中岡 俊裕

The g factor of a single self-assembled quantum dot is tuned by applying an electrical bias voltage. Individual InGaAs quantum dots embedded in a stripe mesa structure sandwiched by Schottky electrodes are studied by photoluminescence measurements. We find that under applied magnetic field a dot with an asymmetric shift for applied bias voltage shows an increase of the exciton g-factor absolute value by about 8%, while most of the dots with relatively symmetric shifts show no significant change. The anomalous g-value increase is discussed in terms of the Kondo effect.

The authors report on transport phenomena in single self-assembled GaN quantum dots using metallic leads with a nanoscale gap. The nitride device works as a single electron transistor. Measurement of the stability diagram at 12 K shows Coulomb blockade regions with a single electron charging energy of about 10 meV.

We probe acoustic phonon mediated relaxation between tunnel coupled exciton states in an individual quantum dot molecule in which the interdot quantum coupling and energy separation between exciton states is continuously tuned using static electric field. Time resolved and temperature dependent optical spectroscopy are used to probe interlevel relaxation around the point of maximum coupling. The radiative lifetimes of the coupled excitonic states can be tuned from similar to 2 ns to similar to 10 ns as the spatially direct and indirect character of the wave function is varied by detuning from resonance. Acoustic phonon mediated interlevel relaxation is shown to proceed over time scales comparable to the direct exciton radiative lifetime, indicative of a relaxation bottleneck for level spacings in the range Delta E similar to 3-6 meV.

We have investigated cavity resonant excitation effects on InGaAs quantum dots (QDs) embedded in high-quality-factor photonic crystal nanocavities. The light emission of the lowest-order cavity mode at 1.06 mu m is enhanced by more than a factor of ten, as compared with nonresonant excitation, when the excitation wavelength is resonant with higher order cavity modes. This result can be attributed to an enhancement in the effective absorption coefficient due to a local enhancement of the excitation light, which couples with the cavity mode. The on-resonant excitation technique selectively and efficiently excites only the QDs in the cavity. On-resonant excitation at energies below the wetting layer band gap energy can achieve stronger light emission from the cavity mode than excitation at energies greater than the wetting layer band gap energy with much less undesirable background emission. It will be shown that this is primarily due to the enhancement in the effective absorption by the cavity resonant effect and the direct carrier generation in QDs.

We have experimentally and theoretically investigated quantum confined Stark effect in hexagonal self-assembled GaN/AlN quantum dots. We have observed a blueshift of up to 100meV for vertical electric field applied against the built-in electric field while we have observed a redshift for the electric field along the built-in field. The experimental result is compared with a charge self-consistent effective mass calculation, taking into account strain, piezoelectric charge, and pyroelectric charge. The tunability of the emission energy and the exciton binding energy is discussed. (c) 2005 Elsevier B.V. All rights reserved.

We have investigated magneto-optical properties of GaSb/GaAs self-assemble type II quantum dots by single dot spectroscopy in magnetic field. We have observed clear Zeeman splitting and diamagnetic shift of GaSb/GaAs quantum dots. The diamagnetic coefficient ranges from 5 to 30 mu eV/T-2. The large coefficient and their large distribution are attributed to the size inhomogeneity and electron localization outside the dot. The g-factor of GaSb/GaAs quantum dots is slightly larger than that of similar type I InGaAs/GaAs quantum dots. In addition, we find almost linear relationship between the diamagnetic coefficient and the g-factor. The linear increase of g-factor with diamagnetic coefficient is due to an increase of spin-orbit interaction with dot size. (c) 2005 Elsevier B.V. All rights reserved.

We have studied excitation wavelength dependence of light emission from InGaAs quantum dots (QDs) embedded in high-quality-factor photonic crystal nanocavities. The light emission of the cavity mode around 1 mu m was very weak with below-band-gap excitation in the InGaAs wetting layer. However, the emission of the lowest-order cavity mode was strongly enhanced when the excitation wavelength was resonant with higher-order cavity modes. This phenomenon can be attributed to the local intensity enhancement of the excitation light which couples with the cavity mode. The on-resonant excitation technique selectively and efficiently excites only the QDs in the cavity without undesirable background emission.

