Selection and ultrastructural observation of a high-CO2-requiring mutant of cyanobacterium Synechococcus sp PCC7942

A high-CO2-requiring mutant of Synechococcus sp. PCC7942 las been isolated after chemical mutagenesis of ethyl methane sulphonate (EMS). It was able to grow at 4% CO2, but not under ambient CO2. The initial screening of the mutant showed that the genetic reversion rate was about 10(-7) and death occurred 2 -3 days after being transferred from 4% CO2 to the ambient air. Its photosynthetic dependence on external dissolved inorganic carbon was higher than that of the wild type cells, but its carbonic anhydrase activity was comparatively low. In the ultrastructural level, various types of aberrant carboxysomes appeared in the mutant cells: rod-shaped carboxysomes, irregular carboxysomes and the "empty-inclusion carboxysomes" with increasing number of glycogen granules surrounding the thylakoids. All these alterations indicated that the mutant was defective in utilizing the external CO2. The induction of carboxysomes by lower levels of CO2 and the biogenesis of carboxysomes are herein discussed.

Direct observation of excitonic polaron in InAs/GaAs quantum dots

We report a direct observation of excitonic polaron in InAs/GaAs quantum dots using the photoluminescence (PL) spectroscopy. We observe that a new peak s' emerges below the s-shell which has anomalous temperature dependence emission energy. The peak s' anticrosses with s at a certain temperature, with a large anticrossing gap up to 31 meV. The behavior of the new peak, which cannot be interpreted using Huang-Rhys model, provides a direct evidence for strong coupling between exciton and LO phonons, and the formation of the excitonic polaron. The strong coupling between exciton and phonons opens a way to coherently control the polaron states.

Direct observation of coherent spin transfer processes in an InGaAs/GaAs quantum well via two-color time-resolved Kerr rotation measurements

A two-color time-resolved Kerr rotation spectroscopy system was built, with a femtosecond Ti:sapphire laser and a photonic crystal fiber, to study coherent spin transfer processes in an InGaAs/GaAs quantum well sample. The femtosecond Ti:sapphire laser plays two roles: besides providing a pump beam with a tunable wavelength, it also excites the photonic crystal fiber to generate supercontinuum light ranging from 500 nm to 1600 nm, from which a probe beam with a desirable wavelength is selected with a suitable interference filter. With such a system, we studied spin transfer processes between two semiconductors of different gaps in an InGaAs/GaAs quantum well sample. We found that electron spins generated in the GaAs barrier were transferred coherently into the InGaAs quantum well. A model based on rate equations and Bloch-Torrey equations is used to describe the coherent spin transfer processes quantitatively. With this model, we obtain an effective electron spin accumulation time of 21 ps in the InGaAs quantum well.

Scanning electron microscopy observation of in-device InAs/AlAs quantum dots by selective etching of capping layers

Self-assembled InAs/AlAs quantum dots embedded in a resonant tunneling diode device structure are grown by molecular beam epitaxy. Through the selective etching in a C6H8O7 center dot H2O-K3C6H5O7 center dot H2O-H2O2 buffer solution, 310 nm GaAs capping layers are removed and the InAs/AlAs quantum dots are observed by field-emission scanning electron microscopy. It is shown that as-fabricated quantum dots have a diameter of several tens of nanometers and a density of 10(10) cm(-2) order. The images taken by this means are comparable or slightly better than those of transmission electron microscopy. The undercut of the InAs/AlAs layer near the edges of mesas is detected and that verifies the reliability of the quantum dot images. The inhomogeneous oxidation of the upper AlAs barrier in H2O2 is also observed. By comparing the morphologies of the mesa edge adjacent regions and the rest areas of the sample, it is concluded that the physicochemical reaction introduced in this letter is diffusion limited.

Observation of the surface circular photogalvanic effect in InN films

A sizable spin-dependent photocurrent related to the interband transition in InN films is observed. The surface charge accumulation layer is suggested to be the origin of the circular photogalvanic current, which is consistent with the result of uniaxial strain experiments and the comparison of front and back incidence. The homogeneous photocurrent demonstrates the existence of spin splitting in the InN surface layer, and the structure inversion asymmetry (SIA)-dominant mechanism indicates a great possibility for the manipulation of spin splitting, which would undoubtedly benefit further research and applications of spintronics. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

Observation of the current-voltage plateau-like structure of resonant tunnelling diode with prewells under different magnetic fields

We have grown resonant tunnelling diodes (RTDs) with different sized emitter prewells and without a prewell. The current-voltage (I-V) characteristics of them in different magnetic fields were investigated. Two important phenomena were observed. First, a high magnetic field can destroy the plateau-like structure in the I-V curves of the RTD. This phenomenon is ascribed to the fact that the high magnetic field will demolish the coupling between the energy level in the main quantum well and that in the emitter quantum well or in the prewell. Secondly, the existence and size of the prewell are also important factors influencing the plateau-like structure.

