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Physicists Carve 'Logarithmic Spirals' Into Steel with Laser Vortexes

The results are beautiful but also might change how we store information.
Image: Perrie et al

The sworls above are more than just excellent woozy patterns, they're the result of beams of polarized light twisted into "logarithmic spirals" and then imprinted onto the surfaces of steel plates. Though first described by Descartes and found pretty much everywhere in nature, logarithmic spirals and their peculiar set of properties had not been created artificially in a lab (with lasers) until Walter Perrie, an engineering researcher at the University of Liverpool, and his group set about carving up metal with optical vortices.

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Perrie's research is published in the current issue of Optics Express.

You know what a logarithmic spiral is if not by that name. An example is the Golden Spiral, which grows in accordance to the Golden Ratio, an unresolvable or irrational number that results in endless self-similar repetitions. It's a shape that looks the same at any scale, basically. Examples in nature include certain seashells, galaxies, corneal nerve arrangements, a hawk circling its prey/an insect circling a light source, and the whorls of a cyclone.

It's hard to imagine light itself as a spiral, but that's just what happens. Light as it travels through space has a polarization, which is the orientation of its constituent electromagnetic waves relative to the beam's direction of travel—a vertically polarized beam features waves that stick straight up like a laser mohawk. A radially polarized beam has waves that all point inward toward the center, while an azimuthal polarization is sort of the opposite: waves that circle the beam's direction of travel, where instead of pointing out or in, they lay flat against (at a tangent to) the beam's direction of travel. It's easier to see:

It's possible to achieve either of those configurations using a waveplate, which is a device that alters the polarization of any light passing through it. And it's possible to get something really cool by creating beams that feature both polarizations at the same time, as is the case with Perrie's logarithmic spirals. We can see easily enough how combining radial waves (like spokes) with azimuthal waves (like the wheel) would result in a spiral.

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It wasn't enough to just make the beams, the Liverpool group used them to machine the "highly complex surface micro-patterns" seen in the images above, basically capturing these strange laser vortices on the surface of a steel plate as "Laser Induced Periodic Surface Structures." The actual imprinting process is known as surface plasmon structuring.

Image: Perrie et al

So, what is the point of all of this? For one thing, the technique might be useful for storing information, as the researchers explained to Physics World. The grooves created on the plates are incredibly tiny, separated only by a distance equal to the wavelength of the beams used. Imagine a record with grooves with the widths of electrons.

But it's also just cool, and an interesting viewpoint into the strange nature of light itself—particularly how the angular momentum of light is shared by the spin of individual electrons (spin angular momentum) and the momentum of the particles themselves as they move through space (orbital angular momentum).

Here, it seems that spin angular momentum is being converted to orbital angular momentum, which is still a pretty exotic and recent notion. "We've got to think seriously about what that really means," Perrie told PW.