Titanium (Ti) is widely used for biomaterials such as an implant for human body due to its excellent anti-corrosion. However, new functions should be added to Ti substrates because a strong revival is slow in a joint with a bone. A useful method is periodic nano structure formation on a material to control cell spreading and promote the osteogenic differentiation of cells. When a femtosecond laser is irradiated on a Ti plate at a fluence of near the ablation threshold, periodic nano structures are formed in the direction of perpendicular to the polarization angle of the laser pulse. In our previous study, it was found that the direction of cell was spread by periodic nano structures formed on Ti plate. For the controllability improvement of the cell spreading direction, it is required to improve the uniformity of periodic nano structures. Since the period of nano structures depend on the target material, irradiation fluence, number of irradiation-pulses, incidence angle, and laser wavelength, it is difficult to control the uniformity. In this study, to improve the uniformity of nano structures, Ti plate was irradiated by two-color double-pulse irradiation with a femtosecond laser at the wavelength of 800 nm. As the results, when the 800 nm and 400 nm pulses had orthogonal polarization directions, high uniformity of periodic nano structure was formed.
We have demonstrated the suppression of ablation rate on a silicon surface irradiated by a double-pulse beam with two color laser in time delays of Δt = -900 - 900 ps. The double pulse beam consists of 810nm with 40fs pulse and 405nm with > 40fs pulse. The fundamental-pulse fluence F810 is kept below ablation threshold (Fth, 810nm = 0.190 J/cm2 ) while the second harmonic pulse fluence F405 are kept above the ablation threshold (Fth, 405nm = 0.050 J/cm2 ). We find that ablation rate of silicon is drastically decreased at delay times of 600ps.
An ultra-intense short pulse laser induces a shock wave in material. The pressure of shock compression is stronger than a few tens GPa. To characterize shock waves, time-resolved velocity measurement in nano- or pico-second time scale is needed. Frequency domain interferometer and chirped pulse laser provide single-shot time-resolved measurement. We have developed a laser-driven shock compression system and frequency domain interferometer with CPA laser. In this paper, we show the principle of velocity measurement using a frequency domain interferometer and a chirped pulse laser. Next, we numerically calculated spectral interferograms and show the time-resolved velocity measurement can be done from the phase analysis of spectral interferograms. Moreover we conduct the laser driven shock generation and shock velocity measurement. From the spectral fringes, we analyze the velocities of the sample and shockwaves.
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