We succeeded in observing the electroluminescence and Stark shift of a single InAs/GaAs quantum dot. in the O-band (O-band is a 1.3 mu m band which has the lowest dispersion characteristics in optical fiber bands). In order to access a single quantum dot, we fabricated a p-i-n diode containing one quantum dot layer with a small ohmic contact area. The electroluminescence of a single exciton (lambda = 1321.6 nm) and biexciton (lambda = 1322.3 nm) were clearly observed at the center of the O-band at 7 K. This result is the longest wavelength attained Lip to now. The Stark shift of single quantum dots was also observed at around 1.32 mu m at 7 K. These results are promising for the realization of electrically driven single-photon emitters at optical fiber bands.

We show that a peculiar excitonic effect and a confining potential in self-assembled hexagonal GaN/AlN quantum dots produce an unconventional quantum-confined Stark effect. In contrast to the conventional quantum-confined Stark shift, the emission line from a single GaN dot under the applied electric field perpendicular to the growth direction blueshifts nearly symmetrically with respect to the direction of the field. The field dependence of the emission lines is reproduced in a charge self-consistent effective mass calculation, taking into account strain, piezoelectric charge, and pyroelectric charge. The unconventional blueshift is attributed to a significant variation of the exciton binding energy, made obvious by a cancellation between the energy shifts of electron and hole confined states.

We have investigated electron filling in single InAs quantum dots (QDs) using a lateral electron transport structure, i.e., nanolithographically defined metallic leads with nanogaps. Elliptic InAs QDs with a diameter of similar to 60/80 nm exhibited clear shell filling up to 12 electrons before the gate leakage became significant. Shell-dependent charging energies and level quantization energies for the s, p, and d states were determined from the addition energy spectra. Furthermore, it was found that the charging energies and the tunneling conductances strongly depend on the shell, reflecting that the electron wave functions for higher shells are more extended in space. (C) 2005 American Institute of Physics.

We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-Q single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

We have investigated the effect of strain on the g-factors of self-assembled In(Ga)As dots by single-dot spectroscopy and an eight-band effective mass calculation taking into account the influence of the strain distribution and the Zeeman effect. The strain and its distribution in and around the quantum dots are varied by thermal annealing or by introducing an strain reducing layer. Thermal annealing produces a graded composition profile due to In-Ga intermixing. The graded composition profile reduces both hydrostatic and biaxial strain near the bottom of the dot; and enhances them near the top. This strain variation results in a large reduction of the absolute hole g-value and a small reduction of the absolute electron g-value. On the other hand, the covering of InAs dots with an In0.17Ga0.83As strain reducing layer decreases mainly the hydrostatic strain. The variation of the strain and the band edge alignment enhance the electron g-value while they reduce the hole g-value. These results should provide insights to control the g-factors in pyramidal self-assembled dots.

We have observed light emission at a wavelength of 1.55 mu m from InAs quantum dots (QDs) embedded in a high-quality-factor photonic crystal (PhC inicrocavity for the first time. The InAs QDs were grown on a (100) GaAs substrate and the photoluminescence measurement was performed at room temperature. The cavity quality factor Q reaches 2700, which is close to the resolution limit of our measurement system. Due to the small mode volume V of the PhC cavity, Q/V increases up to 3400 x (n/lambda)(3). These are the highest values ever reported for microcavities containing QDs emitting in a spectral range around 1.5 mu m. Details of the optical properties of the microcavity, such as polarization and pump power density dependences are also discussed.

We demonstrated the first room temperature continuous wave lasing in InAs quantum-dot microdisk lasers with a standard air-cladding optical confinement structure. The spectrum shows the single strong lasing peak at a wavelength of 1280 nm. The threshold pump power is 410 muW, and the corresponding effective threshold obtained by considering the absorption efficiency is 81 muW. This achievement is mainly attributed to the increase in Q factor by the improved disk shape. (C) 2005 Optical Society of America.

