Cover Story：Papanikolaou A, Tserevelakis G J, Melessanaki K, Fotakis C, Zacharakis G et al. Development of a hybrid photoacoustic and optical monitoring system for the study of laser ablation processes upon the removal of encrustation from stonework. Opto-Electron Adv 3, 190037 (2020). 

The research groups of Dr. G. Zacharakis (Biophotonics and Molecular Imaging laboratory of IESL-FORTH, Greece) and Dr. P. Pouli (Photonics for Heritage Science of IESL-FORTH, Greece), with the help of Prof. C. Fotakis (who initiated in early nineties the Photonics for Heritage Science of IESL-FORTH, currently a distinguished Member of FORTH), have built an apparatus that can enable the detection of MHz frequency range acoustic waves generated during the laser ablation process. On this basis, a methodology has been developed which combines the information derived from the recorded PA signals with the high resolution optical images recorded in laser ablation, in order to determine the critical irradiation conditions (i.e. number of pulses) and thus ensure a controlled and safe elimination of the encrustation layer. In parallel, the developed diagnostic approach allows the understanding and differentiation of the ablation mechanisms that govern IR and UV laser ablation for the involved materials. Initial experiments on technical mock-ups have been able to demonstrate the potential of this novel technique, thus current research refers to tests on real objects/cleaning challenges in order to establish this diagnostic methodology which is expected to open up new perspectives in the monitoring of laser restoration interventions on Cultural Heritage (CH) surfaces.