R. Boscaino

ESR and PL centers induced by gamma rays in silica

We have studied the point defects created by γ irradiation in various types of commercial silica glasses, including both natural and synthetic samples, with different OH content, in the low dose regime (0.05–100 Mrad). We found that the growth rate of E′ centers depends strongly on the silica type, ranging from 2 × 1015 cm−3 Mrad−1 to 6 × 1017 cm−3 Mrad−1. Samples of natural silica are rather susceptible to γ ray exposure, as E′ concentration saturates (typically 5 × 1017 cm−3) for doses as low as a few Mrads. For both synthetic and natural samples, the radiation hardness is higher in wet than in dry silica. Moreover, we found a strict correlation between the concentration of E′ centers and the γ-induced absorption band at 5.8 eV. Finally, exposure to γ rays generates in all the samples a photoluminescence band at 4.4 eV, whose excitation spectrum has a maximum at 4.95 eV. This band exhibits a sublinear growth kinetics in all the investigated samples.

Luminescence of γ-radiation-induced defects in α-quartz

Optical transitions associated with γ-radiation-induced defects in crystalline α-quartz were investigated by photoluminescence excited by both pulsed synchrotron radiation and steady-state light. After a 10 MGy γ-dose we observed two emissions at 4.9 eV (ultraviolet band) and 2.7 eV (blue band) excitable in the range of the induced absorption band at 7.6 eV. These two luminescence bands show a different temperature dependence: the ultraviolet band becomes bright below 80 K; the blue band increases below 180 K, but drops down below 80 K. Both emissions decay in a timescale of a few ns under pulsed excitation, however the blue band could also be observed in slow recombination processes and it afterglows in about 100 s at the end of steady-state excitation. The origin of the observed luminescence bands and the comparison with optical features of oxygen-deficient centres in silica glass are discussed in the framework of different models proposed in the literature.

Experimental evidence of the composite nature of the 3.1 eV luminescence in natural silica

Abstract The 3.1 eV photoluminescence of silica has been investigated on several samples of commercial origin, by measuring both time-resolved and stationary spectra. This structure, known as the β band, displayed the well known peculiarities: 3.1 eV energy position, 0.4 eV full width at half maximum, excitation peak at 5.1 eV and 100 μs decay time. However, an accurate analysis of the experimental data has shown the double nature of the β band in all natural silica samples here studied. The characteristic values of the two components β1 and β2 are: peak energy 3.04 and 3.20 eV, full width at half maximum of 0.44 and 0.35 eV and decay time of 115 and 97 μs respectively. In addition the intensity ratio between the luminescent emissions at 3.1 and 4.2 eV was found to be constant in the whole set of samples. The occurrence of two β components with so close spectral and time peculiarities might be ascribed to fluorescence processes involving the same oxygen deficient centers surrounded by two slightly different environments.

Chapter 1 – Optical absorption, luminescence, and ESR spectral properties of point defects in silica

Publisher Summary This chapter is divided into two parts: (1) In the introductory part, it describes the problems of point defects in a-SiO2, and (2) in the second part it discusses the experimental results. This chapter focuses on the Oxygen-Deficient Centers (ODCs) species in silica. This chapter investigates the ODC defects in a-SiO2 through their optical absorption, photoluminescence, and electron spin resonance activities. The effects of γ-ray irradiation are also investigated to evidence their ability to generate or transform structural defects. The aim of this chapter is to understand the optical activity of such defects to help in the characterization of their structure. The properties of point defects in a wide variety of both natural and synthetic silica types of commercial origin are investigated in the chapter. This chapter outlines the role of structural and dynamic properties of the vitreous matrix in determining the observed spectral properties of different centers. It deals with the theoretical aspects of the mechanism that are able to influence the fine structure of the spectral band profiles of point defects in interaction with the glassy matrix.

