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When an earthquake occurs, the larger the scale of the disaster, the harder it is to support the victims’ needs. In fact, even the most meager support for victims has become quite difficult. Furthermore, it is also known that the greater the damage, the more difficult it becomes to use cell phones, applications, etc. The authors have developed multiple reflectors observable by synthetic-aperture radar (SAR) satellites and differing backscattering coefficients. Using them, we have proposed a method for ascertaining the location and needs of victims during a large-scale earthquake disaster. In this study, we developed an object detection model using YOLOv5 to detect the reflectors from within SAR images. In addition, we constructed a method for managing setup locations in GIS by conferring latitude and longitude information on reflectors obtained through YOLO v5. Through analysis, a model of the proposed reflector detection and identification of setup locations via GIS was developed in this study.

In Japan, where the birthrate continues to decline, various initiatives are underway to promote traffic safety for children. Although these efforts have helped reduce the number of traffic accidents involving children, an examination of the circumstances under which children were killed or injured in traffic accidents in recent years shows that accidents in which children were riding bicycles accounted for the highest percentage of accidents. We investigated the relationship between the ages of children involved in traffic accidents and violations of traffic laws using traffic accident statistics maintained by the Ishikawa Prefectural Police Headquarters. These records revealed that 16-year-olds were most likely to be involved in traffic accidents. Our analysis of the violations of laws and ordinances with respect to the ages of bicyclists involved in car accidents at intersections found that, of all the age categories, elementary school students had the lowest rate of accidents with no violations and the highest rate of accidents involving a failure to stop. Junior and senior high school students had lower rates of accidents involving a failure to stop than elementary school students. Moreover, at non-intersections, driving safety violations were notably higher for accidents involving bicycling elementary school students than for those in other age groups.

An exclusive advantage of semiconductor spintronics is its potential for opto-spintronics, which will allow integration of spin-based information processing/storage with photon-based information transfer/communications. Unfortunately, progress has so far been severely hampered by the failure to generate nearly fully spin-polarized charge carriers in semiconductors at room temperature. Here we demonstrate successful generation of conduction electron spin polarization exceeding 90% at room temperature without a magnetic field in a non-magnetic all-semiconductor nanostructure, which remains high even up to 110 degrees C. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter. We further show that the quantum-dot electron spin can be remotely manipulated by spin control in the adjacent spin filter, paving the way for remote spin encoding and writing of quantum memory as well as for remote spin control of spin-photon interfaces. This work demonstrates the feasibility to implement opto-spintronic functionality in common semiconductor nanostructures. An electron spin polarization of 90% is achieved in a non-magnetic nanostructure at room temperature without magnetic field. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter. Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2015-05532, 2019-04312, 201605091, 2020-04530]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; European Research Council, ERC AdG AMETISTEuropean Research Council (ERC) [695116]; Academy of FinlandAcademy of FinlandEuropean Commission [310985, 323989]; Japan Society for the Promotion of Science (JSPS)Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [16H06359, 19H05507]; JSPSMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [19K15380]

Conventional close visual inspection of bridges has high cost and lack of skilled engineers. New technologies, such as AI, UAV, and robots, can be provided to help the inspection process and substitute previous inspection methods to save labor effort and reduce costs. We develop damage detection system for bridge inspection by adopting image recognition technology based on deep learning. It detects damage from bridge images and provides the accurate outline. Such technology can reduce inspection work by detecting the damage instead of inspectors, and they can focus on important tasks such as damage determination. However, it takes a lot of time to collect and annotate for training images. Although linear damage such as cracks requires a fine outline for each pixel, planar damage such as free lime is presumed to be allowable even at low precise boundaries. If low precise boundaries are allowed, training data is obtained in less time. To determine damage with the same accuracy as close visual inspection, the limits of allowable low precision display need to be determined. This study examined the limis of low precise boundaries for free lime. The bridge engineers compared with the detection output of gradually reduced precision boundaries and investigated the limits of the low precision they allow.

