Mutsuo Ogura - Academia.edu (original) (raw)

Papers by Mutsuo Ogura

Physica E: Low-dimensional Systems and Nanostructures, 2000

Lasing from the ground state electron and heavy-hole-like transition of quantum wire (QWR) is dem... more Lasing from the ground state electron and heavy-hole-like transition of quantum wire (QWR) is demonstrated for the ÿrst time at room temperature, with an oxide-isolated V-grooved GaAs=AlGaAs triple QWR laser grown by ow-rate modulation epitaxy (FME). The lasing peaks at all temperatures (4-300 K) are in reasonably good agreement with both the photon energies of the peaks of the photoluminescence curves and the numerical calculation of the electronic sub-band energy states of the corresponding QWR structure. These results are considered to be responsible for the reduced heterointerface inhomogeneities (the Stokes shift ∼0:3 meV) of the FME grown QWR, giving a low-loss wave guide in the QWR laser.

Physical Review B, 1998

A systematic study of exciton dynamics is presented in quantum boxes formed naturally along the a... more A systematic study of exciton dynamics is presented in quantum boxes formed naturally along the axis of a V-shaped quantum wire, by means of time and spatially resolved resonant photoluminescence. The dependence of radiative lifetimes and relaxation mechanisms of excitons is determined versus the size of the boxes. The radiative recombination rate varies linearly with the length of the box, showing that the exciton has a coherence volume equal to the volume of the box. In a low excitation regime, emission from excited states has not been observed, which would be a consequence of relaxation bottleneck, but there is clear evidence that relaxation via emission of LA phonons depends strongly on the energy separation between the different quantum box levels. ͓S0163-1829͑98͒07839-4͔

Physical Review B, 2001

Near-field optical spectroscopy is used to investigate the effects of disorder in the optical pro... more Near-field optical spectroscopy is used to investigate the effects of disorder in the optical processes in semiconductor quantum wires. We observe photoluminescence emissions from extended, delocalized excitons at low temperatures ͑5 K͒ and low excitation densities. Combining high spectral and spatial resolution, we isolate homogeneous emission lines from excitons delocalized over distances up to 600 nm in the fundamental state. The energies of the emissions are consistent with different quantum spatial confinements along the wire axis. Unlike the photoluminescence originating from localized excitons, these emission lines show a high degree of polarization along the axis of the wire.

Ultramicroscopy, 2002

We report on a distance control system for low-temperature scanning near-field optical microscopy... more We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork-optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2 nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing to employ it in a liquid He environment. The system is extremely compact and allows for several hours of stability at 5 K.

phys. stat. sol. (a), 2002

An overview is presented of the optical properties in single GaAlAs/GaAs quantum wires grown on a... more An overview is presented of the optical properties in single GaAlAs/GaAs quantum wires grown on a V-grooved GaAs substrate studied by high spatial resolution spectroscopy. In these structures there is evidence for strong localization effects hiding completely the expected one-dimensional character of the system. Exciton dynamics are governed essentially by the properties of a zerodimensional system, since the monolayer fluctuations of the wire thickness create local potential minima that naturally form quantum boxes along the free axis of the wire. A systematic study of the exciton relaxation processes and radiative recombination as well as the fine structure of exciton states is presented as a function of the size of the quantum boxes.

Phys. Rev. B, 2003

We report on the study of many-body interactions in a single high quality V-shaped quantum wire b... more We report on the study of many-body interactions in a single high quality V-shaped quantum wire by means of continuous and time-resolved microphotoluminescence. The transition from a weakly interacting exciton gas when the carrier density n is less than 10 5 cm −1 (i.e. naX < 0.1, with aX the exciton Bohr radius), to a dense electron-hole plasma (n > 10 6 cm −1 , i.e. naX > 1) is systematically followed in the system as the carrier density is increased. We show that this transition occurs gradually : the free carriers first coexist with excitons for naX > 0.1, then the electron-hole plasma becomes degenerate at naX = 0.8. We also show that the non-linear effects are strongly related to the kind of disorder and localization properties in the structure especially in the low density regime.

