A. Minafra

Optical spectra and photoluminescence of C60 thin films

Abstract We report on transmission and reflectance spectra of fullerite C60 thin films deposited on quartz substrate. An Urbach tail of states is present below the absorption edge due to the disordered structure of the films. Photoluminescence (PL) spectra have been measured from 10 to 300 K. Recombination of self-trapped excitons and their phonon replicas are present in the PL spectra. Temperature dependence of the integrated intensity of each emission line is also reported.

Optical characterization of fullerite C60 thin films

Abstract Transmission and reflectance spectra of fullerite C 60 thin films deposited on quartz and Si substrates are presented. An Urbach tail of states is present below the absorption edge due to the disordered structure of films. Photoluminescence (PL) spectra have been measured from 10 to 300 K. The recombination line of a self-trapped exciton polaron and its phonon replicas are present in the PL spectra.

UV spectra of graphite microparticles produced by laser vaporization

Abstract Submicronic graphitic particles produced by laser vaporization of a bulk graphite target and sublimated on LiF substrates are investigated. Transmission Electron Microscopy analysis denotes the presence of particles ranging from small clusters to submicronic length scale. Optical absorption and reflectance spectra in the wavelength range from 100 to 2000 nm show that the main contribution to the observed optical features comes from surface plasmon resonance of π-electrons and from interband transitions of σ-electrons of the graphite grains. Interferential effects of light due to the granular structure of the films are also present in transmission spectra. Energy shift of the spectral position of the plasmon resonance is observed either in transmission or in reflectance spectra. This shift is due to a size effect of the graphite particles.

Influence of the substrate temperature on the photoluminescence of C60 films

Abstract The optical and morphological properties of C 60 films deposited on Si(111) substrate at different temperatures are investigated. The photoluminescence intensity is enhanced in the case of larger deposition temperature of the substrate, because of the better film homogeneity.

Photocurrent spectroscopy in n-i-n GaAsAl.3Ga.7As short period superlattice

Abstract Photocurrent spectra of a 50 A period superlattice (SL) of GaAs Al .3 Ga .7 As , were measured in the range 14–300 K in a low electric field regime. The energy gap between the heavy-hole and electron extended states of the SL shows the same dependence on temperature as its bulk components. The decrease of photocurrent in the range 100–300 K indicates transport in the extended states of the SL. Different photo-ionization of DX centres existing in the n-doped layers of Al.3Ga.7As causes an effective asymmetry in the n-i-n structure.

Noise dependence of correlation and structure functions on mean count rate and sampling time

Experimental results for the noise of photon-correlation and photon-structure functions are presented. The noise dependences on the mean count rate and on the sampling time are investigated by means of a specifically built linear fluctuation analyzer, and the results are compared with theoretical predictions. The predicted mean count rate dependence and sampling time dependence of the noise are confirmed by the experimental analysis. Some discrepancies between analytical expressions and experimental results are observed in the crossover predictions. The results show that the structure function has better noise performance for relatively high mean count rates and for long sampling times, which strongly suggests a wider use of this statistical estimator in measurements requiring good accuracy, as in the case of uniform sampling.

A low cost correlator employing a personal computer

The intention of this paper is to describe the design of a 16‐bit correlation setup employing a personal computer (PC) used in intensity fluctuation spectroscopy. It is based on a hardware acquisition board which counts the photons detected in the selected sampling time. The calculation of the correlation coefficients is achieved with a PC by means of a suitable software program. This arrangement permits one to obtain various kinds of information from the recorded data, i.e., it is possible to evaluate other statistical estimators instead of the usual correlation function. Experimental tests of the apparatus have been obtained by measuring the light scattered by a liquid suspension of polystyrene latex spheres.

Strong Localization of Light in Spongy Agglimerates of Carbon Grains

Ultraviolet extinction spectra of spongy agglomerates of amorphous carbon grains have been measured in the spectral range 100-300 nm. These spectra present two dips which can be explained in terms of strong localization of light in a disordered medium. The particular spongy structure of the carbon grains samples determines random fluctuations of the refraction index. When a light beam crosses samples having a granular structure, the fluctuatuions of n produce optical resonant modes which are localized along the disordered medium. Therefore, the transmission is enhanced for particular resonant frequences corresponding to the positive interferences of the light within an agglomeration of carbon grains having, in our experiment, a width of about 100 nm.

Anomalous dips in extinction spectra of disordered carbon grains

Abstract Ultraviolet extinction spectra of spongy agglomerates of amorphous carbon grains have been measured in the spectral range 100–300 nm. These spectra present two dips which can be explained in terms of harmonic resonant modes of electromagnetic waves in a disordered medium. This effect is due to the random fluctuations of the refraction index of our samples which have a disordered granular structure.

Photoluminescence of GaSe(Cu) doped by ion implantation

Abstract Photoluminescence spectra of GaSe doped with Cu atoms by the ion implantation technique are reported. They are similar to spectra of GaSe doped by chemical methods. The analysis of spectra of crystals doped with different Cu concentrations confirms the defect level structure of GaSe(Cu) and also provides further evidence supporting the recombination kinetic model which explains the excitation intensity and temperature dependence of indirect excitonic lines.