Dot-Matrix Hologram Rendering Algorithm and its Validation through Direct Laser Interference Patterning

Abstract

The fight against forgery of valuable items demands efficient and reasonably priced solutions. A security tag featuring holographic elements for anti-counterfeiting is one of them. However, the content and colours of a diffraction image that would be seen by an observer are often counterintuitive in the design stage. Here, we propose an original algorithm based on the conical diffraction formalism, which can be used to describe the variations of a diffraction image with respect to all aspects of observation. We validate the output of the algorithm by comparing it to test holograms, which we have produced by employing direct laser interference patterning (DLIP) in electrochemically grown nickel foil. We have employed a motorized femtosecond laser system to micro-machine arrays of 65 µm × 65 µm sized diffraction gratings with a defined orientation and pitch on the order of 1 µm. Based on completed diffraction efficiency measurements, we determined optimal ablation parameters, i.e. 57.4 mJ/cm2 fluence per pulse and 1100 pulses/pixel. Furthermore, we show how accurate the proposed algorithm is through measured diffraction spectra as well as captured diffraction images of test holograms produced using the obtained parameters. Finally, we showcase anti-counterfeiting tag prototypes with complex holographic effects, i.e. colour reconstruction, animation effects, and image multiplexing. The proposed algorithm can severely shorten the time between design and production of a holographic tag, especially when realizing it via a competitive origination technology—DLIP.

Tailoring of Silver Nanoparticle Size Distributions in Hydrogenated Amorphous Diamond‐Like Carbon Nanocomposite Thin Films by Direct Femtosecond Laser Interference Patterning

“…Up to irradiation by 27k pulses, the width of the laser‐affected line increases up to two times, whereas further increasing number of pulses width additionally increases only up to 1.2 times. Similar results were obtained for nickel foil ablation with the same setup, where only narrow ridges remained at the maximum energy densities applied, and the Gaussian intensity variation within the irradiated spot was stated to be visible as height modulation …”

“…The general equation, governing the pitch Λ, is as follows: Λ = λ /(2 n sin θ ), where λ is the wavelength of the laser, n is the refractive index of the surrounding medium (in our case air, n = 1), θ is the incidence angle . More details about the utilized DLIP setup can be found elsewhere . The patterning beam spot size was 75 × 75 μm 2 , and the total patterned area was 4 × 4 arrays of beam size resulting in nearly uniformly patterned areas of 300 × 300 μm 2 .…”

“…[55] More details about the utilized DLIP setup can be found elsewhere. [44] The patterning beam spot size was 75 Â 75 μm 2 , and the total patterned area was 4 Â 4 arrays of beam size resulting in nearly uniformly patterned areas of 300 Â 300 μm 2 . A laser fluence of 1-17 mJ cm À2 was applied for DLC:Ag samples, 6-145 mJ cm À2 for the Ag sample, and 1-145 mJ cm À2 for the a-C:H sample.…”

“…Similar results were obtained for nickel foil ablation with the same setup, where only narrow ridges remained at the maximum energy densities applied, and the Gaussian intensity variation within the irradiated spot was stated to be visible as height modulation. [44]…”

Development of an Analytical Model for Optimization of Direct Laser Interference Patterning

“…For example, surface functions including tribological, superhydrophobicity, and bacteria-repellency have been already demonstrated by means of DLIP in previous works [6][7][8]. It was also showed that this technology can be used for decorative applications by forming structural colors on the material surface, including in steels and polymers [9][10][11].…”