Fabrication qualities of micro-gratings encoding dependence on laser parameters by two-beam femtosecond lasers interference

摘要

Femtosecond laser interference is a promising tool for micro-fabrication and micromachining of periodical structures on the surface of samples or inside transparent materials, but femtosecond laser pulses are very hard to interfere due to their spectrum widths may reach to several tens of nanometers, and their spectrum widths will be stretched by shorting the duration according to the Fourier transform. We realized two 25 fs pulses interference and encoded micro-gratings on Au-Cr thin films using this interference pattern. The interference patterns of two laser pulses with different pulse durations in sub-hundred femtosecond time domain were calculated to explore the influence of pulse durations on processing qualities of encoded micro-gratings. The results show that, the shorter pulses are preferable to fabricate micro-gratings with fine resolution on intractable materials, and longer pulses are helpful to improve encoding efficiency and contrast ratio of bright & dark interfered fringes. The differences between encoded micro-gratings on Au-Cr thin film using these interference patterns validated our analysis, which are hardly observed when pulse duration is longer than 100 fs mainly because the size of interfered area is larger than the focal spots. Moreover, the distance between two focal spots also has been chosen to identify our calculations, and the experimental results are agreement with the calculations.