We study the limiting dynamics of C 60-doped porous sol-gel glasses by single- and double-pulse pump-probe experiments. We find that, contrary to crystalline films of C60, reverse saturable absorption (RSA) of these solid samples can be described by a 5-level model developed for C 60-solutions. We observe an ultrafast rise time, a large transmission-decrease at high fluence, a shortened singlet-state lifetime as compared to solutions, but nevertheless an important triplet yield. We explain differences with respect to solutions by the absence of stabilizing aromatic solvents and by the interaction of the amorphous environment with the molecules, broadening considerably the spectral features of RSA.

Persistent spectral holes (p-SHs) are induced in the Z1,2 excitonic absorption band of CuBr nanocrystals (NCs) in glass by selective excitation with nanosecond dye-laser pulses at low temperatures. The effect can be observed only in samples containing NCs with mean radius smaller than about 5 nm. The kinetics of p-SHs growth and the long-time relaxation (spontaneous hole-filling) after burning are studied. The burning process may be described as the first order dispersive reaction, which proceeds through a phonon-assisted tunneling between different excited states of the NC/matrix system. The same model apply in the backward reaction (hole-filling) but the tunneling takes place between different ground states of the NC/matrix system and the reaction rate is much lower. The photoproduct of persistent spectral hole-burning reaction is a charged NC which energy states are modified by the Stark effect. The quantum efficiency of burning reaction is quite high 4×10−3 (mean rate constant is 4×107 s−1) while the rate of spontaneous HF reaction is very low (0.5 s−1).

physica status solidi (a)Volume 167, Issue 1 p. R5-R6 Rapid Research Note Blue Electroluminescence from an SiO2 Film Highly Implanted with Si+ Ions P. Knápek, P. Knápek Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorB. Rezek, B. Rezek Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorD. Muller, D. Muller Laboratoire PHASE, UPR 292, CNRS, F-67037 Strasbourg Cedex, FranceSearch for more papers by this authorJ. J. Grob, J. J. Grob Laboratoire PHASE, UPR 292, CNRS, F-67037 Strasbourg Cedex, FranceSearch for more papers by this authorR. Lévy, R. Lévy IPCMS-GONLO, UMR 46, CNRS-ULP-ECPM, F-67037 Strasbourg Cedex, FranceSearch for more papers by this authorK. Luterová, K. Luterová Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorJ. Kočka, J. Kočka Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorI. Pelant, I. Pelant Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this author P. Knápek, P. Knápek Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorB. Rezek, B. Rezek Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorD. Muller, D. Muller Laboratoire PHASE, UPR 292, CNRS, F-67037 Strasbourg Cedex, FranceSearch for more papers by this authorJ. J. Grob, J. J. Grob Laboratoire PHASE, UPR 292, CNRS, F-67037 Strasbourg Cedex, FranceSearch for more papers by this authorR. Lévy, R. Lévy IPCMS-GONLO, UMR 46, CNRS-ULP-ECPM, F-67037 Strasbourg Cedex, FranceSearch for more papers by this authorK. Luterová, K. Luterová Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorJ. Kočka, J. Kočka Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this authorI. Pelant, I. Pelant Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, CZ-16253 Praha 6, The Czech RepublicSearch for more papers by this author First published: 29 January 1999 https://doi.org/10.1002/(SICI)1521-396X(199805)167:13.0.CO;2-YCitations: 12AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume167, Issue1May 1998Pages R5-R6 RelatedInformation

We have studied the stimulated emission from an optically pumped graded index separate confinement heterostructure, realized in the form of a metal-organic vapor-phase-epitaxy-grown single quantum well based on a wide-gap (ZnCd)Se semiconductor. The structure is composed of a central Zn0.78Cd0.22Se quantum well sandwiched between two thicker, zinc-rich (ZnCd)Se layers with a graded cadmium composition varying continuously and monotonously between 0% and 5%. The stimulated emission occurred at similar to 2.49 eV (T =8.5 K), being spectrally redshifted with increasing temperature and disappearing for T greater than or equal to 200 K. The optical gain has been measured using the variable stripe-length method, and values of the gain up to 620 cm(-1) have been achieved. High-resolution spectral studies of the stimulated emission have revealed a fine structure in the emission spectra originating from different localization sites for excitons. We identify the lasing mechanism as due to an inhomogeneously broadened system of localized excitons.

We report on gain profile and spectral composition of stimulated emission from an optically excited MOVPE GRINSCH ZnCdSe-ZnSe quantum well.

We determine dephasing times T2 in free standing porous silicon samples using nondegenerate four-wave mixing. Within the framework of an idealized inhomogeneously broadened two-level system, values of 20 fs for different porosities are obtained. Fast energy relaxation times are found in the range of 1.5–3.5 ps for 64%–73% porosities, respectively. They are attributed to carrier thermalization within the band tails, originating from porous silicon nanocrystallites and their surface states.

