We report the optical spectroscopic results of a single self-assembled In0.75Al0.25As/Al0.3Ga0.7As quantum dot. The polarization-dependent shift of the Zeeman splitting in a single InAlAs QD has been observed. The induced Overhauser field is estimated to be ∼0.16 T in this InAlAs QD and the magnitude is shown to be controllable by the degree of circular polarization of excitation light.

We report the experimental demonstration of single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires is fabricated by epitaxial growth on an electron-beam patterned substrate. Photoluminescence spectra taken on single quantum dots show narrow emission lines. Superconducting single photon detectors, which have a higher sensitivity than avalanche photodiodes in the infrared, enable us to measure auto and cross correlations. Clear antibunching is observed ͓g ͑2͒ ͑0͒ = 0.12͔ and we show a biexciton-exciton cascade, which can be used to create entangled photon pairs. © 2010 American Institute of Physics. ͓doi:10.1063/1.3506499͔Semiconductor quantum dot ͑QD͒ structures are attractive candidates for solid-state single photon and/or entangled-photon pair generation. 1-3 Nanowire QDs ͑NW-QDs͒ are promising candidates for such sources because of the controllability of doping, shape, and material freedom. 4,5 Fine structure splitting is predicted to be absent, which makes NW-QDs ideal for the creation of entangled photon pairs. 6 Single photon emission from a NW-QD has been shown at wavelengths shorter than 1000 nm. 7 However, a single photon NW-QD emitter at telecommunication wavelengths and a detailed study of its emission lines has not been reported, because until recently a single photon detector ͑SPD͒, with a high enough signal to noise ratio at infrared wavelengths and an adequate timing resolution was lacking. In this letter, we report on the fabrication and characterization of a regular array of InAsP QD embedded in an InP NW, emitting around 1.3 m and characterization of the QD photoluminescence ͑PL͒ using superconducting SPDs ͑SSPDs͒. We demonstrate controlled positioning of the NWs by growing them in a regular array. Control of the position is important for uniform growth, which is necessary for uniform QDs. SSPDs offer single photon detection with low dark counts, excellent timing resolution, and decent efficiency in the infrared, without the need for gating. In addition, SSPDs have very short dead times ͑10 ns͒ and no after pulsing. These characteristics enable us to perform auto and cross correlation experiments.Arrays of InAsP QDs embedded in InP NWs are synthesized by selective area metal organic vapor phase epitaxy ͑SA-MOVPE͒. 8 A metal catalyst is usually used ͑i.e., Au͒ to grow NW structures, however with SA-MOVPE a catalyst is not needed, preventing diffusion of the metal into the NW. A ͑111͒ InP wafer is covered by 30 nm of SiO 2 . By electron beam lithography and wet-etching, 40-60 nm diameter openings are created to form NW nucleation-sites. At a growth rate of 3 nm/s, first a 1 m long segment of InP is grown by adding trimethylindium and tertiarybutylphosphine ͑TBP͒ to the MOVPE reactor at 640°C. Subsequently the temperature is lowered to 580°C and arsine ͑AsH 3 ͒ is added to the reactor ͑V/III ratio 340, partial pressure TBP: AsH 3 3:1͒ to grow 8 to 10 nm InAsP to form the QDs. The ...

Clarification of the mechanism of oxytocin (OT)-induced contraction of the uterus seems to be essential for the elucidation of the mechanism of the initiation of labor.Although it has been suggested that estradiol (E) and progesterone (P) are involved in the expression of oxytocin receptors (OTRs), no consensus opinion has been formed on this topic.Thus we recently assessed the effects of E and P on OTR expression using cultures of human uterine cells and we examined the changes in the expressed OTRs following treatment of the cell with exogenous OT. The following results were obtained: OT added and the time after the addition of OT. This change was observed within 60 minutes after the addition of OT to the culture. This decrease in surface OTR concentra tion was suppressed by concanavalin A (ConA), an inhibition of the internalization of cell surface receptors.These results indicate that in humans also, the expression of myometrial OTRs is regulated by change in the E/P ratio. The present study also revealed that OTRs, once expressed, soon disappear from the cell surface in the presence of exogenous OT (due to internalization of OTRs, i.e., dislocation of OTRs from the cell surface to the inside of the cells), and that prolonged exposure to OT even leads to the disappearance of intracellularOTRs. The present study thus suggests that the expression of human myometrial OTRs is 176 S.ADACHI regulated by E and P, and that an agonist-induced desensitization mechanism at the receptor level, similar to that reported for fi-adrenergic receptors, is also operating in this receptor.

