Luminescence, induced absorption and degenerate four-wave mixing experiments are perform on GaN epilayers grown on a sapphire substrate by MOCVD. We measure the nonlinear behavior of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I2 line due to the finite donor density in the sample. Higher intensities of excitation induce collision processes between photo-created quasi-particles. Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, transmission spectra show a red-shift of the absorption edge and of the excitonic resonance. Pulsed degenerate four-wave mixing experiments were performed using the third harmonic of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time constant of 16 ps has been measured, which is independent of the temperature, spacing of the interference fringe and of the intensity of the pump beams.

We proposed and studied a new light emitting material based on silicon nanocrystals (Si-nc). The new material was fabricated using a low cost way by incorporating Si-nc into a sol–gel derived SiO2 matrix. The Si-nc were prepared (i) by electrochemical etching of monocrystalline Si wafers and (ii) pulverizing the obtained porous silicon film. The porous silicon powder was then dispersed in the SiO2 sol. After solidification, we obtained transparent and self-supporting SiO2 layers of about 1 mm thickness containing Si nanocrystals. The sol–gel layers exhibit bright red photoluminescence (PL) under UV lamp excitation at room temperature. This novel method circumvents the usual Si+-implantation step. We present first basic experimental studies of the PL properties of these sol–gel derived SiO2 layers and compare them with that of as-prepared porous silicon. We mention also observation of a non-linear behavior in the PL spectra. We emphasize potential advantages of this technology compared to the standard Si+-implanted SiO2 layers.

Des couches minces epitaxiees de GaN ont ete deposees sur Al 2 O 3 (0001) par la technique d'ablation laser reactive en atmosphere d'azote. Une nitruration prealable du substrat s'est averee indispensable pour obtenir des couches epitaxiees. L'etat cristallin de ces couches minces a ete caracterise par diffraction de rayons X (XRD) et par diffraction d'electrons rasants (RHEED). On a etabli une correlation directe entre les figures RHEED observees et les profils de raies de diffraction X obtenus par la methode de la rocking curve. Cette correlation permet de suivre in-situ la croissance de la couche GaN et d'en controler la qualite cristalline.

We investigate the nonlinear absorption of C60-doped porous sol-gel glasses by single- and double-pulse pump–probe experiments. We find that the reverse saturable absorption (RSA) of these samples can be explained in the frame of a five level system as it is commonly used for C60 solutions. We observe a strong saturation of the RSA at high fluences, especially if the molecules are prepared in the triplet state. In a double-pulse pump–probe experiment we measure the triplet quantum yield for C60 solutions (0.8) and doped glasses (0.25) and the singlet and triplet absorption cross sections. In the first excited singlet state (S1) lifetimes of 1 ns and 65 ps are determined in solutions and glasses, respectively. We find that the dynamics of both depopulation processes, direct S1–S0 relaxation, and intersystem crossing is faster in glasses than in the case where C60 is in solution in a liquid aromatic solvent. We tentatively explain these findings by the absence of a stabilizing solvent and a perturbation of the molecular energy levels due to interaction with the solid glass matrix. We finally determine the lifetime of the triplet states in our glass samples to 2 μs.

A procedure to make optical quality thin films of Zn(x)Cd(1-x)Te by use ofvacuum evaporation of the ternary compound has been developed. Thestarting point was the preparation of the compound that was then usedas the source in a simple vacuum evaporation system. Thecharacteristics of a film containing 85% ZnTe (x =0.85) are presented. Electron microscope, atomic forcemicroscope, x-ray and optical spectral measurements were made. Theindex of refraction was determined at room temperature fromtransmittance measurements in the range of from 580 to 800 nm and wasfound to agree within 1% with values found by others for singlecrystals. We did this by assuming a Sellmeier equation and a knownindex of refraction at infinite wavelength. The calculation alsoyielded the roughness of the film.

This paper reports induced absorption in Cd1−xZnxTe thin films epitaxially grown by pulsed laser deposition on (100)-oriented GaAs substrate. The high optical nonlinearity is adjustable in the visible over a wide spectral region as the concentration x is varied.

A 47-year-old woman developed acute Guillain-Barré-syndrome (GBS) within one day, presenting tetraparesis (muscle strength 1/5 for the upper and 2/5 for the lower extremities) and respiratory failure. On day 5 a cardiac pacemaker was necessary due to severe bradycardia. Electrophysiological studies were performed on days 2, 3, 5, 6, 8, 12, 15, 30 and 40. Our initial findings revealed normal motor conduction velocities and normal spinal motor evoked potentials. However, neither F-waves nor cortical motor evoked potentials could be registered from the upper or lower extremities. The motor conduction of the median nerve remained normal until day 6. The compound motor action potential declined thereafter and disappeared by day 12, indicating axonal degeneration. No axonal degeneration occurred in the lower extremities. The cervical or lumbar magnetic stimulation excites nerve roots at the level of the foramen intervertebrale. Thus, our findings suggest a conduction block between the foramen intervertebrale and the point where the nerve roots enter the spinal cord.

In this paper measurements of induced absorption and its dynamics in C60-doped solid xerogel matrices are presented. The measurements are performed in a pump-test geometry using picosecond pulses generated from a frequency doubled modelocked Nd:YAG laser. An induced absorption leading to optical limiting was observed. Outstanding optical limiting qualities for single shot excitation were found. Furthermore, limiting dynamics are studied. A decrease of the initial laser-induced absorption with a time constant of 150 picoseconds was observed. This is attributed to a transition from excited singlet to triplet electronic states in the C60 molecule. A long-living component due to the resulting triplet state population was also observed.

