Friday, November 30, 2018

Princeton Physicists Design Light-Twisting Plasma Chamber

It was my first day at the cyclotron, and I stood in an underground lab area that smelled like sawdust and solder, sectioned off behind the heavy black vinyl of a laser safety curtain.  I squinted at the chunky piece of glass held delicately between my fingers—the lab chief had called it a quarter wave plate. From certain angles, its translucent surface gleamed like a tinted mirror, reflecting a bizarrely yellow-green hued world.

"...and it makes the photons...circular?" I asked, feeling a little foolish. The wiry lab chief grinned, the corners of his eyes crinkling behind the thick rims of his glasses.
"It makes the photons spin," he replied. "The electric field does a kind of corkscrew as it moves." he punctuated the sentence with an energetic gesture, tracing a helix in the air by moving his hand as he drew circles with his index finger.

A circularly polarized photon corkscrews through space.
Image Credit: Public Domain, via Wikimedia
For me, learning that a wave of light could spin—and that this spin is the same kind of spin that governs the quantum behavior of electrons—was simultaneously humbling and gratifying. It was a point of connection between the abstract and the real, and a reminder that I still had so much to learn.

The more peculiar properties of light can be a challenge to wrap your head around, but those same properties can make it useful in ways you'd never expect: I spent a summer behind that laser curtain, firing those spinning (or circularly polarized) photons through long tubes of helium gas, trying to impart some of the light's angular momentum on the atoms inside, so that they could be used in a kind of MRI that would help search for unknown physical forces.

But as it turns out, there's more than one way to spin a photon: while circularly polarized photons can trade spin with subatomic particles, a beam of light can also carry a different kind of angular momentum, when it takes the form of an optical vortex.

At center left, a regular plane wave beam of light is shown. Above and below it are optical vortex beams, which carry orbital angular momentum.
Image Credit: E-karimi, via Wikimedia (CC BY-SA 3.0)
In circularly polarized light, each individual photon spirals around the axis that it moves along. But in an optical vortex, pictured above, the electric field of an entire beam of light is twisted. Circularly polarized light can transfer spin to subatomic particles, but optical vortices can apply torque to much larger objects.

One of the other defining characteristics of an optical vortex is that, at its center, the electric field of all the photons in the beam cancel out. This leaves an effectively "hollow" core to the laser, which makes it useful for a variety of purposes like trapping small particles, and even propelling them in tiny circles! You might have heard that another method for trapping and manipulating small particles, optical tweezerswon part of this year's Nobel prize in physics—it's fair to think of an optical vortex as being like a wrench, or screwdriver: the angular analog to the tweezer's grasping capability.

Making an optical vortex isn't easy, especially if you want a high-power beam. Working with visible light, it's possible to create an optical vortex using materials similar to those that normal lenses are made of, by carving them into a structure (called a q-plate) that shifts the phase of the light waves by just the right amount. But this is a painstaking process, and an inherently limited one: the way light interacts with materials depends on its wavelength, so you need q-plates with different thicknesses and compositions for different wavelengths of light. On top of that, if you crank up the power of the laser too high, you're liable to melt straight through your delicately-carved component.

There might be a way around the problem, though, according to a trio of scientists from Princeton University's astrophysics department: instead of making their q-plate out of something like glass or crystal, Kenan Qu, Qing Jia, and Nathaniel Fisch have proposed a design that uses a highly unconventional material: magnetized plasma.

When a gas gets hot enough, the atoms in it start to collide with such force and frequency that they can knock one another's electrons free, signaling the gas's transition into a plasma state. These free-floating electrons can conduct current, and give the material a host of other unique electromagnetic properties—earlier this month, for instance, we covered a breakthrough from CERN in the design of "plasma lenses" that could help focus beams of charged particles from accelerators. Applying similar thinking to the problem of making optical vortices, Qu and his colleagues have found a design that should allow them to make a q-plate that functions at a much wider range of wavelengths and intensities than any existing technology.

The design of the plasma q-plate is surprisingly elegant: a cylindrical chamber, which contains the gas that will be turned into a plasma, is surrounded by a series of wire loops of various sizes. When the device is activated, electric current runs through these wire loops—traveling in one direction on one side of the chamber, and in the other direction on the other side to generate a pair of magnetic fields that point opposite directions. High voltage turns the gas into a plasma, and a beam of circularly polarized light—the "spinning photons" discussed earlier—is fired into the cylinder.

(Click to Enlarge)
In the Princeton team's design, a beam of circularly polarized photons (a) travels through a chamber of magnetized plasma (b), where it is converted into an optical vortex (c). The concentric red circles represent coils of conductive wire which generate the magnetic field that shapes the plasma, producing the vortex beam. The direction of the electric field at each point in the beam's cross-section is indicated by the tick marks inside small green circles.
Image Credit: Qu, Jia, & Fisch. Physical Review E
Starting with circular polarization is what lets a relatively simple and symmetrical design like this create the twisted beam of an optical vortex. In a normal laser, circularly polarized photons all spiral through space in-sync, but the tunable magnetic field of the plasma q-plate can de-synchronize them in just the right way to yield an optical vortex. The new design may prove more versatile, as well: the ability to tune the current in the wire coils that control the plasma's magnetic field means a device that can work at a wide range of wavelengths.

The apparatus is still theoretical at this point, but in the future it might have applications from physics and astronomy to telecommunications: optical vortex beams can transmit a theoretically infinite amount of data along a single path, to be separated out into its component signals at its destination. In the real world, of course, there are limitations on that amount, but the prospect is exciting nonetheless.

"Our group does not have the facility to build it, because we are a theory group." says Qu, when asked about the possibility of a prototype. "However," he continues, "we are actively looking for groups that are interested in building it."

The earliest versions of the laser operated in the microwave range, and it was only once the tech was adapted to function at other wavelengths that its true potential began to become clear. Will we see a similar explosion of usefulness happen with vortex beams? What kind of uses will we find for an "optical wrench" at other wavelengths? Only time—and our imagination—will tell.

—Stephen Skolnick

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