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Physicists Untangle Twisted Light

Bessel beams offer the promise of robust optical data transmission, microscale tractor beams, cellular scalpels, and much more.
​Beam cross-sections. Image: A. Grinenko/MacDonald/Courtney/Wilcox/Demore/Cochran/Drinkwater/Opt. Express

​Imagine light as a corkscrew. The face of a spotlight is no longer a face, but an inward spiral, a drillbit made from photons. This is what's known as a Bessel beam, and it's hardly trivial: a laser than can "self-heal" if blocked, that doesn't spread out and lose energy as it travels, and that can even self-accelerate. It's a pretty cool concept, and one that offers a strange future of IRL tractor beams and "optical tweezers."

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Unsurprisingly, a Bessel beam is a bit tricky to implement—well, "tricky" as in "impossible." A true Bessel beam would require infinite energy, but physicists are doing some amazing work in approximating the phenomenon. The latest example comes courtesy of a research team spread between the University of Bristol and the University of Dundee, led by ultrasonic engineer Bruce Drinkwater and biophotonics researcher Mike MacDonald. It may be the best version yet.

Drinkwater and MacDonald ​came up with a new optic-acoustic device that's able to achieve higher-order beams—beams with more and more rings—and do so in a way that's tunable and quickly reconfigurable. That's key if the technology is to make it past the realm of "hey neat" to practical utility. And the trick to making the strangest laser beams in physics is sound: an array of 64 tiny piezoelectric loudspeakers.

Making any sort of laser involves a step called "pumping." This is where some collection of atoms contained in a cylindrical trap is given an energy boost via usually high-intensity bursts of light or electrical discharges. This kicks the electrons swirling around these atoms into higher energy states, from which they eventually fall back down from, releasing a photon (light particle) in the process. The atoms are able to stimulate each other in such a way as to make that release "coherent," with the result being a bunch of photons fired off all with the same wavelength.

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This coherence is what makes a laser. Light from a regular light bulb is noisy, consisting of all sorts of different wavelengths, while a laser beam offers coordination and a whole different sort of power. But that's just the start of it.

In Drinkwater and MacDonald's version, the 64 loudspeakers are arranged around the exterior of a small cylinder filled with water. Crucially, the walls of the cylinder are constructed such that they don't reflect sound, only absorb it. This setup allows for a increased level of control as the cylinder acts as a dynamic lens of sorts. A beam enters the tube as a normal laser beam, but is then shaped by the acoustic array. What exits is a customized Bessel beam, or at least a really good approximation.

"In general, acousto-optic devices, either of bulk or surface wave types have been widely used for a range of optical applications," Drinkwater and MacDonald ​write in the journal Optics Express. "Most notable in the context of the application discussed here, is their use in holographic displays where a high refresh rate is required so that the observer perceives a static image." But there's more to it.

Other applications/potential applications include simple data transmission—a Bessel beam can shoulder more data load without diminishing over large distances—and medicine and biotech, where the beams can achieve entirely new levels of precision, offering the possibility of ​probing the interiors of living cells without killing the cells in the process. A molecular-scale drill: The potential is, ahem, huge.