Role of thermal ionization in internal modification of bulk borosilicate glass with picosecond laser pulses at high repetition rates

Abstract

We study the roles of thermal ionization and electronic damage in the internal modification of bulk borosilicate glass by high repetition rate picosecond laser pulses. Laser-induced plasma generation, nonlinear energy deposition and steady temperature distribution are numerically analyzed. The simulated modified regions show good agreement with the experimental results, thereby revealing the roles of thermal damage and electronic damage in the internal modification. While the elliptical outer structure is recognized as the molten region, we found that the teardrop-shaped inner structure is the damaged zone caused by high-density free-electrons. In the formation of the inner structure, cascade ionization is seeded by thermal ionization instead of multi-photon ionization and dramatically increases the free-electron density to the damage threshold. The contour of the inner structure is found to be corresponding to a characteristic isotherm of around 3000 ~4000 °C

Characterization of Plasma in Microwelding of Glass Using Ultrashort Laser Pulse at High Pulse Repetition Rates

“…When SHG Nd:YAG laser is irradiated, some attenuation of SHG Nd:YAG laser in the plasma region is observed suggesting the plasma is of high density free-electrons [21,22]. However, the quality of the pictures is not good enough to evaluate the absorption property from the pictures, because of fringe patterns appeared in the pictures.…”

“…In addition, the light emission from the hot plasma can compete with the SHG Nd:YAG laser beam transmitted through the band pass filter, since rather strong plasma emission lasts between the fs-laser pulses due to the heat accumulation effect at high pulse repetition rates [14,21]. Thus pictures of the laser-induced plasma taken without SHG Nd:YAG laser are solely shown in the paper, and the evolution of free-electron density in conduction band is estimated by the simulation based on the rate equation model [10,11,22].…”

“…Therefore difficulty is found to use existing pump-probe technique [9,10] for characterizing plasma at high pulse repetition rates. While free-electron density in the plasma is also simulated at high pulse repetition rate [22], process fluctuation is not taken into consideration.…”

Mechanisms of damage formation in glass in the process of femtosecond laser drilling

“…Because the energy of the free electrons is transferred to phonons, the increase in temperature T can be expressed as follows [29]:…”

Control of waveguide properties by tuning femtosecond laser induced compositional changes

“…12 Also more recently, thermal ionization has been analyzed as an important factor in high repetition rate processing of glass. 26 However, the reason for a given ion to move towards or opposite to a given gradient direction remains unsolved. Once a given ion initiates its migration in the presence of a thermal gradient, other species should move in the opposite direction in a high-temperature, highly ionized, molten state to compensate charge/oxygen deficiencies.…”