In Japan, where the birthrate continues to decline, various initiatives are underway to promote traffic safety for children. Although these efforts have helped reduce the number of traffic accidents involving children, examination of the circumstances under which children were killed or injured in traffic accidents in recent years shows that accidents in which children were riding bicycles accounted for the highest percentage of accidents. We investigated the relationship between the ages of children involved in traffic accidents and violations of traffic laws using traffic accident statistics maintained by the Ishikawa Prefectural Police Headquarters. Our analysis of violations of laws and ordinances with respect to ages of bicyclists involved in car accidents at intersections found that, of all the age categories, elementary school students had the lowest rate of accidents with no violations and the highest rate of accidents involving failure to stop. Junior and senior high school students had lower rates of accidents involving failure to stop than elementary school students. Moreover, at non-intersections, driving safety violations were notably higher for accidents involving bicycling elementary school students than for those in other age groups.

A series of assembled PtII complexes comprising N-heterocyclic carbene and cyanide ligands was constructed using different substituent groups, [Pt(CN)2 (R-impy)] (R-impyH+ =1-alkyl-3-(2-pyridyl)-1H-imidazolium, R=Me (Pt-Me), Et (Pt-Et), i Pr (Pt-i Pr), and t Bu (Pt-t Bu)). All the complexes exhibited highly efficient photoluminescence with an emission quantum yield of 0.51-0.81 in the solid state at room temperature, originating from the triplet metal-metal-to-ligand charge transfer (3 MMLCT) state. Their emission colors cover the entire visible region from red for Pt-Me to blue for Pt-t Bu. Importantly, Pt-t Bu is the first example that exhibits blue 3 MMLCT emission. The 3 MMLCT emission was proved and characterized based on the temperature dependences of the crystal structures and emission properties. The wide-range color tuning of luminescence using the 3 MMLCT emission presents a new strategy of superfine control of the emission color.

Image analysis has been increasingly used in damage detection, particularly in the inspection of aging bridges. We adopted the image-analysis-based damage detection technology to study the feasibility of remote inspection support aimed at reducing the number of engineers that are dispatched to bridge sites. The remote inspection support involves uploading bridge images from the bridge site and then issuing directions and instructions to an onsite inspection engineer while a skilled engineer at a remote location verifies the damage detection results in real time. The 5G interface, which can transfer large volumes of data in a short time, was used to upload images, enabling shorter upload times compared with 4G. In addition, by sharing damage conditions in real-time, the engineer at a remote office could ascertain them in detail and make appropriate decisions without going to the bridge site. The damages are complex in aged bridges and their decision requires extensive experience and knowledge of skilled engineers. We determined that 5G-based inspections are highly efficient because directions and instructions can be received from a bridge site in real time in cases where a skilled engineer’s decision is needed.

An understanding of the spin-transport properties in semiconductor barriers is essential to improve the performance of spin-polarized light-emitting diodes (spin LEDs) for future optospintronics integration in information processing. Here, we report on the temperature and bias-voltage dependence of spin-transport properties in an ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dot (QD) spin LED using a combination of spin-dependent electroluminescence (EL) and time-resolved photoluminescence. The QD EL spin polarization increases with an increase in temperature above 125 K; this is attributed to the improved conversion efficiency from spin polarization of electrons to circular polarization of photons of the QDs. We find that both the electric field and temperature can enhance spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier above 200 K. At 298 K, the QD EL spin polarization decreases beyond 2.5 V; this is attributed to the enhanced D'yakonov Perel' spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier caused by the increase in electron temperature. This study provides valuable insights into the spin-relaxation mechanism in the semiconductor barrier during the room-temperature operation of the QD spin LED.

The III–V compound semiconductor quantum dot (QD) photonic devices have attracted considerable attention due to their stable operation at high temperature, low threshold current, and high-speed mod...

