EPIOPTICS 2000

The following sections are included:

• Preface

• CONTENTS

A few basic properties of excitons in quantum wells and in semiconductor microcavities are reviewed at an introductory level. The first lecture treats electron-hole and center-of-mass quantization regimes in wide quantum wells, electron-hole and center-of-mass localization in narrow quantum wells, and excitons bound to interface defects: emphasis is given on the behavior of the oscillator strength, which marks clearly the crossover between the various regimes. The second lecture deals with planar microcavities and the strong coupling regime of exciton-radiation interaction, three-dimensional microcavities and the modification of spontaneous emission rate (Purcell effect), and the possibility of obtaining a vacuum-field Rabi splitting for quantum dots in three-dimensional microcavities.

Spectra for reflectance anisotropy and second harmonic generation of clean Si(100) reconstructed surfaces, based on the polarizable bond model, are presented. The crystal is modeled as an array of point-like polarizable dipoles which are considered to be at the middle of every Si-Si bond. The model incorporates the reconstruction of the surface trough the local field effect. Spectra for the reconstructed surfaces 2 × 1 and c(4 × 2) are presented and are compared with the experiment.

The three-phase theory of SDR (Surface Differential Reflectivity) and RAS (Reflectance Anisotropy Spectroscopy) is briefly outlined in order to show the importance of selection rules in the interpretation of the spectra. Special consideration is given to the rule: which allows us to find out when a given group of electrons have exhausted their oscillator strength.

Experimental results are presented for Ge(111)2 × 1, Si(111)2 × 1, GaAs(100)c(4 × 4), 2 × 4, 4 × 2, that show that the above rule is well obeyed in Si, Ge, GaAs(100)c(4 × 4) while in GaAs(100) 2 × 4 and 4 × 2 (whose RAS spectrum partially overlaps the critical points of the bulk structure) less clear-cut results are obtained. In this case, simulation of the RAS spectra with Lorentz oscillators allows some empirical rules for the analysis of the spectra to be drawn.

The near-field visible photoluminescence spectra of F₃⁺ and F₂ aggregate point defects located in a colored region induced by low energy electrons at the surface of Lithium Fluoride (LiF) films thermally evaporated on glass substrates is measured for the first time. Such spectra, acquired at different points of the irradiated area, show broad visible photo-emission bands ascribed to this type of laser active centers. Several peculiar features, i.e. the spectral distribution and the intensity of the emissions, are under investigation.

Reflection Second-Harmonic Generation on native-oxide-covered porous silicon (p-Si) samples and on their (100) crystalline silicon (c-Si) substrates has been measured. The non-linear reflection coefficient is obtained in absolute units, allowing to estimate surface and bulk terms of the second-order susceptibility (χ⁽²⁾) for both materials. χ⁽²⁾ of p-Si is found to be two orders of magnitude smaller than in c-Si.

X-ray Absorption Near Edge Structure (XANES) spectroscopy and atomic force microscopy was used to study the interaction of nifedipine, a drug used in the treatment of hypertension and of several cardiovascular diseases, with hemoglobin and erythrocytes.

Nifedipine is a dihydropyride compound with calcium channel blocker activity and that seems to have spin trapping properties.

XANES data do not reveal interaction between the drug and hemoglobin but, under extensive X-ray exposure, reveal a protective role of the drug against radical induced oxygen scavenging that was not detected when the drug was added to an erythrocyte suspension. This data suggest a predominant intramembrane location for nifedipine.

The effect of nifedipine on erythrocytes morphology was studied by air operating atomic force microscope revealing several deformations of cells that depend in number and strength by the concentration of drug. Comparison of the nifedipine treated cells with the morphological effect of compound that locates into the erythrocyte membrane, shows that nifedipine-induced deformation cannot be justified by a simple mechanical insertion. A multiple mechanism of nifedipine action should be invoked involving, beyond the insertion of the drug into the membrane, also the calcium channel blocker activity of nifedipine.

We used air operating atomic force microscopy (AFM) and X-ray absorption near edge structure (XANES) spectroscopy to investigate the active site structure and the overall arrangement of the giant hemoglobin from the earthwormEisenia Foetida.

This protein constitute a large macro complex (with weight of about 4.2 MD and dimension in the order of tenth of nanometer) composed by four different heme-containing chain repeated in 144 units and a not well known number of non-heme carrying subunits called linkers.

Here we report the first measures at iron k-edge of the active sites' arrangement in the whole molecule as well as that of its main sub-aggregates, reporting evidence of some spectroscopic markers do not present in the vertebrate's hemoglobin. A role for the linker chains and/or for the interface contact of one heme carrying subunit seems to emerges as well as the presence of Zinc in non-amorphous arrangement into the protein.

