江馬 一弘

The parameters related to the localized states in green-emitting indium gallium nitride (InGaN) have been evaluated by considering the energy diagrams derived by five different methods: (1) the exponential tail of the low-energy side of photoluminescence (PL) spectra, (2) the photon energy dependence of PL decay time, (3) excitation energy dependence of the PL peak energy, (4) the PL excitation spectrum, and (5) the temperature dependence of PL peak energy. The results indicate that the energy diagram of InGaN is divided into four regions: deep localized states, migration region, transition region, and extended states. It is suggested that wider localized states and a narrower transition region are preferable in order to achieve higher PL efficiency. In addition, the dependence of carrier density on PL properties supports the fact of photo-generated carriers forming localized excitons in green-emitting InGaN, although the carriers do not form localized excitons in orange-emitting InGaN and instead exist as localized electrons and holes.

We evaluated detailed optical properties of a lead iodide-based layered perovskite self-organized quantum well with cyclohexenylethyl ammonium cations as an organic layer. From refractive index dispersion obtained by the Kramers-Kronig transformation of reflection spectrum demonstrated formation of exciton with a large binding energy and a large optical density. From the polarized photoluminescence (PL) spectra, fine structures of the exciton were described. The PL excitation spectrum exhibited formation of exciton-polariton in the layered perovskite quantum well.

One of the peculiar features in quantum mechanics is that a superposition of macroscopically distinct states can exist. In optical system, this is highlighted by a superposition of coherent states (SCS), i.e. a superposition of classical states. Recently this highly nontrivial quantum state and its variant have been demonstrated experimentally. Here we demonstrate the superposition of coherent states in quantum measurement which is also a key concept in quantum mechanics. More precisely, we propose and implement a projection measurement onto an arbitrary superposition of two weak coherent states in optical system. The measurement operators are reconstructed experimentally by a novel quantum detector tomography protocol. Our device is realized by combining the displacement operation and photon counting, well established technologies, and thus has implications in various optical quantum information processing applications.

We report the dependence of randomness in sample configuration on stimulated emission phenomena in two-dimensional nanocolumn arrays. From the wavelength selectivity of the photoluminescence, we found that the stimulated emission is apparently related to a distributed feedback mechanism. By comparing the emission behavior between two regularly arranged InGaN/GaN nanocolumn samples with different degrees of randomness, we found that localization effects become prominent if the sample array has any randomness, even in an almost perfect sample. Several modes are localized at different areas in the nanostructures and partially overlap in space, and they compete with each other, especially in a slightly imperfect sample. In addition to the randomness dependence, by observing the wave number space images of the photoluminescence, we confirmed that the stimulated emission phenomena in the crystal arrays are generated by Bragg diffraction at photonic band edges, though the modes have some degree of variability via the sample randomness. (C) 2017 The Japan Society of Applied Physics

It is well known that the surface trap states and electronic disorders in the solution-processed CH3NH3PbI3 perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surfacetrap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (V-oc) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed Voc as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5% in the polymer-modified devices (PCE=15%) with V-oc more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50% humidity).

We developed anew and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film :for fabricating highly efficient perovskite solar cell (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-root VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidiniurn (FA) and methylammonium (MA), mixed cation-halide perovskite (FAPbI(3))(0.85)(MAPbBr(3))(0.15). The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-Voltage performance with high fill factor and suppressed hysteresis, achieving the, best conversion efficiency of 18.5% for a 5 X 5 mm(2) device and 15.2% for a 1.0 X 1.0 cm(2) device.

Formamidine (FA) and cesium (Cs) cations were introduced into quasi-two-dimensional (2D) perovskites as B site cations. The unique crystalline growth of the resulting (n-C6H13NH3)(2)FAPb(2)I(7), which promotes charge transport in photovoltaic solar cells, was confirmed, as was the stability of this material. The photovoltaic properties of (n-C6H13NH3)(2)FAPb(2)I(7) were found to be superior to those of other homologous quasi-2D perovskite compounds.

The effect of the structural properties on the optical characteristics was investigated for In<inf>0.3</inf>Ga<inf>0.7</inf>N nanocolumns (NCs) grown on GaN NCs as a function of GaN column diameter, D<inf>GaN</inf>. With increasing D<inf>GaN</inf>, the photoluminescence spectra changed from single-peak to double-peak emissions at the diameter D<inf>0</inf>where InGaN axial NCs change to InGaN–InGaN core–shell NCs. For the core–shell NCs, the volume recombination probabilities of the InGaN cores did not change with D<inf>GaN</inf>. Whereas the surface recombination probability of the InGaN cores exponentially decreased because of the spontaneous formation of InGaN shells for D<inf>GaN</inf>> D<inf>0</inf>, it drastically increased for D<inf>GaN</inf>≤ D<inf>0</inf>.

