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We report the fabrication of self-assembled, strain-free GaAs/Al0.27Ga0.73As quantum dot pairs which are laterally aligned in the growth plane, utilizing the droplet epitaxy technique and the anisotropic surface potentials of the GaAs (100) surface for the migration of Ga adatoms. Photoluminescence spectra from a single quantum dot pair, consisting of a doublet, have been observed. Finite element energy level calculations of a model quantum dot pair are also presented.When two semiconductor quantum dots (QDs), which can each spatially confine an individual carrier in a discrete energy level, are in close proximity to each other, these carriers begin to interact with each other. Specifically, the wavefunctions of the carriers confined in each QD of the QD pair (QDP) begin to overlap, resulting in an efficient carrier tunneling [1,2], and the wavefunctions may become admixed to form molecular orbital states. Moreover, resonance in the optical transition energies leads to the formation of a coupled QDP via dipole-dipole interactions [3,4,5]. Proposals for using such a coupling in quantum information processing have been brought forth [6,7]. To this end, various semiconductor QDPs have been fabricated and studied, such as coupled QDs grown by cleaved edge quantum well overgrowth [1], vertically-aligned QDs grown by StranskiKrastanow epitaxy incorporating an indium-flush procedure [2,4], and interface QDs formed in a quantum well [3]. In this work we demonstrate the fabrication of self-assembled, laterally aligned GaAs/Al 0.27 Ga 0.73 As QDP structures by droplet epitaxy: Droplet epitaxy is a nonconventional growth technique for the self-assembly of high quality nanostructures with lattice-matched materials, making possible the growth of nanostructures with various structural characteristics, such as QDs [8,9,10], quantum rings (QRs) [11], and concentric double QRs [12,13]. Here we observe the formation of laterally aligned GaAs QDPs by accurately selecting the As 4 flux during crystallization. Within relevant conditions, the nanocrystals show a remarkable shape like a double summit, reflecting the anisotropic surface potentials of the GaAs (100) surface. To gain insight into the interactions between the carriers confined in such a structure, we study the QDP single-structure optical properties. We further consider the electronic structure of a model QDP via energy level calculations requiring no inclusion of strain.The samples are grown by droplet epitaxy using a molecular beam epitaxy system with elemental sources and a valved As source, which enables the accurate control of the As 4 flux.

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Dynamics of photoinduced magnetization rotation was studied in ferromagnetic p-͑Ga,Mn͒As by carefully comparing the temporal profile of the photoinduced polar Kerr rotation with that of the photoinduced reflectivity change. Rotation of ferromagnetically coupled Mn spins is induced by photogenerated hole spins within the excitation pulse width of a 100 fs, whereas relaxation takes place in tens of picosecond as a result of strong damping. Observed results suggest that hole and Mn spins rotate and relax together upon optical excitation. This would be a different type of excitation ͑coupled hole-Mn spin complex͒ appearing in the hole-mediated ferromagnetic system.Recently, magnetization rotation from in-plane to out-ofplane direction by the illumination with circularly polarized cw light has been demonstrated in epilayers of hole-mediated ferromagnetic semiconductor ͑Ga,Mn͒As. 1 The results suggest the occurrence of collective rotation of ferromagnetically-coupled Mn spins induced by the small amount of optically generated hole spins, which can be regarded as novel spin-induced cooperative phenomenon based on the p-d spin-exchange interaction. In this Brief Report, we are concerned with dynamics of the magnetization rotation triggered by the optically injected carrier spins in the ferromagnetic ͑Ga,Mn͒As. Results obtained from photoinduced femtosecond time-resolved polar Kerr rotation ͑fs-TRKR͒ spectroscopy indicate that ferromagnetically-coupled Mn spins are rotated instantaneously by the photogenerated hole spins, and they relax back to the original, in-plane state in tens of picoseconds ͑ps͒ as a result of strong damping. These pictures suggest that the photogenerated hole spins form coupled hole-Mn spin complex, in contrast to the paramagnetic semiconductor systems. 2,3 Ferromagnetic ͑Ga,Mn͒As samples were grown by molecular-beam epitaxy on GaAs/GaAs͑100͒ substrates at the substrate temperature of 250°C. Thickness and Mn contents of two representative samples were 200 nm and x ϭ0.011 ͑Curie temperature T C ϭ30 K), and 100 nm and x ϭ0.068 (T C ϭ90 K), respectively. Hole concentration is inferred to be 10 20 cm Ϫ3 or below for the xϭ0.011 sample and around 10 20 cm Ϫ3 for the xϭ0.068 sample, referring to the existing carrier transport data. 4 Lattice mismatch between the ͑Ga,Mn͒As layers and the substrates yields an in-plane magnetic anisotropy 5 in the present samples.Before excitation, samples are in the virgin state at which magnetization of magnetic domains is primarily in the lateral direction ͓Fig. 1͑a͔͒. Reflecting the hole-mediated ferromagnetism, 6 spin-split density of states ͑DOS͒ of the valence band is expressed by two half parabolas in the spinplane A whose spin quantum axes are along ͉S x,y ;Ϯ1/2͘ ͓Fig. 1͑b͔͒. The magnitude of the splitting is given by the exchange-split energy ⌬E ex . 7 Here, we neglect orbital angular momentum L of holes for simplicity. Linearly polarized probing light reflected normal from the sample surface only exhibits small longitudinal/transverse Kerr rotation. Through the ba...