Phys. Rev. B, 2003

The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by high spatial ... more The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by high spatial resolution spectroscopy. Scanning optical imaging of different generations of samples shows that the localization length has been enhanced as the growth techniques were improved. In the best samples, excitons are delocalized in islands of length of the order of 1 µm, and form a continuum of 1D states in each of them, as evidenced by the √ T dependence of the radiative lifetime. On the opposite, in the previous generation of QWRs, the localization length is typically 50 nm and the QWR behaves as a collection of quantum boxes. These localization properties are compared to structural properties and related to the progresses of the growth techniques. The presence of residual disorder is evidenced in the best samples and explained by the separation of electrons and holes due to the large in-built piezo-electric field present in the structure.

Physical Review Letters, 2005

We study exciton and biexciton spectra in disordered semiconductor quantum wires by means of nano... more We study exciton and biexciton spectra in disordered semiconductor quantum wires by means of nanophotoluminescence spectroscopy. We demonstrate a close link between the exciton localization length along the wire and the occurrence of a biexciton spectral line. The biexciton signature appears only if the corresponding exciton state extends over more than a few tens of nanometers. We also measure a nonmonotonous variation of the biexciton binding energy with decreasing exciton localization length. This behavior is quantitatively well reproduced by the solution of the single-band Schrödinger equation of the four-particle problem in a one-dimensional confining potential.

physica status solidi (c), 2004

Luminescence spectra of V-groove GaAs/AlGaAs quantum wires are investigated by spatially resolved... more Luminescence spectra of V-groove GaAs/AlGaAs quantum wires are investigated by spatially resolved photoluminescence (PL) spectroscopy using a low temperature scanning near-field optical microscope (SNOM). The statistical analysis of these spectra in terms of the autocorrelation function reveals the presence of level repulsion. We find that the comparison between the experimental autocorrelation curve and the theoretical one obtained from the simulation of PL spectra, assuming for each exciton state a Lorentzian lineshape, is unsatisfactory. This is due to the presence of a broad background in the measured spectra, which is absent in the simulated ones. We propose an improvement of the theoretical model in order to include this feature and propose as explanation for the origin of this broad background the coupling between a single exciton state and the phonon thermal bath. The addition of this last element in our model is very important to establish a better quantitative agreement between the simulated and measured autocorrelation functions.

physica status solidi (a), 1997

Physica E: Low-dimensional Systems and Nanostructures, 2006

We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression... more We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression of the band-edge peak, in the photoluminescence spectra of ultra-high-quality modulation-doped AlGaAs/GaAs quantum wires (QWRs). We find that the FES effect is very sensitive to the Fermi energy. The strong FES is observed only in QWRs having a Fermi energy of the order of a few meV, and disappears almost completely when the Fermi energy exceeds 10 meV. These results are expected to spark new research activities, both experimentally and theoretically, on many-body effects in one-dimensional electron gas.

Physica E: Low-dimensional Systems and Nanostructures, 2001

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs... more We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 eV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.

Physica E: Low-dimensional Systems and Nanostructures, 2003

The exciton localization and delocalization is studied in GaAs/GaAlAs V-shaped quantum wires (QWR... more The exciton localization and delocalization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by microscopy and spectroscopy. Scanning optical imaging of di erent generations of samples shows that the localization length has been enhanced as the growth techniques were improved. In the best samples, excitons are delocalized in islands of length of the order of 1 m, and form a continuum of 1D states in each of them. On the opposite, in the previous generation of QWRs, the localization length is typically 50 nm and the QWR behaves as a collection of quantum boxes. These localization properties are compared to structural properties and related to the progresses of the growth techniques. The presence of residual disorder is evidenced in the best samples and explained by the separation of electrons and holes due to the large built-in piezo-electric ÿeld in the structure.

