Hole-spin initialization and relaxation times in InAs/GaAs quantum dots

Abstract

We study, at low temperature and zero magnetic field, the hole-spin dynamics in InAs/GaAs quantum dots. We measure the hole-spin relaxation time at a time scale longer than the dephasing time (about ten nanoseconds), imposed by the hole-nuclear hyperfine coupling. We use a pump-probe configuration and compare two experimental techniques based on differential absorption. The first one works in the time domain, and the second one is a new experimental method, the dark-bright time-scanning spectroscopy (DTS), working in the frequency domain. The measured hole-spin relaxation times, using these two techniques, are very similar, in the order of T h N ≈1 μs. It is mainly imposed by the inhomogeneous hole hyperfine coupling in the hole localization volume. The DTS technique allows us also to measure the hole-spin initialization time τ i . The hole spin is initialized by a periodic train of circularly polarized pulses at 76 MHz; we have observed that τ i decreases as the power density increases, and we have measured a minimum value of τ i ≈100 ns in good agreement with a simple model [seeB. Eble, P. Desfonds, F. Fras, F. Bernardot, C. Testelin, M. Chamarro, A. Miard, and A. Lemaître, Phys. Rev. B 81, 045322 (2010)].

Two-phonon process and hyperfine interaction limiting slow hole-spin relaxation time in InAs/GaAs quantum dots

“…As the temperature increases, the TPAM becomes dominant. Theoretical works 25 K −2 , in very good agreement with the experimental value. The TPAM for electrons is predicted to be much weaker than for holes and then negligible compared to hf effects.…”

“…Moreover, for the fixed experimental conditions, when compared with a standard pump-probe configuration, the DTS technique gives very similar relaxation time while increasing the signal-to-noise ratio. 25 In order to identify the mechanisms at the origin of the hole-spin relaxation, we have measured T h 1 as a function of an external magnetic field along the growth direction (in the range of 0 B 2 T), and also as a function of temperature (4 T 50 K). Figures 2(a) and 2(b) show the measured and fitted second Fourier coefficient of the periodic PCD(T L ) signal, in absence of an applied magnetic field and for a magnetic field B = 2 T, vs f m .…”

“…21 In this paper, we address the slow hole-spin relaxation mechanisms in conditions of low magnetic field (0 B 2 T) and low temperature (2 T 50 K), which have been largely unexplored. 24 A recently proposed experimental method, 25 dark-bright time-scanning spectroscopy (DTS), based on differential absorption measurements and working in the frequency domain, is used to measure T h 1 as a function of a longitudinal magnetic field and of lattice temperature. We experimentally show that the slow T h 1 is limited by two mechanisms in InAs QDs: at T = 2 K, the hf coupling with the nuclear spins of the host material fixes the T h 1 value at ≈1 μs and imposes an increase for very low magnetic fields in the range of tens of mT; for larger fields, T h 1 saturates at ≈2.5 μs.…”

Optical initialization of hole spins inp-doped quantum dots: Orientation efficiency and loss of coherence

“…To compare it to other processes occurring in systems of this type one can use the only characteristics available, namely the total decrease of coherence and the duration of dephasing process which is equal to the laser pulse duration (here, sub-picosecond). Comparing the latter to timescales of other relevant processes occurring in discussed system, like the phonon-assisted 23,24 or hyperfine hole-nuclei coupling driven hole spin relaxation 25 , electron spin dephasing [26][27][28] or relaxation of the positive trion 29 (ranging from hundreds of picoseconds to a few microseconds), we find it to be at least three orders of magnitude shorter. This fact allows us to consider it separately and treat the dynamical spin dephasing as instantaneous on the background of the whole optical spin initialization process.…”

Optical pumping and reversal of hole spin in InAs/GaAs quantum dots

“…T R is the trion lifetime, and T 1 is the long-scale relaxation time of the hole spin. 17 A periodicity condition is also imposed:JðT À L Þ¼Jð0 À Þ,whereT L is the period of the pulsed excitation and the superindex (-) means before the pump pulse. For a resonant excitation, the z component of the photocreated electron spin S z ð0 þ Þ {of the resident hole spin J z ð0 þ Þg,a f t e rar þ -polarized pump pulse with pulse area H, is written as S z ð0 þ Þ¼sin 2 ðH=2Þ½2J z ð0 À Þ=3 À 1 =4 fJ z ð0 þ Þ ¼½1 À sin 2 ðH=2Þ=2 J z ð0 À Þþ3sin 2 ðH=2Þ=4g.…”