Observation of Stress Wave and Thermal Stress in Ultrashort Pulse Laser Bulk Processing inside Glass

Stress dynamics, stress wave and thermal stress, after photoexcitation inside glass a by tightly focused ultrashort laser pulse were investigated by a pump-probe polarization microscope and Transient Lens (TrL) method. The observation by the pump-probe polarization microscope revealed transient birefringence distributions of a laser-induced stress wave and thermal stress. The directions of compressive stress by a stress wave and thermal stress were radial from the photoexcitation, and the amplitudes of the stress estimated from the retardance of the birefringence were as high as several hundreds MPa. The observation by the TrL measurement showed the propagation of a laser induced stress wave as an oscillating TrL signals. From the pulse duration dependence of the oscillation amplitudes of the TrL signals, we found that the threshold and sensitivity to pulse energy of the stress wave become higher at longer pulse duration. We interpreted the pulse duration dependence in terms of different ionization mechanism; multiphoton ionization should be dominant for shorter pulse durations, while avalanche ionization should be dominant for longer pulse duration.