Observation of antiphase states in a passively Q-switched multimode microchip Yb : YAG laser with a GaAs saturable absorber

Antiphase dynamics has been observed experimentally for the laser modes operation in a laser-diode-pumped Q-switched microchip Yb:YAG laser with GaAs as a saturable absorber in the presence of spatial hole-burning. The Q-switched pulses sequences of two modes at different pump power have been obtained. The experimental results have shown that the pulses sequences displayed classic antiphase dynamics. (C) 2003 Elsevier B.V. All rights reserved.

Experimental observation of resonant modes in GaInAsP microsquare resonators

GaInAsP-InP microsquare resonators with InP pedestals are fabricated by two-step chemical etching, and obvious mode peaks are observed in the photoluminescence spectra of the resonators. The mode Q-factors about 500 are obtained for a microsquare resonator with the side length of 7 mu m. The experimental mode interval is in agreement with that predicted by the light ray method based on the cavity length, instead of that of the whispering-gallery (WG)-like modes, which has mode interval twice of that determined by the cavity length. The finite-difference time-domain simulation shows that a little asymmetry may greatly reduce the difference of the Q-factors between the WG-like modes and the other modes.

Observation of intershell and hybridized energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling as a function of excitation power density in InAs/GaAs quantum dots (QDs). In addition to the emission bands of exciton recombination corresponding to the atom-like S, P, and D, etc. shells of quantum dots, it was observed that some extra states, P-' (between the S and P shells) and D-' (between the P and D shells), appear in the spectra with increasing number of excitons occupying the QDs. The emergence of these intershell excitonic levels is an experimental demonstration of strong exciton-exciton exchange interaction and coupling as well as state mixing and hybridization of a multiexciton system in quantum dots.

Observation of electric current induced by optically injected spin current

Linearly polarized light at normal incidence injects a spin current into a strip of two-dimensional electron gas with Rashba spin-orbit coupling. The authors report observation of an electric current when such light is shed on the vincinity of the junction in a crossbar-shaped InGaAs/InAlAs quantum well Rashba system. The polarization dependence of this electric current was experimentally observed to be the same as that of the spin current. The authors attribute the observed electric current to the scattering of the optically injected spin current at the crossing. (c) 2007 American Institute of Physics.

Room-temperature observation of electron resonant tunneling through InAs/AlAs quantum dots

Molecular beam epitaxy is employed to manufacture self-assembled InAs/AlAs quantum-dot resonant tunneling diodes. The resonant tunneling current is superimposed on the thermal current, and together they make up the total electron transport in devices. Steps in current-voltage characteristics and peaks in capacitance-voltage characteristics are explained as electron resonant tunneling via quantum dots at 77 or 300 K, and thus resonant tunneling is observed at room temperature in III-V quantum-dot materials. Hysteresis loops in the curves are attributed to hot electron injection/emission process of quantum dots, which indicates the concomitant charging/discharging effect. (c) 2006 The Electrochemical Society.

Observation of "ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.

Observation of quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells

Quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells are suggested by the bias dependence of the photocurrent spectra of p-i-n photodiodes. Both Stark redshift and blueshift have been observed for the same sample in the different ranges of electric fields applied to the quantum wells. The turnaround point corresponds to a certain electric field (named "critical" field). This phenomenon was generally predicted by Austin in 1985 [Phys. Rev. B 31, 5569 (1985)] and calculated in detail for SiGe quantum structure by Kim recently [Thin Solid Films 321, 215 (1998)]. The critical electric field obtained from the photocurrent spectra is in reasonable agreement with the theoretical prediction. (C) 2000 American Institute of Physics. [S0021-8979(00)03711-7].

Observation of field-driven blue shift in delta-doped Si n-i-p-i multiple quantum wells

We report the observation of the field-driven blue shift at near absorption edge in the photo-current response spectra of delta-doped Si n-i-p-i multiple quantum wells due to the widening of the effective energy gap. This phenomenon differs from the observed results in GaAs/AlGaAs and GeSi/Si superlattices, because the physical mechanisms of forming energy band in these superlattice samples are different. Our experimental results are interpreted satisfactorily by the theoretical calculation. (C) 1999 Elsevier Science Ltd. All rights reserved.

Observation of defects in GaN epilayers

GaN epilayers grown on sapphire substrates nitridated for various lengthy periods were investigated by light scattering tomography (LST) and Raman scattering. In the LST images of the plane-view epilayers, the light scattering defects distribute in [<11(2)over bar 0>] directions. The defect density is lower in epilayer grown on substrate nitridated for a longer period. The defects are believed to be straight threading edge dislocations on {<1(1)over bar 00>} planes. The Raman shift of E-2 mode is larger in the sample grown on substrate nitridated for a longer period. Our results show that the stress is higher in the sample with fewer dislocations.