We report on the enhancement effect of metal nanotriangles (NTs) on the photoluminescence and its time-resolved dynamics of tris(8-quinolinolato) aluminum (Alq3). Regularly arranged Al NTs arrays were fabricated on the quartz substrate by means of nanosphere lithography. The absorption peaks corresponded to the localized surface plasmon (LSP) resonance of Al NTs were observed within a visible light region, and it was significantly shifted towards longer wavelength as increasing refractive index of the covering medium. The typical resonance wavelength of LSP of Alq3/Al NTs fabricated with 350 nm-PSt beads was 600 nm. From the PL measurement 2.6-fold PL enhancement was observed in Alq3 with Al NTs at room temperature, whereas the 4.4-fold PL enhancement was observed for Ag NTs with same size. According to the time-resolved PL data, the coupling of LSP with the excitation light field mainly contribute to the observed emission enhancement for the Al NTs, whereas the emission enhancement by Ag NTs can be originated from the direct coupling of LSP and emission which was suggested from the large contribution of short lifetime component.

Electric-field-effect spin switching with an enhanced number of highly polarized electron and photon spins has been demonstrated using p-doped semiconductor quantum dots (QDs). Remote p-doping in InGaAs QDs tunnel-coupled with an InGaAs quantum well (QW) significantly increased the circularly polarized, thus electron-spin-polarized, photoluminescence intensity, depending on the electric-field-induced electron spin injection from the QW as a spin reservoir into the QDs. The spin polarity and polarization degree during this spin injection can be controlled by the direction and the strength of the electric field, where the spin direction can be reversed by excess electron spin injection into the QDs via spin scattering at the QD excited states. We found that the maximum degrees of both parallel and antiparallel spin polarization to the initial spin direction in the QW can be enhanced by p-doping. The doped holes without spin polarization can effectively contribute to this electric-field-effect spin switching after the initial electron spin injection selectively removes the parallel hole spins. The optimized p-doping induces fast spin reversals at the QD excited states with a moderate electric-field application, resulting in an efficient electric-field-driven antiparallel spin injection into the QD ground state. Further excess hole doping prevents this efficient spin reversal due to multiple electron-hole spin scattering, in addition to a spin-state filling effect at the QD excited states, during the spin injection from the QW into the QDs.

Trihexyltetradecylphosphonium (P-6,P-6,P-6,P-14) salts of luminescent anionic Pt(II) complexes, (P-6,P-6,P-6,P-14)[PtX2(ppy)] (X = Cl-, Br-; ppy = 2-phenylpyridinate), are synthesized and photofunctional thin films with crystal/liquid bi-stability are fabricated. In particular, the chloride complex provides a thin film exhibiting thermo- and mechano-triggered luminescence ON/OFF switching at ambient temperature, which is based on the control of the supercooled liquid phase and bright luminescent crystalline phase. The photophysical properties of the complexes are investigated and compared with those of the bromide complex and tetra(n-butyl)ammonium salts of the complexes. Furthermore, detailed photophysical analysis reveals a large contribution of the charge-transfer character to the ligand-centered (3)pi pi* excited state, which results in intense phosphorescence in the crystal and high-contrast luminescence ON/OFF by the phase transition of the chloride complex.

Enhancement of 4.4-fold photoluminescence (PL) of tris(8-hydroxyquinolinato)aluminum (Alq3) thin film was observed in the presence of 50-nm thick Ag nanotriangles (NTs) compared with bare Alq3 film at room temperature. According to the temperature dependence of integrated PL intensity and the PL lifetime obtained from time-resolved PL measurements, an increase in radiative recombination rate was observed over a wide temperature range. Because the absorption peak originated from localized surface plasmon (LSP) states and the emission spectra are well overlapped, the observed enhancement can be explained by efficient coupling of the LSP of the Ag NTs with the emission from the Alq3.

