Advances in Energy Transfer Processes

The following sections are included:

• PREFACE

• CONTENTS

The purpose of the lectures reported in this article is to investigate the basic interactions that lead to processes by which excitation energy, initially localized in a particular constituent or restricted region of a material, transfers itself to other parts or degrees of freedom of the system. Energy transfer processes are relevant to such various and important fields as spectroscopy, laser technology, phosphors, artificial solar energy conversion, and photobiology.

The subject of the interactions among atoms is introduced by considering first the static and then the dynamic effects of these interactions in a two-atom system. Subsequently the different types of interactions (multipolar, exchange and electromagnetic) in a two-atom system are examined.

After reviewing the different modes of excitation of a solid containing both donors and acceptors, a statistical treatment of energy transfer is presented. Firstly the case of energy transfer without migration among donors, and secondly the case of energy transfer in the presence of such migration are considered.

An introduction to the description of energy transfer processes in condensed matter is given with special emphasis on the definition of the energy– and energy–current–density, relevant modes, coherent/incoherent and Markovian/non–Markovian dynamics.

In this paper different mechanisms for intra- and inter-molecular energy transfer in excited atoms or molecules are discussed and experimental techniques for their investigations are presented. These techniques which involve cw excitation as well as time resolved laser spectroscopy differ for vibrationally excited molecules in their electronic groundstate from those involving electronically excited states. The advantages and limitations of the experimental methods and the amount of information, extracted from them, are outlined.

No abstract received.

In a first part of this contribution we calculate the optical properties of matter using the classical approach of the model Lorentz-oscillators, derive then the dispersion relation of light in matter and introduce the concept of polaritons when going to the quantum mechanical description.

In the second chapter the polariton concept is applied to three-dimensional ordered and amorphous matter like crystals, gases or alloys. The resonances by which we replace the model Lorentz oscillator are exctions, optical phonons, plasmons as well as transitions in the atomic electron system of Na vapour or in ⁵⁷Fe nuclei.

The third chapter is devoted to systems of reduced dimensionality like exciton polaritons in semiconductor quantum wells, surface polaritons or cavity polaritons.

The dielectric mirrors used to form the cavity allow in the fourth chapter an easy introduction of the concept of photonic (or more precisely polaritonic) crystals and ~ bandstructures. This concept will be elucidated with several examples.

The contribution will close with a short conclusion and outlook.

Often we think about scattering processes in solids in terms of classical point-like particles that bounce against each other in instantaneous collisions. In particular the only time scale that appears in the corresponding mathematical description, e.g. in the Boltzmann-equation or in Fermi's golden rule, is the mean free time between collisions. This picture is obviously in contradiction with the quantum mechanical, i.e. the wave-like nature of the collective excitations, which obey the time-dependent Schrödinger equation. Quantum mechanics tells us that any interaction between collective excitations is nothing but an interference phenomenon, including obviously the scattering processes that are responsible for decoherence and 'dissipation'. Intuitively, we can think about the dynamics of such many-body systems in terms of a finite duration of scattering processes which can be described by non-Markovian relaxation. Here we give an introduction into this growing field of quantum kinetics in semiconductors.

Three main upconversion processes in rare-earth ion doped laser solids are presented. They are 1) Excited State Absorption, 2) Energy Transfer Upconversion, and 3) Photon Avalanche Upconversion. Upconversion occurs when luminescence or laser emission is initiated from a level that has energy higher than the pump photon energy. Specific examples of upconversion luminescence and upconversion lasers are given for each of the three processes.

This paper describes some of the laser development efforts that are ongoing in NASA Langley Research Center's Laser Systems Branch. The development efforts are aimed at meeting the solid state laser requirements for measuring atmospheric constituents from various platforms with the ultimate goal of deploying spaceborne instruments. Development of an ultraviolet source for ozone measurements is described, as well as, the advanced 2-micrometer eye-safe laser efforts for coherent winds and potentially DIAL measurements of carbon dioxide. Compositional tuning of a neodymium-garnet material is also presented. The 0.94 µm output being sought from this technique is ideal for making measurement of water vapor from space. An added benefit of the development of all of these laser sources is their capability to measure clouds and aerosols.