Nanotechnology, 2006

InGaAs/AlGaAs V-groove quantum-wire (QWR) arrays were fabricated by holographic lithography and o... more InGaAs/AlGaAs V-groove quantum-wire (QWR) arrays were fabricated by holographic lithography and one-time metalorganic chemical vapour deposition (MOCVD) to evaluate gain-coupled distributed-feedback (GC-DFB) effects in the InGaAs/AlGaAs materials. Using a finite-element method (FEM), mode analysis of the actual cross-sectional structure verified the modal gain of the QWR DFB structure, in which the gain was identified by a single peak at the Bragg wavelength of the grating. In addition, we observed a large gain anisotropy due to the gain/loss coupling in the DFB structure at a wavelength of 914 nm in the emission spectra from the 430 nm pitch QWR grating at room temperature.

Nanotechnology, 2005

Gain-coupled distributed-feedback (GC-DFB) effects in a V-groove In 0.2 Ga 0.8 As/Al 0.2 Ga 0.8 A... more Gain-coupled distributed-feedback (GC-DFB) effects in a V-groove In 0.2 Ga 0.8 As/Al 0.2 Ga 0.8 As quantum-wire (QWR) array are investigated by comparison with those in a GaAs/Al 0.2 Ga 0.8 As QWR array. Temperature-dependent photoluminescence (PL) spectra are measured for both samples, showing that the PL spectra from the QWRs are much stronger than those from the quantum wells (QWs) in the entire temperature region. Then, InGaAs/AlGaAs QWR GC-DFB lasers are fabricated by one-step metallorganic chemical vapour deposition (MOCVD) growth and characterized. As a result, strong nonlinearity in the emission spectra by optical feedback along the DFB directions is clearly observed near the threshold current, indicating that a V-groove InGaAs QWR array is a good candidate for a gain-coupled DFB laser.

Journal of Crystal Growth, 2000

The e!ects of NH OH : H O : H O ("1 : 3 : 50) etching of the initial V-grooved substrate and the ... more The e!ects of NH OH : H O : H O ("1 : 3 : 50) etching of the initial V-grooved substrate and the use of an AlGaAs/GaAs short-period superlattice bu!er layer on improvement of the interface uniformity of V-shaped Al-GaAs/GaAs quantum wires (QWRs) were investigated by atomic force microscopy and micro-photoluminescence spectroscopy. We found that the surface roughness on the initial V-grooved substrates induced during V-groove preparation processes could be greatly reduced by the use of the NH OH etching treatment. Further, by the combined use of the above two techniques, the QWR interface uniformity could be signi"cantly improved, as revealed by both the elongation of the monolayer step islands formed on the (0 0 1) QWR facet and the suppression of step bunching on the (3 1 1)A QWR facet.

Japanese Journal of Applied Physics, 2001

We have investigated terahertz (THz) electromagnetic wave generation from semiconductor quantum n... more We have investigated terahertz (THz) electromagnetic wave generation from semiconductor quantum nanostructures.The samples used in our experiment are a GaAs/AlGaAs symmetric triple-coupled quantum well and a GaAs/AlGaAs crescent-shaped quantum wire. The THz electromagnetic waves were generated by ultrashort optical pulse excitation and detected by a spatial electrooptic (EO) sampling method. Generation of a THz electromagnetic wave, which originats from quantum beats with three frequency components of 0.8, 1.0, and 1.8 THz, was observed in the symmetric triple-coupled quantum well. In a quantum wire, THz electromagnetic wave generation due to the instantaneous polarization accompanied by the creation of excitons by ultrashort optical pulse excitation was observed.

Japanese Journal of Applied Physics, 2002

We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconduct... more We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconductor quantum nanostructures. An InGaAs/AlGaAs multi-quantum-well Bragg structure and an AlGaAs/GaAs crescent-shaped quantum wire structure were used as samples for the control of the exciton-polariton and excitons in the nanostructure. In the multi-quantum-well Bragg sample, a two-stage decay due to the super-radiant exciton-polariton mode is clearly observed under the resonance conditions. Highly sensitive coherent control measurement enables us to observe the dephasing characteristics in the crescent-shaped quantum wire sample, and a very fast dephasing time T 2 of between 200 fs and 300 fs with power dependence was observed.

