S. Matsuzaka

Gate voltage dependence of nuclear spin relaxation in an impurity-doped semiconductor quantum well

We investigated the gate voltage dependence of the nuclear spin relaxation in a Schottky-gated n-GaAs/AlGaAs (110) quantum well by the time-resolved Kerr rotation measurement combined with the nuclear magnetic resonance technique. The Fermi contact hyperfine interaction is enhanced by decreasing the background electron density, as the electrons become localized at impurity site. The energy relaxation time T1 and the decay time of the Rabi oscillation T2Rabi can be controlled by more than a factor of 10 and a factor of ∼2, respectively.

Effect of optical waveguide on photoluminescence polarization in layered material GaSe with millimeter scale

We investigate the photoluminescence (PL) polarization of millimeter-scale GaSe films with 100 nm and 150 ?m thicknesses by excitation with linearly and circularly polarized light. Although free exciton emission predominates at 140 K, multiple peaks appear at 30 K owing to the recombination with impurity and/or localized states. Despite the circularly and linearly polarized excitations, the 100-nm-thick GaSe film constantly shows linear polarization in both band-edge and impurity/localized PLs. By rotating the sample geometry, the observed linear polarization is also rotated. This indicates that linear polarization is sensitive to the sample geometry and originates from the optical waveguide effect.

Nuclear spin coherence time in a strained GaAs quantum well

We investigated the quadrupolar splitting dependence of the nuclear spin coherence time in an n‐(110)GaAs/AlGaAs quantum well by time‐resolved Kerr‐rotation measurements combined with pulsed‐rf nuclear magnetic resonance. In GaAs, all the constituent nuclei have quadrupole moment. Therefore the quadrupolar splitting is induced by electric field gradient, which can be controlled by varying strain. We found that the nuclear spin coherence time is extended as the quadrupolar splitting increases by applying the strain. This suggests that the influences of the other transitions are suppressed with increase of the quadrupolar splitting.

Magnetic Field Dependence of Quadrupolar Splitting and Nuclear Spin Coherence Time in a Strained (110) GaAs Quantum Well

We investigated the magnetic field dependence of quadrupolar splitting and phase relaxation time of nuclear spins in an n-GaAs/AlGaAs (110) quantum well (QW) by optically detected nuclear magnetic resonance (NMR). The NMR spectra show large quadrupolar splitting induced by an internal field gradient in the QW epilayer. At lower magnetic fields, the quadrupolar splitting become much enhanced, resulting in the appearance of the second order magnetic field dependence of the quadrupole interaction in the NMR spectra. It is also shown that the intrinsic coherence time and the effective coherence time for resonant transitions become shorter as a result that the energy splitting between the two levels becomes small and incoherent transition is occurred by irradiation of an rf magnetic field.

Intersubband exchange interaction induced by optically excited electron spins in GaAs/AlGaAs quantum wells

Spin-dependent intersubband excitonic interactions have been investigated in GaAs/AlGaAs quantum wells by two-color pump and probe spectroscopy. We generated spin-polarized electrons in the lowest subband by resonant excitation of the heavy-hole exciton (E1-HH1) and observed polarization-dependent broadening of the second-subband exciton resonance (E2-HH2 and E2-LH1). The exchange interaction between the first and the second-subband excitons is found to play a crucial role in polarization-dependent spectral modulation as well as spin-independent Coulomb screening.

Strain dependence of nuclear spin coherence in a (110)GaAs/AlGaAs quantum well

We have investigated effects of an applied uniaxial strain on nuclear spin coherence in an n-(110) GaAs/AlGaAs quantum well by optically detected nuclear magnetic resonance (ODNMR). The ODNMR line widths associated with the quadrupolar splitting increase with the applied external strain because of the increase of inhomogeneous broadening. It is found that the phase decoherence of nuclear spin ensemble for the transition between quadrupolar-split two levels |?1/2? and |+1/2? is suppressed with increasing the quadrupolar splitting.

Anisotropic g‐factor Dependence of Dynamic Nuclear Polarization in n‐GaAs/AlGaAs (110) Quantum Wells

In this work, we investigated the well width dependence of dynamic nuclear polarization in n‐GaAs/Al0.3Ga0.7As (110) quantum wells. We prepared a set of samples with different well width and thus different g‐factor, and measured the degree of dynamic nuclear polarization in these samples by time‐resolved Faraday rotation measurements. Our calculation, considering simply the projection of time‐averaged electron spins along the external magnetic field, reproduced overall shapes of the nuclear fields as a function of anisotropy of g‐factor.