We have calculated the strain distribution and electronic structures in stacked InAs/GaAs quantum dots (QDs) with the dot spacing 6-0 nm. We used the elastic continuum theory for the strain distribution, and the 8-band k . P theory for the electronic structures. For the triply stacked QDs, the light-hole (LH) component of the hole ground state increases with decreasing the dot spacing. The LH component in the columnar QD (dot spacing 0 nm) reaches 21.1 % which is 4.8 times larger than that in the single QD due to the reduction of the biaxial strain. Further increase of the LH component (up to 28.6%) is obtained in the fivefold-stacked columnar QD. This result suggests a possibility of increase in the TMmode transition in the columnar QDs. (C) 2004 Elsevier B.V. All rights reserved.

We have fabricated single quantum dot light emitting diodes and observed over 1.3 μm electroluminescence for the first time. The linewidth was less than 60 ueV and the intensity was increased linearly with current. © 2005 Optical Society of America.

The microdisk laser is a microcavity laser, which strongly confines the whispering gallery mode and realizes the ultralow threshold lasing and large scale integration. This paper presents two recent topics related with this laser. The first topic is the microdisk with InAs quantum-dot active layer. It is not only promising for ultimate low threshold operation but also has the potential of a high-speed single photon emitter because of the atom-like electron transition in quantum-dots and the large Purcell effect in the microcavity. In this study, we obtained the room temperature operation in this device, for the first time, by photopumping with a low effective threshold of less than 1 milliwatt. The second topic is the photonic molecule consisting of evanescent-coupled multiple microdisks based on GaInAsP materials. As well as the unique behavior of coupled modes in this structure, we also observed the bistable lasing by the nonuniform photopumping, which was considered to be arising from the saturable absorption. Due to its low threshold of 70 microwatts, this device is expected to be an optical memory and flip-flop device by photonic integration.

We fabricated and characterized photonic crystal (PhC) microcavities containing InAs quantum dots (QDs) grown on (100) GaAs substrates. QD emission coupled with a PhC cavity mode at a wavelength of 1.55 μm was observed at room temperature. The cavity quality factor Q and its ratio to the mode volume V, Q/V, reach up to 2700 and 3400 × (n/λ)3, respectively. To our knowledge, these are the highest values for microcavities containing QDs emitting at wavelengths around 1.5 μm. The large enhancement of emission intensity at the cavity resonances was clearly observed. The enhancement factor is ∼10-100, which depends on cavity modes and pump power density.

InAsSb quantum dots were grown on GaAs substrates by two methods. One was grown by conventional molecular-beam epitaxy in which arsenic and antimony were irradiated simultaneously. The other was grown by irradiating previously grown InAs dots with antimony to prevent antimony flux from functioning as a surfactant. Although the photoluminescence spectrum of the dots grown by the conventional method had two peaks, the photoluminescence spectrum of the dots grown by the second method had a single peak. Peaks at wavelengths longer than 1.4 μm were observed in the photoluminescence spectrum of the InAsSb quantum dots grown by the second method. © 2005 The Japan Society of Applied Physics.

We have investigated Zeeman splitting in single self-assembled InAs and InGaAs quantum dots experimentally and theoretically. By measuring photoluminescence from single dots, in a wide spectral region, we have obtained the exciton g factors of quantum dots with various photoluminescence energies. We find that the absolute value of the exciton g factors of InAs dots are smaller than those of the InGaAs dots, which differs from the composition dependence expected from that of the bulk ones. The experimentally obtained g factors are compared with calculated ones based on the eight-band k.p model where the influence of strain and the Zeeman effect are included. We find a good agreement between the calculation and the experiment qualitatively and quantitatively. The calculation reproduces the nontrivial composition dependence of the g factor of the quantum dots. In addition, the eight-band model predicts a size and shape dependence of the electron and hole g factors of the pyramidal quantum dots.

