Advanced evaluation of single-shot ultrafast imaging interferometry for increased resolution

Ultrafast interferometry represents a powerful state-of-the-art technique for time-resolved measurement of phase changes on laser-excited surfaces, providing information on optical property changes and nanometer-resolution topographic changes. Thereby, the most common phase extraction method, the Takeda method, is often limited in spatial resolution. Additionally, for exciting thin films by the pump-probe technique for fluences below the ablation threshold, only small topographic changes of a few nanometers are expected, requiring a high signal-to-noise ratio (SNR), which is achievable primarily only with substantial experimental effort. Hence, in this study, two improvements to the standard evaluation are presented, enabling a two-fold increase in spatial resolution and up to more than one order of magnitude increase in the SNR with just a single measurement. The improvements are achieved by removing the background in the interferograms and averaging the spatial distribution of the calculated phase change itself. The improved SNR enables the detection of sub-nanometer changes in the topography. Additionally, a simple method for extracting the phase change is presented as an alternative to the standard Takeda method. Finally, the improved method is exemplified by a precise measurement of a surface oscillation with an amplitude of only a few nanometers of a 150 nm-thick gold layer upon irradiation with single-pulsed ultrafast laser radiation (wavelength 800 nm, pulse duration 40 fs). The experimentally determined time and amplitude of the oscillation are nearly identical to those from two-temperature modeling combined with hydrodynamics.