Thin epilayers of Cd1-xZnxTe have been deposited on monocrystalline (001)GaAs substrates by a pulsed laser ablation technique. The Cd1-xZnxTe targets used were either commercial polycrystalline wafers or cold-pressed samples obtained by mixing the binary powders CdTe and ZnTe. The morphological and structural quality of these epilayers has been studied for different composition of the Zn content.

Epilayers of Cd1-xZnxTe on GaAs (001) substrates were grown by a pulsed laser deposition technique, using either polycrystalline or cold-pressed targets. The composition and structural quality of these epilayers were investigated by energy dispersive X-ray analysis, and X-ray diffraction. A large and widely tunable induced optical absorption has been observed in these CZT epilayers.

Des couches minces epitaxiees de Cd 1-x Zn x Te ont ete deposees sur des substrats orientes (001)GaAs par la technique d'ablation laser pulse. Les cibles de Cd 1-x Zn x Te de composition x variable utilisees etaient soit des blocs polycristallins ou soit des pastilles pressees a froid obtenues a partir du melange des poudres binaires CdTe et ZnTe. La qualite morphologique et structurale de ces films minces a ete etudiee pour differentes composition x en zinc.

We study the dephasing behaviour of neutral (X) and positively charged (X+) excitons in modulation doped CdTe/Cd0.69Mg0.23Zn0.08Te multiple quantum wells by means of spectrally resolved transient four-wave mixing. The photon echo traces show modulations due to quantum beats or polarisation interferences between both resonances with a period determined by their energy splitting. The spectral resolution of the four-wave mixing signal allows a separate determination of the dephasing times of the two quasiparticles.

Zusammenfassung Eine 47jährige Patientin entwickelte innerhalb eines Tages eine akute Polyradikulitis Guillain-Barré (GBS) mit einer rein motorischen Tetraparese (Kraftgrad Arme 1/5, Beine 2/5) und Beatmungspflichtigkeit. Am 5. Tag war sie tetraplegisch und wegen Bradykardien schrittmacherpflichtig. Bei dieser Patientin wurden elektrophysiologische Untersuchungen am 2., 3., 5., 6., 8., 12., 15., 30. und 40. Tag durchgeführt. Der initiale Befund zeigte eine unauffällige fraktionierte motorische NLG des N. medianus und eine normale zervikale Magnetstimulation bei ausgefallener Medianus-F-Welle sowie ausgefallener corticaler Magnetstimulation. Die gleiche Konstellation fand sich für den N. tibialis. Die Medianus-NLG blieb bis zum Tag 6 normal, danach reduzierte sich das CMAP bis zum kompletten Ausfall ab dem 12. Tag als Hinweis auf eine sekundäre axonale Degeneration. Dagegen änderte sich die elektrophysiologische Konstellation an der unteren Extremität nicht. Da die Exzitation von Nervenwurzeln durch die Magnetstimulation auf der Höhe des Foramen intervertebrale erfolgt, muß die Läsion bei der initialen Befundkonstellation zwischen Foramen intervertebrale und Eintritt der Nervenwurzel ins Rückenmark liegen.

We present measurements of induced absorption and its dynamics in C60-doped solid xerogel matrices. The measurements are performed in a pump-test geometry using picosecond pulses generated from a frequency doubled mode locked Nd:YAG laser at 532 nm. We observe an induced absorption leading to optical limiting. We find outstanding optical limiting qualities for single shot excitation but some degradation above a certain threshold intensity for repetitive pulses. Below this degradation threshold, the material is stable and its limiting dynamics is studied. A decrease of the initial laser-induced absorption with a time constant of 150 ps is observed, down to a very long-lived constant level. We attribute this dynamics of induced absorption to a transition from excited singlet to triplet electronic states in the C60 molecule.

Epitaxial layers of ZnSe and ZnS have been grown on GaAs (100) substrates by low-pressure metal-organic vapor phase epitaxy (MOVPE) and etched out from their substrates, The optical qualify of the layers has been investigated at 2 K by low-intensity reflection, transmission and photoluminescence measurements.At high excitation intensities, the near-band-edge luminescence has been determined in both ZnS and ZnSe samples. Exciton and biexciton contributions have been studied. (C) 1997 Elsevier Science S.A.

Native defects and some common dopants (Mg, Zn, and C) in cubic GaN and AIN are examined by means of ab initio theoretical calculations using two methods: i) the Green's function technique based on the linear muffin-tin orbital method in the atomic-spheres approximation; ii) a supercell approach in connection with the full-potential linear muffin-tin-orbital method.We apply the first method to look mainly at the energetic positions of the defect and impurity states in different charge states and their dependence on hydrostatic pressure.The second method allows us to study lattice relaxations.Whereas small relaxations are found near vacancies and substitutional Mg and Zn, the calculations predict large atomic displacements around antisrte defects and the substitutional carbon impurity on the cation site.

This paper reports observed induced absorption in bulk Cd1−xZnxTe compounds in pump and probe experiments. The results are compared to the numerical simulation of a three-level system. The high nonlinearities, attributed to impurity levels in Cd1−xZnxTe, are tunable over a wide spectral region.

In order to get further informations about the pathophysiology of parkinsonian gait, we examined the EMG-pattern of muscles of the lower extremity [tibialis anterior (TA), medial head of gastrocnemius (CC), vastus medialis (VM), biceps femoris (BF), iliopsoas (IP) and gluteus maximus (GM)] in 20 parkinsonian patients and 15 normal controls during normal gait on a treadmill. The different phases of a step were identified by pressure sensors. The begin of a step-cycle was defined by the increase of heel-pressure. The duration of the EMG-activity was given as percentage of the normalized step-cycle, and the amplitude of the EMG-activity was given as percentage of the maximal isometric activity. 14-16 step-cycles were averaged.