Laser Writing of Multilayer Structural Colors for Full‐Color Marking on Steel

Laser‐induced periodic surface structure (LIPSS) can form structural color on the metal surfaces with high production efficiency and thermal stability, and has been used in various industrial applications such as unfaded color marking and anti‐counterfeiting. Herein, a novel fabrication scheme of multilayer LIPSS is proposed by multiple writing in situ with changing the laser polarization direction, to exhibit an effect of color superposition. To verify this approach, the color formation mechanism of LIPSS on the stainless‐steel surface is analyzed by finite‐element numerical calculation and reveals that the high‐angular dispersion of LIPSS is mainly the result of optical diffraction occurring on the surface of periodic structures. The relationship between the angular dispersion of multilayer LIPSS and laser‐processing parameters is established. Through the proposed multilayer LIPSS coloring technology, vivid full‐color patterns on the steel surface are demonstrated, and the in situ superposition of three‐layer graphs is realized, which can greatly enrich the color levels and be competitive in industrial applications.


Introduction
Laser-induced periodic surface structure (LIPSS) is a selfassembled periodic structure formed by laser irradiation on the surface of metals, [1] semiconductors, [2] dielectrics, [3] and polymer, [4] which has attracted a lot of research interest and found great industrial applications in the past few decades. [5]Diverse microstructures endow LIPSS with the ability to modify the physical and chemical properties of materials surface, including hydrophilic, [6] hydrophobicity, [7] coefficient of friction, [8] corrosion resistance, [9] surface reflectance, [10] and surface coloring. [11]11eÀi,13] However, to date, there is a lack of theoretical numerical research on the formation mechanism of this high-angular dispersion phenomenon.
In this article, different angular color formation models on LIPSS are built by means of finite-element analysis, to numerically explain the LIPSS color-dispersion problem.The results of color distribution at different angles are obtained by solving the wave equation of the transmission process of light on LIPSS.By comparing the results of numerical analysis with the experimental results, we found that the surface color is highly correlated with the observation angle and these results revealed that the high-angular dispersion of LIPSS on the stainless-steel surface is mainly the result of the optical diffraction occurring on the surface of periodic structures.In addition, aiming for more vibrant colors and richer application prospects, we propose a novel fabrication scheme of multilayer LIPSS by multiple writing in situ with changing the laser polarization directions, to present an effect of color superposition based on aforementioned physical model.Comparing with the conventional single-layer LIPSS, the advantage of the multilayer LIPSS metal is that it can greatly enrich the color level, increase the information encoding capacity, and improve the anti-counterfeiting ability.Through this method, we demonstrate vivid color patterns on the steel surface, and realize the in situ superposition of three-layer pictures indicating a triple in information encryption capacity.Notably, the DOI: 10.1002/adpr.202300157Laser-induced periodic surface structure (LIPSS) can form structural color on the metal surfaces with high production efficiency and thermal stability, and has been used in various industrial applications such as unfaded color marking and anticounterfeiting.Herein, a novel fabrication scheme of multilayer LIPSS is proposed by multiple writing in situ with changing the laser polarization direction, to exhibit an effect of color superposition.To verify this approach, the color formation mechanism of LIPSS on the stainless-steel surface is analyzed by finiteelement numerical calculation and reveals that the high-angular dispersion of LIPSS is mainly the result of optical diffraction occurring on the surface of periodic structures.The relationship between the angular dispersion of multilayer LIPSS and laser-processing parameters is established.Through the proposed multilayer LIPSS coloring technology, vivid full-color patterns on the steel surface are demonstrated, and the in situ superposition of three-layer graphs is realized, which can greatly enrich the color levels and be competitive in industrial applications.
multilayer coloring method presented in this work is developed on the basis of laser erasures and rewriting techniques, [1c] and represents a significant advance in laser color marking.

Experimental Section
In our experiments, the surface coloring was carried out by a fiber laser (PicoYL-75-MW) that delivered laser with laser pulse width of 50 ps, central wavelength of 1030 nm, and repetition rate of 25 kHz.Figure 1 depicts the experimental setup of picosecondlaser-processing system.The laser polarization direction was adjusted by rotating a half-wave plate and a linear polarizer.The laser beam was focused with a lens of 100 mm as focal length and the size of the laser-beam diameter on the surface of the sample was fixed at 30 μm.The sample was AISI 304 with thickness of 1 mm and its surface roughness was 8 nm.The steel located at a workbench, which could move along the XYZ three-coordinate direction.By translating the workbench, laser scanning can be realized according to the planned path.As shown in Figure 1, LIPSS can be obtained from metal surfaces, and a novel fabrication scheme of multilayer LIPSS was proposed by multiple writing in situ with changing the laser polarization direction.Speed of horizontal movement of table extends X and Y axes from 0 to 33.3 mm s À1 .It is worth noting that the stainless-steel needs to be cleaned with absolute ethanol or acetone before used.Scanning electron microscope (SEM) images are captured using a field-emission SEM (EVO 18-SEM, Carl Zeiss).T surface reflection spectrums are measured by a spectrometer (LIBS2500PLUS).