The nature of metal oxide scaffold played a pivotal role for the growth of high quality perovskites and subsequently facilitates efficient photovoltaics devices. We demonstrate an effective way to fabricate a low-temperature TiO2 brookite scaffold layer with a uniform and pinhole-free layer for enhancing photovoltaic properties of perovskite solar cells. Various concentrations of TiCl4 were used to modify brookite TiO2 for efficient charge generation and fast charge extraction. We observed that the brookite layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the substrates, whereas TiCl4 treatment further improves the contact of the TiO2/perovskite interface which facilitates charge extraction and drastically influenced charge recombination. The surface treated brookite scaffolds perovskite devices showed an improved performance with an average power conversion efficiency more than 17%. The time resolved photoluminescence showed that the treated samples have obvious effect on the charge carrier dynamics. The striking observation of this study was very low appearance of hysteresis and high reproducibility in the treated samples, which opens up the possibilities for the fabrication of high efficient devices at relatively low temperatures with negligible hysteresis via facile surface modifications. (C) 2017 Author( s).

Emission mechanisms in regularly arrayed InGaN/GaN quantum structures on GaN nanocolumns were investigated, focusing on the spatial emission distribution at the nanocolumn tops and the carrier recombination dynamics. The double-peak emission originated from the dot-and well-like InGaN areas with different In compositions was observed. From the results regarding the spatial emission distribution, we proposed a simple analytical approach to evaluating the carrier recombination dynamics using the rate equations based on the two energy states. The considerable six lifetimes can be uniquely determined from the experimental results. Carrier transfer from the high-to the low-energy state is dominant at high temperatures, producing the increased total emission efficiency of the inner low-energy area. In addition, the internal quantum efficiency should not be simply discussed using only the integrated intensity ratio between low and room temperatures because of the carrier transfer from high-to low-energy states. (C) 2016 The Japan Society of Applied Physics

We consider the phase sensing via a weak optical coherent state at quantum limit precision. A detection scheme for the phase estimation is proposed, which is inspired by the suboptimal quantum measurement in coherent optical communication. We theoretically analyze a performance of our detection scheme, which we call the displaced-photon counting, for phase sensing in terms of the Fisher information and show that the displaced-photon counting outperforms the static homodyne and heterodyne detections in a wide range of the target phase. The proof-of-principle experiment is performed with linear optics and a superconducting nanowire single-photon detector. The result shows that our scheme overcomes the limit of the ideal homodyne measurement, even under practical imperfections.

We have measured photoluminescence spectra in CH3NH3PbBr3 at low temperatures and found a nonlinear emission at the lower energy side of the exciton resonance. Considering that this signal shows a rapid decay, we have assigned it as originating from the exciton-exciton inelastic scattering process called P-emission. The energy difference between this P-emission and the free exciton resonance is 20-30 meV, which is consistent with recent studies of the exciton binding energy. (C) 2016 The Japan Society of Applied Physics

We have measured and analyzed the carrier-density dependence of photoluminescence (PL) spectra and the PL efficiency of InGaN/GaN multiple quantum wells in nanocolumns and in a thin film over a wide excitation range. The localized states parameters, such as the tailing parameter, density and size of the localized states, and the mobility edge density are estimated. The spectral change and reduction of PL efficiency are explained by filling of the localized states and population into the extended states around the mobility edge density. We have also found that the nanocolumns have a narrower distribution of the localized states and a higher PL efficiency than those of the film sample although the In composition of the nanocolumns is higher than that of the film. (C) 2015 AIP Publishing LLC.

We have investigated coherent LO phonon properties in zinc-based II-VI widegap semiconductors, focusing on phonon-plasma coupled modes. By a careful treatment of the time evolution of the signals in ZnS, ZnSe, and ZnTe, we found a frequency upshift as the pump intensity increases. Using a classical coupled oscillator model, we have explained the pump intensity dependence of both the shift and the decay rates by a mixing of highly damped two-photon generated plasma. From the linear dependence between them we can estimate the photo-excited carrier mobilities, leading to a new powerful estimation method to measure the mobility. (C) 2015 Optical Society of America

We have measured the temperature dependence of the reflection and photoluminescence spectra of a lead bromide hybrid material (CH3NH3PbBr3) using high-quality macroscopic single crystals. Single crystals provide clear and sample-independent reflection spectra that permit discussion of the properties of the photoexcited carriers. We have found that excitons still exist in CH3NH3PbBr3, even at room temperature. We have also determined the temperature dependence of the exciton resonance energy and the line width.

We have investigated the photon-generated carrier dynamics of InGaN/GaN nanocolumns whose emission wavelength is in the orange to red region, focusing on the changes in the photoluminescence spectra, emission efficiency and relaxation curves. We have explained these changes by considering the filling of non-radiative centers, emission from localized states, the processes above the mobility-edge density and other non-radiative processes. (C) 2015 The Authors. Published by Elsevier B.V.