The energy transfer processes appearing in excited inorganic compounds have been widely recognized as an important item for the design of new systems with improved performances. Behind the theory that describes most of the possible observed phenomena, it is proposed in this paper to survey some of the difficulties that could appear during the experimental studies of excited inorganic materials. In the energy transfer processes, the major role is often attributed to dopant, but the presented examples, will emphasized how stoechiometry, defects (vacancies), impurities, excitons can strongly modify the expected sample response. In some cases the relations between the preparation mode and the existence of unexpected energy transfer processes will be highlighted.

The present concepts evolved in the context of research and development of artificial photosynthetic systems. Our biosphere depends totally on the action of a porphyrin dye, chlorophyll, for its continued existance, since all food resources find their origin in photosynthesis. Equally, for much of our energy resources we rely on the same process, present or past, as stored in fossil fuels. Naturally, therefore, when it comes to the molecular design of dyes for solar photochemical applications the reference to the porphyrins and similar organometallic complexes based on nitrogen ring structures as prototypes is obvious. However, although nature confines itself to magnesium and iron for its principal pigments, chlorophyll and haemoglobin respectively, the synthetic chemist can access the whole range of metallic elements. The use of ruthenium pyridyl complexes has almost thirty years of development history, and although other compounds have been assessed, such as zinc porphyrins and even prussian-blue analogues, the most suitable dyes today are still modifications of the ruthenium-based pyridyl complexes. The molecular engineering of dyes extends the visible spectrum response, enhances stability and promotes chemisorption to oxide semiconductor substrates while maintaining the energetics and kinetics for efficient charge transfer to function in sensitised electrochemical photovoltaic devices. There is also an overview of the present status of the technology, the materials incorporated in current devices, and their reliability in practical applications especially in situations of thermal stress. The conclusion will present the case for ongoing development of dye-sensitised systems in photovoltaic technology.

Dephasing processes in the optical transitions in low-dimensional semiconductor nanostructures are studied by a novel technique involving a speckle analysis of the secondary emission after a short-pulse resonant excitation. The results are compared with more conventional transient four-wave mixing experiments. A special heterodyne detection technique has made it possible to perform the latter on self-assembled semiconductor quantum dots embedded in a semiconductor optical amplifier.

Energy transfer is examined from a quantum mechanical point of view. Energy transfer parameters, such as self quenching and diffusion, are calculated given some basic spectroscopic parameters, specifically the energy levels of a lanthanide series atoms. When summing over all of the possible positions for the atoms participating in the energy transfer, the importance of having a nearest similar neighbor was observed. Consequently, the participating atoms were divided into two groups; those without a nearest similar neighbor, singlets, and those with a nearest similar neighbor, doublets. Different decay times could be assigned to each type. A series of self quenching experiments were performed with both Nd and Tm in YAG and YLF and the decay times were measured. Results of the experiments support the analysis.

Optical films doped by semiconductor nanocrystals and semiconductor nanocrystals with Eu⁺³ and Tb⁺³ were prepared by the sol-gel method. The newly developed techniques for preparation of thin glass films of silica and zirconia doped by nanoparticles of CdS, CdSe are described. Intensification of rare earths (Eu⁺³, Tb⁺³) luminescence by their incorporation into semiconductor quantum dots of CdS is observed as compared to the same luminescence with the absence of quantum dots. The prepared materials were characterized by the optical absorption, luminescence and X-ray diffraction. The sizes of the particles varied from few tens to hundreds of angstroms depending on parameters of thermal treatment and precursor composition. The deviation from the average size in each case is about 10%. Quantum size effect is demonstrated.

