Effect of Pulse Repetition Rate on Ultrafast Laser-Induced Modification of Sodium Germanate Glass

Abstract

We report an unexpected pulse repetition rate effect on ultrafast-laser modification of sodium germanate glass with the composition 22Na2O 78GeO2. While at a lower pulse repetition rate (~≤250 kHz), the inscription of nanogratings possessing form birefringence is observed under series of 105–106 pulses, a higher pulse repetition rate launches peripheral microcrystallization with precipitation of the Na2Ge4O9 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.

Temperature Relaxation in Glass-Forming Materials under Local Fast Laser Excitations during Laser-Induced Microstructuring

“…Photopolymerization and photodamage by highly focused laser pulses can be used in microchemistry and stereolithography [6,19]. Moreover, the femtosecond laser structuring of glass-forming materials can be used for the optical long-term storage of information [3,[20][21][22][23][24][25][26][27][28][29][30], photonics [18], the fabrication of phase gratings [31], nanogratings [32], and quantum dots that can be used in various devices [33][34][35]. Notably, optical storage based on glass-forming materials has the potential to replace magnetic storage in the quest to provide high-speed, high-capacity, low-power, low-cost, highly secure, and long-term data storage [25][26][27][28][29][30].…”

“…In fact, the laser crystallization method makes it possible to control the local temperature inside the material and avoid random nucleation [36]. The ability to form and control the dynamics of changes in the temperature distribution T(t, r) inside the material is very important for the opportunity to control the morphology of the laser-written structure inside the glass matrix [22,32,37]. Thus, a deep understanding of the thermal processes inside glass-forming materials under fast laser thermal perturbations is required.…”