Japanese Journal of Applied Physics, 2003

A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (Q... more A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (QWR) field effect transistors (FETs) prepared by the flow-rate modulated epitaxy (FME) technique. These are attributed to Coulomb blockade at a subthreshold gate bias, universal conductance steps near the threshold, real space transfer under a forward gate bias and a large source-drain bias condition. Weak carrier coupling between sidewall quantum wells and QWR is responsible for the small step height of the measured universal conductance (G m (G 0 ¼ 2e 2 =h). Shubnikov de Haas oscillation measurements revealed that sidewall quantum wells in the V-groove quantum wire act as additional current paths and are switched or mixed with QWR depending on the gate bias conditions and device geometry. The gate-bias-dependent current path switching is found to be responsible for the large current steps and negative differential resistance (NDR) in the drain current (I DS)-gate bias (V GS) and I DS-drain bias (V DS) relationships, respectively.

IEEE Photonics Technology Letters, 2001

AlGaAs-GaAs quantum-wire (QWR) gain-coupled distributed-feedback (GC-DFB) lasers have been fabric... more AlGaAs-GaAs quantum-wire (QWR) gain-coupled distributed-feedback (GC-DFB) lasers have been fabricated by a single metal-organic chemical vapor deposition growth step on 0.36m pitch V-groove arrays of GaAs. A record low-threshold current of 13 mA is achieved via DFB lasing from QWR gain at room temperature. The consistency of the photon energies of the lasing and the photoluminescence peaks from QWR, and about 4-nm-wide stopband with a large threshold gain difference observed in the near-threshold spectrum are presented as possible evidence for GC-DFB effects in these devices.

Physica E: Low-dimensional Systems and Nanostructures, 2000

Lasing from the ground state electron and heavy-hole-like transition of quantum wire (QWR) is dem... more Lasing from the ground state electron and heavy-hole-like transition of quantum wire (QWR) is demonstrated for the ÿrst time at room temperature, with an oxide-isolated V-grooved GaAs=AlGaAs triple QWR laser grown by ow-rate modulation epitaxy (FME). The lasing peaks at all temperatures (4-300 K) are in reasonably good agreement with both the photon energies of the peaks of the photoluminescence curves and the numerical calculation of the electronic sub-band energy states of the corresponding QWR structure. These results are considered to be responsible for the reduced heterointerface inhomogeneities (the Stokes shift ∼0:3 meV) of the FME grown QWR, giving a low-loss wave guide in the QWR laser.

Physical Review B, 1998

A systematic study of exciton dynamics is presented in quantum boxes formed naturally along the a... more A systematic study of exciton dynamics is presented in quantum boxes formed naturally along the axis of a V-shaped quantum wire, by means of time and spatially resolved resonant photoluminescence. The dependence of radiative lifetimes and relaxation mechanisms of excitons is determined versus the size of the boxes. The radiative recombination rate varies linearly with the length of the box, showing that the exciton has a coherence volume equal to the volume of the box. In a low excitation regime, emission from excited states has not been observed, which would be a consequence of relaxation bottleneck, but there is clear evidence that relaxation via emission of LA phonons depends strongly on the energy separation between the different quantum box levels. ͓S0163-1829͑98͒07839-4͔

Physical Review B, 2001

Near-field optical spectroscopy is used to investigate the effects of disorder in the optical pro... more Near-field optical spectroscopy is used to investigate the effects of disorder in the optical processes in semiconductor quantum wires. We observe photoluminescence emissions from extended, delocalized excitons at low temperatures ͑5 K͒ and low excitation densities. Combining high spectral and spatial resolution, we isolate homogeneous emission lines from excitons delocalized over distances up to 600 nm in the fundamental state. The energies of the emissions are consistent with different quantum spatial confinements along the wire axis. Unlike the photoluminescence originating from localized excitons, these emission lines show a high degree of polarization along the axis of the wire.