Surface plasmon resonance (SPR) of Ag nanoparticles (NPs), and its emission enhancement properties are investigated by time-resolved photoluminescence spectroscopy using tris(8-hydroxyquinolinato)aluminium (Alq3), which is generally applied in organic light-emitting diode, as the emitter. 5-nm-thick Ag island-films were fabricated, and annealed at various temperatures to vary the size of the Ag NPs. The peak position of the SPR band in the absorption spectra of Ag NPs blue-shifts with increasing annealing temperature, that is, the SPR frequency is controlled by the NP size. Comparison of the photoluminescence (PL) spectra of Alq3 with and without Ag NPs annealed at 150 °C indicates 1.2-fold enhancement of PL for Alq3 with Ag NPs compared to the reference Alq3. From the temperature dependence of the PL intensity and lifetime of Alq3 with Ag NPs, we evaluated the degree of enhancement of the radiative recombination rate in the presence of the localized surface plasmon of Ag NPs, in comparison with the competitive non-radiative quenching process.

We have demonstrated the fabrication of homogeneously distributed In0.3Ga0.7N/GaN quantum nanodisks (QNDs) with a high density and average diameter of 10 nm or less in 30-nm-high nanopillars. The scalable top-down nanofabrication process used biotemplates that were spin-coated on an In0.3Ga0.7N/GaN single quantum well (SQW) followed by low-damage dry etching on ferritins with 7 nm diameter iron cores. The photoluminescence measurements at 70 K showed a blue shift of quantum energy of 420 meV from the In0.3Ga0.7N/GaN SQW to the QND. The internal quantum efficiency of the In0.3Ga0.7N/GaN QND was 100 times that of the SQW. A significant reduction in the quantum-confined Stark effect in the QND structure was observed, which concurred with the numerical simulation using a 3D Schrodinger equation. These results pave the way for the fabrication of large-scale III–N quantum devices using nanoprocessing, which is vital for optoelectronic communication devices.

We report the ellipsometric and photoluminescence (PL) properties of Indium zinc oxide (IZO) films, which were grown by radio frequency sputtering under Ar and O2 atmospheres. IZO films grown under an O2 atmosphere (IZO (O2)) showed enhanced PL properties when compared to the films grown under an Ar atmosphere (IZO (Ar)), particularly with respect to the band-edge emission. The enhancement of band-edge emission can be attributed to the reduction in the non-emissive defect states related to oxygen vacancies, which were repaired by sputtering under O2 atmosphere, whereas the PL enhancement in green region is probably due to the formation of the different types of defects under the excess oxygen environment. This was also supported by the results of time-resolved PL measurements, where the band-edge emission of IZO (Ar) showed rapid decay with a 50 ps lifetime, which indicates the dominance of the relaxation pathway to underlying defect states. In contrast, the PL decay profiles of IZO (O2) for band-edge and emissive defect states showed moderate decay with time-constants of 2.3 ns and 5.7 ns, respectively. The exciton relaxation dynamics were sensitive to the presence and its kinds of defect states, which were controlled by the growth conditions.

We report on the spin-conserved electron transport to self-assembled InGaAs quantum dots (QDs) through GaAs/AlGaAs superlattice (SL) by circularly polarized time-resolved photoluminescence spectroscopy of the QD excited states with the selective excitation for the SL minibands. The spin transport properties largely depend on the AlGaAs barrier thickness of the SL. The SL with a thinner barrier demonstrates a quantum spin transport to QDs with highly conserving the electron-spin polarization during the transport process through the SL minibands.

We study effects of p-doping on excited spin dynamics of InGaAs quantum dots (QDs) by circularly polarized time-resolved photoluminescence (PL). At excited states of p-doped QDs, the circular polarization degree (CPD) values are twice higher than those of undoped QDs. We attribute this enhanced CPD at excited states to spin-selective energy relaxation from the excited states to ground state. This can be promoted by the suppression of state filling at the ground state, which is induced by the decrease of PL lifetime due to the resident hole population.

