The spectrum of laser technologies available to hackers has gradually widened from basic gas lasers through CO2 tubes, diode lasers, and now fiber lasers. One of the newer entries is the MOPA laser, which combines a laser diode with a fiber-based light amplifier. The diode’s pulse length and repetition rate are easy to control, while the fiber amplifier gives it enough power to do interesting things – including, as [Ben Krasnow] found, etch hologram-like diffraction gratings onto stainless steel.
Stainless steel works because it forms a thin oxide layer when heated, with a thickness determined by the temperature it reaches. The oxide layer creates thin-film interference with incoming light, letting the laser mark parts of a steel sheet with different colors by varying the intensity of heating. [Ben] wrote a script to etch color images onto steel using this method, and noticed in one experiment that one area seemed to produce diffraction patterns. More experimentation revealed that the laser could consistently make diffraction gratings out of parallel patterns of oxide lines. Surprisingly, the oxide layer seemed to grow mostly down into the metal, instead of up from the surface.
The pitch of the grating is perpendicular to the direction of the etched lines, and varying the line spacing changes the angle of diffraction, which should in theory be enough control to print a hologram with the laser. [Ben]’s first experiment in this general direction was to create a script that turned black-and-white photographs into shimmering matrices of diffraction-grating pixels, in which each pixel’s grating orientation was determined by its brightness. To add a parallax depth effect, [Ben] spread out images into a gradient in a diffraction grating, so that it produced different images at different angles. The images were somewhat limited by the minimum size required for the grating pixels, but the effect was quite noticeable.
Unfortunately, since the oxide layers grow down into the metal, [Ben] doubts whether the laser can etch molds for diffraction-grating chocolate. If you’re interested in more diffraction optics, check out these custom diffraction lenses or the workings of normal holograms.
It’s not that surprising that the oxide layer grows into the metal: it’s the metal oxide!! There is no metal above the surface of the metal, right?
Common oxides like rust have lower density than the underlying metal, so they bulge up and crack on the surface, allowing more oxygen deeper into the bulk, thus progressively destroying the object. On the other hand, stainless steel and aluminum form oxides of similar density (mostly chromium oxide and alumina respectively); their oxide layer is flat and continuous, thus protecting the base metal.
the oxide layer could also grow sideways instead of inwards, which would cause problems when generating colors
It is surprising because most of the oxides we encounter DO grow outward because they are considerably less dense than the base metal.
Examples: Rust is FAR less dense and barely penetrates the surface at all before climbing away (and then flaking/breaking off).
Even Aluminum oxide is more external than internal growth(but only slightly), even though it is much more dense than the base metal. All those extra Oxygen atoms need room.
Stainless steel is a weird outlier both because it oxidizes inward and because the oxide layer is so thin.
Is there a way to selectively etch it based on the differences in oxidization? A quick search says “Certain etchants can selectively target oxidized areas in stainless steel”. If it does etch selectively then it might be able to transfer a useful pattern to make the holographic silicone molds for chocolate.
Correction: The oxide layer thickness on stainless steel is based on the DURATION it stays at or above oxide forming temperature. This aligns with normal chemical reaction intuition. It isn’t as if higher temperatures allow for different oxides.
Making it hotter DOES mean it takes longer before it drops below the useful temperature, but that is a side effect.
Hotter DOES also make the oxide form faster (to a point).
But hotter also causes more stress and damage to the part in general.
I’ve created eerie photo prints on stainless and carbon steel with these effects. I just now found this article.