Long nuclear spin polarization decay times controlled by optical pumping in individual quantum dots

Abstract

Nuclear polarization dynamics are measured in the nuclear spin bistability regime in a single optically pumped InGaAs/ GaAs quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the nonlinear nuclear spin dynamics. We measure nuclear spin polarization decay times in the range of 0.2-5 s, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long ͑Ͼ5 s͒ pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bistability regime otherwise undetectable in cw experiments. DOI: 10.1103/PhysRevB.77.125307 PACS number͑s͒: 72.25.Fe, 72.25.Rb, 72.80.Ey, 73.63.Kv The hyperfine interaction between the magnetic moments of the electron and nuclear spins 1 has been shown to limit the electron spin lifetime 2-7 and coherence 8-10 in semiconductor nanostructures. Optical excitation [11][12][13] and transport 5,9,10,14,15 of spin-polarized electrons in semiconductor quantum dots ͑QDs͒ have been found to lead to dynamic nuclear polarization: Overhauser magnetic fields up to a few Tesla have been detected in nanostructures [16][17][18][19] leading to strong modifications of the confined electron energy spectrum. [11][12][13][16][17][18][19] The control of the nuclear spin has been identified as one of the prerequisites for the coherent manipulation of the electron spin in semiconductor nanostructures. 20 In this context, extended nuclear spin polarization lifetimes in a dot are desirable and, as we show here, can be achieved by increasing the optical pumping time, during which not only the dot but also the surrounding material becomes polarized.In this work, we measure nuclear spin dynamics in the optically induced bistability regime in InGaAs/ GaAs selfassembled quantum dots in external magnetic fields B ext of 1-3 T. [16][17][18] We develop a sensitive time-resolved all-optical method based on the nonlinear behavior of the nuclear spin polarization in the bistability regime and measure the rise and decay time of the nuclear polarization in a single dot. Using this method, we find that the optical pumping time required to reach a nuclear spin polarization of S N Ͼ 0.3 in B ext = 2 T can be as short as 100 ms. Working in a pumpprobe mode, where the dot is free of electrons and holes during the "dark" pump-probe delay time, we find that the nuclear polarization decay in a single dot is strongly dependent on the pumping time: it can vary from 0.2 s for 100 ms pumping to 5 s for Ͼ3 s pumping. This observation indicates the importance of nuclear spin pumping outside the dot, which arises due to nuclear spin diffusion from the dot into the surrounding matrix. Finally, the pump-probe data allow us to reconstruct the whole nuclear spin bistability curve, including the unstable nuclear polarization state, which is otherwise undetectable in cw experiments.Previously, nuclear spin dynamics w...

Nuclear spin effects in semiconductor quantum dots

“…Dynamics of the optically induced spin pumping have been studied in pump-probe experiments 36,38,[61][62][63] . For small B ext (i.e.…”

Quadrupolar spectra of nuclear spins in strained InxGa1−xAs quantum dots

“…39. Guided by the samples used in recent nuclear spin experiments, [13][14][15][16][17][18][19][20][21][22][23][24][25] we center our discussion around a QD of a truncated cone shape 59 with a base (top) diameter 25 nm (10 nm), placed over a 0.5 nm wetting layer, all fully embedded in a GaAs host lattice. The computational supercell contains some 1 800 000 atoms, most of which for the host matrix region, with the InAs QD region having 16 702 In and 15 432 As atoms.…”

“…Our choice for such a high-aspect-ratio QD is again guided by the samples used in relevant experiments. [13][14][15][16][17][18][19][20][21][22][23][24][25] In order to extract the dependence on the aspect ratio and alloy composition of the QDs, different heights and indium mole fractions are considered as well.…”

“…Within the full gamut of possible combinations of self-organized materials, 11 the InAs on GaAs system stands out due to extensive research efforts devoted over more than a decade to their growth and optoelectronic characterizations. 12 For the purposes of controlling the spin dynamics in InAs QDs, several groups [13][14][15][16][17][18][19][20][21][22][23][24][25] have studied various aspects of the optical orientation 26 of an electron spin and its transfer to the nuclear spins via the hyperfine interaction leading to the dynamic nuclear spin polarization. 27 One of the remarkable achievements that paved the way to the coherent manipulation of nuclear spin dynamics has been the nuclear magnetic resonance (NMR) of a single QD.…”

Long-term dynamics of the electron-nuclear spin system of a semiconductor quantum dot

“…The direct transfer of spin angular momentum to the crystal by the dipole-dipole interaction between nuclear spins is the main mechanism for the relaxation of the spin polarization. This interaction is also responsible for the energy diffusion from the quantum dot to the neighboring nuclei 23,24,25 .…”