We study effects of growth temperature of a capping layer on optical spin properties of In 0.5 Ga 0.5 As quantum dots (QDs) by photoluminescence (PL) spectroscopy including behaviors of the degree of circular polarization (CPD). The PL energy of the ground state shifts to lower energy with decreasing capping growth temperature, which is due to the increase of In composition inside QDs. The temperature dependence of PL intensity from the QD excited states shows the larger thermal activation energy with decreasing capping growth temperature, indicating the suppression of thermal escape of electrons from QDs to barriers. In addition, we observe the lower-energy shift of the CPD spectral peak, associated with decreasing PL energy, where the high CPD values can assure the high quality of these QDs grown with various capping growth temperatures.

We study lateral electronic coupling among self-assembled In 0.5 Ga 0.5 As quantum dots (QDs), which is promoted by adjoining tunnel-coupled In 0.1 Ga 0.9 As quantum-well (QW) potentials. The resultant collective electron and its spin dynamics at the coupled excited states in the QDs, virtually via the QW potential, are revealed by means of spin-resolved transient photoluminescence (TRPL). These QD excited states spread over those hybrid nano-systems composed of the coupled QDs and QW, which are elucidated by wavefunction calculations and then comparted to the TRPL results. The existence of the adjoining QW can induce stronger coupling among QDs in the lateral direction, where thicker QWs make stronger inter-dot coupling. We observe that circularly polarized TRPL, reflecting the spin dynamics, is dynamical functions of spin-polarized electron transfer among the coupled QDs and discrete-state filling in the QDs as well as spin relaxation in individual QDs. As a result, temporally stable high spin polarizations higher than 60 % are achieved in the QDs, sustained by selective transfer of minority spins into lower-energy spin states.

We report on a photoluminescence (PL) enhancement in IZO/Ag/IZO sandwiched structure via surface plasmonic effects of 14 nm-thick Ag film. In the presence of Ag thin film, the 2–8-fold enhancement was observed for the broad PL around 2.34 eV, which can be originated from defect states in amorphous IZO film. The results of time-resolved PL spectra suggested that the increase in radiative recombination rate, and the maximum Purcell factor of 19 was estimated from the analysis of the PL decay profiles. The comparison between the results of static- and dynamic-PL measurement suggests that the non-radiative process after the excitation of the surface plasmon of the silver film also affects the total efficiency of the emission enhancement.

Quantum dot photonic devices such as light-emitting diodes (LEDs), laser diodes (LDs), high-speed modulators, and semiconductor optical amplifiers are attractive because of their small threshold voltages, low power consumption, and temperature stability. We have studied and developed a defect-less top–down dry fabrication process for GaAs quantum nanodisk (QND) LEDs with diameters of less than 20 nm and thicknesses of 8 nm by employing a bionanotemplate, neutral beam etching, and asymmetric AlGaAs/GaAs regrowth via metalorganic vapor phase epitaxy. AlxGa1-xAs barriers with high (Al0.3Ga0.7As) and low (Al0.17Ga 0.83As) aluminum contents were used between QNDs in the in-plane and vertical directions, respectively. This permits the control of the deep band energy offset between GaAs QNDs and AlxGa1-xAs barriers, leading to stable room-temperature operation. The temperature dependence of the optical properties of the QND LED was measured by electroluminescence, and we found that the energies and their transient behaviors were strongly affected by the band offset energies between the QNDs and the aluminum-rich barriers. Therefore, we could enhance the optical performances of our symmetric QND LED with low-Al-content barriers by developing asymmetric AlxGa1-x As barriers with low-Al-content vertical barriers and high-Al-content in-plane barriers.

