Abstract
We discuss the dynamics of ultrashort pulsed laser excitation in bulk optical silica-based glasses (fused silica and borosilicate BK7) well-above the permanent modification threshold. We indicate subsequent structural and thermomechanical energy relaxation paths that translate into positive and negative refractive index changes, compression and rarefaction zones. If fast electronic decay occurs at low excitation levels in fused silica via self-trapping of excitons, for carrier densities in the vicinity of the critical value at the incident wavelength, persistent long-living absorptive states indicate the achievement of low viscosity matter states manifesting pressure relaxation, rarefaction, void opening and compaction in the neighboring domains. An intermediate ps-long excited carrier dynamics is observed for BK7 in the range corresponding to structural expansion and rarefaction. The amount of excitation and the strength of the subsequent hydrodynamic evolution is critically dependent on the pulse time envelope, indicative of potential optimization schemes.
Highlights
Laser-induced three-dimensional transformation of bulk transparent materials carries today a strong potential in photonic technologies
The nature and the morphological aspect of optical damage, namely the potential onset of refractive index changes, regular voids or catastrophic runaway is strongly dependent on the deposited energy
The low exposure domain, at pump energies below or close to observable modifications, was regularly investigated and indicates fast relaxation (100 fs) and trapping of free carriers on excitonic states self-trapped due to the electronic polarization of the matrix. 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
Summary
Laser-induced three-dimensional transformation of bulk transparent materials carries today a strong potential in photonic technologies. Laserinduced phenomena can affect the resistance of optical materials to damage and breakdown, posing severe limitations to high power laser installations, fiber communication setups, or optical control elements in harsh environments[2]. These elements justify the strong interest in the fundamentals of laser-induced electronic and structural modifications of transparent materials on macro-, micro- and mesoscopic scales. Ultrafast laser radiation can strongly localize excitation in space and time, determining in turn a range of phenomena: from electronic runaway or transient defects states to macroscopic thermomechanical phenomena[3, 4].
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