Time-Resolved Digital Holography System with High Phase Precision for Detail Observation in Laser Ablation Dynamics

In order to further improve the controllability of ultrashort pulsed-laser processing, it is important to explore the principles of material ablation dynamics itself. Particularly, it is necessary to obtain information on thermal properties and ultrafast thermodynamics in materials to understand the mechanism of ablation crater formation [1] . In recent years, using a technique called time-resolved digital holography (TRDH), some attempts have been made to directly observe the state changes of materials due to laser ablation as a spatial distribution of time-dependent changes in the complex refractive index [2] . This allows us to visualize the transmittance and the optical-phase-delay (OPD) changes for the crater formation dynamics as spatially resolved distribution information of craters. It is expected that thermal changes in materials, which are mainly observed as changes in the real part of refractive index, i.e., OPD, should also be observed with this technique. However, it has been still difficult to observe the thermal change in materials during the ablation process since its refractive index change with temperature change is too slight, e.g. $|{{\Delta }}n| = 1.6 \times {10^{ - 6}}\left[ {{{\text{K}}^{ - 1}}} \right]$ at 564nm in BK7 glass [3] . Here, we report on the construction of a tailor-made, coaxial probing TRDH optical system incorporating a novel interferometer that achieves high spatial resolution and high OPD precision. We evaluate these values and demonstrate time-resolved complex transmittance imaging of single-pulsed laser ablation of glass samples with this optical system.