Kikuo Cho

Optical Absorption Line Shapes Due to Transition from Orbital Singlet to Triplet States of Defect Centers with Cubic Symmetry

Structures in the optical absorption line shapes have been investigated for the transitions from orbital singlet to triplet states of defect centers with cubic environment. In the triplet excited states both the spin-orbit and the linear electron-lattice interactions have been taken into account. Under the classical Franck-Condon approximation, line shapes are specified by the combination of the four coupling constants which represent the strengths of the spin-orbit interaction and the Jahn-Teller interaction with one-, two- and three- dimensional lattice vibrational modes. For all the possible combinations of the four coupling constants, optical absorption line shapes have been numerically calculated by means of Monte Carlo integration method with sufficient accuracy. The results can be applied not only to the dipole-allowed transitions (F-bands, U 2 -bands and C-bands in Tl + -like centers etc.) but also to the dipole-forbidden and vibration-allowed transitions in O h and T h environment. As an example ...

Exciton-Phonon Interaction and Optical Spectra –Self-trapping, Zero-Phonon Line and Phonon Sidebands–

Optical properties of interacting exciton-phonon system have been theoretically studied in terms of a simple model consisting of Frenkel excitons and Einstein oscillators. The problem is characterized by two non-dimensional parameters B and S representing the exciton band width and the interaction energy, respectively. Approximate solutions have been obtained through the diagonalization of the Hamiltonian in special sub-spaces, and they reasonably describe the behavior of the system for all values of B and S . Absorption and emission spectra and the effective mass of the composite particle vary from free exciton type to self-trapped exciton type as we go from B > S to B < S . The energy-momentum relation has been qualitatively studied below the one-phonon continuum, and the following new features have been found in the strong coupling region; (1) complete split-off of the lowest branch in the whole Brillouin zone, and (2) the split-off of the second discrete branch. Furthermore, a clue to the mechanism of...

ABC-Free Theory of Polariton: From Semi-Infinite Medium to Quantum Well

A new theoretical scheme is proposed for polaritons in bounded media, from a semi-infinite system to a quantum well. For each specific model, this theory allows a direct calculation of the electric field E ( r ) of polaritons without referring to any additional boundary condition (ABC). For a thick enough sample, bulk polaritons emerge automatically in the expression of E ( r ). Applications are shown in two simple cases; (i) a single quantum well, and (ii) a semi-infinite medium with simple exciton wave functions.

Excitonic Polariton Dispersion in ZnSe Determined by the Resonant Raman Scattering under Two-Photon Excitation of Excitonic Molecules

The excitonic polariton dispersion for \(\bm{k}//[111]\) is determined by the two-photon-resonant Raman scattering via intermediate excitonic molecule state. Existences of the k -linear effect and the heavy and light masses are clarified. Several levels of excitonic molecules are found.

Theory of Stress and Magnetic Circular Dichroism in Alkali Halide Phosphors

Magnetic circular dichroism (MCD) and stress dichroism (SD) in the absorption spectrum due to ( s ) 2 →( s ) 1 ( p ) 1 transition with electron lattice interaction have been theoretically studied in order to investigate the further consequence of the Jahn-Teller effect in the excited state. In the framework of the classical Frank-Condon approximation, dichroic absorption spectra have bee numerically calculated by means of the Monte Carlo integration over the five-dimensional “interaction-mode” coordinate space. Calculated dichroic spectra are consistent with the existing data for the line shapes of MCD and SD in the Tl + -like centers in alkali halides.

Theory of resonant SNOM (scanning near-field optical microscopy): breakdown of the electric dipole selection rule in the reflection mode

A theoretical study is reported about SNOM with the use of light in resonance with the quantized levels of a mesoscopic sample. The microscopic variation of electromagnetic field and the coherence of matter state are properly treated by solving the microscopic Maxwell equations and Schrodinger equation self consistently. A remarkable effect, i.e., the breakdown of the dipole selection rule, is shown in the reflection mode where a probe tip is used for both illumination of the sample and collection of the signal. Namely, both dipole-active and higher-multipole-active levels give comparable signal intensities for appropriate probe tip positions.

Electronic Structure of the U2-Center in KCl. III. The Effect of Correlation

The magnetic and optical properties of the U 2 -center have been theoretically studied. Starting from the many-electron formulation of the problem, its reducibility to one-body problem is discussed. The reducibility is determined by the approximation employed for the many-electron wave function and the physical quantity considered. Numerical calculation is carried out in terms of the wave function with correlation effect as well as with the overlapping effect between orbital wave functions. It has lead to a good result for the hyperfine (hf) constants of the nearest neighbor ions and the g -shift. But it gives an overestimate for some quantities of farther ions. This result is in contrast with that of the approach from the orthogonalization procedure only. Quadrupole interaction constants and the oscillator strength for the optical U 2 -band are also calculated.

Electronic Structures of the U2-Center in KCl and KBr: II –The Effect of the Configuration Mixing–

A formula for calculating the spin density of the U 2 -center has been obtained by considering the mixing of the configuration having H - and a hole in the host crystal and the effect of the orthogonalization of the core orbitals between neighboring ions. For the numerical calculation of the g-shifts and hyperfine structure constants of the U 2 -center, the problem is reduced to the one-hole picture in which the ground state wave function is expressed by an LCAO molecular orbital. It is pointed out that the effect of the configuration mixing plays an important role in producing the smaller hyperfine splitting due to proton in the U 2 -center than that in the free hydrogen atom, However, the g -shifts and the superhyperfine constants are overestimated by this effect; the effect of the orthogonalization of the core orbitals on these quantities is discussed.

Exciton Quantization and Polariton Interference in Thin Films: Comparison of Different Approaches

In search for an accurate and convenient method to calculate optical properties of excitons in thin films as a non-local response, a comparison is made among the different frameworks available for different regions of film thickness ( d ). A detailed discussion is given about the multi-mode nature of polariton interference in optical spectra. The exciton levels in GaAs films with d >2.5 a B are shown to follow approximately the center-of-mass quantization. The measured reflectance spectra for GaAs films with d ≥4800 A are reasonably interpreted in terms of three-branch polaritons arising from light- and heavy-mass excitons.

Ultra‐Fast Radiative Decay of Spatially Extended Non‐Dipole Type Excitonic State

By means of the non-local theory of the pulse response and the analysis of the exciton-radiation coupled modes, we elucidate a recently observed short response time of degenerate four-wave mixing (DFWM) of weakly confined excitons in a thin film. Especially, it is expected that the excitons with non-dipole type of center-of-mass wavefunctions exhibit a peculiar thickness dependence owing to a spatial structure of the radiation field. The experimental results of the three-beam DFWM of GaAs thin layers strongly support our theoretical explanation of the radiative decay that is beyond the long wavelength approximation.