Optical pumping of nuclear polarization in a single self-assembled
InAlAs quantum dot
S. Adachia*, T. Yokoia, T. Mukumotoa, H. Kumanob, I. Suemuneb, and S. Mutoa
a
Department of Applied Physics, Hokkaido University, N13 W8, Kitaku, Sapporo 060-8628, Japan
b
Research Institute of Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
We report the optical pumping of nuclear spin polarization and its formation time in
a single self-assembled In0.75Al0.25As quantum dot (QD). The nuclear spins in a single
QD (~30000 nuclei) get polarized via hyperfine interaction with electron spins that are
injected spin-selectively by circularly polarized optical excitation. In turn, the created
nuclear spin polarization (i.e. nuclear field) acts back on the electron and shifts its energy
levels. This Overhauser shift has been observed in interface GaAs QDs[1] and in a selfassembled InAlAs QD[2] and InAs QD[3].
Figure shows the Zeeman spectra of the
excitonic emissions split by 645 eV in
linearly-polarized excitation and by 750
eV in circularly-polarized excitation (-)
at 5 T. The difference of the energy
splitting corresponds to the Overhauser
shift and the nuclear field that electron
feels is estimated to be ~ 4.2 T along the
direction of the external magnetic field. In
addition, the temporal sequence of these
Zeeman spectra shows the response of the energy shifting when the excitation light
polarization changes suddenly from linear to circular and vise versa at 0 and 1000
seconds. The polarization switching from circular to linear and vise versa makes that the
averaged nuclear spin polarization rate decreases (increases) from (to) the high spinpolarized state and the transient process will be observable if the formation time of
nuclear spin polarization is longer than the accumulation time. At present, the
accumulation time of CCD for one spectrum needs 10 seconds in order to obtain the PL
with high S/N ratio. Accordingly, the abrupt change in the energy shifting indicates that
the formation of nuclear polarization is established within 10 sec. That gives the upper
limit of the formation time. The large nuclear field up to a few T and the relatively fast
response to the excitation light polarization were observed. Their features as a highspeed switching nano-magnet can be used for qubit conversion between photon qubit and
electron-spin qubit.[4]
* corresponding author e-mail: adachi-s@eng.hokudai.ac.jp
References
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2. T. Yokoi, S. Adachi et al., Phys. Rev. B 71, R041307 (2005), Physica E 29, 510 (2005).
3. B. Eble, O. Krebs, A. Lemaître, K. Kowalik, A. Kudelski et al, cond-mat/0508281 (2005).
4. S. Muto, S. Adachi, T. Yokoi, H. Sasakura et al., Appl. Phys. Lett. 87, 112506 (2005).