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3-Mar-2020 14:00 14:00 Age: -11 days

Optical glass materials

NIKITA SHCHEBLANOV (ECOLE POLYTECHNIQUE)

Lieu et heure: Salle N20bis, 3 eme étage, bâtiment Lavoisier, 14h.

Dr Nikita Shcheblanov (Ecole Polytechnique)

 

Title: Optical glass materials 

Part I (short). Nonlinear photoionization of transparent solids: a nonperturbative theory obeying selection rules

We provide a nonperturbative theory for photoionization of transparent solids, which consistently account for the selection rules related to the parity of the number of absorbed photons (odd or even). We derive closed-form analytical expressions for the photoionization rate within the two band structure model. Our model exhibits excellent agreement with measurements for the frequency dependence of the two-photon absorption and nonlinear refractive index coefficients in sapphire and silica, two highly relevant materials for industrial applications. We demonstrate the crucial role of the interference of the transition amplitudes, which in the semi-classical limit, can be interpreted in terms of interfering quantum trajectories that were disregarded in Keldysh’s foundational work of laser physics [Sov. Phys. JETP 20, 1307 (1965)], resulting in the violation of selection rules.

Part II. Raman spectroscopy of femtosecond multi-pulse irradiation of vitreous silica: experiment and simulation

We report an experimental and numerical study of femtosecond multi-pulse laser-induced densification in vitreous silica (v-SiO2) and its signature in Raman spectra. We compare the experimental findings to recently developed molecular dynamics (MD) approach [Shcheblanov & Povarnitsyn EPL (2016)] accounting for bond-breaking due to laser irradiation, together with a dynamical matrix approach and bond polarizability model based on first-principle calculations for the estimation of Raman spectra. We observe two stages of the laser-induced densification and Raman spectrum evolution: growth during several hundreds of pulses followed by further saturation. At the medium-range, the network connectivity change in v-SiO2 is expressed in reduction of the major ring fractions leading to more compacted structure. With the help of Sen & Thorpe model, we also study the short-range order transformation and derive the interbonding Si-O-Si angle change from the Raman measurements. Experimental findings are in excellent agreement with our MD simulations, and, hence, support bond-breaking mechanism of laser-induced densification. Thus, our modeling explains well the laser-induced changes both in the short-range order caused by the appearance of Si-coordination defects and medium-range order connected to evolution of the ring distribution. Finally, our findings disclose similarities between sheared-, permanently-densified- and laser-induced-glass and suggest interesting future experiment in order to clarify the impact of the thermo-mechanical history on glasses under shear, cold- and hot-compression, and laser-induced densification.

Part III. Vibrational and structural properties of P2O5 Glass: Advances from a combined modeling approach

We present experimental measurements and ab initio simulations of the crystalline and amorphous phases of P2O5 computing the Raman, infrared and vibrational density of states spectra. The calculated spectra are in excellent agreement with experimental measurements and contain the signatures of all the peculiar local structures of the amorphous phase, namely, bridging and non-bridging (double-bonded) oxygens, and tetrahedral PO4 units associated with Qn species (Qn denotes the various types of PO4 tetrahedra, with n being the number of bridging oxygen atoms that connect the tetrahedra to the rest of the network). In order to reveal the internal structure of the vibrational spectrum, we use a mode-projection approach at different symmetries based on the Td symmetry group. The projections of the eigenmodes onto Q2, Q3, and Q4 species yield well defined contributions at frequencies in striking correspondence to the positions of the Raman and infrared bands.

 

 


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