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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*1016[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 report observations of lasing phenomena in the regularly arranged InGaN/GaN nanocolumns. We show several types of laser actions, namely random lasing and distributed feedback lasing resulting from the randomness of the sample configurations.

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.

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.

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.

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.

We study coherent transient phenomena of excitons using femtosecond time-resolved four-wave-mixing (TR- FWM) in two types of high quality thin films of ZnSe; one is a homo-epitaxial film (1.2 μm thickness) and the other is a very thin (50nm) hetero-epitaxial film on GaAs substrate. Free induction decay (FID) behavior of the third-order polarization is clearly observed. We obtained the same values of exciton dephasing time from the temporal measurements, the decay time of FID, and the frequency domain measurements, analyses of reflection spectra. This implies that the excitons in ZnSe films are homogeneous. In the thin film sample, we observe a beat signal in time-integrated and time-resolved FWM. Based on the perturbational calculation, we conclude that the beat originates from the quantum interference of heavy- and light-hole excitons.

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.

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.

The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be understood by considering the density of states and electron occupation number of the conduction band. It was also revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.

In the two-dimensional random system composed of a disordered array of a dielectric cylindrical column ensemble, Anderson localization of light is possible. We show localization parameter maps for the light localization adopting parameters of gallium nitride nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate parametric dependence of the localization characteristics on the light frequency, the radius of the columns, and the filling fraction of the columns. To obtain the maps, we have simulated temporal light diffusion in random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We conclude that the main mechanism for localization varies continuously with the column filling fraction from Mie resonance of single column to Bragg-like diffraction of the column ensemble.

We experimentally demonstrated the coherence transfer/retrieval of time-bin pulse to a semiconductor quantum dot ensemble using a photon-echo technique at the telecommunication wavelength. The interference visibility of the retrieved pulse exceeded 95 percent.

The bound states of n Frenkel excitons ( n = 2,3,…), which are called excitonic n-strings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic n-strings are clarified. The first evidence for the excitonic 2- and 3-strings is provided from comparison of the calculated and observed differential transmission spectrum of an Anthracene-PMDA crystal pumped by femtosecond laser pulses.

In the two-dimensional random system composed of a disordered array of dielectric cylindrical columns, Anderson localization of light occurs. To obtain frequency dependence of the light localization characteristics, we have simulated temporal diffusion of electromagnetic waves in such a random system adopting parameters of actual nanosized semiconductor samples with a high filling fraction of the columns, using the finite-difference time-domain method. We have investigated diffusion length, autocorrelation function of light energy density, and time variation in total energy within the system at several frequencies. We obtain universal behavior of light localization phenomenon as a function of the light localization length and system size, from which we estimate frequency dependence of the localization length. In addition, we show that the frequency dependence of the localization effect depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section of a single scatterer.

We study the lasing behavior and photo induced quenching of dye doped polystyrene microspheres with diameter ranging from 10 μ to 92μ. Lasing of the spherical cavity modes is clearly observed. Emission from individual sphere placed on a glass plate is examined under the pumping of nanosecond pulses which are tuned to the absorption band of the dye. Intensity of each lasing line is strictly controlled by additional irradiation of monochromatic laser which can couple to resonance modes of the cavity. The line intensity plummets down when the additional beam frequency matches to the resonances of the modes which have the same radial profile of relevant mode.

We report a time-to-space conversion technique using a material which has a large χ(3)(≃10−6 esu) and a fast response time (

Steady-state and time-resolved photoluminescence (PL) measurements on vapor deposited films of a non-ionic bisazomethine dye have been performed. In the films, it is possible to control the ratio between J-aggregate and crystalline phases of the dye by means of exposure to chloroform vapour, and thus the origin of PL can be determined from comparison between several films. In the films, PL was emitted exclusively from the crystalline phase. Although the origin of PL was not from the J-aggregate phase, some features of the observed PL were very similar to those reported for J-aggregates of ionic dyes. We also found that these features in the vapor deposited films were caused by a competition between free excitons and excitons trapped at defect sites.

Ultrafast and slow photo-carrier dynamics in InN films were investigeted using femtosecond transient measurements. We found the slow component disappears under the optimized condition due to the equilibrium of band filling and bandgap renormalization effects.

We studied resonant generation of coherent phonon in ZnTe by a nondegenerate EO sampling measurement. The pump energy dependence of the initial amplitude can be explained by the intense absorption of the pump pulse.

We report the observation of extremely efficient energy transfer (greater than 99%) in an organic-inorganic hybrid quantum-well structure consisting of perovskite-type lead bromide well layers and naphthalene-linked ammonium barrier layers. Time-resolved photoluminescence measurements confirm that the transfer is triplet-triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier. Using measurements in the 10-300 K temperature range, we also investigated the temperature dependence of the energy transfer.