We fabricated a microdisk laser with five-stacked InAs quantum-dot (QD) active region, and demonstrated the lasing operation from 3 K to room temperature by femtosecond pulsed photopumping. At room temperature, the threshold power was estimated to be 0.75 mW, when the influence of the surface recombination at the disk edge was neglected. The lasing wavelength was 1.2-1.3 mum, which corresponded to excited states of the QDs. The temperature dependence of the threshold, slope efficiency, lasing wavelength, and linewidth are explained by the rapid increase in nonradiative recombination and internal absorption at critical temperatures of 200-230 K. (C) 2004 American Institute of Physics.

We have investigated the carrier relaxation in closely vertically stacked InAs quantum dots by time-resolved photoluminescence (PL) and micro-PL measurements. The PL decay and the excitation spectrum in the closely stacked dots are much different with those in single layer dots. The PL decay in the stacked dots strongly depends on the PL energy. The decay time in the lower energy side of the PL increases with the number of stacked dot layers. These suggest the existence of a cascadelike relaxation channel via nonresonant tunneling between the stacked dots. The nonresonant tunneling is consistent with the results of micro-PL measurement which allows us to access single columns of the stacked dots. A broad near-resonant absorption in a single column of the dots is explained on the basis of the nonresonant tunneling. (C) 2004 American Institute of Physics.

We have fabricated a micromachined air-bridge in which InGaAs self-assernbled quantum dots are embedded. Electrostatic voltage applied between the air-bridge and the substrate pulls the air-bridge down. The deformation of the bridge produces additional strain on the matrix and the dots modifying the electronic states. The modification has been detected through a photoluminescence peak shift of a single dot. The effect of the deformation on the electronic states is evaluated with the aid of theoretical calculation using a finite element method. The good agreement between calculated and experimental energy shifts demonstrates that the energy shift is due to the strain induced by the bridge deformation. Based on the result, we discuss the capability of this micromachined device with quantum dots to control the zero-dimensional electronic system and to measure small deformation and strain.

We have achieved a large increase (by a factor of up to 3200) in photoluminescence emission from InAs/AlAs quantum dot structures using the GaAs insertion layer and by hydrogen passivation at room temperature. The hydrogen passivation can lead to the reduction in a number of nonradiative recombination centers, and thus can increase the photoluminescence efficiency. After the treatments, the linewidth and the peak position of the luminescence were negligibly changed, indicating that the samples were not damaged during the plasma treatments. In time-resolved photoluminescence measurements, the decay time of hydrogenated samples was about twice as long as those of as-grown samples due to the passivated defects in surrounding barrier materials.

We carried out optical selective excitation of individual self-assembled quantum dots by using phase-modulated pulses. Based on scattered photoluminescence excitation resonances in individual QDs, the excitation pulses modulated in the spectral region allows for addressing individual ground states emission. The photoluminescence spectra including several QDs showed intensity changes according to both the modulation energies and phases. The results also suggested the individual control of selective QDs even in collective excitation. (C) 2003 Elsevier B.V. All rights reserved.

Self-assembled GaSb quantum dots (QDs) with a photoluminescence wavelength longer than 1.3 mum were successfully grown by suppressing the replacement of As and Sb on the surface of the Gash QDs. This result means that GaSb can thus join InAs or GaInAs as a suitable material for QD lasers for optical communications. (C) 2003 Elsevier B.V. All rights reserved.

We have investigated the carrier relaxation dynamics in single columns of tenfold stacked vertically aligned InAs quantum dots by micro-photoluminescence measurement. The excitation spectrum in the stacked dots is much different from that in the single dot characterized by the existence of a zero-absorption region and sharp multiple phonon emission lines. We have observed a broad continuum absorption far below the wetting layer band edge in the spectrum of the single columns although we have confirmed the existence of a zero-absorption region in the same sample with reduced number of dot layers to almost single, realized by surface etching. The broad absorption feature suggests the existence of additional carrier relaxation channels through non-resonant tunneling between the dots. (C) 2003 Elsevier B.V. All rights reserved.

We demonstrate an approach to manipulate the quantum states of single self-assembled quantum dots via strain. We fabricate a micromachined air-bridge with microelectromechanical systems (MEMS), in which quantum dots are embedded. The air-bridge is deformed by electrostatic force, which produces additional strain on the dots to modify the confining potential. Our method with MEMS technique will allow functional manipulation of the electronic states through the direct modification of the confining potential. (C) 2004 American Institute of Physics.

