Haruki Sanada

Synthesis of GaAs nanowires with very small diameters and their optical properties with the radial quantum-confinement effect

We report the synthesis and optical properties of GaAs nanowires with very small diameters. We grew the GaAs nanowires by using size-selective gold particles with nominal diameters of 5, 10, 20, 40, and 60 nm. The diameter-controlled nanowires enable us to observe blueshifts of the free exitononic emission peak from individual nanowires with decreasing gold-particle size due to the two-dimensional radial quantum-confinement effect. We also analyze the absorption and emission polarization anisotropies of these bare GaAs quantum nanowires.

Proposal of spin complementary field effect transistor

Spin complementary field effect transistor is proposed on the basis of gate-controlled persistent spin helix (PSH) states. Uniaxial effective magnetic field in the PSH state creates coherent spin propagation with or without precession. By the gate control of the Rashba spin-orbit interaction, the PSH state can be reversed to the inverted PSH state. Switching between two PSH states enables complementary output depending on the channel direction. Our proposed device could be a reconfigurable minimum unit of the spin-based logic circuit.

Exciton and biexciton emissions from single GaAs quantum dots in (Al,Ga)As nanowires

We report the optical properties of GaAs quantum dots embedded in (Al,Ga)As nanowires grown by the vapor–liquid–solid method. We used the micro-photoluminescence (PL) technique to observe PL peaks, which are assigned as exciton and biexciton emissions from single quantum dots. In addition, unusual features appear in the excitation power dependence of the energies and linewidths of the two PL peaks. The PL also depends on the optical polarization axis, indicating that the nanostructures have a highly asymmetrical shape. The results show that our method is a promising way of engineering the positions and optical properties of GaAs/(Al,Ga)As nanostructures.

Photoluminescence Dynamics of GaAs/AlAs Quantum Wells Modulated by Surface Acoustic Waves

We have investigated the dynamics of photoluminescence (PL) of GaAs/AlAs quantum wells under strain and piezoelectric modulation introduced by a surface acoustic wave (SAW). Measurements performed using a microscopic excitation spot show that the efficiency of PL quenching due to exciton ionization and the subsequent sweeping of free electrons and holes by piezoelectric potential varies significantly with quantum well thickness. This variation is attributed to the well-thickness dependence of carrier mobility and diffusivity. The relative timing between carrier generation pulse and SAW-induced band structure modulation also changes the delay of PL quenching and PL decay time under the SAW field.

Drift transport of helical spin coherence with tailored spin–orbit interactions

Most future information processing techniques using electron spins in non-magnetic semiconductors will require both the manipulation and transfer of spins without their coherence being lost. The spin–orbit effective magnetic field induced by drifting electrons enables us to rotate the electron spins in the absence of an external magnetic field. However, the fluctuations in the effective magnetic field originating from the random scattering of electrons also cause undesirable spin decoherence, which limits the length scale of the spin transport. Here we demonstrate the drift transport of electron spins adjusted to a robust spin structure, namely a persistent spin helix. We find that the persistent spin helix enhances the spatial coherence of drifting spins, resulting in maximized spin decay length near the persistent spin helix condition. Within the enhanced distance of the spin transport, the transport path of electron spins can be modulated by employing time-varying in-plane voltages.

Spatially modulated photoluminescence properties in dynamically strained GaAs/AlAs quantum wells by surface acoustic wave

Spatially resolved photoluminescence (PL) spectra and polarization anisotropy were investigated in GaAs/AlAs dynamic wires, which were formed by applying a surface acoustic wave (SAW) on GaAs/AlAs quantum wells along the [110] or [1-10] direction. A synchronized excitation method clearly demonstrates that the band gap energies are spatially modulated by the travelling-SAW-induced strain. It is found that both the spatial PL modulation and anisotropic polarization properties depend on the SAW direction. The spatial modulation of the polarization anisotropies and their dependence on the strain-induced valence band mixing are also discussed theoretically.

Creation of charged excitons with two-color excitation method and initialization of electron spin qubit in quantum dots

An optical pumping method for creating charged excitons in semiconductor quantum dots is demonstrated using a two-color excitation method. This method employs two laser sources whose energies are in the resonant and barrier excitation conditions, respectively. The photoluminescence (PL) spectra of a single quantum dot clearly vary from exciton-originated PL to charged exciton-originated PL as the excitation conditions are changed. This method is applied to the initialization process of an electron spin qubit without a magnetic field, which provides a simple and effective way of implementing quantum computing with spin qubits.

Photoluminescence dynamics in GaAs/AlAs quantum wells modulated by one-dimensional standing surface acoustic waves

The effects of standing surface acoustic waves (SAWs) on carrier dynamics in GaAs/AlAs quantum wells are investigated by spatially and time-resolved photoluminescence (PL) spectroscopy. We found that the PL spectra vary considerably depending on the position and the phase of the standing SAW field. The PL spectra are characterized by oscillations in the PL intensity and emission energy due to the motion of free carriers and excitons driven by the piezoelectric fields as well as by the strain-induced band-gap gradient. It is also demonstrated that the positions of the nodes and antinodes of the standing SAW are precisely controlled.

Drift-Induced Enhancement of Cubic Dresselhaus Spin-Orbit Interaction in a Two-Dimensional Electron Gas

We investigated the effect of an in-plane electric field on drifting spins in a GaAs quantum well. Kerr rotation images of the drifting spins revealed that the spin precession wavelength increases with increasing drift velocity regardless of the transport direction. A model developed for drifting spins with a heated electron distribution suggests that the in-plane electric field enhances the effective magnetic field component originating from the cubic Dresselhaus spin-orbit interaction.

Comparison of electrical and optical detection of spin injection in L10-FePt/MgO/GaAs hybrid structures

We have investigated comparative experiments for spin injection into semiconductor in an ordered L10-FePt/MgO/n-GaAs hybrid structure using electrical and optical detection methods. Spatial-resolved Kerr rotation microscope image clearly demonstrates accumulation of perpendicularly oriented spins in an n-GaAs channel at zero magnetic field. On the other hand, electrical three-terminal Hanle measurement shows shorter spin lifetime than that of the optical measurement. It suggests that the spin lifetime obtained from three-terminal Hanle method originates from spins at the MgO/GaAs interface but not in the bulk GaAs channel.