Exciton spin relaxations in bulk GaN were directly observed with sub-picosecond's time resolution. The obtained spin relaxation times of A-band free exciton are 0.47 ps - 0.25 ps at 150 K - 225 K. The spin relaxation time of the acceptor bound exciton at 15K is measured to be 1.1 ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin relaxation time of A-band free exciton is found to be proportional to T-1.4, where T is the temperature. The fact that the spin relaxation time in GaN is shorter than that in GaAs, in spite of the small spin-orbit splitting, suggests that the spin relaxation is dominated by the defect-assisted Elliot-Yafet process.
We report on the fast spin flip process in quantum wells and quantum dots. The utilization of spin degree of freedom will open the door to novel functional devices. In the GaAs/AlGaAs quantum ells, the spin relaxation time is found to be about several ten picoseconds at room temperature. InGaAs/InP quantum wells whose bandgap correspond to 1.5 micro meters have the fast spin relaxation of several picoseconds. Regarding to GaAs/AlGaAs multiple-quantum wells, we observed that Dyakonov-Perel process governs the spin relaxation at room temperature. All optical switching devices using this fast spin relaxation process were proposed and demonstrated. In the quantum dots, we have found that anitferromagnetic coupling between quantum dots exist at temperatures lower than 50-80K. The electron spin flips within 70-200 ps after the carrier generation. The spin relaxation time under the antiferromagnetic order is extended to 10-12 ns, an order of magnitude longer than that in isolated quantum dots.
Electron-spin relaxation has been investigated in a variety of quantum confined structures including GaAs-based quantum wells and quantum wires as well as InP-based quantum wells, and it has been shown to exhibit ultrafast relaxation in the picosecond, and even in the femtosecond, range. This ultrafast relaxation has been used together with the exciton absorption nonlinearity as an novel principle of all-optical switching device which can avoid the speed limit due to the slow carrier lifetime in conventional switching devices. Experiments on GaAs-based quantum-well spin switches have shown 4-ps gate switch operation. Also, experimental analyses have shown the feasibility of these devices at a high contrast ratio (13 dB) and high repetition rate (40 GHz). The application of this device to a demultiplexer has been proposed.
We demonstrate picosecond signal recovery in all optical gate operation using a type II tunneling bi-quantum well (TBQ) etalon. Type II TBQ consists of a series of GaAs wells, AlGaAs barriers, and AlAs layers. The recovery time from excitonic absorption bleaching in GaAs wells is governed by tunneling of electrons out of the well through AlGaAs barriers into AlAs layers. The type-II TBQ etalon with 1.7 nm barriers shows 17 ps-recovery.
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