Influence of Laser Parameters on the Surface Color of Stainless Steel
Figure 2 shows the laser coloring of a large-area stainless-steel surface.Under natural lighting conditions, the surface of the metal plate presents rainbow-like colors at a fixed viewing angle (Figure 2a), and when the viewing angle is changed, full colors covering red, green, and blue can be vividly observed on the stainless-steel surface.As the observation angle increasing, the color changes from a longer wavelength of red to shorter wavelength of color like blue or purple.Figure 2b,c shows the phoenix patterns of LIPSS on stainless steel under different viewing angles.The same samples show different color distributions.Figure 2d shows the microscopic morphology of LIPSS under optical microscope.The LIPSS has a structural period of about 1 μm, which is approximately the incident laser wavelength, while its direction is perpendicular to the laser polarization direction.LIPSS surface stripes are uniformly distributed in space.And its Fourier analysis is shown in Figure 2e, which also illustrates that LIPSS exhibits a spatial distribution with an approximate sinusoidal function.
To study the effects of laser-processing parameters on metal surface coloring, we take the photographs of the stainless steel with the LIPSS as functions of the laser power and observation angle every 5°(Figure 3a).With the observation angle increasing, the color of the surface changes from red to blue.As the laser power increases from 80 to 120 mW, the color becomes brighter with the increase of color saturation, indicating an Helmholtz-Kohlrausch effect. [14]However, when the laser power continues to increase above 120 mW, the surface color becomes dim.Meanwhile, the color saturation and brightness of the painted surface shows a great correlation with its microstructure.Figure 3b-d shows the SEM images of stainless-steel surface irradiated by laser with a power of 80, 110, and 140 mW, respectively, despite showing similar periods, their morphologies are different apparently.When the laser power is low (80 mW), as shown in Figure 3b, the stripe-like structure only accounts for a very small proportion due to the low laser power, and most of the surface area remains a flat metal surface, which corresponds to the slight color change in Figure 3a. Figure 3c shows the SEM image of the surface when the laser power is 110 mW, which shows that the surface is clearly seen to be covered by uniform LIPSS (⊥) whose direction is perpendicular to the laserbeam polarization with a spatial period of 950 nm.In addition to low-spatial-frequency LIPSS (LSFL) (⊥), high-spatial-frequency LIPSS (HSFL) (‖) is also observed, oriented parallel to the polarization direction of the laser beam, with a spatial period of 330-550 nm.The formation of LSFL (⊥) is generally explained by the interference mechanism, [15] while the mechanism of the HSFL (‖) is still under debate. [16]Thereby, at a higher power of 110-120 mW, LIPSS forms clear and vivid colors on the stainless-steel surface, as marked by the dotted box in Figure 3a.When the laser power continued to increase to 140 mW, as shown in Figure 3d, local excessive ablation or even melting occurs on the metal surface, forming irregular stripes on the metal surface, [17] and the color also becomes dim under such processing conditions.To summarize, different laser powers result in the formation of different morphologies on the surface, leading to different color distributions in the final LIPSS.
To confirm the color of LIPSS under white light, LIPSS was prepared on stainless steel with a laser power of 120 mW, a speed of 20 mm s À1 , and laser pulse energy of 4.8 μJ.A spectrometer was used to detect the spectrum of reflection light with the angle of incident white light of 45°(Figure 4a).The measured spectrum is shown in Figure 4b, showing that the color gradually changed from red to blue with the increase of observation angles.It is worth emphasizing that Figure 4b depicts the proportionality of the distribution of the received spectra, the data have been normalized and there is no proportionality between the vertical coordinates corresponding to the different lines.As shown in Figure 4c, the spectral results of different observation angles (30°-70°) are put into the Commission Internationale de l'Eclairage (CIE) chromaticity diagram software to calculate to get different color distributions, and the color distributions in the diagram are wider, and all kinds of colors can be obtained from the red of the long wavelength to the purple of the short wavelength.

