R. Girlanda

Entangled photon pairs from the optical decay of biexcitons

Abstract We show that it is possible to transfer the exciton–exciton Coulomb correlation to photons, producing thus pairs of near-gap photons with a high degree of quantum entanglement. The photon pairs emerge from the spontaneous optical decay of biexcitons into two polaritons. The pair intensity-correlations, calculated in the low density limit for a CuCl slab, exhibit quantum features which can be observed by coincidence detection.

Selection rules for direct creation of biexcitons by giant two-photon absorption

Abstract The selection rules for direct creation of biexcitons by two-photon absorption processes are discussed on symmetry basis. A detailed analysis is performed for CuCl and CdS crystals, with the energy levels given by Bassani et al. It is found that only Гbiex1 states can be created by giant two-photon absorption when the photons have the same frequency. The polarization dependence is given for the other cases.

Core excitons in corundum

Abstract New photoabsorption spectra of thin films of α-Al2O3 measured by synchrotron radiation at the LII–III edges of aluminum are presented. The theory of the intermediately bound exciton in large-gap materials is applied to describe quantitatively the sharp peak observed below the photoconduction threshold. The doublet splitting of the lowest exciton is computed in terms of the ratio of the exchange integral to the spin-orbit splitting of the core 2p states. The exciton binding energy and intensity agree fairly well with the experiment.

Silicon-based organic-inorganic microcavity and its dispersion curve from angle-resolved photoluminescence

We report an experimental study of a planar organic-inorganic microcavity consisting of a porous silicon distributed Bragg reflector, a single layer of acridina orange as active material and a top aluminum reflector. By tuning the cavity resonance energy around the maximum of the organic material photoluminescence, we found an intense emission and a spectral narrowing of the emission band to about 45 meV. The angle-resolved photoluminescence spectra enable us to determine the microcavity dispersion curve. The very good agreement with the theoretical dispersion provides a precise determination of the refractive index of the organic material.

Electronic properties of the layer semiconductor InSe

Abstract The three-dimensional band structure of InSe is calculated by the tight-binding and pseudopotential methods. The two band structures have many features in common and both agree with most optical and photoemission data. The high ionicity of this compound is reflected in the low dispersion of the energy bands and is evidenced by a calculation of the total valence charge density.

Optical properties studied using linear-muffin-tin orbital theory

A new method is presented for calculating optical properties within the framework of the self-consistent linear muffin-tin orbital (LMTO) band-structure theory. We show how Greens second identity along with the commutation relation between the position and Hamiltonian operators may be used to calculate, independently of the gauge, the velocity matrix elements required for optical properties. The method has been applied to calculate the imaginary part of the dielectric function epsilon 2(E) for FeSi and Si. Notably, the new expressions for matrix elements are also valid for Hamiltonians in which non-local potentials are included.

Optical properties of the semiconductor 7, 7′, 8, 8′, tetracyanoquinodimethane (TCNQ)

SummaryThe reflectivity and absorption spectra of 7, 7′, 8, 8′ tetracyanoquinodimethane thin films, in the visible and near-ultraviolet range, have been measured. Samples have been grown and characterized by means of elemental microanalysis and differential scanning calorimetry (DSC) in order to obtain the purity degree. The experimental optical spectra are interpreted on the basis of a calculation of the electronic molecular energy levels by the complete neglect of differential-overlap method in a new parametrization scheme.RiassuntoSono stati misurati gli spettri di riflettività e di assorbimento di film sottili di 7, 7′, 8, 8′ tetracianochinodimetano nella regione spettrale del visibile e del primo ultravioletto. Il grado di purezza dei campioni è stato determinato mediante microanalisi elementale e calorimetria differenziale a scansione (DSC). Gli spettri sperimentali sono stati interpretati sulla base di un calcolo di livelli elettronici con il metodo CNDO seguendo un nuovo schema di parametrizzazione.РезюмеБыли измерены спектры отражательной способности и поглощения тонких пленок 7, 7′, 8, 8′ тетрацианоквинодиметана в видимой области и в области ближнего ультрафиолета. Проведено выращивание образцов. Проведены элементный микроанализ и дифференциальная сканирующая калориметрия, чтобы определить степень чистоты. Экспериментальные оптические спектры интерпретируются на основе вычисления электронных молекулярных энергетических уровней в новой схеме параметризации.

Imaging spectroscopy of quantum wells with interfacial fluctuations: A theoretical description

We present a theoretical approach for the simulation of scanning local optical spectroscopy in disordered quantum wells (QWs). After a single realization of the disorder potential, we calculate spectra on a mesh of points on the QW plane, thus obtaining a three-dimensional matrix of data from which we construct two-dimensional spectroscopic images of excitons laterally localized at interface fluctuations. Our simulations are in close agreement with the experimental findings, and contribute to the interpretation of spatially resolved spectra in QWs.

Band structure and electronic properties of FeSi and α‐FeSi2

The results of self‐consistent linear muffin‐tin orbital calculations for the ordered end members of the FeSi1+x series, i.e., FeSi (x=0) with a CsCl structure and α‐FeSi2 (x=1) with a tetragonal structure, predict semimetallic behavior for FeSi and metallic behavior and higher stability for α‐FeSi2. The valence band density of state features of the α‐FeSi2 differ slightly from those of the cubic fluorite (γ‐FeSi2). The analysis of the valence band photoemission spectra taken on silicide thin films epitaxially grown on Si(111) substrates estimates the composition of the mono‐ and disilicide phases present in the annealed films but does not determine the proposed formation of the distorted fluorite structure (α‐FeSi2) well below its bulk stabilization temperature of 950 °C.

Fluorescence spectra and electronic-energy levels of the semiconductor 7, 7′, 8, 8′ tetracyanoquinodimethane (TCNQ)

SummaryThe luminescence of solutions of TCNQ together with their absorption spectra have been measured and compared with the optical properties of powders, as deduced by diffuse reflectance spectroscopy. The experimental results are interpreted on the basis of the electronic-energy levels in the molecule obtained by a CNDO calculation scheme.RiassuntoSono stati misurati gli spettri di luminescenza e di assorbimento di soluzioni di TCNQ e confrontati con le proprietà ottiche delle polveri cosí come dedotte da misure di riflettanza diffusa. I risultati sperimentali sono stati interpretati sulla base di un calcolo di livelli di energia elettronici effettuato con il metodo CNDO.