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.

Spin-orbit effects in non-magnetic semiconductors allow for the manipulation of electronic spins in the absence of an applied magnetic field. Here, the authors exploit a persistent spin helix state in single quantum wells to enhance the coherence length of electronic drift transport.

Acoustically induced spin-orbit interactions revealed by two-dimensional imaging of spin transport in GaAs

Magneto-optic Kerr microscopy was employed to investigate the spin-orbit interactions of electrons traveling in semiconductor quantum wells using surface acoustic waves (SAWs). Two-dimensional images of the spin flow induced by SAWs exhibit anisotropic spin precession behaviors caused by the coexistence of different types of spin-orbit interactions. The dependence of spin-orbit effective magnetic fields on SAW intensity indicates the existence of acoustically controllable spin-orbit interactions resulting from the strain and Rashba contributions induced by the SAWs.

Cavity-QED assisted attraction between a cavity mode and an exciton mode in a planar photonic-crystal cavity

The photoluminescence spectra from a quantum-dot exciton weakly-coupled to a planar photonic-crystal cavity is experimentally investigated by temperature tuning. Significant resonance shifts of the cavity mode are observed as the cavity mode spectrally approaches that of the exciton mode, showing the appearance of cavity-to-exciton attraction or mode pulling. Cavity-mode spectral shifts are also found theoretically using a master equation model that includes incoherent pump processes for the coupled exciton and cavity, pure dephasing, and allows for photon emission via radiation modes and the leaky cavity mode. Both experiments and theory show clear cavity mode spectral shifts in the photoluminescence spectra, when certain coupling parameters are met. However, discrepancies between the experimental data and theory, including more pronounced spectral shifts in the measurements, indicate that other unknown mode-pulling effects may also be occurring.

Cavity mode emission in weakly coupled quantum dot--cavity systems

We study the origin of bright leaky-cavity mode emission and its influence on photon statistics in weakly coupled quantum dot - semiconductor cavity systems, which consist of a planar photonic-crystal and several quantum dots. We present experimental measurements that show that when the system is excited above the barrier energy, then bright cavity mode emissions with nonzero detuning are dominated by radiative recombinations of deep-level defects in the barrier layers. Under this excitation condition, the second-order photon autocorrelation measurements reveal that the cavity mode emission at nonzero detuning exhibits classical photon-statistics, while the bare exciton emission shows a clear partial anti-bunching. As we enter a Purcell factor enhancement regime, signaling a clear cavity-exciton coupling, the relative weight of the background recombination contribution to the cavity emission decreases. Consequently, the anti-bunching behavior is more significant than the bare exciton case - indicating that the photon statistics becomes more non-classical. These measurements are qualitatively explained using a medium-dependent master equation model that accounts for several excitons and a leaky cavity mode.

Quality factor control and lasing characteristics of InAs/InGaAs quantum dots embedded in photonic-crystal nanocavities

We demonstrate lasing action with a high spontaneous emission factor and temperature insensitivity in InAs/InGaAs quantum dots (QD) embedded in photonic crystal nanocavities. A quality factor (Q) of over 10,000 was achieved by suppressing the material absorption by QDs uncoupled to the cavity mode. High Q cavities exhibited ultra low threshold lasing with a spontaneous emission factor of 0.7. Less frequent carrier escape from the QDs, which was primarily favored by high potential barrier energy, enabled low threshold lasing up to 90 K.

Transport and lifetime enhancement of photoexcited spins in GaAs by surface acoustic waves

We demonstrate spin transport and spin lifetime enhancement in GaAs quantum wells induced by the traveling piezoelectric field of a surface acoustic wave (SAW). Spin transport lengths of about 3 microm corresponding to spin relaxation times during transport over 1 ns are observed, which are considerably longer than the exciton spin diffusion lengths in the absence of a SAW. The slow spin relaxation is attributed to a reduced electron-hole exchange interaction, when the carriers are spatially separated by the lateral potential modulation induced by the SAW.

[A case of sinus arrest during a surgical operation]

A 75-year old obese female was scheduled for mastectomy due to breast tumor. At the preoperative examinations, she had premature atrial contraction on electrocardiogram. But she had no clinical signs or symptoms regarding sick sinus syndrome. She was evaluated as ASA risk II and arranged for neuroleptanesthesia. During the operation, she developed severe bradycardia and asystole lasting for 7 seconds, which was not effectively controlled by atropine sulfate. Then cardiac rhythm resumed spontaneously and she recovered with no neurological deficits. On the postoperative examination of 24 hr-Holter ECG, she showed bradycardia and sinus arrest and she was diagnosed to have sick sinus syndrome.