EPIOPTICS-7

PREFACE.

CONTENTS.

The theory of surface optical properties is reviewed including the solution of light-propagation equations at surfaces and methods for electronic-structure calculations. Realistic calculations of surface optical properties within the single-particle approximation are discussed. GaAs(110) ans Si(100) are considered as test cases.

We present the use of Reflectance Anisotropy Spectroscopy (RAS) as a probe to study the deposition of organics We started the study of ordered layers deposited onto disordered substrates by Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques, nevertheless we believe that the goal is the application of RAS to OMBE (Organic Molecular Beam Epitaxy), to monitor the growth process in real time and in situ, similarly to the deposition of inorganic semiconductors. After a short introduction, we discuss an example testing the RAS sensitivity to the structure of the growing layers: in particular, we report recent data about the polarization dependence of the optical reflectivity for LS PdC₁₀OAP porphyrin layers. Deposition has been carried out at two values of the surface pressure (Π₁=30 mN/m; Π₂=10 mN/m), corresponding to different layer structures. The RAS spectra measured in two cases are successfully explained in terms of the particular morphological characteristics of the layer.

The experimental characterization of surfaces is often obtained through spectroscopic techniques like photoemission, inverse photoemission, electron energy loss spectroscopy and optical techniques as Reflectance Anisotropy and Differential Reflectivity, etc. It is hence very important to describe accurately the electronic excitations with highly reliable and efficient ab-initio approaches. In the last two decades Density Functional Theory has proven to be a very powerful tool for electronic ground-state properties, but shows significant deviations from the experiments when excited states are involved. The use of many-body Green’s functions theory, with DFT calculations as zero order approximation, is now the state of the art to obtain quasi-particle excitation energies and optical spectra both in bulk and at surfaces. In this paper we will present the current status of this theoretical and computational approach, showing results for several semiconductor surfaces.

First-principle calculations of the linear optical properties of the nonpolar (110) surface of cubic Boron and Gallium Nitride are presented. We analyze the reflectance anisotropy (RA) spectra in relation to the better known spectrum of the GaAs(110) surface and to the existing tight-binding results for GaN(110). Previous results for the RA spectrum of GaN(110) obtained in tight-binding scheme are confirmed. The present first ab-initio results for BN(110) show the important role played by surface states transitions at the onset of the RA spectrum.

A theoretical review of surface second harmonic generation from semiconductor surfaces based on the longitudinal gauge is presented. The nonlinear susceptibility tensor χ is split into two terms, one that is related to inter-band one-electron transitions, and the other is related to intra-band one-electron transitions. The equivalence of this formulation to the transverse gauge approach is shown and the possibility of confirming its numerical accuracy is discussed. Also, the calculation of the surface second harmonic radiated intensity R within the three-layer-model is derived. With χ and R one has a complete description of this fascinating optical phenomena.

The phenomenology of second harmonic generation at interfaces is outlined and experimental techniques described. The use of surface steps to obtain single domain growth, by self-assembly, of low dimensional nanostructures on planar silicon is discussed and it is shown that SHG appears particularly promising for characterising anisotropic low dimensional systems.

Liner and nonlinear optical properties of oligothiophenes self-assembled monolayers (SAM) are studied by optical second harmonic interference technique. Linear polarizability and first hyperpolarizability as functions of the number of thiophene rings are calculated using a self-consistent approach for linear and nonlinear optical properties. Both of these dependencies can be described by a power law, a nonzero first hyperpolarizability points to the asymmetry of delocalized electron system that might be due to an adsorption of thiophene molecules on a metal substrate.

Thin Al films on Si(111) have been investigated by optical second-harmonic generation (SHG) in combination with synchrotron radiation photoemission spectroscopy. Valence band spectra of Al films show quantization of the sp-band in the energy range from the Fermi level down to the surface state at a binding energy of 4.6 eV. Quantization effects in SHG are observed as oscillations in the signal as a function of film thickness with an oscillation period that depends on the pump frequency of the laser light. The relation between film thickness and photon energy of SHG resonances for Al films indicates different types of resonances involving both quantum well and substrate states.

This paper describes femtosecond infrared - visible sum frequency generation (SFG) as a technique to obtain information about the structure and dynamics of molecules at surfaces and interfaces. The use of ultrashort laser pulses allows for experiments in both the time- and frequency-domain, which are demonstrated to provide complemantary information about the molecular structure at the surface. In addition, from the time-resolved experiments, energy transfer rates (typically in the (sub-)picosecond regime) can be inferred, both from the metal substrate into the adsorbed ad-layer, as well as between the adsorbed molecules.

