Arnaud Zoubir and Rainer Kling, Alphanov
Lasers can interact with matter in a variety of ways. Depending on the wavelength, the energy of the laser beam and a whole set of other parameters, such as pulse duration or repetition rate, laser photons have countless ways of mingling with the atoms, molecules and electrons weaving the fabric of matter. Scientists also have found countless ways to use these interactions, from spectroscopy to microscopy, from laser cooling to nuclear fusion. High-power lasers, in particular, can be used for industrial processes to cut, drill, weld, engrave and even clean materials for applications as diverse as the palette of materials they can address.
More recently, scientists have found that femtosecond lasers can reshape matter in very peculiar ways. The instrument has a good reputation: Pulses emitted are so short that machining of materials operates in nonthermal conditions, enabling unprecedented accuracy (down to 1 µm), and offering burr-free contours, almost no heat effects, and the possibility to machine all types of material. It can not only improve existing processes, but also enable new functionalities: Applied at the surface of different materials (e.g., metal or silicon) with particular parameters, femtosecond pulses enable a texturation with nanometer dimensions.
For instance, under certain conditions, metallic surfaces can be shaped to exhibit very high water repellence (superhydrophobicity), similar to the leaves of the lotus flower; thus, “the lotus effect” (Figure 1). It also is expected that this confers interesting tribological properties to the treated surfaces, which can find applications in aeronautics or micromechanics.
Figure 1. Laser-generated texturation demonstrating superhydrophobicity (the “lotus effect”): (a) and (c)
represent the natural equivalent on the lotus leaf and the corresponding nanostructures observed by SEM. Photo courtesy of ©luckypic/Fotolia.
An interesting phenomenon that scientists have observed when shining a femtosecond laser on a metallic surface is that matter self-arranges under the beam, turning boring flat surfaces into wavy structures consisting of humps and valleys called “ripples,” or LIPSS (laser-induced periodic surface structures). When the period obtained between ripples is in the same range as the wavelength of the visible light, diffraction leads to coloring effects, as with the wings of some butterflies and other bugs that nature has blessed with similar structures. The period and orientation of these man-made ripples can be controlled by the wavelength and polarization of the laser light. At the macroscopic scale, the ripples act essentially as diffraction gratings that disperse the wavelengths of light into vivid rainbowlike colors (Figure 2). This can effectively be used for decorative effects with various materials or for anti-counterfeiting markings on expensive goods.
Figure 2. Laser-generated grating structures (ripples) on metallic surface: (a) and (b) show the natural equivalent on a beetle (Calosoma sycophanta) and the corresponding nanostructures observed by SEM. Photo courtesy of ©Samuele Gallini/Fotolia.
With various sets of laser parameters, the ripples can be accentuated until they change shape and become spikes, bringing whole new optical properties to the surface exposed to the laser beam: The texture obtained exhibits characteristic features at the micro- and nanoscale, and these spike structures act as little photon traps that absorb all the light that falls on them. The result is a superblack coating with a reduction in reflectance of more than 95 percent, compared with the native reflective surface of a metal (Figure 3).
Figure 3. Superblack coating obtained on stainless steel by femtosecond laser processing: (a) and (b) illustrate the natural equivalent found in a moth’s eye and the corresponding nanostructures observed by SEM. Photo courtesy of ©pelooyen/Fotolia.
This effect is comparable to the “moth eye” effect. The eyes of a moth indeed exhibit similar “spike” features at the nanoscale that suppress reflection between air and the cornea, granting the nocturnal insect one of the best night-vision systems nature can produce. Not surprisingly, these nature-inspired structures have successfully been applied to sensors such as IR detectors and photovoltaic panels to increase their photon-conversion efficiency.
From a more mundane point of view, these structures produce a deep-black finish that can easily be shaped to create decorative effects and patterns with high precision and uniformity.
As with other laser processes, laser texturation offers many advantages: Besides being noncontact, it is a one-step process, removing the need for pre- and post-treatment.
So far, most works on femtosecond laser texturation have been performed with Ti:sapphire lasers. However, for these processes to get out of the lab and find industrial applications, more-compact and -robust lasers must be used. Also important is the processing speed that relates to the laser average power. Recent developments make use of ytterbium-based lasers that allow for higher repetition rates and more rugged design than the Ti:sapphires. With such lasers, processing speed in the range of a few seconds per square centimeter is commonly obtained, opening the door to several industrial applications.
Another advantage of this technique is that it does not rely on the use of harmful chemicals commonly used in chemical and electrochemical processes for coloring metals. In fact, the metal surface is not even coated by another material and so does not undergo any chemical reaction. It is merely reshaped in such a way that it no longer reflects light. Unlike paint, powder coats or anodizing, the coating is of the same chemical nature as the substrate and it is engraved in it, so it won’t ever flake off or induce allergic reactions to the skin when applied to jewelry.
With a more environmentally friendly approach (a "greener" black, so to speak) and faster processing times, the process is beginning to draw interest in industrial applications including automotive, detectors, advanced optical systems and luxury goods. It can be used effectively for marking various objects or whenever a metallic object needs coating to give it a high-contrast color or enhance light absorption. High-end brand marking, inscription of Datamatrix or other traceability codes, or simply marking decorative features with long-lasting stability fall well within the reach of this new laser process. Stray-light control and enhanced detector sensitivity or emissivity are other applications of this laser-produced black coating.
Mimicking nature has led to many technological innovations, including honeycomb structures for lightweight construction. On the nanoscale, nature provides a tremendous diversity of examples, such as self-cleaning surfaces with the lotus effect or the wall-climbing abilities afforded by gecko feet. Femtosecond lasers seem to be the appropriate tools to realize and exploit these features for industrial applications. In laser machining, the ripples and spike seem to be just the beginning of the nano age.
Meet the authors
Arnaud Zoubir is the business development manager at Alphanov in Talence, France; email: [email protected]. Rainer Kling is the business unit manager of laser micromachining at Alphanov; email: [email protected].