The population and coherent dynamics of excitons in InAs quantum dots were investigated using transient pump-probe and four-wave mixing spectroscopies in the telecommunications wavelength range. The sample was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to stack over a hundred QD layers, which resulted in a significant enhancement of nonlinear signals. By controlling the polarization directions of incident pulses, we precisely estimated the radiative and non-radiative lifetimes, the transition dipole moment, and the dephasing time while taking into account their anisotropic properties. The measured radiative lifetime and dephasing time shows large anisotropies with respect to the crystal axes, which results from the anisotropic nature of the transition dipole moment. The anisotropy is larger than that for InAs quantum dots on a GaAs(100) substrate, which seems to reflect a lack of symmetry on an (311)B substrate. A quantitative comparison of these anisotropies demonstrates that nonradiative population relaxation and pure dephasing are quite small in our QDs.

We observed coherent phonon oscillations in zinc blende and wurtzite wide‐gap semiconductors (ZnTe, ZnS, ZnO). We clarified the different relaxation mechanism of the coherent oscillations between LO phonon in zinc blendes and non polar mode in ZnO. Resonant effect of coherent phonon generation is also studied.

We perform a transient four‐wave mixing experiment to investigate the initial dynamics of the time evolution of exciton coherence in highly‐stacked InAs quantum dots. For short time delays, an oscillatory behavior with a period of 31 ps is observed only in the weak excitation regime. The excitation density and polarization dependence of this oscillation cannot be explained by the usual quantum beat phenomena such as biexcitonic and bright excitonic beats.

We measure the polarization dependence of four‐wave mixing (FWM) signals in highly‐stacked InAs quantum dots on an InP(311)B substrate. Dephasing time T2 and FWM signal intensity show large anisotropy for the crystal axes. The measured T2 is determined only by the exciton population lifetime, and thus the polarization dependence of T2 reflects the difference of the transition dipole moments related to bright exciton states with orthogonally linear polarizations.

We have investigated the detailed ultrafast electronic relaxation dynamics and coherent nonlinearity at optical communication wavelength in GaN/AlN multiple quantum wells (MQWs), using spectral‐integrated and — resolved two‐color pump‐probe techniques in a wide energy range. First, we found that the ISBT relaxation dynamics consists of the ultrafast (∼140 fs) intersubband relaxation and slow (∼1.3 ps) carrier cooling process. Second, we have observed coherent spectrum oscillations, which show that the ISBT dephasing time is more than 100 fs at a room temperature. Finally, we have observed the polarization‐dependent absorption in waveguide structures.

We have investigated the relaxation process of photo‐excited carriers in anatase TiO2 films, in which the concentration of the oxygen vacancy is controlled by annealing in ultra high vacuum. A broad luminescence band peaked at 2.32 eV is ascribed to a recombination of self‐trapped excitons (STEs). The luminescence decay curve was composed of several time constants corresponding to the direct and indirect formation of STEs. We have confirmed that the longer annealing, i.e. higher concentration of the oxygen vacancy, leads to an increase in the indirect STEs.

We have investigated the 2s‐2p splitting of excitons in an organic‐inorganic hybrid quantum well material, (C4H9NH3)2PbBr4, by measuring one‐photon and two‐photon excitation spectra. Generally, energy level of 2s is lower than that of 2p for usual Wannier excitons because of the polaron effect. For the hybrid system, however, we have found that the energy level of 2p is lower by 20 meV than that of 2s. This large and opposite splitting of 2s‐2p excitons can be explained by considering image charge effect.

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.

Photoluminescence and spin-flip Raman scattering of excitons in an organic-inorganic quantum well material ${({\mathrm{C}}_{4}{\mathrm{H}}_{9}\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Pb}{\mathrm{Br}}_{4}$ have been investigated at low temperature. The estimated exchange energy of the exciton is greatly enhanced by strong confinement and by image charge effects, and ranges from $28\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}32\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. These values are one or two orders of magnitude larger than those for quantum wells made from the more usual inorganic materials. The ratio of the oscillator strength of the dipole-allowed exciton to that of the dark (dipole-forbidden) exciton is estimated to be of order 800.

We have investigated the ultrafast coherent dynamics of the intersubband transition in GaN/AlN multiple quantum wells, using a spectral resolved two-color pump-probe technique. We have found a significant spectrum change as a function of the delaytime τ. For negative delaytimes corresponding to τ 0, no measurable spectrum change has been observed. From the analysis of these results, we have estimated that intersubband transitions dephasing time is more than 100 fs measured at room temperature.

We investigate the dephasing of excitons in InAs self-assembled quantum dots by using a transient four-wave-mixing technique. A used sample is specially designed to compensate the strain. We observe long-lived coherence of excitons at 5 K which corresponds to the dephasing time longer than a nanosecond, where the photon energy of the excitation pulse is 0.874~eV. We find that a pure dephasing due to exciton-phonon interactions dominates in the exciton dephasing rather than the population decay and the exciton-exciton interaction in the weak excitation region, by analyzing the population lifetime and the polarization-dependent dephasing time.

This study analyzed the influence of the spin relaxation on the excitonic nonlinearity using a 7-level model, and found that it is important to clarify whether coherence is conserved in spin relaxation process or not. If the coherence is perfectly conserved, the nonlinearity will not have size dependence. On the other hand, if the coherence is not conserved, the nonlinearity has coherent size dependence. In order to clarify this point three-pulse four-wave mixing with several polarization configurations at the exciton resonance was performed.

