I. Akai

Exciton transitions in the hexagonal CuI microcrystallites grown on polymers

Abstract Quantum confinement effects and nonlinear luminescence of excitons in CuI microcrystallites on the substrates of poly-methyl methacrylate have been studied. Exciton transitions due to the hexagonal structure appear in the disk-shaped microcrystallites. The confinements of the relative motion of excitons in the hexagonal structure induce remarkable blue-shifts of the transition energy. From the size dependence of their confinement energies, reduced mass ratios of the hexagonal excitons were obtained. Under high-density excitation, a new luminescence band grows super-linearly at the energy position lower by ∼30 meV than the exciton resonance. The origin of the luminescence was assigned to an annihilation of bi-excitons in the hexagonal structure.

PHOTOLUMINESCENCE AND DYNAMICS OF EXCITONS IN Alq3 SINGLE CRYSTALS

The relaxation dynamics of photo-excited states in single crystals of Alq3, utilized as organic electroluminescent (EL) devices, have been investigated. The photoluminescence (PL) peak of Alq3 changes depending on the exciting photon energy and the temperature. Nevertheless, a PL peak appears at the same energy position as the EL peak for excitations at energies above the transition edge and a new PL band has been resolved at energies below the EL peak for excitation at the lowest part of the absorption tail. However, for low temperatures and for excitation in the intermediate energy region of the absorption tail, a PL peak appears at the lower energy side. This PL peak shifts into the same energy as the EL peak with increasing temperature. This behavior is explained by a model which takes account of two luminescent states and one non-luminescent intermediate state. In the intermediate energy region, the temperature dependence of the PL for the excitation is also explained by a thermal activation process mediated by the non-luminescent state. The lifetimes of the three states obtained from the temporal profiles of the PL are understood consistently on the basis of this model.

Space-Resolved Spectra Due to Highly Mobile Excition at the Stacking Fault in BiI3 Crystals

The study of exciton transfer along the stacking fault interface in BiI 3 crystals is presented by using a newly designed space-resolved spectroseopy method. The resonant luminescence lines of the excitons with exact zero Stokes shift were observed in the space-resolved spectra at a point of more than 300 µm away from the exciting laser spot. Analyzing the spatial distribution of the resonant luminescence intensities on the assumption of exciton diffusion, we obtained unusually large diffusion coefficients of more than 10 5 cm 2 /sec at 4.2 K. This result indicates the existence of highly mobile excitons. The spatial distribution of the resonant luminescence intensities becomes smaller with decreasing temperature. The temperature dependence is discussed based on the weakly bound exciton model.

Picosecond studies of cascade relaxation processes among the stacking-fault exciton states in layered BiI3 single crystals

Abstract The luminescence decay processes of the three ground-state excitons localized at a stacking fault with quasi two-dimensional symmetry in layered BiI 3 crystals were investigated in the picosecond time domain. The characteristic time behaviour of these luminescences were quantitatively analyzed on the basis of the cascade relaxation processes among these exciton states. Lifetimes of 0.2, 0.40 and 0.80 ns for the exciton states in descending order of energy were obtained. The anomalous intensity ratios of the luminescence lines are discussed in terms of efficient cascade processes and k -selective radiation processes.

Optical properties of nanostructures in layered metal tri-iodide crystals

Abstract The optical response of excitons confined in characteristic nanostructures in layered metal tri-iodide crystals introduced by some irregular stackings from the bulk structures is reviewed. In BiI 3 a specific stacking fault takes place during crystal growth constructing macroscopic planar defects. In this space conspicuous localized exciton transitions occur below the intrinsic absorption edge. Another stacking disorder introduced by applying external stress in this crystal brings about a new nanostructure domain of symmetry D 3d different from that of bulk symmetry C 2 3i . The optical transitions due to new structures appear in the lower energy region as an absorption and luminescence line series. The similar nanostructures are induced in SbI 3 crystals under the hydrostatic pressure. In these nanostructures, the electronic structure is analyzed by a model based on the confined excitons in a nanoscale disk-like shape space. The magnetic field effect confirms the structure in the wave function-size scale. The nanoscale disk-like structure of BiI 3 in CdI 2 matrices is also obtained by a hot wall technique and mixed crystal annealing, which is realized by observing the size distribution with an electron microscope. In a BiI 3 disk in CdI 2 Stokes shifted photoluminescence bands appear. The Stokes shifts of the luminescence bands are understood by considering the size-dependent exciton-phonon interaction. In these nanostructures large optical nonlinearity under the intense laser field was obtained.

The photocalorimetric spectra in layered BiI3 single crystals

We have studied photocalorimetric spectra (PCS) of characteristic exciton transitions in BiI 3 crystals. Our PCS measuring system has the sensitivity of less than 10 -12 J for the detection of photo-thermal heating after photo-excitation. The photocalorimetric (PC) spectra are analyzed quantitatively by comparing the net absorbance spectra obtained from the simultaneous measurements of reflection and transmission for the first time. Above the absorption edge, the PCS signal intensity well corresponds to the total amount of absorbed light energy. For the excitation at stacking fault excitons, the PC spectrum shows sharp structures, but the amount of PCS signal is only a small fraction of the absorbed light energy. This result corresponds to the intense resonant luminescence and cascade-type relaxation among these exciton states.

Nonlinear Optical Spectra of the Confined Excitons in Stress Induced Nanostructures in BiI3

We report the optical nonlinear response of the confined excitons in nanostructures induced by external stress in a layered crystal BiI3. The absorption intensity of the confined excitons is saturated by intense laser excitation. We clarify the size dependence of the saturation density which reflects the nanostructure size. The saturation density of the confined exciton in large domains is lower than the the case of small domains. By pump-probe measurements using a sub-picosecond laser pulse, we examine the dynamical process of the confined excitons in the disks with various sizes. From the temporal behavior of the absorption saturation, it becomes clear that the exciton life times in small disks are shorter than the large ones. A new nonlinearly growing luminescence due to the process by a inelastic recoiling of excitons is detected.

Excitons localized in mesoscopic domains produced by stacking fault in bil3

Abstract Absorption line series WJ(J=I,II,III,…) appeared below the indirect, edge in BiI3 has been studied. The series is assigned to the excitons confined in mesoscopic domains produced by deformation faults characteristic of layered materials. Each line consists of four structures which correspond to x+iy, x-iy, z, and pure triplet states originated from the bulk cationic excitons. The energy separations between these four states are depend on the size of mesoscopic domains in which an exciton is confined. By introducing a disk-like shape model of the mesoscopic domain, the experimental results are analyzed on the basis of anisotropic quantum size effects on the cationic excitons. The relative motion of the exciton is found to be strongly affected by the localization of the translational motion in the direction perpendicular to the confinement axis.

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ∼105 W/cm2, excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.

SIZE EFFECTS OF EXCITONS OF BiI3 IN LAYERED MATRICES

The size effects on the exciton absorption and luminescence spectra are studied in nanostructure of BiI3 embedded in the matrices of bulk BiI3 and CdI2 crystals. The quantum-confined exciton states are observed as the absorption lines of the series WJ (J=I, II, III, …). The results are compared with a model calculation of the confined excitons in the disks with thickness Lz and radius R. The disk-size distribution is directly observed with a high-resolution electron microscope. The relaxed luminescence from the BiI3 disks in CdI2 matrix is analyzed by the model of the confined excition interacting with acoustic phonons by taking into account the size-dependent Stokes shift and the distribution of the disk size.