Analysis on the Coloring Mechanism
To understand the mechanism of LIPSS color formation and figure out the reason for the color change with the observation angle, in this article, the finite-element analysis is used to solve its coloration at different angles, and three conditions was considered before modeling: 1) the formation of oxide film on the metal surface caused by the thermal effect of the laser; [18] 2) the periodic structure formed by the interference between the incident laser pulse and the surface plasmonic wave; [19] and 3) the periodic structure and its oxide layers generated by laser are uniformly distributed in space.Thus, the effect of LIPSS on the metal surface color is considered in two parts, including the effect of the metal surface oxide film on the surface color, and the effect of the periodic structure on the metal surface color.Here, to address the role of these two parts on coloring mechanism, we established a numerical model based on the numerous research results on the LIPSS.Typically, as shown in Figure 5a, the bottom layer is metal; the metal surface is LIPSS structure, presenting the absolute value of the sine function fitted to the metal surface in numerical calculations with a period of 1 μm (approximate to the laser wavelength used in the experiment), a structure height of 200 nm, [20] an oxide layer thickness of 50 nm; [21] then above is the vacuum environment; the left side of the model is the light source; and this finiteelement simulation simulates the LIPSS coloring mechanism, so the light source is selected as white light, with wavelength of 380-780 nm, and an incident light of 45°.The reflections light  is on the right side.Diffraction occurs when light is partially absorbed in the metal LIPSS, and light of different wavelength is reflected to the right side with uneven distribution.We calculate this distribution by light's transmission process as following equation.
where E is the electric field, ε r is the refractive index of the metal, ω is the frequency, μ is 1 in vacuum, and ε 0 is the dielectric constant.
The incident light is diffracted at the LIPSS, resulting in different distributions of different wavelengths of light received at different locations at the receiving end.The angle and energy distribution of the reflected light on the metal surface after the LIPSS is calculated by the following equation.
where I x is the x-component of the reflected light energy and I y is the y-component of the reflected light energy.The effect of the periodic structure and angle on the reflectivity of the metal surface is shown in Figure 5b.The periodic structure causes a redshift of reflection spectra with the increase of observation angle, which is consistent with aforementioned experimental measurement in Figure 4a.The influence of the oxide layer on its coloring is shown in Figure 5c.The oxide film produces an interference effect, and the coloring effect of the oxide layer is not significant and regular compared with that with only periodic structure.Thus, although the laser pulse may form an oxide film on the metal surface during the coloring process, the high-angular dispersion of LIPSS on the stainless-steel surface is mainly the result of the optical diffraction occurring on the surface of periodic structures.

Overlapping LIPSSs and Multilayer Coloring Application
As a single-laser scan can only form LIPSS in a single direction, we cannot observe any color in a direction parallel to the structure of the LIPSS.To achieve a full-color marking on steel in all directions, we propose a novel fabrication scheme of multilayer LIPSS by multiple writing in situ with changing the laser polarization direction, to present an effect of color superposition.As shown in Figure 1, by changing the polarization direction of the laser, different directions of LIPSS are formed on the surface of the stainless steel.When incident light strikes this LIPSS surface, it will not appear that the incident light is parallel with all stripes, which means the situation that no color appear in one direction will not occur.Diffraction can occur on LIPSS in at least one direction, and when incident light is diffracted on LIPSS in more than one direction, the colors of reflection light from LIPSS in different directions are not the same and will appear to be superposed.As shown in Figure 6a-c, for the color distribution of the same sample presented at different observation angles.Bidirectional arrows represent laser polarization direction.The angle between the two layers of the structure in the figure is 45°, and the graph can be seen as a superposition of two layers of colored patterns, and two different shades of color can be seen at certain angles, as shown in Figure 6a-c, where the color is a superposition of yellow and red, a superposition of cyan and blue and a superposition of purple and red, respectively.It is worth noting that the overlapping sections do have a different color than the separate areas.
For example, the color in overlapping areas in Figure 6c is between blue and green, giving the observer the impression of combining two colors, while the color in two separate areas are blue and green, respectively.Figure 6d-f illustrates the three-layer LIPSS on the steel surface, the polarization angle of each two-layer periodic structure is 60°.Compared with the results of two-layer scanning, three-layer LIPSS shows more vivid colors.In Figure 6d, the color is a superposition of blue-violet, green, and red, with the change of observation angle.And the color in overlapping areas shows the combination of different colors in visual characteristics.In Figure 6e, the color is a superposition of light blue, cyan, and yellow.Notably, the superposition of light blue, cyan, and light green in Figure 6f indicates that the range of hue changes is substantially expanded by multilayer superposition in the same area of stainless-steel surface.Thus, the multilayer LIPSS can greatly enrich the color level, making a more vivid color and richer color hue in visual characteristics.
Figure 7 shows the SEM morphology of the bilayer LIPSS sample, which was processed by two linearly polarized laser beams with different angle between the polarization directions.As shown in Figure 7, the angles between the two LIPSS are 90°a nd 60°, respectively.More different SEM morphology of the bilayer LIPSS sample can be seen in Figure S1, Supporting Information.Therefore, when the interference of light occurs on the surface of this sample, the two different directions of the stripes cause the different interference light to appear at different angles so color will be observed in all observation angles.Moreover, since the light received by the observer is a combination of interfering light from two different stripe directions, the observed color could be more vivid.
In addition, based on study of the multiple-layer LIPSS, we also demonstrate the in situ superposition of multiple-layer graphs for visual art and anti-counterfeiting applications, as shown in Figure 8.To illustrate the ability of LIPSS to construct artistic patterns, Figure 8a,b shows the "flower" printed on the polished steel plate by the multilayer LIPSS, which are illuminated at a different angle of 0°and 45°, displaying a great color-map change from yellow-green to green-blue.More details of this visual art can be seen in Figure S2 and S3, Supporting Information.In addition, multilayer LIPSS can be used for anti-counterfeiting, encoding multilayer quick response (QR) code, as shown in Figure 8c,d.We use two laser beams with mutually orthogonal polarization directions to induce two spatially complementary LIPSSs, and obtain complementary patterns of two mutually perpendicular micro-stripe directions, as shown in Figure 8c,d.At the same 90°viewing angle and mutually perpendicular lighting direction, the colored areas in Figure 8c are dim in Figure 8d, and vice versa.The colored areas in Figure 8c,d are added together to cover exactly the whole square area with an area of 20 Â 20 mm 2 .It can be seen that the QR code information in the two sets of Figure 8c,d is exactly opposite, and the coding information is also different,  proving that the technology has great prospects in the field of anti-counterfeiting.More details of these anti-counterfeiting applications in QR can be seen in Figure S4 and S5, Supporting Information.

