Bessel Beams: The Coolest Physics Phenomena That Technically Don't Exist

Bessel Beams are impossible to create, can't be destroyed, and don't diffract. In other words, physics has discovered yet another thing that makes no sense.

Bessel beams bear the name of Friedrich Bessel, a man whose life straddled the late eighteenth and early nineteenth centuries. And yet he helped create something that seems like it should be from the 24th century. Right now, Bessel beams are used as optical tweezers in microbiology — literally, they are beams of light that can move tiny things like cells. But someday they'll probably be used as tractor beams. It's an impressive feat for things that don't really exist.

What the hell are Bessel beams?

Bessel beams are lasers that behave very differently from ordinary lasers. Consider how the typical laser pointer behaves, creating a small red dot where you point it. Instead of a single point on a wall, Bessel beams create a bullseye: one dot surrounded by concentric rings. The number of rings is some indication of the strength of the beam. Many commercial Bessel beam devices create beams with about eleven rings. The ideal Bessel beam would have an infinite amount - because an ideal Bessel would use an infinite amount of energy. Since infinite energy isn't available, the true Bessel beam doesn't actually exist. Right now, we're working with knock-offs, but we're still doing impressive work.

Bessel Beams: The Coolest Physics Phenomena That Technically Don't Exist

Unlike a typical laser beam, a Bessel beam does not diffract and get larger as the beam gets farther from its point of origin. Your pen-sized laser pointer creates a tiny dot on your wall because the wall is relatively nearby. If your laser pointer were focused on the moon, and its beam were visible, you wouldn't see a red dot; you'd see an entire red moon. That household laser will spread so widely that the beam would be wider than the Moon's diameter. Bessel beams don't do that. In an ideal Bessel beam, although the rings around the dot spread out, the central beam itself stays focused. Real-world Bessels don't stay focused forever, but they do hold together a great deal more than the average laser.

One of the most prized attributes of Bessel beams is the fact that the central core of the beam can be blocked, without interrupting the beam. The laser essentially self-heals by using the rings which were not blocked. It's the optical terminator. It's also the idea building block for a tractor beam because it won't drop its load just because a stray space rock zooms through its beam.

How do people make Bessel beams?

Bessel beams are lasers: beams of "coherent" light whose waves are all in phase and moving in the same direction. A standard laser is made by exciting a substance inside a tube, rather like the tube in a neon sign, causing its electrons to jump up a level. When they fall back down they emit photons of one specific wavelength - making the laser one color. But how does their light become coherent? At one end of the covered laser tube is a mirror, bouncing all light off of it, and at the other end a mirror that lets only a certain percentage of light through - and generally only the light that's coming at it from the right angle. The photons bounce off the walls of the tube until they're all in phase and moving in the same direction, at which point a certain percentage of them leave in a directed, focused stream.

Bessel Beams: The Coolest Physics Phenomena That Technically Don't Exist

Creating a Bessel beam laser meands adding more technology. The laser is put through an axicon lens - a conical lens that directs the different photons of the laser towards a central point. Since light travels in waves, this means that at least two different waves are all hitting each other at the focus point beyond the lens. When light waves collide, two peaks amplify while a peak and a trough cancel out. A Bessel beam is a beam constructed when the peaks of different streams of light hit each other at the right angle to form the central core. The rings around the core are the troughs and peaks of more light waves interfering with each other. (Often scientists will include other elements to create Bessel beams, such as devices that reflect light to the right spot on the lens, or amplify the beam.)

What are Bessel beams used for?

Right now they're used for working with living cells. Most laser beams have to be focused for peak intensity, and so they require incredible precision. Cells have to be positioned just at the most focused part of the beam. Any error and they'll be killed. Bessel beams, with their continuous focus, allow scientists more leeway when using things like optical tweezers, or making optical injections into a living cell.

Bessel beams have recently demonstrated their use in physics. Scientists have made a tractor beam from ordinary lasers, but these can only be used to push objects away from the beam, and pin objects down. That's because all the momentum comes from the direction of the stream of light. Bessel beams, on the other hand, have the ability to reconstruct themselves on the far side of objects. This allows scientists to reach to the other side of an object and pull it back towards the stream of light, Star Trek style.

Via University of St. Andrews twice, Sciner.com, and Medical Answers.