A. Serpi

RECOMBINATION IN HgGaInS4 SINGLE CRYSTALS

Abstract Photoelectronic processes in HgGaInS4 have been studied by photoconductivity, thermally stimulated conductivity and photoluminescence. An exponential distribution of traps with a slope of 23 meV/decade as well as a further electron trap system with an activation energy of 70 meV have been localized below the bottom of the conduction band. Radiative electron transitions are shown to occur mainly from exponentially distributed traps to an acceptor level characterized by an activation energy of 220 meV.

Fundamental optical constants of CdIn2S4

Abstract The reflectivity of CdIn2S4 has been measured at 300 K and 77 K in the 2–10 eV energy range. The analysis of the low temperature data by means of the Kramers Kronig relations led to the determination of the optical constants. The results have been compared with recent band structure calculations and a substantial agreement between theory and experiment has been found.

Ultraviolet excitation of photoluminescence of porous silica under vacuum conditions

Abstract The optical properties of porous sol–gel silica are investigated by measuring the absorbance spectrum in the vacuum ultraviolet energy range and the excited photoluminescence. The emission spectrum shows a luminescence with two maxima at about 3.5 eV and

Vacuum ultraviolet absorption spectrum of photorefractive Sn-doped silica fiber preforms

Vacuum ultraviolet (VUV) absorption data have been obtained up to 8.3 eV on Sn-doped silica fibre preforms. Measurements have been carried out before and after exposure to 248 nm radiation from KrF excimer laser. The absorption spectrum is composed of three structures peaking at about 4.9, 5.8 and 7 eV, with the absorption edge at about 8.2 eV. The main effect of 5 eV irradiation is the decrease of the spectral components at 4.9 and 7 eV, whereas a small increase of absorption intensity is only observed just below the band at 4.9 eV. According to the observed negative absorption changes in the whole region of point-defect bands one would expect a negative refractive index change, contrary to the positive change previously reported in optical fibres of the same composition. Structural modifications accompanying the defect photoconversion process are suggested to be responsible for positive refractive index changes.

Vacuum ultraviolet absorption of silica samples

Abstract Optical absorption analysis in the ultraviolet and vacuum ultraviolet region from 4.0 to 9.0 eV has been performed on different silica materials. We analysed natural and synthetic silica samples, both as grown and irradiated with different irradiations, neutrons, γ, and ultraviolet photons. Regardless of irradiation we always observed in the spectrum of irradiated samples the growth of the band at 5.8 eV, a band attributed to the E′ paramagnetic center. Comparing this absorption under different irradiation, we suggest the B2 and E absorption bands are not correlated and therefore cannot be assigned to the same defect.

Low temperature time resolved photoluminescence of the 3.1 and 4.2 eV emission bands in Ge-doped silica

Abstract Time resolved photoluminescence (PL) has been performed on Ge-doped silica preform in the temperature range 10 to 295 K. Under pulsed KrF laser (5 eV) the well known α and β emissions have been recorded at different delays from excitation. An accurate analysis of the time resolved spectra taken at different temperatures has shown the composite property of the two PL structures. At room temperature α components (α1, α2) are peaked at 4.09 and 4.26 eV with a decay time of about 10 ns. The peak energies of β1 and β2 components are calculated at 3.03 and 3.21 eV with lifetimes of 111 and 94 μs, respectively. As temperature is decreased, α1 and α2 display the normal behaviour increasing in intensity down to 125 K; on the contrary, in the same temperature range, β1 and β2 undergo a quenching of their intensities. Taking into account their mutual spectral characteristics, α1 has been correlated to β1 and α2 to β2. The two sets of emission bands are tentatively attributed to a single center stabilized in different environments of the glassy matrix.

Low temperature reflectivity and optical properties of red mercury iodide

Abstract The reflectivity of red HgI 2 in the spectral range of 2–10 eV at 100 K with E⊥c is studied and the optical constants are deduced by means of the Kramers and Kronig relations. It is evidenced that the valence band is split in two subbands. Furthermore reflectivity spectra on good natural surfaces at 15 K in the two polarizations have been measured, and a high energy excitonic series, characterized by a large value of the Rydberg, is found.

Excitation pattern of the blue emission in Ge-doped silica

Abstract The luminescence of Ge-doped glassy SiO 2 has been investigated at room temperature using a deuterium lamp as the excitation source in the ultraviolet and vacuum ultraviolet energy range. Two emission bands are observed around 3 and 4 eV as the excitation increases from 4.5 to 8 eV. The peak shift and bandwidth broadening of the emission around 3 eV indicate the overlapping of several bands. We carried out a Gaussian best-fit approach of the detected spectra and extracted the excitation spectra of three distinct emission: a 2.9 eV band with two excitation channels at 4.7 and 5.3 eV and no contribution above 6 eV and the well-known composite β-band (3.1 eV). The latter is resolved into two Gaussian components whose excitation spectra present three main excitations around 5.1, 6.5 and 7.3 eV.

Absorption spectrum of Ge-doped silica samples and fiber preforms in the vacuum ultraviolet region

We report absorption measurements in the vacuum ultraviolet (VUV) spectral region on Ge-doped silica samples grown through chemical vapor deposition (CVD) and fiber preforms grown through modified chemical vapor deposition (MCVD) with GeO2 doping concentration from 0.3 to 4.0 mol%. We observed in all spectra an absorption band at 5.15 eV and a structure at about 7.0 eV. We observed differences in the absorption spectrum between Ge-doped silica samples and fiber preforms in the shape and relative amplitude of optical absorption bands. The intensity of defect-related bands depends on the Ge concentration and distribution. Differences in Ge concentration and distribution are due to the preparation methods and growing conditions.

Photoelectronic transitions in CuGa2.5In2.5S8

Abstract Photoluminescent and photoconductive properties of CuGa2.5In2.5S8 in the temperature range 77–300 K are presented. The analysis of the optical response allows us to outline a scheme for the energy levels in the fundamental optical gap. Their role in the photoexcitation processes is also discussed in the framework of the proposed model.