We report the dependence of stimulated emission phenomena on randomness in a collection of regularly arranged InGaN/GaN nanocolumns. By comparing the stimulated emission behavior for two samples with different degrees of randomness, we found that localization effects are inevitable even in the almost perfect sample, and several modes partially overlapping in space will compete with each other. The ultrafast dynamics of stimulated emission under femtosecond pulsed laser excitation were also investigated, and the emission peaks were observed to change with time. (C) 2015 The Authors. Published by Elsevier B.V.

An inclusion complex consisting of a boronic acid fluorophore (C1-APB) and beta-cyclodextrin (beta-CyD) acts as a supramolecular sugar sensor whose response mechanism is based on photoinduced electron transfer (PET) from the excited pyrene to the boronic acid. We have investigated the PET process in C1-APB/CyD complexes by using time-resolved photoluminescence (TRPL) measurements at room temperature, and have succeeded in estimating the electron-transfer time to be about 1 ns. We have also studied the effects of CyDs on the PET process by comparing two kinds of CyDs (alpha-CyD, beta-CyD) under different water-dimethylsulfoxide (DMSO) concentration conditions. We found that the CyDs interacting with the boronic acid moiety completely inhibits PET quenching and increases the monomer fluorescence intensity.

Broadband light sources play essential roles in diverse fields, such as high-capacity optical communications, optical coherence tomography, optical spectroscopy, and spectrograph calibration. Although a nonclassical state from spontaneous parametric down-conversion may serve as a quantum counterpart, its detection and characterization have been a challenging task. Here we demonstrate the direct detection of photon numbers of an ultrabroadband (110 nm FWHM) squeezed state in the telecom band centred at 1535 nm wavelength, using a superconducting transition-edge sensor. The observed photon-number distributions violate Klyshko's criterion for the nonclassicality. From the observed photon-number distribution, we evaluate the second-and third-order correlation functions, and characterize a multimode structure, which implies that several tens of orthonormal modes of squeezing exist in the single optical pulse. Our results and techniques open up a new possibility to generate and characterize frequency-multiplexed nonclassical light sources for quantum info-communications technology.

The first near-field optical imaging of light localization in a GaN nanocolumn system was performed. The sample used was a randomly arranged GaN nanocolumn with high aspect ratio. We attached an InGaN single quantum well at the apex of each GaN nanocolumn as an illuminant antenna and observed luminescence from the illuminant using an aperture-type scanning near-field optical microscope. By this technique, we directly obtained optical images of luminescence and its spatial distribution for the GaN nanocolumn system. These images, along with histogram analysis, excitation wavelength dependence, and numerical calculations, offer evidence of Anderson localization of light. (C) 2014 The Japan Society of Applied Physics

We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.

We propose quantum receivers with optical squeezing and photon-number-resolving detectors (PNRDs) for the near-optimal discrimination of quaternary phase-shift-keyed coherent state signals. The basic scheme is similar to the previous proposals [e. g., Izumi et al., Phys. Rev. A 86, 042328 (2012)] in which displacement operations, on-off detectors, and electrical feedforward operations were used. Here we study two types of receivers, one of which installs optical squeezings and the other uses PNRDs instead of on-off detectors. We show that both receivers can attain lower error rates than that in the previous scheme. In particular, we show the PNRD-based receiver has a significant gain when the ratio between the mean photon number of the signal and the number of the feedforward steps is relatively high, in other words, when the probability of detecting two or more photons at each detector is not negligible. Moreover, we show that the PNRD-based receiver can suppress the errors due to dark counts, which the receiver with the on-off detector cannot do with a small number of feedforwards. DOI: 10.1103/PhysRevA.87.042328

We have experimentally compared the excitonic properties of hybrid multiple quantum wells, (C6H5-C2H4NH3)(2)PbBr4 and (C4H9NH3)(2)PbBr4, using photoluminescence, reflection, and photoluminescence excitation measurements. We focused on the contribution of the image charge effect (ICE) to the excitonic energy structure in these materials which have different dielectric constants of the barrier layers. We have found that the binding energies of the 2s and 3s excitons are considerably enhanced by ICE, while the contribution of ICE to the 1s excitons is smaller because of the small Bohr radius, which is comparable to the well width. DOI: 10.1103/PhysRevB.87.125421

Ultrabroadband photon-number-resolving detection is demonstrated for parametric fluorescence ranging over 150 nm in the telecom window, using a Ti transition edge sensor. Our results violate Klyshko's classical limit for even photon numbers.

Optical properties of InN grown by the epitaxial lateral overgrowth (ELO) method have been studied using photoluminescence (PL) and excitation-correlation (EC) measurements. The PL spectrum is analyzed by free-electron recombination band (FERB) model, which shows that the ELO sample has a very low background carrier concentration (n=5.5*10(16)[cm(-3)]). EC measurements show that the dependences of the band gap renormalization and Auger effect on the carrier concentrations are similar in spite of the different physical origins.