Since the size of the protein falls in a range detectable by scanning probe microscopy we report and discuss preliminary data on the structure of the overall protein bound to different ligands.

The polarization dependence of the optical reflectivity for sapphyrin layers deposited by Langmuir-Blodgett technique onto a gold substrate has been measured. The experimental results show that characteristic spectra are related to layers of different thickness. An interpretation of the main spectral features in terms of the anisotropic optical properties of the sapphyrin molecule is presented.

With the aim of evaluate the optical resolution related with our Scanning Near-field Optical Microscope (SNOM) operating in the External Reflection-Collection configuration we performed measurements on two different samples and observed optical details much smaller than λ/2 that is the limit imposed by diffraction. The level of resolution depends on the specimen and on the wavelength of the external light source.

The near–field photoluminescence of CsPbBr₃ nanocrystals in CsBr-PbBr₂ co-evaporated thin films is measured. Room temperature emission spectra acquired at different points of the sample area irradiated by an Ar⁺ laser (λ = 455nm) show substantial differences in the emission pattern, namely in the 520nm emission peak presence, which belongs to the exciton emission of CsPbBr₃ phase. Such an observation points out a marked local variation of the chemical composition and the presence of different ternary phases, probably the CsPbBr₃ and Cs₄PbBr₆ ones.

We present a theoretical description of surface second harmonic generation from semiconductor surfaces based on a Green function approach. The nonlinear susceptibility tensor χ is written as a function of its 1ω and 2ω transitions and classified according to the bulk or surface character of the states involved in the one-electron transitions. Several examples are presented and compared to experimental spectra.

The oxidation of the GaAs(001)-4 × 2 Ga-rich surface has been investigated by Reflectance Anisotropy Spectroscopy, up to one monolayer coverage. The main negative signal at 2.2 eV exhibits a strong sensitivity to oxygen uptake. This finding suggests that surface states related to Ga dimers are involved in the optical transitions. This conclusion is consistent with the fact that the 2.2 eV peak is out of the region of bulk optical transitions in GaAs. After monolayer oxidation, a residual signal (about half of the initial value) is observed at 1.9 eV. This residual signal is tentatively explained with a model whereby an oxygen molecule bonds to only one dangling bond (out of two) in a Ga dimer. Furthermore, a minor positive peak is observed at 3.8 eV, exhibiting the same dependence upon oxygen exposure as the main negative peak.

Investigations of homogenous and surface waves scattering by gallium nanoparticles (NPs) and 1D surface defects have been performed in a whole hemisphere. A model of the interaction between 1D defects and surface waves has been built, simulations were then performed and found in good agreement with experimental results. This model has also been extended to the interactions between NPs and surface waves and gives a direct possibility of comparing the results of homogeneous and surface waves scattering by these NPs.

We studied the surface electronic structure of a PbS sample, cleaved in ultra-high-vacuum environment, with surface differential reflectivity (SDR) and angle-resolved photoelectron spectroscopy (ARUPS). The ARUPS spectra show the presence of two surface states along the two angular directions [001] and [011]. SDR shows the existence of three optical transitions at energies of approximately 2.1, 2.7 and 3.1 eV. These transitions have been assigned to different points in the surface Brillouin zone.

This paper describes the spectroscopic-ellipsometric characterization of multilayer diffraction gratings, which are the key optical components of optochemical sensors and photodetectors. The conditions of the surface-plasmon excitations in Au-Cr-GaAs and Al_xO_y-Al-GaAs structures are analyzed in terms of differential formalism proposed by Maystre and Chandezon. The present study demonstrates the potentialities and the way of optimization of the conditions of photon–plasmon coupling at the multilayer diffraction gratings.

Electrical properties of lattice-mismatch induced defects in InGaAs/GaAs heterostructures have been studied by means of electron-beam induced current (EBIC) in a scanning electron microscope and deep-level transient spectroscopy (DLTS). DLTS measurements, carried out with p-n junctions formed at the heterostructure interface, revealed one electron trap and one hole trap induced by the lattice mismatch. The electron trap has been attributed to electron states associated with threading dislocations in the ternary compound close to the interface. On the other hand, the hole trap has been ascribed to defects associated with the lattice-mismatched interface of the heterostructure.

The localized states of light flux propagating in the array of parallel plane nonlinear optical waveguides are investigated. It is shown that the problem is reduced to a model of coupled anharmonic oscillators and all the parameters of such a model are found exactly ….