This study has observed coherent LO phonon oscillations in ZnS for the first time using a noncollinear optical parametric amplifier (NOPA) as a light source. Merits using NOPA are not only tunability in a visible region but also shorter pulse width than those of conventional optical parametric amplifiers. Therefore, NOPA is suitable for observing coherent phonon oscillations near the exciton resonance to investigate the interaction between phonons and excitons.

This study investigates the detail of ultrafast intersubband relaxation dynamics in GaN/AlN quantum wells. The experiment uses a two-color pump-probe technique at room temperature. Results show the temporal traces of the change in the transmittance of the probe pulses.

In this study, the fine structure of the excitons is investigated in (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/ and (C/sub 4/H/sub 9/NH/sub 3/)/sub 2/PbBr/sub 4/ using photoluminescence (PL) measurement under magnetic field and spin-flip Raman scattering (SFRS) measurement.

Summary form only given. Among many conjugated polymers, polythiophene and its derivatives are attractive material for the nonlinear optical devices because they are stable for both oxygen and moisture. However, the optical excitation dynamics in polythiophenes is not sufficiently investigated. In this study, we report time-resolved photoluminescence experiments on highly ordered and disordered films of polythiophene derivative to understand the excitation dynamics.

Coherent exciton-exciton interactions have been recognized to be very important in nonlinear optical responses of semiconductor nanostructures. Polarization dependent four-wave-mixing (FWM) techniques have been widely used to study exciton-exciton interactions and dephasing processes of excitons. In this paper, we have investigated the polarization dependence of degenerate and nondegenerate FWM signals and discussed the two-exciton states in (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/. This material is a self-organized quantum-well system, in which a two-dimensional network of corner-sharing octahedra is sandwiched between organic barrier layers of alkylammonium chains.

We have investigated the ultrafast relaxation dynamics of intersubband transition (ISBT) in GaN/AlN, using a two-color pump-probe technique, in a wide energy range around the optical communication wavelength. We suggest that the origin of the signal depends on the relation between the pump and probe pulse energies. We have observed an ultrafast induced absorption signal and a slow negative component which are due to the absorption of electrons during intra -subband scattering and a carrier cooling process with a hot-phonon effect, respectively. Moreover, we clarify the origin of the inhomogeneous broadening width of the ISBT and of the intrinsic absorption width of ISBT from the detailed analyses of the result. We have reproduced the relaxation dynamics by a numerical calculation to confirm this interpretation of ISBT relaxation dynamics.

We report a clear experimental evidence of the image charge effect on excitons in the optical and electroabsorption spectra of an inorganic-organic quantum-well crystal ${({\mathrm{C}}_{6}{\mathrm{H}}_{13}\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Pb}{\mathrm{I}}_{4}$. The ns $(n\ensuremath{\geqslant}2)$ Wannier series are ideal two-dimensional excitons whose binding energies are enhanced by both the spatial confinement and image charge effects. Exceptionally large binding energy of the $1s$ exciton (361 meV) is also due to the prominent image charge effect. We also observed unique blueshifts of the $1s$ excitons under the electric field perpendicular to the quantum-well layers, for which we demonstrated with calculations that the image charge effect plays a significant role.

We have investigated intersubband relaxation dynamics in GaN/AlN multiple quantum wells by the two-color pump-probe technique. We have clarified the ultrafast relaxation scenario which includes thermalization and cooling processes, and found that relaxation is influenced by a hot-phonon effect.

We studied biexciton dynamics of an layered perovskite-type quantum-well material (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/. We estimated biexciton binding energy to be 60 meV. The equilibrium temperature of biexciton is 160K which is much higher than the lattice temperature.

We have observed intersubband transition dynamics in GaN/AlN MQWs at 1.55 /spl mu/m. The relaxation dynamics consists of ultrafast component of /spl sim/170 fs and slower component of /spl sim/1.1 ps. We discussed the origin of these dynamics.

We have estimated degenerated and non-degenerated nonlinear susceptibilities in GaN/AlN MQW by one- and two-color pump-probe technique. We have discussed the origin of the nonlinear susceptibility in the framework of intersubband absorption saturation.

Photoinduced effects in a single crystal of bilayered manganites, La2-2xSr1+2xMn2O7 (x=0.38), were investigated in a wide range of temperatures by pump-probe measurement at a photon energy of 1.6eV. In a ferromagnetic metallic state, significant enhancement of positive rise in differential reflectivity with a slow relaxing time of hundred picoseconds was observed just below Tc=127K, indicating that the reflectivity change with the slow relaxation time constant is induced by laser heating. We have also observed an unconventional fast relaxing component that has a time constant of the order of ten picoseconds. This fast relaxing component, whose absolute value has an asymmetric peak at Tc, is presumably due to short-range correlation of Jahn-Teller distortion., Comment: 13 pages, 4 figures, accepted to Solid State Communications

We have observed intersubband transition dynamics in GaN/AlN. The relaxation dynamics consists of ultrafast and slower components. The origin of these dynamics is discussed