Transient Newton rings in dielectrics upon fs laser ablation

TL;DR

This paper reports the first observation of transient Newton rings in dielectrics during femtosecond laser ablation, providing new insights into ablation mechanisms and a novel method for their investigation.

Contribution

It demonstrates the occurrence of transient Newton rings in dielectrics, challenging previous assumptions and opening new avenues for studying dielectric ablation processes.

Findings

Transient Newton rings observed in sapphire and lead-oxide glass.

The rings dynamically change with delay time, indicating interference effects.

This phenomenon was previously thought impossible in dielectrics.

Abstract

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.