To achieve long-wavelength emissions from quantum structures grown on GaAs substrates, InAsSb quantum dots were grown using two methods and compared. The first was conventional molecular-beam epitaxy in which arsenic and antimony were irradiated at the same time. In the second, antimony flux was irradiated after the InAs quantum dots were grown to prevent the flux from working as a surfactant. Clear and sharp emissions around 1.3 mu m were observed at 77 K from InAsSb quantum dots grown with the second method. However, the photoluminescence spectrum from conventional InAsSb alloy split into two peaks. This was probably due to size fluctuations in the InAsSb quantum dots. This indicates that antimony irradiation after InAs quantum dots are grown is a suitable technique to grow long-wavelength InAsSb dots while maintaining dot uniformity.

We have fabricated bowed airbridges in which self-assembled InGaAs quantum dots are embedded. Strong strain distribution induced in the bowed airbridge and the effect on the electronic states of the quantum dots are investigated through the measurement of the photoluminescence from the individual dots and the theoretical calculation. A finite element calculation shows the strain in the bowed airbridge to distribute from tensile to compressive along the growth direction. The strain effect on the electronic states of the dots is probed through the photoluminescence peak shift following the deformation of the GaAs matrix of the dots from a wall-shaped structure to the bowed airbridge. The magnitude of the peak shift varies systematically with the position of the quantum dot along the growth direction, clearly reflecting the strain distribution in the bridge. The energy level shift following the deformation is calculated by solving the three-dimensional Schrodinger equation taking into account the strain distribution around the dots embedded in the bridge. The calculation, which agrees well with the experiment, demonstrates that the characteristic strain distribution around the dot embedded in the bowed airbridge modifies not only the energy levels, but also the wave functions. The electron and hole wave functions are modified differently, mainly due to the opposite contribution of the biaxial strain to the hydrostatic ones. (C) 2003 American Institute of Physics.

We have fabricated freestanding wire (air-bridge) structures with a bowed shape by introducing a strain layer to vary the strain around quantum dots. The photoluminescence peak energy shift following the shape change of the bridge can be observed for individual InGaAs quantum dots. We find systematic dependence of the peak shift on the dot position along the growth direction. The dependence of the peak shift is explained by strain distribution in the bridge. The strain distribution in the bridge as well as in the dot is calculated using a finite element method. Using the strain data, the electronic structures of the dots embedded in the bridge structures are calculated within the effective mass approximation. The calculated energy shifts agree well with the experimental ones.

Phonon-assisted transitions in single self-assembled InGaAs quantum dots are investigated by photoluminescence (PL) and PL excitation (PLE) spectroscopy. Under nearly resonant conditions of the excitonic ground state, a broad sideband is observed in both PL and PLE. The adiabatic description with the localized acoustic phonons well explains the observed spectra.

The electronic structure of pyramid-like InAs/GaAs quantum dots (QDs) covered with an InGaAs strain-reducing layer is analyzed numerically, focusing on the dependence of the transition energy on the strain-reducing layer thickness. The transition energy of the QD is calculated by the effective mass approximation, taking into account the change of the strain distribution induced by the strain-reducing layer. A large transition energy redshift of more than 60 meV caused by the strain reduction in the QD is obtained, as the strain reducing layer thickness increases. Furthermore, it is found that when strain-reducing layer thickness becomes large, the transition energy redshift saturates. The calculation explains the reported experimental results correctly, which indicates that the strain reducing layer enables control of operation-wavelength over a wide range in various optical devices.

Strain effect on optical anisotropy of quantum dots has been investigated by changing the surrounding matrix of the dots. Optical anisotropy can be induced by lateral patterning of the matrix of the dots, although such anisotropy is absent in the as-grown dots. A reduction of the optical anisotropy is observed by changing the laterally patterned structure into a free-standing structure or an air bridge. The optical anisotropy is mainly attributed to strain asymmetry in the fabricated structures. The presence of the strain asymmetry is confirmed by the observation of a doublet fine structure in spectrally resolved photoluminescence of single quantum dots. (C) 2002 American Institute of Physics.