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. © 2011 American Institute of Physics. ͓doi:10.1063/1.3583453͔Studies on localized spins in semiconductor quantum dots ͑QDs͒ have been attracting considerable interest. This is because the discrete electronic levels involved in the optical transitions serve the fascinating applications in which QDs are used as emitters of single, indistinguishable, and entangled photons.1 For these applications, it is crucial to study the polarization of the emitted photons associated with exciton annihilation. For ideal QDs as artificial atoms, the relevant eigenstates are bright excitons with the angular momentum of Ϯ1, and the circularly polarized photons ͑ + or − ͒ are to be absorbed ͑emitted͒ to ͑from͒ the eigenstates. However, actual QDs have the anisotropic distributions of shape and strain, and as a result, the confinement potential symmetry is reduced from D 2d to C 2v or lower. It is well known that the shape anisotropy induces the change in the emission polarization as well as the level splitting as an exciton fine structure.2-5 Also, QDs formed by self-assembly in the Stransky-Krastanov ͑SK͒ growth mode is believed to have a large strain originating from the QD formation process, and the strain with the anisotropic distribution more or less remains inside a QD even after QD formation is complete. Consequently, the emission polarization is affected by the anisotropic exchange interaction ͑AEI͒ and the straininduced valence-band mixing ͑SI-VBM͒. The former originates from the QD shape asymmetry and the latter comes mainly from the in-plane anisotropic relaxation of strains. Accordingly, the polarization of the QD emissions is one of valuable probes for the origin of the QD symmetry lowering. Since the strain anisotropy may be different largely from QD to QD, the investigation of the polarization is necessary for the individual QDs.In this work, we investigated the polarization of the photoluminescence ͑PL͒ in single InAlAs QDs in order to probe the interactions that reduce the QD potential symmetry. By comparing the positive trion PL with the neutral exciton PL in the same single QD, the intrinsic parameters for AEI and SI-VBM were deduced. Since III-V semiconductor QDs generally have a smaller strain anisotropy than that in II-VI QDs, 6,7 the precise estimation of the composite of AEI and SI-VBM in III-V semiconductor QDs have not been reported so far. The simple method pr...

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We report the hysteresis of optically-pumped nuclear spin polarization and the degree of circular polarization of photoluminescence on the excitation power and electron spin polarization in single InAlAs quantum dots. 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 of the exciton and exciton complexes from the same single quantum dot under an external magnetic field of 5 T. However, the degree of circular polarization shows different behaviors between a positively charged exciton and a neutral exciton or biexciton; further, only 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 electronnuclear 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 after the abrupt rise indicates the almost complete cancellation of the external magnetic field by the nuclear field created within the width that is decided by the correlation time between the electron and the nuclei spin system. PACS numbers: 73.21.La, 78.67.Hc, 71.35.Pq,71.70.Jp Recently, research on electron-nuclear spin interaction has been revived in view of its applications. This is because semiconductor quantum dots (QDs) enhance the electron-nuclear spin interaction (hyperfine interaction) due to their strong 3D confinement of the electron wavefunction, and the enhanced interaction gives the possibility of aligning nuclear spins in one direction up to several tens % in a single QD through the optical pumping. In fact, a large rate of nuclear spin polarization and the resultant large effective nuclear field up to several tesla were observed recently in interface GaAs QDs 1,2 , selfassembled InAlAs QDs 3,4 and InGaAs QDs 5,6,7 . Because of the ultralong coherence, nuclear spin is expected to contribute to applications such as a long-lived quantum memory at the nuclear level 8 and qubit conversion by using the nuclear field 9 . Beyond such potential applications for quantum information processing, nuclear magnetic ordering and optically induced ferromagnetic ordering of spin systems are of surpassing interest in fundamental physics. Therefore, the control of nuclear spins in nanostructures has both fundamental as well as practical significance.In this study, we investigated the optical pumping of nuclear spin polarizations in a single self-assembled InAlAs QD. An abrupt rise in and the hysteresis of the Overhauser shift in addition to the degree of circular polarization (DCP) in the photoluminescence (PL) of positively charged excitons were clearly observed in the excitation power and excitation polarization dependences. Additionally, with the aid of this abrupt change, the sign of the electron g-factors in the z dire...