Ultramicroscopy, 2002

We report on a distance control system for low-temperature scanning near-field optical microscopy... more We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork-optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2 nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing to employ it in a liquid He environment. The system is extremely compact and allows for several hours of stability at 5 K.

phys. stat. sol. (a), 2002

An overview is presented of the optical properties in single GaAlAs/GaAs quantum wires grown on a... more An overview is presented of the optical properties in single GaAlAs/GaAs quantum wires grown on a V-grooved GaAs substrate studied by high spatial resolution spectroscopy. In these structures there is evidence for strong localization effects hiding completely the expected one-dimensional character of the system. Exciton dynamics are governed essentially by the properties of a zerodimensional system, since the monolayer fluctuations of the wire thickness create local potential minima that naturally form quantum boxes along the free axis of the wire. A systematic study of the exciton relaxation processes and radiative recombination as well as the fine structure of exciton states is presented as a function of the size of the quantum boxes.

Phys. Rev. B, 2003

We report on the study of many-body interactions in a single high quality V-shaped quantum wire b... more We report on the study of many-body interactions in a single high quality V-shaped quantum wire by means of continuous and time-resolved microphotoluminescence. The transition from a weakly interacting exciton gas when the carrier density n is less than 10 5 cm −1 (i.e. naX < 0.1, with aX the exciton Bohr radius), to a dense electron-hole plasma (n > 10 6 cm −1 , i.e. naX > 1) is systematically followed in the system as the carrier density is increased. We show that this transition occurs gradually : the free carriers first coexist with excitons for naX > 0.1, then the electron-hole plasma becomes degenerate at naX = 0.8. We also show that the non-linear effects are strongly related to the kind of disorder and localization properties in the structure especially in the low density regime.

Phys. Rev. B, 2003

The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by high spatial ... more The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by high spatial resolution spectroscopy. Scanning optical imaging of different generations of samples shows that the localization length has been enhanced as the growth techniques were improved. In the best samples, excitons are delocalized in islands of length of the order of 1 µm, and form a continuum of 1D states in each of them, as evidenced by the √ T dependence of the radiative lifetime. On the opposite, in the previous generation of QWRs, the localization length is typically 50 nm and the QWR behaves as a collection of quantum boxes. These localization properties are compared to structural properties and related to the progresses of the growth techniques. The presence of residual disorder is evidenced in the best samples and explained by the separation of electrons and holes due to the large in-built piezo-electric field present in the structure.

Physical Review Letters, 2005

We study exciton and biexciton spectra in disordered semiconductor quantum wires by means of nano... more We study exciton and biexciton spectra in disordered semiconductor quantum wires by means of nanophotoluminescence spectroscopy. We demonstrate a close link between the exciton localization length along the wire and the occurrence of a biexciton spectral line. The biexciton signature appears only if the corresponding exciton state extends over more than a few tens of nanometers. We also measure a nonmonotonous variation of the biexciton binding energy with decreasing exciton localization length. This behavior is quantitatively well reproduced by the solution of the single-band Schrödinger equation of the four-particle problem in a one-dimensional confining potential.

physica status solidi (c), 2004

Luminescence spectra of V-groove GaAs/AlGaAs quantum wires are investigated by spatially resolved... more Luminescence spectra of V-groove GaAs/AlGaAs quantum wires are investigated by spatially resolved photoluminescence (PL) spectroscopy using a low temperature scanning near-field optical microscope (SNOM). The statistical analysis of these spectra in terms of the autocorrelation function reveals the presence of level repulsion. We find that the comparison between the experimental autocorrelation curve and the theoretical one obtained from the simulation of PL spectra, assuming for each exciton state a Lorentzian lineshape, is unsatisfactory. This is due to the presence of a broad background in the measured spectra, which is absent in the simulated ones. We propose an improvement of the theoretical model in order to include this feature and propose as explanation for the origin of this broad background the coupling between a single exciton state and the phonon thermal bath. The addition of this last element in our model is very important to establish a better quantitative agreement between the simulated and measured autocorrelation functions.

