T. Karasawa

Optical Properties of Extrinsic Two-Dimensional Excitons in BiI3 Single Crystals

Interface excitons caused by stacking faults in BiI 3 , crystals have been investigated on absorption, photoluminescence and its excitation spectra. Characteristic three sharp absorption lines (called R , S and T ), appear close to the fundamental absorption edge. The temperature dependence of the line shape shows a typical feature of exciton-phonon interaction in the case that K =0 at the bottom of the exciton band. Resonant luminescences of these lines have also very narrow line widths, and no Stokes shifts can be detected. The luminescence line shape of phonon side band shows Maxwellian-like having the line width proportional to thermal energy but accompanied by a fine structure which reflects a quasi two-dimensional band nature of this exciton system. The high density excitation effects of these states reveal a specific interaction among the R , S and T excitons.

Color Center Formation in KI and NaCl Crystals by Pulsed Electron Beam

It has been confirmed that F centers in KI and NaCl single crystals are produced with a time constant shorter than 20ns and 10ns, respectively, between 10K and 270 K under the pulsed electron beam from a linear accelerator. The F band due to these F centers is located at 1.87 eV and 2.75 ev in KI and NaCl, respectively which is the same position, along with its half width, as that by a conventional x-raying coloration, and the height of the band stays almost constant after 20 µs or more indicating no recombination of F and H centers. The triplet state of the self trapped exciton in KI and NaCl is not directly connected to the F center formation as well as in KCl and KBr.

Intrinsic Luminescence in KBr Crystals between 1.8 K and 80 K

Intrinsic luminescence and optical absorption from the triplet state of the self trapped exciton in KBr crystals have been observed by a pulsed electron beam and by a N 2 laser. The π emission at 2.28 eV decays with two components of ∼3 ms(τ S ) and 100 µs(τ F ) at 1.8 K. τ F component arises from a transition from B u (a combined state of B 3 u and B 2 u ) to A g , and τ S component from A u → B u →A_g i n t h e s e l f t r a p p e d e x c i t o n a s t h o s e i n K I r e p o r t e d b y f i s c h b a c h et al . T h e e n e r g y s e p a r a t i o n b e t w e e n B_u a n d A_u i s 1.6 m e V . I n a d d i t i o n t o t h e a b o v e , t e m p e r a t u r e d e p e n d e n c e o f t h e y i e l d o f t h e 2.28 e V e m i s s i o n a n d o f t h e s e l f t r a p p e d e x c i t o n s a t t h e t r i p l e t s t a t e i s d i s c u s s e d i n c o n n e c t i o n w i t h t h e F c e n t e r f o r m a t i o n .

Secondary emission of indirect exciton in BiI3

Resonant secondary emissions of BiI3 crystals in indirect exciton edge are investigated at liquid helium temperature by using a tunable dye laser. The emission spectra depend critically on the excitation energy, and consist of 1st-, 2nd- and 3rd-order resonant Raman lines and hot-luminescence bands of LO phonon assisted exciton recombinations. From an analysis of Raman lines, the associated phonon energies and the exciton effective mass are estimated. Preliminary discussion on the hot-luminescence is also given.

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.

Photo-induced magnetic polarons in low-dimensional dilute magnetic semiconductors

Abstract The temperature dependence of exciton magnetic polaron energies was determined from the pico-second time-resolved photoluminescence (PL) spectra, and have shown characteristic dynamical behavior of exciton localization process in bulk-Cd1−xMnxTe (x=0.1–0.2). The overall behavior appears different from that reported previously for the bound- or localized-exciton magnetic polarons (EMP) in the same material. In symmetric diluted magnetic semiconductor (DMS) quantum wells (QWs), CdTe/Cd1−xMnxTe, studied here by steady-state and transient-PL, it is plausible that the exciton magnetic polaron formation occurs after localization by the nonmagnetic disorders. Temperature and magnetic-field dependencies (up to 15 T) of PL have been studied in Cd1−xMnxTe−CdTe–Cd1−yMgyTe asymmetric single QWs. These show the phenomena similar to those expected in the exciton ‘bifurcation’ effect due to magnetic polaron formation on one side of the diluted magnetic barrier. The doublet PL lines observed are attributed to symmetric and asymmetric exciton states. Biexcitons are observed in DMS-QWs, and confirmed by the polarized two-pulse degenerate four-wave-mixing (DFWM) experiment.

Secondary emission due to recombination of excitons near indirect band bottom. II. Experimental and theoretical analysis in BiI3

For pt.I see ibid., vol.16, p.4719 (1983). Experimental and theoretical studies are presented on the secondary emission (SE) due to recombination of excitons near the indirect band bottom in BiI3. The authors have made systematic measurements to observe the excitation energy, temperature and sample dependences of SE spectra, and obtain a consistent interpretation on these experimental results applying the theory developed in preceding work. The observed SE spectra consist of the Raman scattering (RS) lines, involving resonant higher-order Raman processes, and the hot luminescence (HL) bands in the scheme. Under the excitation near the indirect absorption edge, the spectral structure of SE reflects the relaxation processes of excitons which are governed by the intraband scattering by acoustic phonons and a non-radiative decay due to crystal imperfections. It is shown that the two parameters characterising these two processes are uniquely determined by fitting the calculated intensities of RS lines to the observed ones, and the computed spectral lineshape of HL using these parameter values reproduces experimental results satisfactorily. The relationship between RS and HL intensities is also discussed by comparing the observed and calculated intensity ratios.

Zone-folding effects on phonons and excitons in polytypic BiI3 single crystals

Abstract Resonant Raman scattering (RRS) of new mode phonons in BiI3 crystals containing stacking faults is reported. Under excitation with laser light tuned to a characteristic sharp absorption line, new mode Raman lines show the resonance behavior. The new phonon modes and the origin of the sharp absorption line can be interpreted in terms of folding back effects of the Brillouin zone for phonons and excitons in a polytypic structure

Resonant Raman Scattering and Hot Luminescence at the Indirect Exciton in BiI3

Secondary emission (SE) spectra under excitation around the indirect exciton edge of BiI 3 crystals are studied at liquid helium temperatures (LHeT). The spectra consist of one-phonon non-resonant Raman scattering lines, and two-and multi-phonon resonant Raman scattering (RRS) lines associated with hot luminescence (HL) bands due to phonon assisted exciton recombinations. Detailed results on excitation-energy and temperature dependences of the spectra are presented. The RRS cross sections are analyzed on the basis of exciton damping processes near the exciton band bottom, and the HL line shapes are discussed on quasi-thermal equilibrium distribution under the same relaxation mechanisms.

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.