Direct grating writing using femtosecond laser interference fringes formed at the focal point

Af emtosecond pulse laser has been used for the machining of gratings primarily due to its ability of direct ablation and its capability to fabricate sub-wavelength structures. In this paper, we present a direct ablation technique for the fabrication of gratings by interfering femtosecond pulses in a specially designed optical configuration. This technique ensures that gratings are formed only at the focal point. The grating line-width can be varied with minor adjustment to the optical configuration. This configuration not only simplifies the optical set-up, but also immunizes the system to extraneous and inherent vibrations, thus enabling gratings of good edge acuity. With this technique, we have successfully fabricated planar gratings of different line-widths on a silicon substrate. Methods of adjusting the grating period are demonstrated. The effects of pulse number and the laser threshold on the grating quality have been qualitatively studied using SEM analysis. This method offers a new approach of the fabrication of a grating on a surface by direct ablation. Also, the feasibility of effectively writing internal gratings in transparent materials is discussed.