Thermally induced structural changes of GeSe2 glasses have been studied by Raman scattering from room temperature to crystallization temperature. The Raman spectra in the glassy state change with temperature in an almost reversible manner. The degree of the spectral changes differ among samples although the spectra are identical at room temperature. Depending on the degree of the spectral changes, it is found that the GeSe2 glasses crystallize into three different phases, the beta, alpha, and phi phases ("trifurcated crystallization"). The trifurcated crystallization from apparently the same glassy state is explained in terms of the inhomogeneity in distribution of Ge-Ge "wrong" bonds.

GexSe1-x glasses have been investigated by Raman scattering. The ability of crystallization is obtained by studies of heating glasses from room temperature up to 1000 K. To understand the low-frequency dynamics of the network, we propose a fractal-structure view and a qualitative degree of the fragility. Transition with the connectivity for each studied property is universally related to the stiffness transition, derived from the concept of the constraint theory.

We have observed resonant Raman enhancement of stretching vibration of Se-Se bonds relative to breathing vibration of GeSe4/2 tetrahedra at room temperature and 15 K in GeSe2 chalcogenide semiconducting glass. At 15 K, structural changes by illumination, which cause photodarkening, induce excess resonant enhancement. After the structural changes are saturated, we can observe a reversible change with the excitation power in resonant Raman spectra. It is found that the origin of the the reversible power dependence is the same as that of the excess resonant enhancement. Both of them are attributed to three-fold coordinated Se atoms forming Se-Se bonds, which cause the photodarkening.

We have investigated the structural relaxations by Raman scattering (10-350 cm(-1)) in GexSe1-x glasses (0.07 less than or equal to x less than or equal to 0.35). Around and above the glass transition temperature, strong quasielastic scattering is observed in the low-frequency spectra (10-90 cm(-1)) of floppy glasses (x less than or equal to 0.20). On the other hand, in rigid glasses (x greater than or equal to 0.23), the strong quasielastic contribution cannot be detected in our spectra. We discuss the transition of the relaxational dynamics from floppy glasses to rigid ones in terms of the constraint counting theory. The distinctness of the transition demonstrates a self-organization of the network.

The relationship between the local structures and the electronic stares of layered crystalline GeSe2 are studied by resonant Raman scattering. We observe 34 Raman-active modes of symmetry A(g), and find a selective resonant enhancement of the modes. Almost all the modes are enhanced around 2.7 eV except the two modes. This selective enhancement is attributed to the resonant effect with the localized exciton. We find the essential difference in the atomic motions between the resonant and nonresonant modes. Such behaviors are explained by a structural model of the localized exciton.

Relaxation processes of the photoexcited states in layered crystalline GeSe2 are studied by time-resolved photoluminescence measurement. Two photoluminescence bands, P1 and P2, are clearly resolved from their decay kinetics. It is found that one of the relaxation pathways to the P2 band arises from a band-edge exciton state. The P2 band shows no polarization dependence, in spite of the fact that the photoexcitation of the exciton shows a strongly anisotropic optical character, We discuss the relaxation process on the basis of the luminescence characteristics and the structural model of the band-edge exciton quasi-localized at edge-sharing tetrahedra, (C) 2000 Elsevier Science B.V. All rights reserved.

We have investigated the effect of thermal annealing on dynamics of photoluminescence (PL) in vacuum-evaporated a-GeSe2 films with thickness of 15 mu m. The intensity of PL increases over 200 times after annealing at 250 degrees C, which makes it possible to take a first investigation on both decay time and fatigue effect. The disordered amorphous structure of the a-GeSe2 film, containing wrong bonds, Ge-Ge and Se-Se, has been reconstructed to a stable structure by diminishing the number of the wrong bonds and the related strain or internal pressure by thermal annealing. (C) 2000 Elsevier Science B.V. All rights reserved.