physica status solidi (a), 1997

Physica E: Low-dimensional Systems and Nanostructures, 2006

We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression... more We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression of the band-edge peak, in the photoluminescence spectra of ultra-high-quality modulation-doped AlGaAs/GaAs quantum wires (QWRs). We find that the FES effect is very sensitive to the Fermi energy. The strong FES is observed only in QWRs having a Fermi energy of the order of a few meV, and disappears almost completely when the Fermi energy exceeds 10 meV. These results are expected to spark new research activities, both experimentally and theoretically, on many-body effects in one-dimensional electron gas.

Physica E: Low-dimensional Systems and Nanostructures, 2001

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs... more We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 eV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.

Physica E: Low-dimensional Systems and Nanostructures, 2003

The exciton localization and delocalization is studied in GaAs/GaAlAs V-shaped quantum wires (QWR... more The exciton localization and delocalization is studied in GaAs/GaAlAs V-shaped quantum wires (QWRs) by microscopy and spectroscopy. Scanning optical imaging of di erent generations of samples shows that the localization length has been enhanced as the growth techniques were improved. In the best samples, excitons are delocalized in islands of length of the order of 1 m, and form a continuum of 1D states in each of them. On the opposite, in the previous generation of QWRs, the localization length is typically 50 nm and the QWR behaves as a collection of quantum boxes. These localization properties are compared to structural properties and related to the progresses of the growth techniques. The presence of residual disorder is evidenced in the best samples and explained by the separation of electrons and holes due to the large built-in piezo-electric ÿeld in the structure.

Nanotechnology, 2006

InGaAs/AlGaAs V-groove quantum-wire (QWR) arrays were fabricated by holographic lithography and o... more InGaAs/AlGaAs V-groove quantum-wire (QWR) arrays were fabricated by holographic lithography and one-time metalorganic chemical vapour deposition (MOCVD) to evaluate gain-coupled distributed-feedback (GC-DFB) effects in the InGaAs/AlGaAs materials. Using a finite-element method (FEM), mode analysis of the actual cross-sectional structure verified the modal gain of the QWR DFB structure, in which the gain was identified by a single peak at the Bragg wavelength of the grating. In addition, we observed a large gain anisotropy due to the gain/loss coupling in the DFB structure at a wavelength of 914 nm in the emission spectra from the 430 nm pitch QWR grating at room temperature.

Nanotechnology, 2005

Gain-coupled distributed-feedback (GC-DFB) effects in a V-groove In 0.2 Ga 0.8 As/Al 0.2 Ga 0.8 A... more Gain-coupled distributed-feedback (GC-DFB) effects in a V-groove In 0.2 Ga 0.8 As/Al 0.2 Ga 0.8 As quantum-wire (QWR) array are investigated by comparison with those in a GaAs/Al 0.2 Ga 0.8 As QWR array. Temperature-dependent photoluminescence (PL) spectra are measured for both samples, showing that the PL spectra from the QWRs are much stronger than those from the quantum wells (QWs) in the entire temperature region. Then, InGaAs/AlGaAs QWR GC-DFB lasers are fabricated by one-step metallorganic chemical vapour deposition (MOCVD) growth and characterized. As a result, strong nonlinearity in the emission spectra by optical feedback along the DFB directions is clearly observed near the threshold current, indicating that a V-groove InGaAs QWR array is a good candidate for a gain-coupled DFB laser.

