole pairs by subpicosecond optical excitation. The excitons are initially formed in large-wave-vector states. At low temperatures, these nonthermal excitons relax in =400 ps to the K =0 states, which couple directly to light by interaction with other excitons and acoustic phonons. This leads to a slow rise of exciton luminescence and an unusual dependence of this rise time on temperature, excitation density, and excitation energy.The optical properties of excitons in quantum wells have been the subject of intense research in recent years for fundamental' and applied reasons. Many fundamental properties of excitons have been investigated by using ultrafast laser spectroscopy. For example, the dynamics of exciton ionization and the ac Stark effect of excitons induced by an intense optical field ' have been investigated using excite-and-probe spectroscopy. The homogeneous linewidth of excitons, and the influence of temperature and various collisions on this linewidth, have been investigated by four-wave-mixing experiments. The recombination dynamics of excitons has been investigated by time-resolved luminescence spectroscopy.In spite of this intense interest in excitons, one important aspect of excitons, the dynamics of formation of bound states of excitons following photoexcitation of electron-hole pairs, has remained essentially unexplored.The recent results of Kusano et al. are largely dominated by extrinsic effects. The complex process of formation of intrinsic excitons has been identified as an important problem since the introduction of the concept of excitons in solids, but has not been addressed either experimentally or theoretically. Photoexcited electron and hole (either from a geminate or a nongeminate pair) can form an exciton by interaction with acoustic and optical phonons, and also by carrier-carrier interactions. The relaxation of photoexcited pairs within the bands proceeds simultaneously with the exciton formation process. Also, the excitons can be initially formed in the ground as well as excited states, and in the singlet as well as triplet states (corresponding to the orthohydrogen and parahydrogen states). Furthermore, the excitons are very likely created with a large total momentum wave vector E, corresponding to the center-of-mass motion of excitons in the quantumwell planes. The relaxation of these nonthermal excitons into the singlet K =0 state (the only state that can directly couple to the photons) must also be considered. It is clear that an understanding of these aspects of excitons is a fundamental importance in the physics of elementary excitons in solids.We present in this article the first results on the dynamics of exciton formation and relaxation, which provide insight into many facets of exciton physics that have not been considered before. All results presented in this letter deal with intrinsic excitons. We probed exciton formation dynamics in GaAs quantum wells as a function of temperat...

Quantum dots ͑QDs͒ of high symmetry ͑e.g., C 3v ͒ have degenerate bright exciton states, unlike QDs of C 2v symmetry, making them intrinsically suitable for the generation of entangled photon pairs. Deviations from C 3v symmetry are detected in real QDs by polarization-resolved photoluminescence spectroscopy in side-view geometry of InGaAs/AlGaAs dots formed in tetrahedral pyramids. The theoretical analysis reveals both an additional symmetry plane and weak symmetry breaking, as well as the interplay with electron-hole and hole-hole exchange interactions manifested by the excitonic fine structure. DOI: 10.1103/PhysRevB.81.161307 PACS number͑s͒: 78.67.Hc, 71.70.Gm, 73.21.La, 78.55.Ϫm Semiconductor quantum dots ͑QDs͒ exhibit atomiclike energy spectra potentially useful in the area of quantuminformation processing. The indistinguishable radiation paths of the biexciton cascade decay have been proposed as the source of polarization-entangled photons.1 In the conventional QD fabrication process the nucleation of strained InAs QDs occurs spontaneously on the ͑001͒ plane of Zincblende crystals. The symmetry of these QDs is thus limited by the crystal to C 2v .2 The resulting anisotropy of the confined exciton breaks the degeneracy of its bright states, which prohibits entanglement and produces a fine structure splitting ͑FSS͒ characterized by the emission of two linearly polarized photons of unequal energies. Nevertheless, entangled photon pairs from such QDs have been detected by means of careful preselection of particular QDs, 3,4 by spectral postselection, 5 at the price of losing photons, or by the heavy use of external magnetic fields to restore the intermediate level degeneracy. 6 In the quest of more efficient QD sources of entangled photons, it was recently predicted that replacing the conventional GaAs barriers by InP significantly reduces the exciton FSS in such InAs self-assembled QDs. 7 Until now, however, studies of the FSS of neutral and charged exciton complexes have been limited to QDs of C 2v or lower symmetry. [2][3][4][5][6][7][8][9][10][11] In this Rapid Communication, we experimentally and theoretically investigate the FSS in QDs with high symmetry. Zincblende QDs of C 3v symmetry can ideally be achieved by choosing ͓111͔ as the crystallographic direction of crystal growth instead of the conventional ͓001͔ direction. For this growth geometry, including the lack of inversion symmetry in the crystal and the effects of strain and piezoelectric fields, the minimal symmetry is C 3v as long as the QD heterostructure has symmetrical shape. Here we utilize InGaAs/AlGaAs QDs that allow the simultaneous study of the FSS of dominating heavy-hole ͑hh͒ and light-hole ͑lh͒ excitons as well as a hybrid hh-lh trion by side-view polarization-resolved photoluminescence ͑PL͒ spectroscopy. We show how these trion states can probe a small symmetry breaking in otherwise ideal C 3v QDs due to exchange interactions.The polarization properties of the exciton fine structure depend on the symmetries of the initial and fi...

