The coloration of stainless steel surface due to the formation of spatially periodic structures induced by laser pulses of nanosecond duration is demonstrated. The period of microstructures corresponds to the laser wavelength, and their orientation angle depends on the adjustment of laser polarization. The marking algorithm for the development of authentication patterns is presented. Such patterns provide several levels of protection against falsification (visual, colorimetric and structural) along with high recording speed and capability of automated reading.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Modification of micro- or nano-scale topographies of metals is essential to various fields of both science and industry, such as laser marking (including color marking) [1–5], biomedical applications [6–9] and control of tribological performance . Moreover, marking of products is important not only for identification purposes but also authentification, i.e., as an advanced technology of counterfeit protection. The technology of protection against falsification must meet the following requirements: it should be difficult to copy and easy to visually identify. Several different methods exist: encrypted microparticles , holograms , micro-printing , serialized barcodes , UV, IR printing , color shifting ink or film , taggant fingerprinting . The combined methods are the most advanced, which include “open” (visible to the naked eye) and “hidden” informational patterns used for product authentication. Holographic methods  based on holographic labels are highly popular. Stickers are placed on products using glue line, and holographic image is often partially destroyed during its removal due to the film structure of the pattern. Rainbow holograms [19,20] are used in a wide range of security applications such as credit cards, banknotes, and quality merchandise. However, in some cases, these methods do not provide reliable protection against falsification.
Papers [1,2] demonstrate an effect of metals coloring using light diffraction on small-scale laser-induced periodic surface structures (LIPSS) obtained under Ti: Sapphire laser system action. Paper  shows the formation of ripples with a period of 660 nm, which are used for steel surface coloring by separating the image into pixels with different LIPSS orientation. Reading the obtained colored images was carried out using a scanner, and the method was proposed for protection against products falsification. However, the stability of femtosecond laser systems is still not high enough to be used in large-scale manufacturing, which makes it difficult to widely implement this method.
In our previous work  a method of coloring the metals via LIPSS using fiber laser with nanosecond pulse duration was demonstrated, which also allows controlling the structures orientation by changing the polarization and incident angle of the laser beam. Currently, fiber lasers are widely used in the industry not only because of their beam quality but also because of their high reliability, efficiency, and low operating expenses.
The purpose of this work is to develop the technology of the products protection against falsification based on the creating of protecting patterns containing hidden information on the metal parts with nanosecond pulses of the fiber laser.
2. The method of authentification patterns formation
Stainless steel, being a common metal for industrial applications, was used in our experiments. AISI 304 stainless steel plates (thickness 0.5 mm, arithmetic mean roughness Ra = 0.02 μm, maximum height Rz = 0.18 μm) was cleaned with ethanol before laser treatment.
The technology of metal products protection against falsification is based on the laser-induced formation of periodic surface structures with given orientation, reading them and comparing to a previously recorded pattern [Fig. 1].
The formation of periodic structures was performed via laser irradiation in air using the commercially available system based on pulsed fiber ytterbium laser with a wavelength of λ = 1.06 µm, which generates pulses with a duration of τ = 100 ns at a repetition rate in the range of f = 20-99 kHz. A laser beam with a focal diameter of d = 50 µm was moved along the surface of the sample with a speed of Vsc = 1–250 mm/s. Laser intensity I was ranged from 1·107 to 3·107 W/cm2. The exposure was carried out by both irradiating every point by a series of pulses (single point exposure, the distance between two successive points is about 50 μm, i.e., without overlapping) and scanning the sample surface (scanning exposure) with a scanning step along X/Y axis Mx/y = Vsc/f.
The difference between images recorded with different LIPSS orientation angle is noticeable with a naked eye, however, for an automated check of product authenticity a flatbed scanner can be used. A sample is being placed on a glass plate, along which a scanning slide with a white light source moves. The light from the source falls onto the sample at a certain angle, decomposes into a spectrum due to diffraction on LIPSS and goes into a narrow diaphragm with a CCD sensor behind it.
During the first stage, an authentification template is developed; its structural elements consist of LIPSS with orientation, that depends on laser exposure parameters (single point exposure or scanning exposure, polarization). A half-wave waveplate is used to control polarization. During the second stage, a formation of authentification pattern with given colorimetric parameters (LIPSS orientation) is performed, by assigning a color from the template to each pixel. During the third stage, the pattern is read with a scanner, and the colors of pattern elements are compared to the chosen colors of the template.
Obtained samples were studied using optical (Axio Imager A1.m, Carl Zeiss) and atomic force (NanoEducatior, NT-MDT) microscopy. Epson Perfection V370 photo scanner was used to read the recorded structures.
