Single electron spins in individual quantum dots (QDs) are measured off-resonance using Kerr rotation. The QDs investigated here are MBE-grown simple interface fluctuation QDs, also called natural QDs. Figure 1 shows a schematic of the sample structure. The QD layer is centered within an optical cavity with a quality factor of ~ 150. In addition, the QDs are embedded within a diode structure enabling controllable charging of the QDs with a bias voltage. The front gate also acts as a shadow mask with 1 μm apertures which are used to isolate single QDs as well as identifying their specific position.
The spins are measured using Kerr rotation pump-probe spectroscopy. A circularly polarized pump laser optically injects spins into the conduction band due to the optical selection rules of the GaAs quantum well. Then, when a linearly polarized probe laser reflects off the sample its polarization axis will rotate by an angle which is proportional to the spin along the direction of the light. Using Kerr rotation allows for the spin-state to be probed non-resonantly, thus minimally disturbing the system.
The photoluminescence (PL) from a single QD is shown in fig. 2a, black line. The different peaks are identified as recombination from the neutral exciton (X0), negatively-charged exciton (X-), and biexciton (XX) -states. The Kerr rotation signal as measured over this energy range is shown in the blue circles (fig. 2a) using continuous wave pump and probe lasers. In order to isolate the spin-dependent signal lock-in techniques are used and in addition, the pump excitation is switched between right and left circularly polarized light. The signal is obtained from the difference in the Kerr rotation angle at the two helicities. A clear odd-Lorentzian feature is observed centered at the X- energy; fig. 2b shows a more detailed view where the solid line is a fit to the data. Comparing the energy of the Kerr rotation feature and the X- transition energy as a function of bias (as shown in fig. 2c), it is observed that the two energies agree well and show the same quantum-confined Stark shift. This odd-Lorentzian feature centered at the X- transition energy is exactly the Kerr rotation signal expected to arise from a single electron spin in a QD.
To learn more about our studies, please refer to: "Nondestructive Optical Measurements of a Single Electron Spin in a Quantum Dot", J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, Science 314, 1916 (2006).