Structures on human fingernail surface processed by use of a focused femtosecond laser pulse

Processing of a human fingernail surface by a tightly focused femtosecond laser pulse is investigated. The processed structure in the fingernail surface is strongly dependent on focus position and irradiation energy of a laser pulse. A variety of structures depending on the focus position were observed. A sudden change in the size of the processed structure according to the irradiation pulse energy is also observed. From a linear theoretical estimation based on the diffraction of a laser beam, we found that the sudden change is mainly due to the diffiraction pattern generated by the circular aperture of an objective lens. The processing feature is investigated by comparing the structures processed in a fingernail with those processed in glass.