We report laser-induced transient grating experiments performed at room temperature on self-supporting p-type porous silicon films with different porosities. With this technique the diffusion of the photocarriers can be studied with a time resolution of some tens of picoseconds. The gratings were created by two interfering pulses of the second-harmonic radiation from an Nd3+:yttrium-aluminum-garnet YAG laser (2.3 eV) and their decay in time was studied by a time-delayed pulse of the fundamental YAG laser frequency (1.15 eV). The observed grating decay time is very fast (hundreds of picoseconds) and shortens with decreasing porosity. Diffusion constants D=45, 24, and 5 cm2 s−1 have been found for the porosities of 64%, 68%, and 73%, respectively. To explain these high values of D we consider a simple kinetic model which takes into account two different types of carriers, delocalized and trapped ones.

We have studied the stimulated emission of MOVPE-grown quantum wells of Zn0.78Cd0.22Se, sandwiched between two thicker, zinc-richer layers. The gain of these samples has been measured, as a function of temperature, using the variable strip-length method. Its mechanism has been shown to be the radiative recombination of an inhomogeneously broadened exciton system.

The authors report on the first experimental observation of transient laser-induced gratings in self-supporting porous silicon membranes with different porosities. The observed grating decay in time is very fast (hundreds of picoseconds) at room temperature and shortens both with decreasing porosity and grating period. A preliminary microscopic model for this behaviour is suggested. The third-order non-linear optical susceptibility χ(3) (532 nm) in porous Si was found to be χ(3) ≈ 7 × 10−11 e.s.u..

Samples of CdS doped silica thin films have been prepared with weight concentrations ranging from 0.5 to 20% (CdO/SiO2). Nanocrystal structure and size distribution are determined by Transmission Electron Microscopy (TEM) and image analysis. The mean size is tile same for all concentrations (4 nm) and is related to the matrix porosity. The linear absorption shows characteristic features of the excitonic level and the gap blue shift due to the quantum confinement. Non-linear optical properties are studied by Degenerate Four Waves Mixing (DFWM) to measure the third order susceptibility chi((3)) which is high (typically 10(-7) esu) and depends linearly of the volume fraction of the semiconductor particles.

Nanocrystallites of CdS of constant mean radius are embedded in various concentrations (up to 29%) in pure silica glass obtained by a sol-gel technique. Doped glass samples as well as thin glass layers deposited on microscope slides are prepared and structurally characterized by electron microscopy. The linear optical properties (absorption coefficient and index of refraction) of these CdS-doped glasses are measured as functions of the CdS volume fraction in the samples. Their third order nonlinear optical susceptibility is also determined by degenerate four-wave mixing experiments near resonance. The values obtained for samples with different CdS volume fraction are compared to those of bulk CdS platelets.

Bulk Cd0.13Zn0.87Te monocrystals are studied by nonlinear optical experiments at room temperature near their absorption edge. Time-resolved test and pump experiments allow us to determine separately the refractive index and absorption changes induced by the pump pulse by measuring the spatial intensity distribution of the test pulse.

The spectral and temporal behavior of absorption bleaching and of self-diffraction in ZnxCd1−xTe polycrystalline films with a nominal composition x=0.87 have been investigated. It has been found that the optical transmission increases 15 times at a wavelength of 575 nm for a laser light intensity of 0.2 GW/cm2. In a pump and test experiment, the buildup and the relaxation times T1 of the bleaching were measured. Their values, between 10 and 40 and 20 and 45 ps, respectively, depend on the wavelengths of the pump and the test beams. The saturation intensity Is was also determined (between 1.5 and 6 GW/cm2). This absorption saturation effect can be related to the high density of crystallite grain surface states as well as to other imperfections in the polycrystalline films studied. The high density of these states can explain the large linear absorption and the ultrafast relaxation of the photogenerated carriers. Then, in self-diffraction experiments, the phase relaxation time T2=(10±5) ps of the localized surface states has been determined. The long duration of T2 and the short energy lifetime T1 indicate that elastic scattering processes are highly suppressed in polycrystalline material when compared to monocrystals, while their coherence time is increased.

The biexciton luminescence and the optical gain due to CuCl quantum dots are presented for different confinement regimes under picosecond excitation at low temperature. Gain is observed in the spectral region below the Z3 exciton resonance and can be attributed to the induced radiative recombination of biexcitons into excitons. Its spectral shape shows, as the luminescence does, two bands called A and B which are separated by 9 meV. The temporal behavior of the gain is recorded and shows a fast (160 ps) and a slow (6 ns) component for both bands.

Photoluminescence spectra of ZnSe epitaxial layers grown by MOVPE on GaAs substrates are investigated at 4.2 K under high excitation using a XeCl excimer laser (308 nm). It is demonstrated by lineshape fitting that at the highest excitation densities (300 kW cm −2 – 3 MW cm −2 ) the emission spectra are due to an electron-hole plasma recombination, strongly influenced by stimulated emission. No emission band attributable to exciton-exciton inelastic scattering is found under these excitation conditions.