R. Kaji

Optical anisotropy and photoluminescence polarization in single InAlAs quantum dots

We have investigated the optical anisotropy in individual self-assembled quantum dots. The linear polarization analysis of the positive trion photoluminescence reveals the effect of the strain-induced valence band mixing since the positive trion has the spin-paired holes and therefore exchange interaction has no influence. Meanwhile, the neutral exciton indicates the complex polarization states due to both the in-plain asymmetries of the dot shape and the strain distributions. The experimental and theoretical polarization analysis has been performed for tens of InAlAs quantum dots and the correlation between the important parameters was investigated.

Decoherence of exciton complexes in single InAlAs quantum dots measured by Fourier spectroscopy

We report the single-photon Fourier spectroscopy of exciton and exciton complexes in single self-assembled InAlAs∕AlGaAs quantum dots. A Michelson interferometer was inserted in the photoluminescence path for measuring the coherence length of the time-averaged emission of neutral excitons and their complexes. The effects of exchange interactions and fluctuations in the surrounding excess charges were estimated by comparing the coherence times and the excitation energies of the excitons and their complexes.

Hysteretic response of the electron-nuclear spin system in single In 0.75 Al 0.25 As quantum dots: Dependences on excitation power and polarization

Optically pumped nuclear spin polarization in single InAlAs quantum dots was investigated in detail with regards to the dependences on the excitation power and electron spin polarization. By increasing (or decreasing) the excitation power at a particular excitation polarization, an abrupt rise (or drop) and a clear hysteretic behavior were observed in the Overhauser shift of the photoluminescence under an external magnetic field of

Precise measurements of electron and hole g-factors of single quantum dots by using nuclear field

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Effects of valence band mixing on hole spin coherence via hole-nuclei hyperfine interaction in InAlAs quantum dots

. Those features appear equivalently for the exciton and exciton complexes of the same quantum dot since the created nuclear field is equally effective. However, the degree of circular polarization shows different behaviors between a positively charged exciton and a neutral exciton or biexciton; further, only the positively charged exciton exhibits the precisely synchronized change and hysteretic behavior. It is suggested that the electron spin distribution is affected by the flip-flop of electron-nuclear spins. Further, the hysteresis is observed as a function of the degree of circular polarization of the excitation light, and its dependence on the excitation power is studied. The saturation of the Overhauser shift indicates the almost complete cancellation of the external magnetic field by the nuclear field.

Population dynamics in epitaxial Er2O3 thin films grown on Si(111)

We demonstrated the cancellation of the external magnetic field by the nuclear field at one edge of the nuclear polarization bistability in single InAlAs quantum dots. The cancellation for the electron Zeeman splitting gives the precise value of the hole g factor. In combination with the exciton g factor that is obtained from the Zeeman splitting for linearly polarized excitation, the magnitude and the sign of the electron and hole g factors in the growth direction are evaluated.

Dispersions of hole and electron g-factors in single InAs quantum rings evaluated using optically induced nuclear spin polarization

The effects of valence band mixing on the hole spin coherence in self-assembled InAlAs quantum dots are investigated. The valence band mixing induces not only optical anisotropy in the quantum dot emissions but also heavy hole spin dephasing via the hyperfine interaction with the lattice nuclei. We evaluated the degree of valence band mixing for a number of In0.75Al0.25As/Al0.3Ga0.7As quantum dots from experiments. The magnitude of valence band mixing does not show a clear dependence on the photoluminescence energy, and it is less than 0.25 in our sample. Although the direct measurement with experimental methods has not been carried out at this stage, the effect of valence band mixing on the hole spin coherence is discussed in detail by the calculations.

Discrimination of quantum dots using an optically created nuclear field

We grow single crystal erbium-oxide (Er2O3) epitaxially on a Si (111) substrate by using molecular beam epitaxy and investigate the population dynamics in Er3+ ions for the coherent manipulation of the population in Er2O3. Sharp and discrete Stark energy levels of the 4I13/2 manifold as small as 200 μeV are observed with inhomogeneous broadening caused by the uniform crystal field of the epitaxial Er2O3. We also experimentally determine the time constant of the resonant population transfer between spatially distant Er3+-ion sites, which is limited to the manipulation time of the population in the Er2O3 crystals. Using selective excitation of the Stark level in the 4I13/2 manifold, we obtain the energy transfer times between spatially distant Er3+ ions, and they are about 2 μs between sites whose crystallographic symmetry is different and 10 μs between sites whose symmetry is the same.

Mechanism of concentration quenching in epitaxial (Er x Sc 1-x ) 2 O 3 thin layers

The electron and hole g-factors in individual InAs/GaAs quantum rings were evaluated using the bistable responses of the optically induced nuclear spin polarization. Although the dispersions of the hole and exciton g-factors were larger than that of the electron g-factor in the individual quantum rings, a strong correlation between the hole and exciton g-factors was clearly observed. Part of the dispersion of the measured hole g-factor was explained well by considering the effect of strain-induced valence band mixing.

Hole g-factor anisotropies in individual InAs quantum rings

We investigated an optically created nuclear field in a single InAlAs quantum dot and demonstrated that the nuclear field can be used to discriminate whether photoluminescence lines originate from the same dot or a different dot. Since the nonlinear response of the nuclear field is sensitive to the electron g factor and correlation time of a coupled electron-nuclear spin system, the resultant Overhauser shift is their good measure for individual quantum dots. This method provides a simple and convenient alternative to the standard photon cross-correlation method.