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Low dark current InAs/GaAs quantum-dot infrared photodetectors ͑QDIPs͒ are demonstrated. The dark current is reduced by over three orders of magnitude by using a thin AlGaAs current blocking layer. This thin AlGaAs layer reduces the dark current much more than the response signal. The responsivity at 0.5 V is 0.08 A/W with a peak detection wavelength at 6.5m. The corresponding detectivity is 2.5ϫ10 9 cm Hz 1/2 /W 1/2 , which is the highest detectivity reported for a QDIP at 77 K.

Background Studies comparing long-term outcomes between general anesthesia (GA) and local anesthesia (LA) for STN-DBS in Parkinson's disease (PD) are lacking. Whether patients who received STN-DBS in GA could get the same benefit without compromising electrophysiological recording is debated. Methods We compared five-year outcomes for different anesthetic methods (GA vs LA) during STN-DBS for PD. Thirty-six consecutive PD patients with similar preoperative characteristics, including age, disease duration, and severity, underwent the same surgical procedures except the GA (n=22) group with inhalational anesthesia and LA (n=14) with local anesthesia during microelectrode recording and intraoperative macrostimulation test. Surgical outcome evaluations included Unified Parkinson's Disease Rating Scale (UPDRS), Mini-Mental Status Examinations, and the Beck Depression Inventory. Stimulation parameters and coordinates of STN targeting were also collected. Results Both groups attained similar benefits in UPDRS part III from STN-DBS (GA 43.2 ± 14.1% vs. LA 46.8 ± 13.8% decrease, p=0.45; DBS on/Med off vs. DBS off/Med off) and no difference in reduction of levodopa equivalent doses (GA 47.56 ± 18.98% vs. LA 51.37 ± 31.73%, p=0.51) at the five-year follow-up. In terms of amplitude, frequency, and pulse width, the stimulation parameters used for DBS were comparable, and the coordinates of preoperative targeting and postoperative electrode tip were similar between two groups. There was no difference in STN recording length as well. Significantly less number of MER tracts in GA was found (p=0.04). Adverse effects were similar in both groups. Conclusions Our study confirmed that STN localization with microelectrode recording and patient comfort could be achieved based on equal effectiveness and safety of STN-DBS under GA compared with LA.

We report on a systematic correlation between the fine-structure splitting and polarization anisotropy of excitons in InAs/GaAs quantum dots, but with an unexpected reversal in order of the polarization eigenaxes. Such an anticorrelation is explained by a large valence-band-mixing induced splitting due to the shape and strain anisotropies. The strength and phase of valence band mixing are also found to play an important role in the tuning of fine structure splitting using an in-plane magnetic field.

We report on the magnetic response of negative trions X − in single self-assembled InAs/GaAs quantum dots. Unlike the conventional quadratic diamagnetic shift for neutral excitons, the observed X − diamagnetic shifts are small and nonquadratic. In particular, we also observed a reversal in sign of the conventional diamagnetic shift. A theoretical analysis indicates that such anomalous behaviors for X − arise from an apparent change in the electron wave function extent after photon emission due to the strong Coulomb attraction induced by the hole in its initial state. This effect can be very pronounced in small quantum dots, where the electron wave function becomes weakly confined and extended much into the barrier region. When the electrons gradually lose confinement, the magnetic response of X − will transit gradually from the usual quadratic diamagnetic shift to a quartic dependence, and finally into a special paramagnetic regime with an overall negative energy shift.The diamagnetic shift of confined excitons has long been used as a measure of the spatial extent of the excitonic wave functions in various semiconductor nanostructures such as quantum wires, 1-4 quantum dots ͑QDs͒, 4-6 as well as quantum rings. 7 In a weak magnetic field B, the exciton diamagnetic shift is expected to exhibit a quadratic B dependence, i.e., ⌬E = ␥B 2 , with a diamagnetic coefficient ␥ proportional to the area of the excitonic wave function, which reflects both the spatial confinements and interparticle Coulomb interactions. 8 For excitons ͑X͒ strongly confined in QDs, where the single-particle energies dominate over the Coulomb energies, ␥ is a measure of the spatial confinement of the QDs, while the magnetic responses of interparticle Coulomb energies only appear as correction terms to the overall diamagnetism. This simple picture is also true for biexcitons ͑XX͒ and trions ͑X + or X − ͒ strongly confined in QDs, except for a slight but systematic difference in ␥ due to the different magnetic responses of interparticle Coulomb energies. 6 On the other hand, when excitons are weakly confined in QDs such that Coulomb energies dominate over the single-particle energies, ␥ becomes a measure of the magnetic response of Coulomb energies. For exciton complexes weakly confined in QDs, the diamagnetic behaviors are expected to be more complicated because of the more elaborate Coulomb interactions. In particular, it has been theoretically predicted that a weakly confined negative trion ͑X − ͒ in large-size QDs would exhibit a negative magnetic dispersion, 5 based on the interpretation of the reported weak negative magnetic dispersion for X − in two-dimensional systems. 9,10 However, such an unusual behavior of X − has not yet been observed in past works focused on charged trions in QD system, 11,12 probably because the condition to observe this anomalous behavior is critical as we shall explain in this Brief Report.In this Brief Report, we report on the magnetic response of negative trions ͑X − ͒ in single InAs/GaAs self-assembled QDs. We ...