We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C4H9NH3)(2)PbBr4 and (C6H5-C2H4NH3)(2)PbBr4, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.

We conducted numerical and experimental studies on Anderson localization of light in two-dimensional random systems of semiconductor columns. We investigated finite size scaling of the localization effect as a function of localization length and system size from the calculation. We therefore obtained a localization parameter map. We also report observations of random lasing in the semiconductor samples. We show that the occurrence of the laser action have a strong relationship to the localization length by comparisons of the map with the experimental results

The optical properties of InGaN/GaN multiple-quantum-well (MQW) nanocolumns whose emission wavelength is in the yellow-to-red region (2.1 ∼ 2.3 eV) have been investigated using time-resolved photoluminescence (PL) measurements. The analysis of the PL decay curves using a stretched exponential function shows that the radiative and nonradiative rates in the red region are almost the same as those in the yellow region. Long-lived migrating carriers also contribute the strong PL intensity in the red region. © 2012 WILEY-VCH Verlag GmbH &amp Co. KGaA, Weinheim.

The quantum receiver is an important tool for overcoming the standard quantum limit (SQL) of discrimination errors in optical communication. We theoretically study the quantum receivers for discriminating ternary and quaternary phase-shift-keying coherent states in terms of average error rate and mutual information. Our receiver consists of on-off-type photon detectors and displacement operations without electrical feedforward operations. We show that for the ternary signals, the receiver shows a reasonable gain from the SQL even without feedforward. This scheme is realizable with the currently available technology. For the quaternary signals feedforward operation is crucial to overcome the SQL with imperfect devices. We also analytically examine the asymptotic limit of the performance of the proposed receiver with respect to the number of feedforward steps. © 2012 American Physical Society.

The quantum receiver is an important tool for overcoming the standard quantum limit (SQL) of discrimination errors in optical communication. We theoretically study the quantum receivers for discriminating ternary and quaternary phase-shift-keying coherent states in terms of average error rate and mutual information. Our receiver consists of on-off-type photon detectors and displacement operations without electrical feedforward operations. We show that for the ternary signals, the receiver shows a reasonable gain from the SQL even without feedforward. This scheme is realizable with the currently available technology. For the quaternary signals feedforward operation is crucial to overcome the SQL with imperfect devices. We also analytically examine the asymptotic limit of the performance of the proposed receiver with respect to the number of feedforward steps.

The optical properties of InGaN/GaN multiple-quantum-well (MQW) nanocolumns whose emission wavelength is in the yellow-to-red region (2.1 similar to 2.3 eV) have been investigated using time-resolved photoluminescence (PL) measurements. The analysis of the PL decay curves using a stretched exponential function shows that the radiative and nonradiative rates in the red region are almost the same as those in the yellow region. Long-lived migrating carriers also contribute the strong PL intensity in the red region. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

歪補償量子ドットにおける励起子ラビ振動を観測した。

The possibility of Anderson localization of light in random dielectric systems has been discussed over the last three decades [1]. However, theoretical and experimental studies for the light localization in random media are much more difficult than those in photonic crystals. Therefore, observation of the light localization has been realized only recently [2]. As a result, it is fair to say the wave localization phenomenon is not well understood. © 2011 IEEE.

Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)(2)(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6](4-) octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)(2)PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In disordered materials, the combination of multiple light scattering and optical interference induces the localization of light. This phenomenon is called Anderson localization which is widely observed in electron systems with random potentials. Experimental studies of Anderson localization of light have been performed over the last three decades [1]. However, most of those studies attempted to secure the evidence of light localization by macroscopic observation of the light scattered by disordered materials. Recently, we proposed the direct observation of light localization using GaN nanocolumn samples by near-field scanning optical microscopy (NSOM), and presented the evidence of the light localization by a histogram analysis of NSOM image and by showing the wavelength dependence of the position of the localized spots in NSOM images [2,3]. We also observed random lasing in GaN nanocolumn samples and discussed the lasing property from a viewpoint of Anderson localization [4]. In this study, we extracted the two-dimensional (2D) spatial dependence from the NSOM image to estimate the localization length in GaN nanocolumns. © 2011 IEEE.

Anatase TiO2 is well known to exhibit high photocatalytic activity. It is believed that photo-excited carriers create activated species by reacting with water and oxygen at the crystal surface, and such activated species decompose organic pollutants. However, the relaxation and diffuse process of the photo-excited carriers is not understood sufficiently. In this study, we investigated such dynamics of photo-excited carriers by transient absorption spectroscopy in anatase TiO2 epitaxial thin film. Moreover, the transient absorption signal and photoluminescence (PL) results are compared for the first time. © 2011 IEEE.