Improved electron-photon spin conversion efficiency is a key component of technological platforms for optospintronics integration in information processing; this concept is based on optical devices trans-mitting and receiving spin information superimposed on light. Semiconductor quantum dots (QDs) are the most promising materials for optospintronic devices; however, in addition to their weak room-temperature luminescence, their electron-photon spin conversion efficiencies are lower than 50%. Here, we present semiconductor QDs embedded in quantum wells (QWs) containing quasi-QDs. The proposed semiconductor hybrid nanosystem with gradual quantum dimensionality reduction demonstrates luminescence one order of magnitude stronger than that of conventional QDs and an electron-photon spin conversion efficiency of almost 80% at room temperature. Optical characterization reveals that efficient carrier capture, suppressed depolarized-spin reinjection, and quasi-three-dimensional quantum confinements in the QWs facilitate the highly efficient electron-photon spin conversion. This study constitutes a significant advance towards the realization of QD-based spin-functional optical devices for electron-spin-based quantum information platforms.

Asymmetric spin relaxation induced by the residual electron spin in semiconductor quantum dots (QDs) adjacent to a superlattice (SL) was studied using spin- and time-resolved photoluminescence under the selective photoexcitation of the SL miniband states. Spin-polarized electrons were photoexcited in the SL barrier and then injected into the QDs through spin-conserving tunneling. The spin-polarized electron transport and the faster transport of the electrons as compared to the holes generate the residual majority electron spins in the QDs. A reversal of the optical spin polarity was observed at the ground state of the QDs, depending on the excitation powers. A rate equation analysis considering the individual spin-flip times between spin-split QD states indicates that the polarity reversal originates from the asymmetric spin-flip process at the excited state of the QDs. The asymmetric spin relaxation is associated with the selective relaxation of the spin-flipped electron and hole to the unoccupied ground state, which is induced by the existence of the residual majority electron spin at this state. In addition, we observed a clear recovery of the optical spin polarity by eliminating the existence of the residual electron spin through heavy p-doping. These findings are important to attain a fundamental understanding of the spin relaxation mechanism within the QDs and provide an insight into the manipulation of the optical spin polarity by controlling the residual electron spins in the QDs.

The suppression of a thermally excited electron-spin relaxation in InGaAs quantum dots (QDs) using p-doped capping layers toward enhanced room-temperature (RT) spin polarization has been demonstrated, in which the electron-spin polarization in QD excited states (ESs) was measured through time-resolved spin-dependent photoluminescence. We revealed that the p-doping of QDs can enhance the emission intensity of QD-ES by approximately twofold to threefold over a wide temperature range. An electron-spin relaxation time of 106 ps was observed at 293 K for p-doped QDs, which is approximately three times longer than the radiative lifetime of 36 ps, relative to the shorter electron-spin relaxation time of 71 ps for undoped QDs. The increased electron-spin lifetime was mainly attributed to the suppressed relaxation of the electron spin reinjected from the p-doped capping barrier after thermal escape from an ES, where the D'yakonov-Perel' spin relaxation in the barrier was potentially weakened through impurity scattering. These results suggest that InGaAs QDs with p-doped capping layers have a significant advantage for use in spin-functional optical active layers with a higher spin polarization toward RT.

It is essential to understand which mechanism of localized surface plasmon (LSP) emission enhancement is to be utilized when combining plasmonic metal nanostructures with an emissive material to produce an optoelectronic device. Herein, we report on the size dependence and time-resolved dynamics of photoluminescence (PL) enhancement using three sizes of aluminum nanotriangles (Al NTs) on emission by tris(8-hydroxyquinolinato)aluminum (Alq3) for elucidating its enhancement mechanism. Al nanostructures were fabricated using a nanosphere lithography technique. Two-dimensionally aligned polystyrene (PSt) beads with different diameters were used as mask templates for obtaining the Al NTs. The absorption peaks red-shifted with increasing Al NT size, and hence, were attributed to the LSP resonance. From PL of Alq3 with and without Al NTs, 2–3 fold PL enhancements were observed among the samples with different Al NTs sizes. The significant shortening of the PL lifetime was observed only in Alq3/Al NTs using Ps500, indicating the effective coupling of LSP of Al NTs and the emission, which is also evidenced from the overlapping the PL peak and the LSP absorption. In contrast, the PL emission enhancement due to LSP coupling with the absorption process of Alq3 was dominant in Alq3/Al NTs using Ps200, suggested from the similarity of the PL decay curve with that of bare Alq3, and the excitation energy matching with LSP resonance. From these observation, we can distinguish the LSP coupling process (absorption or emission) from the presence or absence of lifetime enhancement in the time-resolved PL measurements, offering a guideline for designing the optical devices incorporating the metal nanostructures.

