In-process monitoring of femtosecond laser-induced periodic nanostructures on glass by using anti-reflective property

Abstract

Laser-induced periodic surface structure (LIPSS) in nanometer scale formed by femtosecond (fs) laser pulses depends strongly on laser parameters such as fluence and superimposed pulse number, as well as the surface condition of target materials. Process control based on in-process monitoring is one of the solutions to form the nanostructure stably and homogeneously. However, it is difficult to monitor the nanostructure in the process, because it is much smaller than the light wavelength. This paper reports on a new technique for in-process monitoring of the periodic nanostructure using its anti-reflection property. As a target, we used a synthetic quartz plate. The linear-polarized 1030-nm, 250-fs laser pulses from a Yb fiber laser amplification system operated at a repetition rate of 20 kHz were focused with an objective lens and scanned on the target surface. Microscopic images of the target surface with coaxial epi-illumination and transillumination were acquired with two CMOS cameras. From these images, the surface reflectivity and transmittance were evaluated. After the ablation experiment, the surface morphology was observed with a scanning electron microscope. The surface of which transmittance increased as reflectivity decreased, had a line-like periodic nanostructure with a period of ~200 nm and a depth of ~1 μm. On the other hand, the surface of which both transmittance and reflectivity decreased did not have the nanostructure. These results demonstrate that an observation technique using anti-reflection property is much more effective in monitoring fs-laser-induced nanostructure on glass in the process.