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

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}_{N}^{h}\ensuremath{\approx}$1 \ensuremath{\mu}s. 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 ${\ensuremath{\tau}}_{i}$. The hole spin is initialized by a periodic train of circularly polarized pulses at 76 MHz; we have observed that ${\ensuremath{\tau}}_{i}$ decreases as the power density increases, and we have measured a minimum value of ${\ensuremath{\tau}}_{i}\ensuremath{\approx}$100 ns in good agreement with a simple model [see B. Eble, P. Desfonds, F. Fras, F. Bernardot, C. Testelin, M. Chamarro, A. Miard, and A. Lema\^{\i}tre, Phys. Rev. B 81, 045322 (2010)].