The temperature dependence of interdot spin-transfer dynamics at laterally coupled excited states (ESs) in high-density InGaAs quantum dots (QDs) was studied using spin- and time-resolved photoluminescence spectroscopy. At low temperatures below 100 K, temporary suppression of electron-spin polarization decay due to selective relaxation of minority spins from emissive ESs to lower-energy states in neighboring QDs was observed. In the temperature range from 20 K to 140 K, thermal activation of electron spins from lower-energy QD states to higher-energy states via interdot transfer prevents the aforementioned selective relaxation of minority spins, leading to a faster decay of electron-spin polarization during light emission. At high temperatures above 140 K, reinjection of depolarized electron spins from barriers after thermal escape from QD ESs accelerates the further decay of the electron-spin polarization, wherein the electron spins can be activated via ladderlike interdot transfer. These findings indicate that the suppression of reinjection of electron spins from barriers in a high-density QD system is crucial for maintaining high electron-spin polarization during light emission at high temperatures. Published under license by AIP Publishing.

The optical spin properties of size-modulated quantum nanocolumns (QNCs), which are composed of 9 layers of vertically coupled InGaAs quantum dots (QDs), have been studied by circularly polarized time-resolved photoluminescence spectroscopy of QD excited states with barrier excitation. High spin polarization at the emissive state is one of the essential elements in the development of spin-functional optical devices. Coupling of QD excited states can enhance the spin polarization if only minority spins are effectively removed from the emissive excited states. In this study, size-modulated QNCs with the increasing size toward the upper layer were grown, and we revealed that the combination of QD size modulation and electron wavefunction coupling in the stacking direction can greatly enhance spin polarization during light emission from the smaller-sized QD layers. We observed a temporal spin amplification of more than 80% at coupled excited states. This enhancement is derived from the size-modulation-induced selective transfer of minority spins to the larger-sized QD layers, which have abundant excited states where electron spins are transferred. In addition, we found that QNCs can retain high spin polarization even at high excitation spin density. Our findings of spin amplification during light emission will provide QNC systems suitable for spin-functional optical devices.The optical spin properties of size-modulated quantum nanocolumns (QNCs), which are composed of 9 layers of vertically coupled InGaAs quantum dots (QDs), have been studied by circularly polarized time-resolved photoluminescence spectroscopy of QD excited states with barrier excitation. High spin polarization at the emissive state is one of the essential elements in the development of spin-functional optical devices. Coupling of QD excited states can enhance the spin polarization if only minority spins are effectively removed from the emissive excited states. In this study, size-modulated QNCs with the increasing size toward the upper layer were grown, and we revealed that the combination of QD size modulation and electron wavefunction coupling in the stacking direction can greatly enhance spin polarization during light emission from the smaller-sized QD layers. We observed a temporal spin amplification of more than 80% at coupled excited states. This enhancement is derived from the size-modulation-induced...

