We have demonstrated the nanostructure fabrication on the Si surface by a 150 fs, 800 nm femtosecond laser pulse. The nanohole size of about 100 nm can be formed presumably by a near-field optical enhancement effect induced by the particle illumination. The nanohole size is nearly independent of the irradiated pulse number. It is found that the optical enhancement factor was about 7 in this experiment. The hexagonally arrayed nanoholes were fabricated. The diameter of the fabricated nanohole was about 90 nm and the depth was about 9 nm. In addition, using the double pulse fs laser, we demonstrated a low-loss waveguide fabrication in fused silica. A low-optical-loss waveguide is fabricated under the conditions that the first pulse energy and teh second pulse energy in the double pulse mode are 30 jJ and 160 jJ, respectively, nad the pulse time interval is 3 ps. The weak first pulse would act as a pre-conditioner of the fused silica and then the second subsequent pulse effectively induces the uniform refractive index change. This new femtosecond double pulse fabrication technique will be very promising for low-loss photonic device fabrication for photonic networks.
One of the technical issues of waveguide fabrication technique inside transparent materials using femtosecond laser has been an optical loss which may be caused by a nonuniformity of the modified volume. In this paper we report on the optical waveguides fabrication inside fused silica glasses using double pulse femtosecond lasers to solve this issue. We investigate writing conditions of optical waveguides with adjusting pulse intervals and relative fluence of the double pulses. The pulse separation time of the double pulses is varied from 500 fs to 200 ps. The better optical property of the optical waveguides under the conditions that the first pulse energy and the second subsequent pulse energy of the double pulse mode are 30 nJ and 160 nJ, respectively and pulse separation time is 3 ps. This optical loss is smaller than that of the waveguides fabricated using a single pulse femtosecond laser.
Permanent refractive index change in bulk glasses induced by femtosecond laser has opened up new ways for photonic devices fabrication. In this technique, temporal profile of femtosecond laser is found to be important because it is a nonlinear material processing. In this paper we investigate temporal profile dependence on the optical waveguides properties written by tightly focused femtosecond laser and loosely focused femtosecond laser. With tight focus writing, optimization of pulse interval of pulse train with a few ps decreased the optical loss of the fabricated waveguides. With loose focus writing, longer femtosecond laser increased the length of the induced refractive index change. 2-D Bragg diffraction gratings were well fabricated using this loose focus method.