Lotarev Sergey V, Fedotov Sergey S, Pomigueva Alyona I, Lipatiev Alexey S, Sigaev Vladimir N
Department of Glass and Glass-Ceramics, Mendeleev University of Chemical Technology, Miusskaya pl. 9, 125047 Moscow, Russia.
Nanomaterials (Basel). 2023 Mar 29;13(7):1208. doi: 10.3390/nano13071208.
We report an unexpected pulse repetition rate effect on ultrafast-laser modification of sodium germanate glass with the composition 22NaO 78GeO. While at a lower pulse repetition rate (~≤250 kHz), the inscription of nanogratings possessing form birefringence is observed under series of 10-10 pulses, a higher pulse repetition rate launches peripheral microcrystallization with precipitation of the NaGeO phase around the laser-exposed area due to the thermal effect of femtosecond pulses via cumulative heating. Depending on the pulse energy, the repetition rate ranges corresponding to nanograting formation and microcrystallization can overlap or be separated from each other. Regardless of crystallization, the unusual growth of optical retardance in the nanogratings with the pulse repetition rate starting from a certain threshold has been revealed instead of a gradual decrease in retardance with the pulse repetition rate earlier reported for some other glasses. The repetition rate threshold of the retardance growth is shown to be inversely related to the pulse energy and to vary from ~70 to 200 kHz in the studied energy range. This effect can be presumably assigned to the chemical composition shift due to the thermal diffusion of sodium cations occurring at higher pulse repetition rates when the thermal effect of the ultrashort laser pulses becomes noticeable.
我们报道了脉冲重复率对超快激光改性22NaO 78GeO成分锗酸钠玻璃的意外影响。在较低的脉冲重复率（≤250 kHz）下，在一系列10 - 10个脉冲作用下可观察到具有形式双折射的纳米光栅的写入，而较高的脉冲重复率会引发外围微晶化，由于飞秒脉冲的热效应通过累积加热，在激光照射区域周围会析出NaGeO相。根据脉冲能量的不同，对应纳米光栅形成和微晶化的重复率范围可能会重叠或彼此分开。无论是否发生结晶，已发现从某个阈值开始，纳米光栅中的光学延迟随脉冲重复率出现异常增长，而不是像之前报道的其他一些玻璃那样，延迟随脉冲重复率逐渐降低。延迟增长的重复率阈值与脉冲能量成反比，在所研究的能量范围内从70 kHz变化到200 kHz。这种效应可能归因于在较高脉冲重复率下，当超短激光脉冲的热效应变得明显时，由于钠离子的热扩散导致的化学成分变化。