In this work we report about the fabrication and characterization of heterostructure organic electroluminescent devices based on the system N,N’-diphenyl-N,N’-bis(3-methyl phenyl)-1,1’-biphenyl-4,4’-diamine (TPD) and tris (8-hydroxy-quinoline) aluminum (Alq₃). We present the electric and photometric characterization of Alq₃/TPD diodes, using ITO (Indium-Tin-Oxide) and Al or Al/LiF as transparent anode and metallic cathode, respectively, and discuss the results.

He⁺ ions with an energy of 1.5 -2 MeV and doses ranging from 6 × 10¹³ to 2.5 × 10¹⁶ ions/cm² were used to produce primary and aggregate defects in Lithium Fluoride (LiF) crystals. The optical properties of the implanted samples and their dependence on ion dose were studied by means of optical absorption spectroscopy and Scanning Near-field Optical Microscopy (SNOM). The results show the formation, for high doses, of macrodefects such as lithium intrinsic nano-aggregates that could play an important role in the realisation of non-linear optical waveguides.

The localization of nonlinear waves propagating in an anharmonic medium along an array of identical plane-parallel defect layers (optical waveguides) is investigated. The formation of a “supersoliton” in periodic layered or waveguide structures perpendicularly to the direction of its propagation is theoretically described. The results obtained can be useful for investigations of layered optical media used in fiber optics (coaxial lightguides) and optic delay lines, magnetic multilayer systems which are promising for the creation of elements for data storage and readout, etc.

Atomic force microscopy (AFM) is a well established technique for high resolution morphological studies and, in fact, it has been applied to characterise surface useful for microelectronics, electrochemistry and other solid state physic’s research field. However, it have been cleared since the first days of AFM that such a technique is very attractive for biological studies too. This is because AFM characteristics allow to observe conducting and non conducting sample, overcoming the limitation of Scanning Tunneling Microscopy (STM), with resolution well below the diffractive limit of the conventional optical microscopes. Other major advantages of AFM for biological studies is that the microscope can work in liquid environment, so that the soft and delicate biological samples can be imaged in nearly physiological conditions.

In this paper we present some applications of Atomic force microscopy (AFM) and of lateral force microscopy (LFM) in the field of cellular biology, with particular emphasis on apoptosis, characterization of the effect of metal doping on cell, as well as a study of RBCs pathology and aging.

Coherent synchrotron sources open the way to powerful new radiology approaches. We briefly review the conceptual background of such approaches, then we present some examples of applications with particular emphasis on materials science systems.

Magneto-optical phenomena associated with cyclotron and magnetoplasma oscillations in a homogeneous two-dimensional (2D) electron system are reviewed. Both uniform and non-uniform magneto-optical resonances in 2D electron system are considered. The phenomena under consideration can be observed on gallium arsenide heterostructures at terahertz frequencies.

The magnetization induced effects in the nonlinear optical response of magnetic media leads to very strong and novel nonlinear magneto optical effects that appear to be very sensitive to magnetic interface properties, yielding to very attractive applications for the study of magnetic thin films and multilayers.

The problem of radiative decay of coherent polariton modes in a two-dimensional (2D) excitonic system is analyzed here. We show, theoretically that inhomogeneous plane electromagnetic waves are spontaneously emitted from such a system. The dispersion branches of radiative exciton-polaritons penetrate into the slow-wave region of the ω−k_∥ dispersion plane. The slow radiative polariton frequency has an appreciable dependence on in-plane wavevector, which appears to be much stronger than was previously expected for radiative polaritons. We conclude that slow radiative polaritons can be observed in time-resolved photoluminescence experiments on 2D excitonic systems at grazing angles of emission. The slow polaritons can be identified by the angular dependence of their emission frequency.

The development of femto-second lasers has opened the way to create external stimuli such as optical or magnetic field pulses that are much shorter than fundamental time scales like spin-lattice relaxation or precession times and that can be used to change the magnetization direction on an ultra-fast time scale. In this chapter one of such approaches is discussed: precessional dynamics and switching by exploiting specifically designed magnetic field pulses, generated by photo-conductive switches.