Physicists Put “Backspin” on Laser Light

Like a pool shark developing trick shots, scientists are always finding ways to bend the rules. Now, physicists from the Shanghai Institute of Optics and Fine Mechanics (SIOM), part of the Chinese Academy of Sciences, have demonstrated a technique that lets them change the dynamics of reflection. By using an intense vortex beam—a special arrangement of photons superimposed on one another to create a rotating, hollow “tube” of light—the researchers coaxed the reflected beam out of the plane of incidence, a rather extraordinary trick. Their work is slated to appear soon in the American Physical Society’s journal Physical Review Letters.
Just about everyone knows the laws of reflection. Even if you’ve never heard it formally stated, or don’t recognize the mathematics involved in a rigorous description, a few minutes playing around with a mirror and a laser pointer is enough to gather an intuitive understanding of the standard behavior of reflected light: the angle of incidence is equal to the angle of reflection. This even holds in other circumstances, like a game of pool—rebounding a cue ball off the edge of the table opens up a world of shots that would otherwise be impossible, and the reliability of this interaction makes the ricochet a powerful technique.
Ordinarily, on a pool table, everything happens on the same plane. When a ball without any spin on it hits a wall at a 45 degree angle, it will bounce off at that same angle—this is an inevitable consequence of the laws of conservation of momentum. Putting some sidespin on the ball can change this rule without violating the conservation laws, since the ball’s linear momentum is no longer the only factor at play in the interaction. However, throughout all of this, the ball remains in the plane of incidence, i.e. the plane of the table.
The SIOM team’s accomplishment is something akin to pulling off a “jump shot” on a ricochet—getting the ball to leave the table entirely when it impacts the wall. This is possible thanks to the spin of the vortex beam as well as some special properties of the reflective surface. Specifically, the reflective surface in this case is a high-density plasma. 
While it might be odd to think of a plasma as being reflective—common plasmas include the flame of a candle, bolts of lightning, or the sun—it makes sense when you consider the similarities between plasmas and more familiar reflective materials. The surface of a metal is shiny and reflective largely thanks to its conductivity. When a photon—an electromagnetic wave—strikes a mirrored surface, electrons in that surface are free to move in response to the wave. In moving, these electrons create an electromagnetic wave of their own, and this is the wave that we see as a reflection of the original light.
In a plasma, which is simply a gas that’s gotten so hot its electrons have been liberated from their home atoms, electrons are similarly free to move. This allows plasmas to exhibit a variety of surprising optical properties, depending on their temperature, composition, and other factors.
Although the scope of this work might seem limited at first (after all, how often do you run into hyperdense plasmas or optical vortex beams?), it has the potential to provide insights into, for example, the behavior of the sun—which has a number of properties that still perplex scientists. Knowing that the usual rules of reflection don’t always apply in dense plasmas might help future scientists develop models that are better at explaining the behavior of stars, or help crack the problem of sustaining fusion in a laboratory. 
Stephen Skolnick

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