Fig. 1. Schematic diagrams of dual-period LIPSS and its application. (a) Sketch of femtosecond laser line scanning in ITO thin films. The red arrows represent the moving direction of the laser beam. D is the distance between two fabricated lines; (b) Sketch of structural colors. The structures will show different colors under bright-field and dark-field illumination; (c) Sketch of anti-counterfeiting based on structure colors from dual-period LIPSS. The LIPSS structures display the same color under bright-field illumination and different colors under dark-field illumination

Fig. 2. The effect of laser energy and the distance between fabricated lines. When the laser scanning speed is 0.5 mm/s, the images of bright-field (a) and dark-field (b) corresponding to different laser scanning line spaces D and processing energy. The size of the rectangle is 40 μm × 100 μm; (c) The SEM image of processed structures

Fig. 3. Analysis of structural color formation. (a) The SEM image of the structures when D is 1.5 μm; (b) The result of the Fourier transformation of (a); (c) Schematic diagram of the simplified physical model of the LIPSS structures; (d) Simulated bright-field and dark-field reflection spectra; (e), (f) are the simulated electric field intensity distributions under dark-field and bright-field illumination, respectively

Fig. 4. Characterization of structural colors. The scanning speed of the laser beam is 0.5 mm/s, the space between lines is 1.5 μm. The dark-field (a) and bright-field (b) micrographs are obtained by processing the snowflake pattern with different laser energy, and the numbers ①~⑥ in (b) correspond to the laser processing energy: 0.95 mW~1.25 mW, the energy interval is 0.06 mW; (c) The correspondence of bright-field and dark-field colors on the chromaticity diagram CIE 1976, "+" is the color corresponding to bright-field, and "о" is the color corresponding to dark-field; (d) The red and blue curves represent the experimentally measured scattered and reflected spectra, respectively, and ①~⑥ correspond to the numbers in the bright-field image

Fig. 5. Experimental results of microscale anti-counterfeiting structural colors. In (a)~(f), the laser scanning speed is 0.5 mm/s, the scanning line space D of the pattern “clouds” is 2 μm, and the scanning line space D of raindrops is 1.5 μm. The processing energy of (a)~(c), (d)~(f) are 1.0 mW and 1.2 mW, respectively. In the figure, (a, d), (b, e), (c, f) are bright-field, dark-field and SEM image results, respectively, and the left and right images of the SEM show part of the structure inside the pattern “clouds” and “raindrops”, respectively; (g) and (h) are the experimental results of QR code hiding; (i) is the experimental result after introducing disturbance information