arXiv subjects

Roberto Boscaino

Publications and source records attributed to Roberto Boscaino.

4 recordsLinked to original sources

arXiv2010

Evidence of delocalized excitons in amorphous solids

We studied the temperature dependence of the absorption coefficient of amorphous SiO$_2$ in the range from 8 to 17.5~eV obtained by Kramers-Kronig dispersion analysis of reflectivity spectra. We demonstrate the main excitonic resonance at 10.4~eV to feature a close Lorentzian shape red-shifting with increasing temperature. This provides a strong evidence of excitons being delocalized notwithstanding the structural disorder intrinsic to the amorphous system. Excitons turn out to be coupled to an average phonon mode of 83~meV energy.

cond-mat.mtrl-sci↗

arXiv2008

Photoluminescence dispersion as a probe of structural inhomogeneity in silica

We report time-resolved photoluminescence spectra of point defects in amorphous silicon dioxide (silica), in particular the decay kinetics of the emission signals of extrinsic Oxygen Deficient Centres of the second type from singlet and directly-excited triplet states are measured and used as a probe of structural inhomogeneity. Luminescence activity in sapphire ($α$-Al$_2$O$_3$) is studied as well and used as a model system to compare the optical properties of defects in silica with those of defects embedded in a crystalline matrix. Only for defects in silica, we observe a variation of the decay lifetimes with emission energy and a time dependence of the first moment of the emission bands. These features are analyzed within a theoretical model with explicit hypothesis about the effect introduced by the disorder of vitreous systems. Separate estimations of the homogenous and inhomogeneous contributions to the measured emission linewidth are obtained: it is found that inhomogeneous effects strongly condition both the triplet and singlet luminescence activities of oxygen deficient centres in silica, although the degree of inhomogeneity of the triplet emission turns out to be lower than that of the singlet emission. Inhomogeneous effects appear to be negligible in sapphire.

cond-mat.dis-nn↗

arXiv2008

Isoelectronic series of oxygen deficient centers in silica: experimental estimation of homogeneous and inhomogeneous spectral widths

We report nanosecond time-resolved photoluminescence measurements on the isoelectronic series of oxygen deficient centers in amorphous silica, Si-ODC(II), Ge-ODC(II) and Sn-ODC(II) which are responsible of fluorescence activities at $\sim$4 eV under excitation at $\sim$5 eV. The dependence of the first moment of their emission band on time, and that of the radiative decay lifetime on emission energy are analyzed within a theoretical model able to describe the effects introduced by disorder on the optical properties of the defects. We obtain separate estimates of the homogeneous and inhomogeneous contributions to the measured emission linewidth and we derive homogeneous spectroscopic features of the investigated point defects (Huangh-Rhys factor, homogeneous width, oscillator strength, vibrational frequency). The results point to a picture in which an oxygen deficient center localized on a heavier atom features a higher degree of inhomogeneity due to stronger local distortion of the surrounding matrix. For Si, Ge, Sn related defects the parameter $λ$, able to quantify inhomogeneity, results to be 65, 78 and 90$%, respectively.

cond-mat.dis-nn↗

arXiv2008

Homogeneous and inhomogeneous contributions to the luminescence linewidth of point defects in amorphous solids: Quantitative assessment based on time-resolved emission spectroscopy

The article describes an experimental method that allows to estimate the inhomogeneous and homogeneous linewidths of the photoluminescence band of a point defect in an amorphous solid. We performed low temperature time-resolved luminescence measurements on two defects chosen as model systems for our analysis: extrinsic Oxygen Deficient Centers (ODC(II)) in amorphous silica and F+ 3 centers in crystalline Lithium Fluoride. Measurements evidence that only defects embedded in the amorphous matrix feature a dependence of the radiative decay lifetime on the emission energy and a time dependence of the first moment of the emission band. A theoretical model is developed to link these properties to the structural disorder typical of amorphous solids. Specifically, the observations on ODC(II) are interpreted by introducing a gaussian statistical distribution of the zero phonon line energy position. Comparison with the results obtained on F+ 3 crystalline defects strongly confirms the validity of the model. By analyzing experimental data within this frame, we obtain separate estimations of the homogenous and inhomogeneous contributions to the measured total linewidth of ODC(II), which results to be mostly inhomogeneous.

cond-mat.dis-nn↗