Optical Bit Recording in a Human Fingernail

Optical bit recording in a human fingernail is demonstrated by using a femtosecond laser processing technique. A bit is formed in a fingernail by irradiation of a focused femtosecond laser pulse. We have investigated the dependence of the irradiation pulse energy on the diameter and length of the formed bit. The bit diameter and length are theoretically explained based on a Gaussian beam irradiation model. Furthermore, the achievable recording density is estimated from the measured bit diameter and bit length. We have found that the pulse energy required to form a bit depends on the recording depth and the surface condition of the fingernail.