Journal of Crystal Growth, 2000

The e!ects of NH OH : H O : H O ("1 : 3 : 50) etching of the initial V-grooved substrate and the ... more The e!ects of NH OH : H O : H O ("1 : 3 : 50) etching of the initial V-grooved substrate and the use of an AlGaAs/GaAs short-period superlattice bu!er layer on improvement of the interface uniformity of V-shaped Al-GaAs/GaAs quantum wires (QWRs) were investigated by atomic force microscopy and micro-photoluminescence spectroscopy. We found that the surface roughness on the initial V-grooved substrates induced during V-groove preparation processes could be greatly reduced by the use of the NH OH etching treatment. Further, by the combined use of the above two techniques, the QWR interface uniformity could be signi"cantly improved, as revealed by both the elongation of the monolayer step islands formed on the (0 0 1) QWR facet and the suppression of step bunching on the (3 1 1)A QWR facet.

Japanese Journal of Applied Physics, 2001

We have investigated terahertz (THz) electromagnetic wave generation from semiconductor quantum n... more We have investigated terahertz (THz) electromagnetic wave generation from semiconductor quantum nanostructures.The samples used in our experiment are a GaAs/AlGaAs symmetric triple-coupled quantum well and a GaAs/AlGaAs crescent-shaped quantum wire. The THz electromagnetic waves were generated by ultrashort optical pulse excitation and detected by a spatial electrooptic (EO) sampling method. Generation of a THz electromagnetic wave, which originats from quantum beats with three frequency components of 0.8, 1.0, and 1.8 THz, was observed in the symmetric triple-coupled quantum well. In a quantum wire, THz electromagnetic wave generation due to the instantaneous polarization accompanied by the creation of excitons by ultrashort optical pulse excitation was observed.

Japanese Journal of Applied Physics, 2002

We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconduct... more We have demonstrated ultrafast coherent control of excitons and exciton-polaritons in semiconductor quantum nanostructures. An InGaAs/AlGaAs multi-quantum-well Bragg structure and an AlGaAs/GaAs crescent-shaped quantum wire structure were used as samples for the control of the exciton-polariton and excitons in the nanostructure. In the multi-quantum-well Bragg sample, a two-stage decay due to the super-radiant exciton-polariton mode is clearly observed under the resonance conditions. Highly sensitive coherent control measurement enables us to observe the dephasing characteristics in the crescent-shaped quantum wire sample, and a very fast dephasing time T 2 of between 200 fs and 300 fs with power dependence was observed.

Japanese Journal of Applied Physics, 2003

A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (Q... more A series of nonlinear conductance phenomena is investigated in the GaAs V-grooved quantum wire (QWR) field effect transistors (FETs) prepared by the flow-rate modulated epitaxy (FME) technique. These are attributed to Coulomb blockade at a subthreshold gate bias, universal conductance steps near the threshold, real space transfer under a forward gate bias and a large source-drain bias condition. Weak carrier coupling between sidewall quantum wells and QWR is responsible for the small step height of the measured universal conductance (G m (G 0 ¼ 2e 2 =h). Shubnikov de Haas oscillation measurements revealed that sidewall quantum wells in the V-groove quantum wire act as additional current paths and are switched or mixed with QWR depending on the gate bias conditions and device geometry. The gate-bias-dependent current path switching is found to be responsible for the large current steps and negative differential resistance (NDR) in the drain current (I DS)-gate bias (V GS) and I DS-drain bias (V DS) relationships, respectively.

IEEE Photonics Technology Letters, 2001

AlGaAs-GaAs quantum-wire (QWR) gain-coupled distributed-feedback (GC-DFB) lasers have been fabric... more AlGaAs-GaAs quantum-wire (QWR) gain-coupled distributed-feedback (GC-DFB) lasers have been fabricated by a single metal-organic chemical vapor deposition growth step on 0.36m pitch V-groove arrays of GaAs. A record low-threshold current of 13 mA is achieved via DFB lasing from QWR gain at room temperature. The consistency of the photon energies of the lasing and the photoluminescence peaks from QWR, and about 4-nm-wide stopband with a large threshold gain difference observed in the near-threshold spectrum are presented as possible evidence for GC-DFB effects in these devices.