Color Centers in alkali halides are a well known class of point defects, whose optical properties have been studied carefully since long times, in particular their efficient emissions after optical excitation in the absorption bands. However, there are a certain number of intrinsic and extrinsic factors which can greatly influence the emissions up to a complete absence of them. In general these phenomena, which are often referred to as quenching effects, occur during the optical cycle when energy exchanges between different types of color centers, and color centers and lattice vibrations of crystals, take place. Among them, the decrease of the quantum efficiency on the density number of color centers, i.e. concentration quenching, occupies a special place both for basic and applied research. In this lecture, the historical background of the emission and emission quenching of the F center will be reviewed with emphasis on the concentration quenching. Afterwards, the luminescent properties of color centers, like F_A, , will be described in details, and it will result that, while the concentration quenching effect of the F center is enough well understood, this is not the case of the previous more complex centers. Indeed, the energy transfer phenomena which decrease the luminescence intensity are not well known yet, an aspect still awaiting to be unraveled both from the experimental and theoretical point of view.

In this contribution, mechanisms leading to energy transfer between ions in solid materials and the importance of energy transfer processes for the application of luminescent materials are discussed.

The main objective of the lecture is to investigate the energy transfer processes among dopant ions in inorganic crystals by both four-wave mixing spectroscopy and fluorescence dynamics techniques. Four-wave mixing has been demonstrated to be an effective technique for characterizing long range energy migration and optical dephazing phenomena. In addition, measurements of the fluorescence decay dynamics of active ions are used to characterize the total energy transfer properties between sensitiser and activator ions.

The set-up used in these experiments will be described in the approach of the production of the laser-induced grating signal in solids. Two nonlinear laser beams are focused inside the sample and the path difference is adjusted in such a way that the two beams interfere creating a sinusoidal intensity pattern. Since the wavelength of the laser beam is resonant with an absorption transition of the active ions, optical absorption creates a similar spatial sinusoidal distribution of the excited ions that acts like a population grating. The laser-induced grating signal carries all of the informations about the physical processes influencing the population grating. Signal strengths and decay times are measured as functions of laser-beam crossing angles and temperature.

We shall show the examples of two different activator ions in Garnets: Cr³⁺ transition metal ion and Nd³⁺ rare earth ion which are characterized by two kinds of spectroscopy. The results will be discussed.

As for other fluorescence light emitters, Rare-Earth (RE)-doped solids usually follow two well known principles:

i) Emission takes place generally through both incoherent spontaneous emission and coherent stimulated emission with a ratio between them given by the statistical approach of Einstein. In absence of population inversion, incoherent spontaneous emission normally overcomes the stimulated one.

ii) Both type of above emission obey Stokes law stating that excitation photons are at higher energy than emitted photons.

In the following lecture, we want to discuss what we shall call "unconventional cases" where at least one of the two above principle is not respected. It shall be shown that RE-doped solids may deviate rather easily from above principle giving rise under strong excitation density to unconventional emissions of the coherent spontaneous or of the anti-Stokes types.

In a first part, the conditions necessary for observation of coherent spontaneous emission are discussed. Comparison with stimulated emission are described in order to discriminate both types one from the other. Recent results obtained on RE-doped crystalline powders are presented as examples and discussed in terms of the coherent back scattering effect.

In a second part, anti-Stokes emissions in RE-doped solids are finally described with emphasis on the distinction between cooperative and energy transfer effects.

No abstract received.

The present status of physics is compared with that at the end of the 19th century. A general issue seconded by mass media is that all fundamental laws of Nature have been discoverd, so that we are not far from the theory of everything, in terms of which all phenomena can be understood. Is this the end of fundamental science? We shall show by specific examples in particle physics and astrophysics that we are far from this presumption; in fact, many fundamental questions are still open and the present theoretical models cannot be tested completely, either because the experimental devices are not available, or because the appropriate mathematical tools are not known.

No abstract received.

