Abstract
Femtosecond (fs)-laser direct writing is a powerful technique to enable a large variety of integrated photonic functions in glass materials. One possible way to achieve functionalization is through highly localized and controlled crystallization inside the glass volume, for example by precipitating nanocrystals with second-order susceptibility (frequency converters, optical modulators), and/or with larger refractive indices with respect to their glass matrices (graded index or diffractive lenses, waveguides, gratings). In this paper, this is achieved through fs-laser-induced crystallization of LiNbO3 nonlinear crystals inside two different glass matrices: a silicate (mol%: 33Li2O-33Nb2O5-34SiO2, labeled as LNS) and a borosilicate (mol%: 33Li2O-33Nb2O5-13SiO2-21B2O3, labeled as LNSB). More specifically, we investigate the effect of laser scanning speed on the crystallization kinetics, as it is a valuable parameter for glass laser processing. The impact of scanning energy and speed on the fabrication of oriented nanocrystals and nanogratings during fs-laser irradiation is studied.Fs-laser direct writing of crystallized lines in both LNS and LNSB glass is investigated using both optical and electron microscopy techniques. Among the main findings to highlight, we observed the possibility to maintain crystallization during scanning at speeds ~5 times higher in LNSB relative to LNS (up to ~600 µm/s in our experimental conditions). We found a speed regime where lines exhibited a large polarization-controlled retardance response (up to 200 nm in LNSB), which is attributed to the texturation of the crystal/glass phase separation with a low scattering level. These characteristics are regarded as assets for future elaboration methods and designs of photonic devices involving crystallization. Finally, by using temperature and irradiation time variations along the main laser parameters (pulse energy, pulse repetition rate, scanning speed), we propose an explanation on the origin of (1) crystallization limitation upon scanning speed, (2) laser track width variation with respect to scanning speed, and (3) narrowing of the nanogratings volume but not the heat-affected volume.
Highlights
As the world is progressively evolving towards a photonic future, there is a need for miniaturization and functionalization of photonic devices
We investigate the role played by scanning speed (v) vs. the energy during the crystallization process, and draw our attention to nano-crystallization orientation and selforganized phase separation
For laser in silicate (LNS) at 0.5 μJ/pulse no crystallization could be detected at scanning speeds above ~50 μm/s, and there was a sharp transition between crystallized lines to no crystallization
Summary
As the world is progressively evolving towards a photonic future, there is a need for miniaturization and functionalization of photonic devices (including photonic chips, wavelength converters, lenses, retardation waveplates, waveguides, etc.) [1] In this context, femtosecond (fs)-laser irradiation in glass materials is an attractive way to functionalization [2,3]. The glass response to fs-laser is manifold, and includes, for instance, formation of defects, densification, fictive temperature changes, stress fields, crystallization, and so forth. The latter (crystallization) is a promising pathway in the functionalization of optical devices. SiO2 with B2 O3 into the glass system is not expected to change the thermal diffusivity by Sciglass software, are ρ (LNS, 20 °C) = 3482 kg/m3, C (LNS, 20 °C) = 629.2 J/(kg∙K), much
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