Westlake News ACADEMICS

Full-color Inkless Printing with Ultrafast Lasers

08, 2023

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Inkless printing is a big step to replace the wasteful inkjet-based colour printing processes that are leaving a huge of carbon footprint year after year. The structural colors, arising from light scattering, absorption, diffraction or interference by micro/nanostructures or thin films, are promising to achieve inkless printing. For practical applications, structural colors with wide gamut, long-stability, large-scale and viewing angle-insensitive key parameters that are decisive for the adoption in decoration technologies need to be simultaneously achieved. However, the widely adopted conventional nanofabrication techniques, such as electron beam lithography, focused ion beam milling and nanoimprinting lithography, are facing the nanoscale and macroscale processing barrier. Production of large-scale coloured surfaces with these techniques is incompatible with the demands of low-cost mass manufacturing.

As an alternative, surface coloring by ultrafast lasers can exhibit much higher throughput, which is appealing to overcome this barrier. Laser coloring generally contains three approaches that are based on different mechanisms: plasmonic colors from randomly self-organized metallic nanoparticles, diffractive colors from laser-induced periodic surface structures, and interfering colors from thin films including transparent oxide layer and FP-cavities. Nevertheless, these techniques suffer from either narrow gamut or low stability.

In a new paper published in Nature Communications, a team of scientists, led by Professor Min Qiu from Westlake University, China, have demonstrated a new laser-coloring scheme to solve these challenges. In their experiments, TiAlN-on-TiN hybrid films, coating on various substrates, are utilized as special printing papers. The thickness of TiAlN and TiN is 60 nm and 50 nm, respectively. Angle-robust structural colors with unprecedented large-gamut of ~90% sRGB are obtained. The highest printing speed reaches 10 cm2/s and the highest resolution exceeds 10000 dpi. The durability of the laser-printed colors is confirmed by fastness examination, including salt spray, double-85, light bleaching, and adhesion tests. These features render their technique to be competitive for industrial applications.

These scientists summarize the underlying mechanisms this new technique.

The colors come from laser-induced oxidation on the TiAlN surface. The pristine TiAlN-TiN hybrid films behave as broadband absorbers, when producing a thin transparent oxide layer on the absorbers, the absorption spectra and thus the observed colors will be changed. By controlling the accumulated laser fluence to modulate the oxidation depth, Various vibrant colors, spanning from red, orange, yellow, green, blue to purple, can be observed.

More importantly, as these coatings are much thinner than the visible light wavelength, the phase accumulation due to the propagation through the film is small. Therefore, they display viewing angles-insensitive colors. The authors further explain the advantages of their technique from the standpoints of printing speed, durability and viewing angle dependence:

Such ultrathin films do not only significantly decrease the material cost and growth time, but also results in small dependence of colors on the incidence angles and runs against intuition given our everyday experience with thin film interfering colors.

Using the conventional nanofabrication techniques to generate colorful patterns on thin film absorbers generally requires multiple steps of contact photolithography with alignment. However, a single step is sufficient printing colors by ultrafast lasers.

When using a high laser repetition rate of 175 kHz, the printing speed reaches to 10 cm2/s, and the highest printing resolution exceeds 10,000 dpi, which is one order of magnitude higher than the conventional inkjet printers.

We have also performed various aging tests for laser-written structural colors through ultraviolet light bleaching, salt fog and high-temperature high-humidity corrosion, and adhesion test. After exposing in these extreme environments for 120 h, the adhesion of the structural colors remains to be the highest level, and the caused color differences are fairly small.

In order to confirm that the printing processes are highly reproducible. The scientists have fabricated 64 identical samples, as shown in Figure 1. These samples have been selected as gifts for the first-year undergraduate students at Westlake University.

Figure 1. Photograph of 64 identical samples that are printed with the same laser parameters.