In semiconductors, localized vibrational mode spectroscopy has been used successfully in identifying the sites occupied by intrinsic defects, light impurities and complexes formed by them. The measurements are most valuable when combined with other complementary techniques. We present an overview of both theoretical and experimental requirements for the observation of localized vibrational modes of impurities and defects. Our discussion is restricted to the infrared absorption and Raman scattering techniques with emphasis on the characteristics of modes caused by light impurities such as beryllium-, boron-, silicon-, and carbon- in GaAs and AlAs grown by molecular beam epitaxy (MBE). Direct evidence for C_As acceptors, and the pairing of intrinsic defects with Si and Be in MBE grown GaAs at low temperatures is demonstrated. A theoretical treatment based on Greens 0function method is illustrated by the calculations of isotopic fine structure of B_As lines in GaAs and the modes of nearest-neighbor and second nearest-neighbor pair defects in GaAs, AlAs and Al_xGa_1-xAs.

The general non-radiative energy transfer coupled equations are solved for the case of doped crystalline materials. In this work the exact solution is reported and the microscopic and macroscopic dopant, donor and acceptor, emissions are calculated. Our calculations are compared to major models such as the Forster-Dexter, Yokota-Tanimoto and Burshtein. It is shown that our modeling unifies and extends the predictions obtained by those authors. Our calculations consider the discreteness of the crystalline materials, and more than one interaction can be assumed to drive each of the processes that are taking place among pair of dopants.

Pure and doped optical waveguides are of interest for integrated optics applications. Furthermore, fundamental studies about structural and optical properties of materials in thin films form take advantage of both the guiding of the light and the fluorescence properties of the dopants…

The beginning of the 1990's saw the advent of the rare-earth doped fiber amplifier in optical communication systems. Erbium-doped fiber amplifiers, which operate in the important 1.55um third telecommunication window, appeared as the ideal medium: low loss, high gain and possibility of multiplexing, etc.

Nowadays, the increasing needs (Internet, cellular network, etc.) impose a new step in performances and more specifically in the gain bandwidth, to permit the amplification of more than one hundred signals simultaneously in only one fiber.

Using data from literature or supplied by Alcatel, we study some relevant parameters that have to be taken into account in view of the expected improvement, and what are the relevant techniques that permit their study.

In this century, luminescence of rare earth ions has been well studied. Since the application of luminescence from rare earth ions in fluorescent tubes, colour television and X-ray phosphors, numerous papers have appeared on 4fⁿ-4fⁿ and 4f^n-15d-4fⁿ emissions. One kind of rare earth luminescence is still relatively unknown: charge transfer luminescence. This transition is the reverse of the well-known charge transfer absorption. Until now only three papers have reported on luminescence from the charge transfer state (CTS) of a rare earth ion [2, 3, 4] although the CTS is important for applications. For example in the red phosphor used in fluorescent tubes (Y₂O₃:Eu³⁺), UV radiation from the Hg-discharge is efficiently absorbed in the CTS of the Eu³⁺ - ion. After nonradiative decay to the lower ⁵D_J states, luminescence occurs. In Eu³⁺ luminescence from the CTS can not be observed, because there will always be fast relaxation to the lower 4f levels…

To improve the contrast of a TV screen, not only the phosphor brightness, but also the daylight reflectivity of a screen has to be considered. The relation of contrast on the one hand and luminance, as well as reflectivity on the other hand, can be quantified by the Luminance-Contrast-Performance (LCP):

In order to reduce the reflectivity of the white phosphor powders (e.g. ZnS:Ag,Al, Y₂O₂S:Eu), each individual phosphor particle is covered with a pigment. The pigment colour should correspond to the emission of the respective type of phosphor, for instance, CoAl₂O₄ for blue emitting ZnS:Ag,Al or Fe₂O₃ for for red emitting Y₂O₂S:Eu.

The pigment particles (e.g. CoAl₂O₄, Fe₂O₃) have to be quite small in size (80 to 120 nm) to guarantee a sufficient adhesion on the phosphor particles. In addition, other ingredients like inorganic oxides or organic polymers can be required to achieve a good adhesion and to establish a homogeneous thin layer of pigment particles on the phosphor surface.

