Optical Spectroscopy of Dynamic Nuclear Spin Polarization in Single InAs Quantum Dots

[摘要] Dynamic nuclear spin polarization is investigated experimentally in the single InAs/GaAs quantum dot system, revealing a strong non-linear feedback mechanism between the spin carriers generated via resonant optical excitation and the nuclear spin ensemble. As measured using resonant Raman scattering, two distinct nuclear spin ensemble configurations are observed that depend strongly on the optical excitation frequency and exhibit hysteresis with respect to the laser frequency scan direction. One regime is characterized by an Overhauser (OH) field that tracks with the excitation frequency, while in the other regime the OH field shifts to avoid a forbidden energy range as the excitation laser is brought into resonance. OH field shifts as large as 794 mT are measured; the fluctuations of the field are also observed as abrupt jumps in Raman scattering energies and CW pump-probe optical response. The resonant Raman scattering technique utilized in this thesis allowed for sub-100 nuclear spin sensitivity, and could be pushed to the single spin sensitivity level with a modest improvement of the scanning etalon spectrometer system.The nuclear spin environment and its coupling to the quantum dot electron-trion system are also investigated using time-domain techniques including Ramsey fringes (free induction decay) on the electron spin and electro-optic modulator (EOM) pulsed pump-probe experiments. The Ramsey fringes are strongly distorted as a function of rotation pulse delay time, and the qualitative nature of the distortion depends on which electron spin state is probed. Fourier transforms of the Ramsey fringes reveal a broadening of the electron spin frequency distribution beyond what is expected for the quasi-static OH field case. Coherent population trapping of the electron spin state results in a (19.8 +- 2.0)% reduction of the OH field distribution width for pumping times of 5 ms and a duty cycle of 70%. EOM-pulsed pump-probe experiments demonstrate DNP-induced non-linear feedback for experimental repetition periods between 100 ns and 3 us; one possible implication is that the time-averaged electron spin polarization plays a significant role in DNP phenomena in the time-domain. Overall, this thesis presents a comprehensive study of dynamic nuclear spin polarization in the QD electron-trion system in both high-resolution CW and time-domain experiments.

[发布机构] University of Michigan

[效力级别] quantum dot

[关键词] dynamic nuclear spin polarization;quantum dot;spectroscopy;single photon;Physics;Science;Physics