Fig. 4: Laser-ablation-based lithography with a 1^st-order lens.

a Sketch for laser-ablation lithography on a chromium (Cr) film by focusing a femtosecond laser with a 1^st-order lens with a focal length of 1 mm. b Optical microscope images of slits fabricated by using the different parts of optical needle. The out-of-focus distance Δz = z-f ranges from Δz = −8 μm to Δz = 13 μm. The exposure time for all the slits is 200 ms. c The widths of fabricated slits in (a) and their theoretical values. RC: Rayleigh criterion (\(0.515\lambda /{NA}\)). The error bars are formed by averaging the x- and y- width of half of measured peak intensity of optical needle. d Widths of slits fabricated by using traditional objective with exposure time of 200 ms. The incident pulse energy is decreased to enable the slit width close to those fabricated with high-order lens because traditional objectives have higher efficiency than our high-order lens. Insert: optical microscope image of fabricated slits with traditional objective. e Measured widths (triangles) of fabricated double slits at the different exposure time from 100 ms to 600 ms. The fitted data reveal a linear dependence between the width and the exposure time, which is obtained at a single pulse energy of 0.2 mJ with a repetition rate of 25 Hz. Insert: Optical microscope images of fabricated double slits by addressing the exposure time.