Almost all PDP manufacturers use (Y,Gd)BO₃:Eu as red primary because it has a high efficiency under VUV excitation. A drawback of this phosphor is that the orange colour point for appropriate video operation is too large. This is caused by the relatively intense line at 594 nm, which corresponds to the magnetic dipole transition ⁵D₀→⁷F₁ while the emission lines of the electric dipole transition ⁵D₀→⁷F₂ at 612 and 627 nm are weaker…

Organic materials are suitable candidates for nonlinear optics (NLO), because of their synthetic diversity, ease of fabrication and low cost. Second-order NLO materials, used for sum-frequency generation and electro-optical switching, often consist of NLO chromophores which are oriented with respect to each other in a fashion to avoid centrosymmetry. In this study, wedge-shaped amino-acid based dendnmers are used as a backbone for NLO-molecules. Since they are synthezised via the convergent method, the periphery can be functionalized with different NLO chromophores having a controlled mutual orientation. The second-order NLO properties of several generations of different types of dendnmers are investigated using hyper-Rayleigh scattering in solution.

For high speed optical communication system with data speed higher than 200 Gb/s sub-picosecond pulse dynamics, coherent effects become important.

We have, experimentally and theoretically, investigated the pulse distortion of a 150 fs pulse due to amplification in a 250 um long InGaAsP ridge waveguide, working at 1.52 mm, for different input pulse energies…

Semiconductor quantum dots ("solid-state atoms") are promising candidates for quantum computers and future electronic and optoelectronic devices. Quantum dots are zero-dimensional electronic systems and therefore have discrete energy levels, similar to atoms or molecules. The size distribution of quantum dots, however, results in a large inhomogeneous broadening of quantum dot spectra.

Work on self-assembled InGaAs/GaAs quantum dots will be presented. Properties of atom-like single-dot states are investigated optically using high spatial and spectral resolution. Single-dot spectra can be used to probe coherence times of exciton states and relaxation processes, both of which are important for future applications.

In 1994 Bhargava et al. [1] reported that nanocrystalline ZnS:Mn²⁺ can yield both high luminescence efficiencies and a decay time shortening from ms to ns at the same time. These spectacular results suggested that doped semiconductor nanoparticles form a new class of luminescent materials with a wide range of applications in, for example, displays, sensors and lasers…

We consider I₂ molecules isolated in a Kr-matrix as a model system for condensed phase dynamics. The I₂. molecule is weakly coupled to the surrounding Kr-lattice serving as a bath held at 15 K…

Recently we have started a project to study the dynamics of polymeric electrolyte materials. We Will concentrate on the system of polyethylene oxide), CH₃O(CH₂CH₂O)_nCH₃ with dissolved LiCF₃SO₃ (lithium triflate, LiTf). In particular we are pumping these materials with a selective heat pulse in the form of an intense infrared pulse tuned to the n(CH)-stretch region near 2900 cm^-1 based optical parametric oscillator (OPO) system with wavelength extension. We will then observe changes in absorption of the system in the entire infrared region to determine the energy transfer dynamics of this system using step-scan Fourier-transform time-resolved infrared spectroscopy with a 10 ns resolution. In particular we will be observing the triflate bands at 750 cm^-1 to see changes in coordination between the triflate- ions and Li⁺ ions. We will also be concentrating on conformational changes observed in the PEO bands from 770-910 cm^-1 These studies will be greatly facilitated by our extensive previous studies of the PEO-LiTf system including smaller PEO oligomers [Freeh 1994]. We are particularly interested in studying conformational changes in the polymeric system which are induced by the infrared pulse and the resulting changes in ionic conductivity of the Li⁺ ions.