Photon emission with nonclassical photon statistics is discussed with a single InAlAs quantum dot. The deterministic single-photon generation in which the emitted photon wavelength matches well to the highly sensitive wavelength region of highly efficient, low-noise Si-single-photon detectors and also to an atmospheric transmission window is demonstrated. Competing transition processes between neutral and charged exciton species originating from an exclusive formation in the same single quantum dot are clarified. It was found that suppressing the charged exciton formation is possible by a quasi-resonant excitation for a deterministic monochromatic single-photon generation. Polarizationdependent photoluminescence spectroscopy clearly indicates the preservation of photon polarizations between photons emitted by biexciton/exciton recombinations. Furthermore, the deterministic polarization-correlated photon pair generation with biexciton-exciton cascaded transition occurring in a single quantum dot is directly confirmed by the polarized second-order photon correlation measurements. This indicates a longer polarization flip time than the exciton lifetime, which is an essential requirement for the deterministic Einstein-Podolsky-Rosen photon pair generation under the present biexciton-exciton cascaded transition scheme.

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. By combining with the exciton g-factor that is obtained from the Zeeman splitting for linearly polarized excitation, the magnitude and sign of the electron and hole g-factors in the growth direction are evaluated.Semiconductor quantum dots (QDs) exhibit a variety of confinement-related optical and electronic properties useful for opto-electronic device applications such as QD lasers and detectors. In particular, broad efforts are currently underway to develop new techniques for controlling spin degrees of freedom in QDs for quantum information processing. A key quantity for the spin manipulation is a g-factor, which is a coefficient connecting magnetic dipole moments with the spin degrees of freedom. Therefore, the knowledge of electron and hole g-factors and their control are important. For example, zero electron g-factor is required to convert the photon qubit into the electron spin qubit 1 while the system with a large gfactor is preferable for controlling spin qubit in terms of the energy selectivity. The g-factors of self-assembled QDs have been obtained by optical measurements and transport measurements. Generally, the electron g-factor is deduced from transport measurements while the exciton g-factor, which is the sum of an electron and a hole g-factors, is deduced from optical measurements. In the optical measurements, since the photoluminescence (PL) is generated by the annihilation of an electron and a hole, it is usually difficult to independently obtain an electron or a hole g-factor of QDs. In addition, sensitivity of the g-factors to the spatial confinement has been predicted by theoretical studies and partly confirmed in the experiments 2 . The obtained values of the g-factors are much different from bulk ones possibly due to size quantization, strain, and other effects, however those effects are difficult to evaluate nondestructively and noncontactly for individual QD. Therefore, the direct probing method of the electron or hole g-factor is required for individual QD target.In this study, we demonstrate the precise measurements of electron and hole g-factors in single InAlAs QDs by using the optically induced nuclear field. The measurement principle is based on the fact that nuclear field is effective only on electrons and can compensate the external magnetic field. We first show that the nuclear field exactly cancels the external magnetic field at one edge of nuclear bistability. Recently, we proposed to use the cancellation of external and nuclear field for photon-spin qubit conversion to dispense with the zero gfactor engineering 3 . The cancellation at the bistability indicates that the condition for the qubit conversion is automatically realized there. Then, the measured Zeeman splitting corresponds to that of not an exciton but a ...