Eiichi Hanamura

Very large optical nonlinearity of semiconductor microcrystallites

We analyze theoretically the oscillator strength and the third-order optical polarizability X 13, due to excitons in semiconductor microcrystallites. The nonlinear optical polarizability is shown to be greatly enhanced for an assembly of such microcrystallites as the exciton is quantized due to the confinement effect and the excitons in a single microcrystallite interact strongly enough to make the excitons deviate from ideal harmonic oscillators.

Excitonic Molecule. I. Calculation of the Binding Energy

The binding energy of the excitonic molecule, a complex consisting of two electrons and two positive holes, is calculated as a function of the mass ratio m e / m h by avariational method. It is shown that the excitonic molecule is stable for any value of the mass ratio. For m e = m h , the binding energy is estimated to be 0.00684 \(m_{\text{e}}e^{4}/{\varepsilon_{0}}^{2}\hbar^{2}\). Some results about the wave function are also presented.

Condensation effects of excitons

Abstract The theory of the electronic excitations in a highly excited semiconductor is presented. The relaxation processes, the formation of excitons and excitonic molecules, the interaction among the various forms of electronic excitations, as well as their optical and thermodynamical properties are analyzed. At low temperatures one expects condensations into the quantum statistically degenerate phases of the excitonic molecules and of the electron-hole plasma. The physical properties of these low temperature phases are investigated. Possibilities and previous attempts to observe the Bose-Einstein condensation in excitonic systems are discussed critically. The experimental observations of the electron-hole liquid phase transition are reviewed.

Anisotropic excitonic molecules in CdS and CdSe

Abstract Luminescence lines due to excitonic molecules are observed in CdS and CdSe. The line shape is analyzed by taking account of the broadening due to elastic collision. It is pointed out that the anisotropy of the effective hole masses makes the molecular binding energies much larger than evaluated by using the geometrical means of the hole masses.

Theory of the High Density Exciton. I

The boson creation and annihilation operators of excitons are introduced and the fermion Hamiltonian of high density electrons and holes is expanded in terms of these boson operators. Then a nonlinear integral equation for the high density excitons is derived and it is solved in two ways: 1) by the perturbational method and 2) exactly for a simple model of the Coulomb interaction in the random phase approximation and the effective mass approximation for the two band model of a conduction band and a valence band. As a result, the cutting down effects of both the band edges due to formation of high density excitons are shown to overcome the exchange self-energy of the electrons and the holes composing the high density excitons.

Binding energy of the excitonic molecule

Abstract The binding energy of the excitonic molecule is calculated as a function of the mass ratio m e m h . It is found that the excitonic molecule should be stable for any value of the mass ratio.

Coexistence of Local and Band Characters in the Absorption Spectra of Solids I. Formulation

The problem of coexistence of the local and band aspects in the fundamental absorption spectra (or the impurity-induced infrared absorption of lattice vibrations) is formulated with the use of the Greens function method. By a suitable decomposition of the hamiltonian of the electron-hole relative motion (or the dynamical matrix for the lattice vibrations), one can derive a line shape expression in which coexist the both aspects, namely, the metastable excitons (or the quasi-local modes) on the one hand and the Van Hove singularities on the other hand. Their interference results in the antiresonance of the quasi-local states and the metamorphism of Van Hove singularities.

Quantum wells with enhanced exciton effects and optical non-linearity

Abstract Exciton effects are studied theoretically for a quantum well of a semiconductor sandwiched by barriers with a smaller dielectric constant and a larger energy gap. The exciton binding energy increases markedly so that the radiative decay rate of the exciton and the non-linear optical susceptibility are also shown to be enhanced.

Theory of Many Wannier Excitons. I

We present the method by which we can describe the many exciton system transparently by making the best use of the boson like character of Wannier excitons. Basing upon this method, the total energy of this system and the number of bose-condensed excitons are calculated as a function of the total concentration of excitons, and we show how the boson like character of Wannier excitons is reflected on the emission spectrum from this system and on the absorption spectrum in the presence of these excitons at finite density.

Excitonic Molecule. IV. Optical Properties

The optical properties of the excitonic molecule are clarified, basing upon the electronic structure of the excitonic molecule derived in the preceding paper. The luminescence, the optical conversion of a single exciton into the excitonic molecule and the giant two-photon absorption due to the excitonic molecules are theoretically discussed and compared with the available experimental data.