The diamagnetic shifts of neutral excitons and biexcitons confined in single self-assembled In͑Ga͒As/GaAs quantum rings are investigated. Unlike quantum dots, quantum rings reveal a considerably large biexciton diamagnetic shift, about two times larger than that of single excitons. Based on model calculations, we found that the inherent structural asymmetry and imperfection, combined with the interparticle Coulomb interactions, is the fundamental cause of the more extended biexciton wave function in the quantum rings. The exciton wave function tends to be localized in one of the potential valleys induced by structural imperfections of the quantum ring due to the strong localization of hole and the electron-hole Coulomb attraction, resembling the behavior in single dots. Our results suggest that the phase coherence of neutral excitons in quantum rings will be smeared out by such wave function localizations.

Magneto-transport measurements are performed on the two-dimensional electron system (2DES) in an AlGaAs/GaAs heterostructure. By increasing the magnetic field perpendicular to the 2DES, magnetoresistivity oscillations due to Landau quantisation can be identified just near the direct insulator-quantum Hall (I-QH) transition. However, different mobilities are obtained from the oscillations and transition point. Our study shows that the direct I-QH transition does not always correspond to the onset of strong localisation. * ctliang@phys.ntu.edu.tw and ochiai@faculty.chiba-u.jp PACS numbers: 72.15.Rn, 71.70.Di, The insulator to quantum Hall (I-QH) transition in a two-dimensional electron system (2DES) at low perpendicular magnetic fields B has attracted much attention [1, 2,3,4,5,6,7,8,9,10,11]. Theoretically, the direct I-QH transition from the insulator to an integer QH state of ν = 1 is forbidden in an infinite, non-interacting 2DES with arbitrary amount of disorder, where ν is the Landau level filling factor [1, 2,3]. In such a system, the only allowed state at B = 0 is the insulating one, and the 2DES undergoes the I-QH transition to enter the ν = 1 QH state [12,13]. Realistically, however, only systems of finite sizes are available, and the effects of the electron-electron (e-e) interaction are significant in some 2DESs [4,5,14,15,16,17,18]. As a result, the 2DESs may experience the direct I-QH transition from the low-field insulator to QH states of higher filling factors [2,3,4,8,16,17,18]. Such a transition can be related to the zero-field metal-insulator transition, to which e-e interaction cannot be ignored [4]. Given that most 2DESs show metallic behavior at B=0, the investigation of the direct I-QH transition at low B should be conducted in low-mobility 2DESs [1,12].

We present detailed experimental results of the temperature dependence of continuous wave and time-resolved photoluminescence ͑PL͒ spectroscopy in self-assembled InAs/GaAs quantum dot and quantum ring nanostructures. A dramatic increase in PL decay time of the excited and ground states is observed in InAs quantum rings at high temperature. We speculate that the longer PL lifetime in quantum rings is due to the interplay among the dark states, ground states, and the reduced wave function overlapping between electrons and holes. A rate equation model is proposed to interpret the observed temperature dependence of the ground state exciton lifetime.