A detailed study of upconversion processes in Cs₃Er₂X₉ (X = Cl, Br, I) crystals and in the diluted systems Cs₃Lu₂Cl₉ : 1% Er³⁺, and Cs₃Y₂l₉ : 1% Er³⁺ is presented. Efficient two-, three-, and four-step upconversion excitation along the sequence ⁴I_15/2 → ⁴I_13/2 → ⁴I_9/2 → ⁴S_3/2 → ²H_9/2, leading to luminescence throughout the visible and near-ultraviolet is demonstrated using a 1.54 mm excitation wavelength. This stepwise excitation is possible due to the low phonon energies and, consequently, the significantly longer lifetime of the ⁴l_9/2 intermediate state in these systems relative to oxides and fluorides. The absorption and upconversion luminescence intensities increase along the isostructural series X= Cl, BR, I as a result of the decreasing energy of the electric-dipole allowed 4f-5d transitions and, thus, their increasing influence on the parity forbidden 4f-4f transitions. The excitation mechanisms in the chloride systems are investigated by time-resolved spectroscopy and the respective dynamics is studied by a rate-equation model. In the diluted sample ⁴I_9/2 → ⁴S_3/2 excited-state absorption plays a major role and occurs within 3 cm^-1 of the ground-state absorption, whereas the dynamics in the concentrated system is dominated by energy-transfer upconversion in all excitation steps. Of the 35 most likely energy-transfer upconversion processes, eight are found to contribute significantly to the excitation mechanisms in the concentrated system.

For applications like mercury-free fluorescent lamps and plasma display panels, new VUV phosphors are needed to convert VUV radiation into visible light. To enhance the efficiency of VUV phosphors 'quantum cutters' can be used: a quantum cutter has a quantum efficiency higher than 100%. Some rare earth ions are known to have such properties, but losses in the IR and/or UV parts of the spectrum are too large to use them for practical purposes…

We investigate the lattice dynamics of cubic CdS/ZnSe superlattices (SLs) by means of Raman scattering and Far-Infrared spectroscopy. This superlattices are the special interest because it is one of a few superlattice systems formed of II-VI semiconductor materials with no common anion ore cation, also called AB/CD superlattices. The superlattices of usually 200 periods have been grown on a GaAs(001) substrate by molecular-beam epitaxy using CdS and ZnSe compound sources…

II-VI semiconductors provide an excellent system for studying exciton-exciton (X-X) interaction. In particular, ZnSe quantum wells with a quarternary ZnMgSSe barrier are interesting because both electrons and holes are confined in the well leading to a very large exciton binding energy. We are investigating multiple quantum wells with wellwidths in the range of 5-8 nm…

II-VI semiconductors provide an excellent system for studying exciton-exciton (X-X) interaction. In particular, ZnSe quantum wells with a quarternary ZnMgSSe barrier are interesting because both electrons and holes are confined in the well leading to a very large exciton binding energy. We are investigating multiple quantum wells with wellwidths in the range of 5-8 run…

Fundamental research on nanocrystalline semiconductors in the past 20 years have yielded very interesting fundamental results, for example on quantum size effects. In many articles potential applications are mentioned, but until now no applications have been realized. A possible application is to use luminescent nanoparticles in electroluminescent devices. For this application efficiently luminescent nanoparticles are required…

For the past few years we have been studying the transport properties of Si metaloxide-semiconductor field-effect transistors (MOSFETs) which have unusually high mobilities (up to 70,000 cm²/Vs) Surprisingly, in this system we observe clear evidence for a metal insulator transition at temperatures below 1 K. This transition is occurs at a critical 2D-electron density (n) of around n_c = 1011 cm^-2…

Recently, a lot of attention has been paid to the investigation of modifications in the spontaneously emitted radiation inside small-sized resonators. Starting from the first structures working in the millimetric and infrared ranges, the interest has turned towards the scale of optical wavelengths, which is widely used in the field of communications and optoelectronics…