Conclusion
In this article, the color formation mechanism of LIPSS is established by finite-element analysis method and experimental study, with the conclusion that the high-angle dispersion of LIPSS on stainless-steel surface is the result of laser-induced periodic structural fluctuations.The relationship between LIPSS morphology and laser parameters, as well as the optimal laserprocessing condition on stainless steel, has been also obtained by experiments.Based on this knowledge, we have established a fabrication technology of multilayer LIPSS by multiple writing in situ with changing the laser polarization direction, to exhibit an effect of color superposition and promote the capability of LIPSS in anti-counterfeiting and information encoding.Through this method, we demonstrate vivid color patterns on the steel surface, and realize the in situ superposition of three-layer graphs.Therefore, the proposed multilayer LIPSS coloring technology could greatly enrich the color levels, increase the information encoding capacity, and improve the anti-counterfeiting ability, which makes it competitive in industrial applications.

Figure 1 .
Figure 1.Schematic of the setup for ultrafast laser surface coloring with enlarged fabrication scheme and the enlarged picture on the right illustrates the manufacturing schemes of single-layer, double-layer, and triple-layer coloring.

Figure 2 .
Figure 2. Picosecond-laser-induced surface coloring of stainless steel; the red bidirectional arrow represents the direction of laser polarization: a) stainless-steel surface coloring results, b) phoenix patterns by laser-induced periodic surface structure (LIPSS) under viewing angles of 30°, c) phoenix patterns by LIPSS under viewing angles of 60°, d) microscope of LIPSS on the stainless-steel surface, and e) Fourier analysis of (d).

Figure 3 .
Figure 3.Effect of laser power on the color and microstructure of stainless-steel surface.a) Photographs of the stainless-steel sample marked with LIPSS as functions of the laser power and observation angle; scanning electron microscope (SEM) images of stainless-steel surface irradiated by laser with a power of b) 80 mW, c) 110 mW, and d) 140 mW.

Figure 4 .
Figure 4. a) Schematic diagram of reflection spectrum measurement with a write light incident angle of 45°, b) the measured reflection spectrum of the irradiated stainless-steel surface, and c) the CIE results of spectrum with different reflection angle from 30°to 70°.

Figure 5 .
Figure 5. Simulation results of metal surface color presentation.a) Simulation model setup and cross-sectional electric field, b) reflectance of metal LIPSS at different reflection angles when the incidence angle is 45°, and c) reflectance of metal LIPSS with metal oxide at different reflection angles when the incidence angle is 45°.

Figure 6 .
Figure 6.a-c) Laser surface coloring on stainless steel with two-layer LIPSS and d-f ) three-layer LIPSS; bidirectional arrows represent laser polarization direction.

Figure 7 .
Figure 7. SEM morphology of the double-layer LIPSS sample.Bidirectional arrows represent laser polarization direction.Two linearly polarized laser beams with an angle of a) 90°and b) 60°, respectively.

Figure 8 .
Figure 8. Printed "flower" on the polished steel plate by the double-layer LIPSS at an illumination angle of a) 0°and b) 45°, respectively; printed QR codes by the double-layer LIPSS using two laser beams with polarization directions of c) 0°and d) 90°, respectively.