Vincenzo Di Michele a réalisé son doctorat en cotutelle entre l’Université Jean Monnet de Lyon et l’Université de Palerme sous la direction de Youcef Ouerdane et Sylvain Girard (Lyon) et de Marco Cannas (Palerme).

Transient Optical Phenomena Related to Point Defects in Pure and Doped Silica

The goal of the present PhD thesis is to shed light into the transient defect’s behavior, studying their generation mechanisms in pure or doped amorphous silica, upon ionizing X-rays and high intensity femtosecond laser pulses. We focused on their optical properties, characterizing the whole photocycle associated with the defect involved in the radiation matter interaction processes. Both bulk glasses and optical fibers are investigated, with a variety of spectroscopic techniques, to evaluate the impact of the induced defects in wide-band gap materials. The first chapter offers an overview on the silica glass, with particular attention in the defect generation under ionizing irradiation and through non-linear photoionization. Chapter 2 is devoted to the concepts at the basis of the phenomena observed along this PhD thesis, introducing the experimental techniques used to investigate the selected samples. Chapter 3 describes our samples as well as the various experimental facilities and setups exploited along these three years of work. Chapter 4 reviews the main results of our investigation about the origins of the pulsed and steady state X-ray radiation- induced attenuation in different classes of optical fibers. Chapter 5 focuses on our characterization of the interaction processes between amorphous silica and high intensity femtosecond laser pulses. For this, we combined post mortem investigation of femtosecond inscribed waveguides with in situ characterization of the unstable defects recovering just after the irradiation. The last chapter, chapter 6, presents a new and promising experimental approach to study the defect’s photocycle under linear absorption conditions and with a sub-picosecond time resolution. Our method takes advantage from the possibility to tune, in a wide spectral range, the excitation wavelength in transient absorption (TA) measurements. As example of the benefit of this approach, we discuss the photocycles of the Non-Bridging-Oxygen-Hole-Center and of the Germanium-Lone-Pair- Center under ultraviolet photo-excitation, following the femto-picosecond dynamics of these photoluminescent defects as a function of excitation wavelength in single-photon absorption conditions.