Electric field control of spin polarity in spin injection into InGaAs quantum dots (QDs) from a tunnel-coupled quantum well (QW) was studied. The degree of freedom of the spin state in high-density QDs will play an important role in semiconductor spintronics such as a spin-functional optical device, where it is crucial to establish spin injection and manipulation by electric fields. To solve this subject in a layered device structure, electric field effects on spin injection from a 2-dimensional QW into 0-dimensional QDs were studied. Spin-polarized electrons were photo-excited in a QW and then injected into QDs via spin-conserving tunneling. After the injection, parallel spin states to the initial spin direction in the spin reservoir of QW were observed in QDs as a result of efficient spin injection, by circularly polarized photoluminescence indicating spin states in the QDs. Moreover, reversal of spin polarity was clearly observed at QD ground states, depending on the electric fields applied along the QD-QW growth direction. The tunneling rate of an electron is different from that of a hole and largely depends on the electric field, owing to electric field induced modifications of the coupled QD-QW potential. This results in negative trions in the QDs with anti-parallel spins to the initial ones in the QW, which is evidently supported by a significant effect of p-doping. The polarization degrees of both spin polarities can be optimized by excitation-spin density, in addition to the electric field strength.

Spin transport properties from the GaAs/AlGaAs superlattice (SL) to InGaAs quantum dots (QDs) are studied by circularly polarized time-resolved photoluminescence spectroscopy of QD excited states with the selective excitation of SL miniband states. For the SL with a thinner barrier, we observe an effective carrier transport in SL owing to the stronger overlap of wavefunctions of adjacent quantum wells and a simultaneous highly efficient carrier injection into QDs. Moreover, the SL with a thinner barrier demonstrates a quantum spin transport to QDs maintaining high spin polarization during the transport process.

Interdot carrier and spin dynamics were studied in a two-dimensional array of high-density small quantum dots (SQDs) of InGaAs with an average diameter of 16 nm and a sheet density of 1.2 x 10(11) cm(-2), in which 24 nm diametric large QDs (LQDs) were embedded as local potential minima. We observed a delayed photoluminescence (PL) rise from the lower-lying LQD states and a considerably faster PL decay from the higher-lying SQD states, indicating carrier transfer from the two-dimensionally coupled SQDs into the LQDs. In addition, inverse carrier tunneling from the LQDs into the SQDs was thermally induced, which is characterized by the thermal activation energy between the LQDs and SQDs. Moreover, circularly polarized transient PL behavior from the SQD states exhibits a suppression of the spin polarization decay in the initial time region, depending on the excited spin density. This tentatively suppressed spin relaxation can be quantitatively explained by selective interdot transfer of minority-spin electrons from the SQDs into LQDs, when the majority spin states in both QDs are sufficiently populated by excited spins. These findings indicate that the high-density SQDs behave as the main emitters with suppressed spin relaxation, while the scattered LQDs with lower potential behave as the receivers of minority-spin electrons.

We have studied GaAs and AlGaAs barriers for the purpose of improving spin-transport performance in spin-polarized light-emitting diodes (LEDs) based on self-assembled quantum dots (QDs) of InGaAs. In the spin-LED utilizing a spin-functional optical active layer of In-based self-assembled QDs, growth temperatures of top barriers of GaAs and AlGaAs were reduced to suppress indium diffusion from the QDs into the barriers after forming the QDs. We show a significant improvement of spin-transport property as well as of carrier-transport one with increasing growth temperature of the Al 0.1 Ga 0.9 As barrier from 580 to 640 °C, while luminescent spectral energy and shape of the QDs are not markedly affected.

We have grown ultrahigh-density self-assembled quantum dots (QDs) of InGaAs with sheet densities up to 2.5×1011 cm−2 and lateral diameters down to 10 nm, where the dot density increases with increasing As pressure during dot growth under optimum growth conditions. A ground-state photoluminescence (PL) spectrum shows a spectral width of 47 meV for the highest-density sample. Optical excitation-density dependences of the PL intensity and time profile are studied. The PL intensity from QD excited states increases with increasing excitation power, originating from a state-filling effect in QDs, which is directly confirmed by a plateau-like behavior on the PL decay curve. We find that the filling effect is significantly suppressed in the above ultrahigh-density dot ensemble, which suggests potential applications to superior energy-saving lasing and spin-functional optical devices.