|
The irradiation of a crystal with zinc blende
structure (such as GaAs) in an external
magnetic field with circularly polarized light creates spin-polarized
photoelectrons in the conduction band. The band structure and possible
transitions due to excitation with circularly polarized light is
shown on the left. This non-equilibrium polarization
can be transferred to the nuclear spin system via the hyperfine
interaction and results in a dramatic increase of the nuclear polarization.
While in optically detected NMR (ODNMR) only nuclei close to donors are
detectable, this is not a limiting factor when the nuclear
polarization is observed directly. In fact, in
this case also the nuclei far from the impurities that are polarized
via nuclear spin diffusion processes contribute to the detected signal.
The entire process can be summarized in three steps: 1) The polarized light creates polarized photoelectrons in conduction band and shallow donor or impurity states; 2) The nuclei in a distance of the order of one Bohr radius (~100 ) to the donor are polarized by the contact with the non-equilibrium electron spin system; 3) Spin diffusion causes polarization of the nuclei far from the donors. Due to the very different timescales, these processes can be analyzed independently. |
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