We have studied the effect of valence band mixing on the optical properties of semiconductor quantum wires by analyzing the luminescence polarization. Large polarization anisotropy is observed and directly compared to the effects predicted by a k ? p model calculation of the valence band structure. [S0031-9007(97) PACS numbers: 78.55. Cr, 71.35.Cc, 73.20.Dx, 78.66.Fd The electronic structure of spatially confined electrons in low-dimensional systems has been attracting considerable interest. In particular, the electronic and optical properties of semiconductors can be tailored in artificial nanostructures due to quantization of the electronic energy and emergence of strong excitonic features [1]. Enhancement of excitonic effects may be achieved in semiconductor quantum wires (QWRs) due to modifications of electron-hole Coulomb correlations and divergence of the one-dimensional (1D) joint density of states [2,3]. The band structure of 1D semiconductor QWRs has been studied theoretically and the prominent role of valence band mixing was described in model systems [4,5]. Unlike the case of quantum wells, the admixture of heavy hole (hh) and light hole (lh) states leads to modified energies of the optical interband transitions, which are tunable by the lateral confinement potential, and a redistribution of the oscillator strength. Moreover, it gives rise to intrinsic polarization anisotropy of the optical absorption spectra providing a unique way to gain insight into the nature of 1D valence band structures.Despite the interest spurred on by these theoretical expectations the observation of the predicted features, i.e., optical anisotropy and sharp optical resonances, have been hampered by the technological challenge in producing QWR structures with two-dimensional (2D) confinement for both the ground state and excited states and with sufficiently small level broadening. The effect of 2D quantum confinement has been evidenced in the luminescence of QWR structures prepared by different approaches [6][7][8][9]. Study of the valence band mixing requires, however, the resolution and identification of several 1D valence subbands, imposing stronger constraints on the optical quality. Furthermore, anisotropy of optical interband transitions can also occur in the presence of strain, in samples with a strong surface corrugation, and in (110)-oriented quantum wells [10]. Initial observation of optical anisotropy in the absorption or emission of most types of QWR structures have been in fact affected by some of these invasive effects and, thus, cannot be directly compared to the predicted effects of 2D confinement on valence-band mixing.In this Letter we report the observation of valence band mixing effects in the optical spectra of high quality 1D semiconductor QWRs. The origin of the optical transitions and their hh and lh character have been identified by performing photoluminescence experiments with circularly polarized light. The observed anisotropy in the linearly polarized excitation spectra of these wires is shown t...

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