It has been demonstrated recently that the solid-state tensile deformation of flexiblecoil polymer matrices leads to excellent uniaxial orientation of embedded rodlike conjugated polymer molecules. Due to their highly polarized fluorescence, these binary blend films are interesting candidates for applications in efficient display devices. We studied such systems at the level of single conjugated guest molecules by means of roomtemperature Scanning Confocal Optical Microscopy…

We have investigated the process of fluorescence quenching from the ¹D₂ state of Pr³⁺ ions in fluoride and oxide crystals and in fluorophosphate glasses due to energy transfer among Pr³⁺ ions. The fluorescence of the ¹D₂ level shows a strong concentration quenching even at moderate Pr³⁺ concentrations. The time evolution of the decays from the state is consistent with a dipole-dipole energy transfer mechanism. Anti-Stokes emission from the ³P₀ level following excitation of the ¹D₂ state is also studied for different Pr³⁺ concentrations. Analysis of the upconverted fluorescence decays supports that the mechanism responsible for the upconversion process is energy transfer. However, this process seems to be complex enough to allow for the use of a single model which could explain the behavior observed.

Introduction of molecular order into the active layer of an organic light-emitting diode (OLED) is a promising approach to control luminescence as well as charge transport properties. For instance charge carrier transport is very sensitive to the degree of molecular order, packing and orientation, and mobilities can be considerably enhanced by proper molecular engineering. Organic molecular beam deposition (OMBD) has been shown to be a very powerful tool to build structures with well-defined molecular order if the deposition conditions are properly chosen. Especially the choice of the substrate, substrate temperature and deposition rate is crucial…

It has been known for some time that amorphous solids exhibit anomalous behaviors when compared with their crystalline counterparts. Among the various models proposed to explain these anomalous behaviors, it is worth pointing out the Soft Potential Model (SPM), which gives a unified description of these properties in terms of soft anharmonic potentials, the so-called Soft Potentials…

The achievement of full color displays is one of the main challenging tasks on the field of luminescent materials, such as the stable and efficient white photoluminescence sol-gel based silicates and the set of white light emission complexes based on lightemitting diodes recently reported…

GaN, its allows, QWs and MQWs have gained an important place among shortwavelength optical emitters and high temperature electronic devices. A major obstacle for the realisation of such devices from Metal-Organic Vapour Phase Epitaxy (MOVPE) grow GaN film is an apparent difficulty to achieve p-type material due to highly n-type residual conductivity. Several groups reported their ability to dope GaN using Mg as a dopant, in nitrogen environment to active Mg dopants. A strong blue emission is present when GaN films are heavily doped with Mg.

Controlled n-type conductivity can be obtained using Si as dopant. It is now that Si replaces Ga in the lattice and acts as a single donor. Besides the near-band-gap emission centred at 3.461 eV, which becomes broader with increasing Si concentration also transitions at lower energies are observed.

In the present work we analyze these emissions by time-resolved spectroscopy and steady-state spectroscopy.

In rare-earth doped compounds the non-linear optical behavior "photon avalanche" has been observed ^[11]. For the first time in a transition-metal doped compound the phenomenon has been found in CsCdCl₃:Ni^{2+ [12]}…

Although the optical properties of ZnO bulk crystals are well understood, there has been a great interest to study the optical properties of thin ZnO films, because they reveal novel luminescence properties and show lasing at room-temperature at moderate excitation intensities…

Chlorophyll a is the major pigment in higher plant photosynthesis, being responsible for both light absorption and light induced charge separation. The photoelectrochemical behavior of chlorophyll a species P740 (polymerized water adduct of chlorophyll a, absorbing at 740nm) was investigated using chlorophyll a thin films, electrodeposited on both Sn02 and Sn02/Ti02 electrodes. Anodic photocurrents have been observed at both Sn02 and Sn02/Ti02 electrodes with chlorophyll a film electrodeposited, under potential-controlled condition…

• FIRST ROUNDTABLE DISCUSSION

• SECOND ROUNDTABLE DISCUSSION

• SUMMARY OF THE COURSE (C. Klingshim)

• LIST OF PARTICIPANTS

• INDEX