Femtosecond lasers have become a powerful tool for 3D space-selective crystallization of glasses. A laser-induced cumulative heating effect required for crystal growth is usually considered to take place only at pulse repetition rate over 100 or 200 kHz and 200 kHz is known as the lowest repetition rate at which femtosecond laser-induced crystallization has been reported so far. We for the first time demonstrate precipitation of LaBGeO5 crystals in lanthanum borogermanate glass using a femtosecond laser emitting 1030 nm, 300 fs, 110 μJ pulses with adjustable repetition rate below 100 kHz. For the applied laser, minimal repetition rate enabling nucleation of ferroelectric LaBGeO5 crystals inside the glass was shown to be 9 kHz at maximal pulse energy of 110 μJ and growth of a crystalline line from the formed seed crystal was obtained starting from 8 kHz though smooth homogeneous oriented line which might be regarded as quasi-single-crystalline could be grown at 25 kHz or higher and corresponding pulse energy of 18 μJ. Thus, the pulse repetition rate sufficient for a cumulative heating effect and a stable crystal growth was reduced by an order of magnitude as compared to earlier publications due to relatively high pulse energy. Possibility and efficiency of cumulative heating and crystal growth and average time required for forming the seed crystal have been studied for various combinations of the pulse energy and the repetition rate. Obtained crystalline features have been studied by micro-Raman spectroscopy and Raman mapping which confirmed growth of stillwellite-like LaBGeO5 phase and orientation of its polar axis along the direction of the crystalline line.
Femtosecond laser-induced modification of Au-doped phosphate glass at different pulse repetition rates using an Yb
femtosecond amplifier emitting pulses of up to 120 μJ energy at 1030 nm with adjustable repetition rate up to 100 kHz is
presented. At the repetition rate as low as 1 kHz, only refractive index modification and formation of red color centers
are observed. Increasing the average output laser power resulted in increase of color intensity, but this color could be
easily erased by the heat treatment at 300˚C for 20 min indicating athermal nature of these laser-induced effects. By
contrast, at the repetition rate from 25 kHz to 100 kHz, cumulative heating effect was demonstrated and allowed to
realize for the first time one-stage laser-induced growth of Au nanoparticles in the solid glass accompanied by the
appearance of a thermally stable red color. Z-scan analysis of nonlinear properties of the studied glass samples is carried
out. Details of Au nanoparticles growth and phosphate glass modification by the tightly focused femtosecond laser beam
are discussed.
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