Research paperFabrication of holographic optical elements on silver by nanosecond IR laser source
Graphical abstract
Introduction
Laser engraving of diffraction optical elements (DOE) on metals has attracted a lot of attention recently [[1], [2], [3], [4], [5], [6]]. Apart from the interesting physics and materials science involved, there is a lot of technological interest in exploiting DOEs [[7], [8], [9]] especially for security and anti-counterfeiting purposes, considering intellectual property rights crimes are worth more than USD 450 billion annually worldwide [10]. Diffractive security markings have been fabricated using laser engraving in metals including stainless steel [11], copper [12,13], and aluminum [14] to name a few. A special case of DOEs are the holographic optical elements (HOEs) where their reconstruction is a meaningful hidden picture, e.g. text or a logo. This hidden information is encrypted through the design of a Computer Generated Holograms (CGH) [15] which is essentially a phase mask that is computed numerically. HOEs are therefore well suited for security and anticounterfeiting applications [[1], [2], [3]]. In its simplest form, by engraving unique HOE, authentication is possible by illuminating the HOE with a laser beam (e.g. using a laser pointer) and comparing the reconstruction with manufacturers' data sheet. Despite the advances in laser based encryption methods that range from simple hologram engraving to more sophisticated approaches that employ phase masks as the hard keys [16] or orbital angular momentum beams as light decoders [17], the actual fabrication of HOEs in a cost effective manner still poses a challenge especially for processing highly reflective precious metals [18]. This paper concentrates on silver metal processing, as real example of treating precious metals. Similar results, despite not presented here, were obtained on gold samples too.
In previous published works by other groups, UV ns sources are used to obtain smaller pixel size resulting to better hologram reconstruction quality. The use of IR ns lasers for micropatterning will provide lower performance, due to higher material reflectivity at this wavelength [18]. Additionally, the operation wavelength limits the minimum achievable feature size due to the diffraction limit, while the nanosecond duration of the pulses induces pronounced thermal effects that adversely affect structural quality of the engraved features. However, IR sources have two emphatic advantages over UV sources: cost-effectiveness and compatibility to large scale industrial processes. Therefore, it is highly desirable to develop laser engraving and patterning techniques specialized for metals, that employ IR ns laser sources. Ultrashort IR laser sources have been also investigated by us, giving interesting results, but their cost, physical dimension and weight limit their applicability in entering markets like the jewelry market.
Laser processing of silver include basic well known laser marking, engraving and cutting applications [18,19], as well as more advanced like colorization [20,21] or synthesis of silver nanoparticles in e.g. liquid environment [22]. The particulars of silver laser ablation have been explored both for fs and ns sources [23,24]. However, there are no accounts in the literature on laser engraving holographic elements on bulk silver and on other reflective precious metals.
In the present contribution we adopt a novel approach that involves the use of IR lasers to engrave HOEs on silver. We employed a modified commercially available ns fiber laser emitting at 1070 nm that is commonly used for jewelry applications. As it will be shown in the following, it is possible to engrave pixel features of the order of 15 μm which suffices for amplitude holograms of moderate complexity. This can be realized by controlling the conditions that exert thermal effects, namely the power, pulse duration, number of pulses, time delay between consecutive single pulses as well as number of passes. Also, this is facilitated by the fact that silver has high ablation threshold.
Section snippets
Equipment and design of HOEs
A specialized laser system and software control were developed by Sisma [25], allowing single pulse operation and pulse duration control for the purposes of the present work. The laser system is based on a 1 mJ energy pulsed Ytterbium Master Oscillator Power Amplifier (MOPA) fiber laser source emitting at 1070 nm, with adjustable pulsewidth between 4 and 200 ns. The laser beam is delivered onto the sample by means of a galvo mirrors scanner and a f-theta lens (f = 100 mm). A dedicated control
Fabrication of HOEs
We engrave the HOEs on the surface of a silver medallion shown in Fig. 2. The silver sample is highly polished to produce mirror-like surfaces (Ra < 5 nm). Presence of scratches on the surface lowers the HOE efficiency, resulting in an unclear if not indiscernible laser reconstructed image. Our preliminary tests with UV sources on silver samples, have shown that, contrary to what happens with harder materials such as stainless steel [2], it is not possible to precisely control the pixel depth,
Discussion
We have attributed the difficulty in micro-engraving silver with ns IR laser sources on the change of the optical properties of the surface of silver upon irradiation and consequent heating.
Fig. 4 shows the change in the reflectance spectra of heat-treated silver samples. Two different pure silver samples that come from the same foil (mean thickness 200 μm), were heat treated at two temperatures, 400 °C and 800 °C for 10 min and then cooled in air. The reflectance is measured using UV–Vis LAMDA
Conclusions
In conclusion, we demonstrated for the first time to our knowledge the fabrication of HOEs on silver using a ns laser fiber station operating at 1070 nm wavelength. In doing so, we used a single-pulse-to-pixel approach that allowed efficient control over the optical characteristics of the reconstructed image. Our results are of immediate importance to anti-counterfeiting applications with potential for elaboration to other application fields.
Declaration of Competing Interest
The authors declare that they have no known conflict of interests that could have appeared to influence the work reported in this paper.
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