Design of Radiation-Hardened Rare-Earth Doped Amplifiers Through a Coupled Experiment/Simulation Approach

We present an approach coupling a limited experimental number of tests with numerical simulations regarding the design of radiation-hardened (RH) rare earth (RE)-doped fiber amplifiers. Radiation tests are done on RE-doped fiber samples in order to measure and assess the values of the principal input parameters requested by the simulation tool based on particle swarm optimization (PSO) approach. The proposed simulation procedure is validated by comparing the calculation results with the measured degradations of two amplifiers made with standard and RH RE-doped optical fibers, respectively. After validation, the numerical code is used to theoretically investigate the influence of some amplifier design parameters on its sensitivity to radiations. Simulations show that the RE-doped fiber length used in the amplifier needs to be adjusted to optimize the amplifier performance over the whole space mission profile rather than to obtain the maximal amplification efficiency before its integration in the harsh environment. By combining this coupled approach with the newly-developed RH RE-doped fibers, fiber-based amplifiers nearly insensitive to space environment may be designed in the future.

Weak hyperfine interaction of E′ centers in gamma and beta irradiated silica

We report on the effects of photon (γ) and electron (β) irradiation in a dose range extending from 100 to 5×109 Gy in a variety of silica samples studied by electron paramagnetic resonance. The E′ centers and a weak intensity satellite signal of their resonance line were generated both in γ- and in β-irradiated samples. We investigated the dependence of their intensity on the irradiation dose. Evidence of the existence of a common generation mechanism for the related paramagnetic point defects is found. These defects are induced mainly through the conversion of precursors except at very high doses, where the direct activation from the unperturbed matrix is concurrent. Our data support the model attributing the satellite signal to the weak hyperfine structure of the E′ center arising from interaction with a second nearest neighbor nuclear spin.

Evidence of delocalized excitons in amorphous solids.

We studied the temperature dependence of the absorption coefficient of amorphous SiO2 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 redshifting with increasing temperature. This provides a strong evidence of excitons being delocalized notwithstanding the structural disorder intrinsic to amorphous SiO2. Excitons turn out to be coupled to an average phonon mode of 83 meV energy.

Twofold coordinated Ge defects induced by gamma-ray irradiation in Ge-doped SiO 2

We report an experimental study by photoluminescence, optical absorption and Electron Paramagnetic Resonance measurements on the effects of exposure of Ge-doped amorphous SiO2 to γ ray radiation at room temperature. We have evidenced that irradiation at doses of the order of 1 MGy is able to generate Ge-related defects, recognizable from their optical properties as twofold coordinated Ge centers. Until now, such centers, responsible for photosensitivity of Ge-doped SiO2, have been induced only in synthesis procedures of materials. The found result evidences a role played by γ radiation in generating photosensitive defects and could furnish a novel basis for photosensitive pattern writing through ionizing radiation.

Photoluminescence band at 4.4 eV in oxygen-deficient silica: temperature effects

We report experimental results on the spectral properties and time behaviour of the 4.4 eV photoluminescence (PL) band in oxygen-deficient silica [Formula: see text]. Our measurements, performed both at T = 300 K and T = 10 K, show that at room temperature the PL features are independent of the particular excitation energy (5.0 eV, 6.8 eV and 7.6 eV) whereas at low temperature, upon excitation at 7.6 eV, the decay of the PL emission is faster than for lower excitation energies. This shortening of the PL lifetime is consistent with previously reported data, which were explained by hypothesizing an interconversion mechanism between two structural configurations of the same oxygen defect. Nevertheless, our results do not support the proposed mechanism and we tentatively suggest a different interpretation of the experimental data.

Ultraviolet emission lifetime in Si and Ge oxygen deficient centers in silica

We have measured the temperature dependence, in the range 10–295 K, of the lifetimes of the ultraviolet emissions associated with twofold coordinated Si and Ge, in pure and Ge-doped silica under excitation by synchrotron radiation. The Si-related fluorescence, centered at 4.4 eV, has a single exponential decay and its lifetime decreases from 4.0 ± 0.2 to 3.5 ± 0.2 ns on increasing the temperature. The luminescence at 4.2 eV, associated with the Ge defect, decays with a single exponential dependence below 150 K and with a more complex dependence at higher temperatures, its lifetime reducing from 7.8 ± 0.2 to 3.4 ± 0.2 ns. These features give evidence of the effectiveness of the thermally activated intersystem crossing in the relaxation of the excited singlet states, which is larger in the Ge-related center.