3. Results and discussion
3.1. LIPSS formation
Single point exposure was performed with the following laser processing conditions: pulse duration τ = 100 ns, repetition rate f = 20 kHz, intensity I = 1.7·107 W/cm2 and number of pulses N = 20, 40, 60 and 80 [Figs. 2(a) and 2(b)].
Pictures show that with an increase in the number of laser pulses, ordered LIPSS that are formed transform into pseudo-ordered ones. LIPSS formation is attributed to the excitation of surface electromagnetic waves , which are generated by the interference between the radiation scattered from the irregularities of surface roughness and the incident electromagnetic wave. It results in the formation of non-homogeneous temperature distribution on the surface of the metal, that is defined by corresponding “intensity field” of the interfering waves. At given laser irradiation parameters, experiments show that even at N = 20 pulses [Fig. 2(a)], the cavities appear on the surface, and the forming liquid phase is pushed from the bottom of the cavity to the edges due to evaporation recoil pressure . At N = 20 pulses LIPSS can be observed near the edges of the cavity [Fig. 2(a)]; their direction is perpendicular to the polarization of laser radiation. At N = 40 [Fig. 2(a)], microstructures form a continuous one-dimensional diffraction grating with a period of approximately 1 µm, which corresponds to the laser wavelength. Light diffraction on LIPSS obtained under this processing conditions is the most pronounced. Further increasing the number of light pulses (at N > 40) leads to the formation of microirregularities [Fig. 2(a)]. It appears that the overlap of interference fields of surface plasmon-polaritons excited under each pulse determines the appearance of such structures . Another reason could be an excitation of ripple waves [24,25].
With a decrease in scanning step, LIPSS formation can be observed during a line-by-line scanning of the surface with a series of laser pulses, which was demonstrated in our previous work . Corresponding microimages and the cross-section profile of steel surface after scanning exposure are presented in Figs. 2(c) and 2(d).
3.2. Development of authentification template and design of protection pattern
Development of the authentification template is an important part of protection pattern formation technology. The template is used to obtain the original colorimetric data for every structure, which is essential for the following authentification of protection pattern. Since LIPSS orientation angle plays a key role in the formation of various colors. Since LIPSS orientation angle plays a key role in the formation of various colors, the authentification template consists of structural elements with different orientations of structures. Chromaticity coordinates are assigned to each orientation according to International standard CIE 1931 . Figure 3 shows the examples of authentification template recorded for single point exposure [Fig. 3(b)] and scanning exposure [Fig. 3(a)] resulting in the appearance of LIPSS with orientation angles from 0° to 180° (in increments of 15°).
A combination of obtained templates was used to create “ITMO University” patterns in single point exposure mode [Figs. 4(a)–4(d)] and LLC “ProColorit” logo in scanning exposure mode [Figs. 4(e)–4(h)]. It is important to note, that both methods can be simultaneously used during recording, which allows obtaining advanced protection patterns on metal products.
Areas of the same color are determined in the chosen image, which is compared to the authentification template colors. Then the recording method is chosen, and laser exposure is performed. Colors assessment was carried out using CIE chromaticity coordinates, color difference parameter ΔEab* was calculated similarly to . It was found that ΔEab < 5, therefore the obtained colors are basically the same.
Figure 5 shows images of protection pattern made under different illumination angles [Fig. 5(a)] and corresponding scanned images [Fig. 5(c)]. Both recording methods were used to obtain the protection pattern [Figs. 5(b) and 5(d)]. Protection pattern recording speed is 0.3-0.6 cm2/min.
In this paper, a method of metal surface marking based on LIPSS generation is proposed. Using the effect of light diffraction on LIPSS, markings that change color under the different viewing angles were obtained. This method has practical importance as a protection against falsification and marking of steel and other metals products. By fine-tuning the structural elements (obtained small-scale spatially periodic structures), it is possible to get the contrast and readable colors, which allows one to combine structural elements and create protection patterns with hidden information. The following levels of protection against falsification can be introduced: visual (change of color under different viewing angles); colorimetric (exact match of colorimetric coordinates in comparison to authentication template ΔEab* < 5); structural (recording method: single point exposure and scanning exposure modes).
The efficiency of the suggested method is comparable to other methods , but it can be enhanced by using higher repetition rates of laser pulses, which will be demonstrated in the further works. An additional advantage is that LIPSS are formed under the action of nanosecond pulses of a fiber laser, which is an industry-ready commercially available.
Ministry of Education and Science of Russia (agreement #14.578.21.0197, RFMEFI57816X0197).
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