Time-resolved phase distributions of laser-induced breakdowns generated by two pulses in water are observed with a
pump-probe interference microscope. When the two pulses are overlapping temporally, the optical interference of the
two pulses are resulted in fluctuation of pulse energy used to breakdown. When shock waves emitted from breakdowns
are overlapped, the pressure of overlapping point is addition of the pressure of each shock wave. When two pulse is
overlapping spatially, even temporally separated, a nonlinear enhancement of shock wave is observed. It may be caused
by an interaction between a plasma and a post pulse.
We investigate the morphology of a glass surface processed by tightly focused femtosecond laser pulse. Processing of a cavity with submicrometer-sized diameter is performed with irradiation laser pulse energy near a destructive threshold in air. In many cases, the cavity is surrounded by a ring-shaped protrusion, debris, and small droplets. In order to reduce the debris and the thermal destruction, we propose to process with coating a transparent material on a target material. PMMA (Poly-methyl methacrylate) is used as the transparent material. A thick PMMA film reduces dissolution and vaporization that is caused by an interaction between a high-density hot vapor plume and the target material. Furthermore, the dissolution is reduced because a low energy part in a laser pulse is reduced by sharpening the beam shape with the self-focusing of the laser pulse in the thick PMMA film. As the results, submicrometer-sized cavity that debris and the thermal destructive area are reduced dramatically is produced.
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 diffraction 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.
A shape of a structural change inside human fingernail produced by a focused femtosecond laser pulse drastically changes depending on the irradiated pulse energy. The structural changes are observed as a dark region by a microscope with transmitted illumination. When the energy is close to the threshold for producing the structural change, it had a linear shape, and at higher energy, it had conical shape. We observed a phenomenon of increase of fluorescence intensity at the structural changes. The fluorescence intensity is higher than its surrounding auto-fluorescence of a fingernail. The spectrum of the increased fluorescence coincided with the auto-fluorescence spectra of a fingernail and a pure keratin. The increase of fluorescence intensity was also observed with the fingernail heated by a drying oven. It is suggested that the fluorescence increase of the structural change is most likely caused by locally heating caused by femtosecond laser pulse irradiation. We demonstrate the fluorescence increase of the structure is useful to read out three-dimensionally recorded data inside human fingernail. Furthermore, we demonstrate the fluorescence can be observed over a half year.
Optical data storage in nails using a femtosecond laser system is demonstrated. A bit is formed as a structural change of the nail by irradiation of a femtosecond laser pulse. The recorded bit is observed using a conventional microscope. The size of the bit increases with increasing irradiation pulse energy. The pulse energy necessary to form a bit depends on the recording depth and the surface condition of the nail.