PACS 72.25.Rb, 78.47.+p, 78.67.Hc Exciton spin relaxation in self-assembled InAlAs quantum dots were investigated by three-pulse four-wave mixing under resonant conditions. The concept of the spin grating holds well for quantum dots and the measurements combined with optical heterodyne detection at 10 K demonstrates that the exciton spin relaxation last up to a few ns and the time constant is ∼5 times larger than the exciton recombination time in average.In the past several years, spectroscopic studies of electronic confinement effects in quantum dots (QDs) have evolved into a very active research field not only in basic physics of their atomlike entities but also in applications aiming at quantum information processing. Especially, the broad effort is now under way to develop new techniques for controlling spin degrees of freedom in QDs. These efforts are stimulated in part by some proposals to use the spin systems as quantum bits in solid-state quantum computing [1][2][3]. In order to use spin as an information carrier, long spin decoherence times are indispensable. The carrier spin dynamics in semiconductor bulk and quantum wells (QWs) have been extensively investigated both theoretically and experimentally [4]. As compared to these higher dimensional structures, the exciton spin relaxation in QDs has been predicted to be slow down drastically reflecting atomlike discrete energy states [5,6]. In fact, the long spin-relaxation time of several hundreds ps or more have been reported in the time-resolved photoluminescence (TRPL) measurements [7,8].Four-wave mixing (FWM) is known to be a powerful tool for the carrier dephasing dynamics as well as carrier population dynamics [9]. Also, FWM permits us to measure the spin relaxation via the decay of spin grating generated by the cross-linear pump polarizations. Therefore, information about spin-conserving and spin-nonconserving scatterings will be obtained. The spin grating has been successful in bulk and QWs [10][11][12], however, it was difficult to be applied to QDs so far because of the weak signal intensity due to the low areal coverage of QDs.In this work, the exciton spin relaxation and dephasing dynamics under resonant condition in selfassembled InAlAs/AlGaAs QDs were investigated by three-pulse FWM measurements at 10 K. FWM combined with heterodyne detection technique (OHD) made it possible to observe the spin relaxation processes, and therefore the method adds a new and simple way for the research of spin relaxation process in QDs.The QD samples grown in a molecular beam epitaxy system and have 15 In 0.75 Al 0.25 As QDs layers separated by a 20-nm thick Al 0.3 Ga 0.7 As layer. The QDs are formed using the spontaneous island formation in the initial stages of the Stranski-Krastanow growth mode during the epitaxy of strained InAlAs on AlGaAs layers. The AlGaAs cap terminates the heterostructure. The sample was held in a closed-cycle He cryostat at 10 K through the measurements. A self mode-locked Ti:sapphire laser with the pulse repetition
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