Marcus Ardron

Direct CO 2 laser-based generation of holographic structures on the surface of glass

A customized CO(2) laser micromachining system was used for the generation of phase holographic structures directly on the surface of fused silica (HPFS(®)7980 Corning) and Borofloat(®)33 (Schott AG) glass. This process used pulses of duration 10µs and nominal wavelength 10.59µm. The pulse energy delivered to the glass workpiece was controlled by an acousto-optic modulator. The laser-generated structures were optically smooth and crack free. We demonstrated their use as diffractive optical elements (DOEs), which could be exploited as anti-counterfeiting markings embedded into valuable glass-made components and products.

Tamper-proof markings for the identification and traceability of high-value metal goods

A customized UV nanosecond pulsed laser system has been developed for the fast generation of tamper-proof security markings on the surface of metals, such as stainless steel, nickel, brass, and nickel-chromium (Inconel) alloys. The markings in the form of reflective phase holographic structures are generated using a laser microsculpting process that involves laser-induced local melting and vaporization of the metal surface. The holographic structures are formed from an array of optically-smooth craters whose depth can be controlled with ± 25nm accuracy. In contrast to conventional security markings, e.g., engraved serial numbers, etched part numbers and embossed polymer holographic stickers, which are only attached to the metal products as an adhesive tape, the phase holographic structures are robust to local damage (e.g. scratches) and resistant to tampering because they are generated directly on the metal surface. This paper describes a novel laser-based process for security marking of high-value metal goods, investigates the optical performance of the holographic structures, and demonstrates their application to watches.

Nanosecond pulsed laser generation of holographic structures on metals

A laser-based process for the generation of phase holographic structures directly onto the surface of metals is presented. This process uses 35ns long laser pulses of wavelength 355nm to generate optically-smooth surface deformations on a metal. The laser-induced surface deformations (LISDs) are produced by either localized laser melting or the combination of melting and evaporation. The geometry (shape and dimension) of the LISDs depends on the laser processing parameters, in particular the pulse energy, as well as on the chemical composition of a metal. In this paper, we explain the mechanism of the LISDs formation on various metals, such as stainless steel, pure nickel and nickel-chromium Inconel® alloys. In addition, we provide information about the design and fabrication process of the phase holographic structures and demonstrate their use as robust markings for the identification and traceability of high value metal goods.

Ultrafast laser ablation giving unstructured surface roughness prior to the emergence of LIPSS

Since the first published observations of laser induced period surface structures (LIPSS) [1], such features have been produced by many research groups but aspects of the mechanism behind the emergence of these features remain unclear. Surface plasmons provide a periodic field which is understood to be involved in the formation of LIPSS as described in various publications [2-7]. However, when the incident laser energy is normal to the target surface LIPSS are formed despite there being no incoming vector component along the target surface, and hence no periodic energy structure to excite surface plasmons. A reasonable explanation is that if the surface is sufficiently rough, it will scatter the light and thereby provide periodic energy components parallel to the target surface. In this case spatial frequency components of the surface roughness could act as gratings to satisfy k vector matching to the plasmons [8]. LIPSS formation would therefore be expected to be strongly dependent on the initial surface roughness.