Surface deformation analysis on anthracene crystal by AFM detection of surface acoustic wave induced by femtosecond laser pulse

Yu Hau Ye^1*, Yuka Tsuri^1,2, Yoichiroh Hosokawa^1,2

¹Division of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara, Japan
²Medilux Research Center, Nara Institute of Science and Technology, Ikoma, Nara, Japan

* Presenter:Yu Hau Ye, email:ye.yu_hau.yx3@naist.ac.jp

When an intense femtosecond laser pulse is focused on an object, it generates an impulse that propagates along the surface as surface acoustic wave. We previously investigated the wave propagation on crystals as vibrational responses, which is detected using an atomic force microscope (AFM), for elucidating the mechanical properties on crystal surface. In this study, we applied this method to evaluate the deformation of crystal surface with micrometer-scaled spatial resolution. Ti:Sapphire femtosecond laser pulses (150 fs) were introduced through a 20× objective lens into an inverted microscope, focusing on the mm-sized anthracene crystal to induce not only vibrations but also deformations on the crystal surface. The first vibrational response was detected as a baseline using an AFM attached to the inverted microscope. Subsequently, the crystal surface (50 µm × 50 µm) was scanned by laser with the energy of 60 nJ/pulse which was less than ablation threshold, inducing invisible deformation. Then, the vibrational responses were remeasured, which were successfully distinguished from the baseline by applying machine learning classification. This result indicates that the laser scanning induces invisible deformations on the crystal surface, which can be distinguished by the vibrational responses but not by transmitted image and topography. This method holds potential for high precision detection of molecular alignments and mechanical properties of crystal surfaces in future studies.

Keywords: femtosecond laser, organic crystal, atomic force microscope, machine learning, surface