Ultrafast laser induced electronic and structural modifications in bulk fused silica

Abstract

International audienceUltrashort laser pulses can modify the inner structure of fused silica, generating refractive index changes varying from soft positive (type I) light guiding forms to negative (type II) values with void presence and anisotropic sub-wavelength modulation. We investigate electronic and structural material changes in the type I to type II transition via coherent and incoherent secondary light emission reflecting free carrier behavior and post-irradiation material relaxation in the index change patterns. Using phase contrast microscopy, photoluminescence, and Raman spectroscopy, we determine in a space-resolved manner defect formation, redistribution and spatial segregation, and glass network reorganization paths in conditions marking the changeover between type I and type II photoinscription regimes. We first show characteristic patterns of second harmonic generation in type I and type II traces, indicating the collective involvement of free carriers and polarization memory. Second, incoherent photoemission from resonantly and non-resonantly excited defect states reveals accumulation of non-bridging oxygen hole centers (NBOHCs) in positive index domains and oxygen deficiency centers (ODCs) with O 2 ions segregation in voidlike regions and in the nanostructured domains, reflecting the interaction strength. Complementary Raman investigations put into evidence signatures of the different environments where photochemical densification (bond rearrangements) and mechanical effects can be indicated. NBOHCs setting in before visible index changes serve as precursors for subsequent compaction build-up, indicating a scenario of cold, defect-assisted densification for the soft type I irradiation regime. Additionally, we observe hydrodynamic effects and severe bond-breaking in type II zones with indications of phase transition. These observations illuminate densification paths in fused silica in low power irradiation regimes, and equally in energetic ranges, characterized by the onset of thermo-mechanical effects

Femtosecond laser induced phenomena in transparent solid materials: Fundamentals and applications

“…In this case, the relaxation of the electronic excitation cannot induce high temperature for the local heating, since this temperature is below the material softening threshold. Under these conditions, the thermomechanical effects seem negligible and the laser-generated defects determine structural transitions corresponding to a denser packing in the matrix and resultant refractive index changes [95,96]. The birefringence of type II modification induced by focused femtosecond laser radiation is attributed either to laser-induced stress, or to the formation of self-assemblied nanogratings with subwavelength periodicity orientated perpendicularly to the laser light polarization leading to anisotropic reflection, as shown in Figs.…”

Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses

“…These evolve to Frenkel pairs and the Si-O bond scission leads to nonbridging oxygen hole center (NBOHC) and E centers accompanying a soft isotropic increase of the refractive index upon pulse accumulation. It was argued before [7,22] that, most probably, the weak positive index changes regime results from defect-induced matrix densification without visible thermal assistance in driving the structural metastability. Photoinscription in this range leads to type I index changes and low-loss waveguides [1] .…”

“…It is therefore expectable that this characteristic morphology modification involves transitions via lower viscosity states (even though perhaps not yet a fully developed liquid phase) and the onset of the void is a result of the mechanically induced rarefaction [24,25] . Photoluminescence signatures indicated the presence of oxygen deficiency centers (ODC) in the vicinity of the void interfaces and the onset of molecular oxygen [22] . These support present scenarios of molecular decomposition that assume that enough energy is provided to the glass matrix to facilitate the onset of gas-phase transformation in void-like domains [20,28] .…”

“…Alternative pathways of structural reorganization of the matrix in the presence of significant bond-breaking, favored by excessive heat and pressure, may contribute to the absence of NBOHC. A sketch of defect redistribution in the high-energy damage area [22,29] is given in Figure 2. The photoluminescence spectrum is taken under conditions of excitation at 325 nm, resonant with NBOHC states.…”

Calculation of nonlinear optical damage from space-time-tailored pulses in dielectrics

“…Much work has been devoted to investigating material modification and damage in transparent dielectrics due to such waveforms with duration on the order of femtoseconds. [1][2][3][4][5][6][7][8] More recent studies have explored more exotic pulses that approximate Airy or Bessel spatial profiles, Airy time shapes, or some combination of these. [9][10][11][12][13] These waveforms have attracted interest because of their self-healing, accelerating, and/or non-di↵racting properties.…”