  We report the appearance of transient Newton rings in dielectrics (sapphire
and lead-oxide glass) during ablation with single fs laser pulses. Employing
femtosecond microscopy with 800 nm excitation and 400 nm illumination, we
observe a characteristic ring pattern that dynamically changes for increasing
delay times between pump and probe pulse. Such transient Newton rings have been
previously observed in metals and semiconductors at fluences above the ablation
threshold and were related to optical interference of the probe beam reflected
at the front surface of the ablating layer and at the interface of the
non-ablating substrate. Yet, it had been generally assumed that this phenomenon
cannot be (and has not been) observed in dielectrics due to the different
ablation mechanism and optical properties of dielectrics. The fact that we are
able to observe them has important consequences for the comprehension of the
ablation mechanisms in dielectrics and provides a new method for investigating
these mechanisms in more detail.
