Photo-acoustic sub-micrometer modifications of glass by pair of femtosecond laser pulses

We present time-resolved studies of glass densification created by an acoustic phenomenon: collision of the transverse and longitudinal sound waves inside glass. Localization of the permanent denisfied region has a lateral cross section similar to 0.4 mu m and is approximately half of the wavelength of femtosecond laser pulses which were used to generate breakdown and launched shock waves inside glass. Controlled time delay between two closely spaced irradiation spots reveals dynamics and relaxation (electronic, thermal, stress) of glass after excitation. The observed phenomenon is important for femtosecond direct laser writing and recording of waveguide couplers using multiple beams.