Saturday, July 23, 2016

Winding Light Takes New Paths

Light travels in a straight line. If that ceased to be the case, reflections, shadows, and really the whole world would make a lot less sense. During the past several years, however, scientists have created beams of light that curve as they travel, called accelerating beams. This crazy-sounding development could have wide ranging applications in fundamental research and practical technology, such as allowing visible light or information to be sent around obstacles.

Most of these beams travel in convex paths, like the trajectory of a bullet bending under gravity, with a few specific exceptions. However, in work published last week in Physical Review A, a team of researchers from Sun Yat-sen University in China demonstrate a technique for designing beams with a variety of other trajectories.

Accelerating beams self-bend. In other words, scientists don’t change the path of the beam with mirrors, lenses, or other optical tools—an accelerating beam bends itself. Although this seems to contradict our fundamental knowledge of how light travels, the reality is more subtle.

In a laser beam, the light waves are all in sync. To put this in more technical terms, the waves have the same wavelength and they are in phase with one another (NASA has a nice visual explanation). This isn’t the case in accelerating beams. Accelerating beams are created when light waves with different phases and amplitudes (brightness) interfere in very specific ways. Each wave travels in a straight line, but together they add up to a beam whose brightest region follows a curved path.

After their experimental debut in 2007, accelerating beams attracted a lot of attention. Scientists began studying and creating them in earnest. Most of the work so far has led to accelerating beams that travel along convex paths, which curve outward. To open up the technology to more applications, research groups are working on ways to create beams that can travel on a wider variety of paths.

The researchers at Sun Yat-sen University, led by Yujie Chen, recently developed a new theoretical method that can be used to wind light beams along a range of different, non-convex paths. This new method builds on a common technique that leads to convex paths, but incorporates another step that breaks through the limits on path shape.

This new method also connects individual paths to specific initial conditions. In other words, if you want the beam to travel on a certain path, the theory provides you with the starting place. It tells you how different phases and amplitudes should be distributed along your starting beam so that the light waves interfere in a way that produces the result you want.

Next, the team tested their method experimentally. In a nutshell, their experiment sent a laser beam through optical tools that created the distribution of phases and amplitudes they wanted, and recorded what happened when the waves interfered.

The process sounds pretty straightforward, but in reality things aren’t quite so simple. Chen and his team initially used a tool called a bulk spatial light modulator (SLM) to control the properties of the laser beam and generate accelerating beams, but they were not satisfied with the results. So, they turned their attention to a technology called integrated optics, which hasn’t been used in quite this way before.

Using integrated optics, they were able to replace the bulk SLM by a quartz glass plate just 2cm x 2cm x 0.05cm. They divided the central area of the plate into over 350,000 pixels and etched specific patterns into them. These patterns controlled the phases of the light waves that passed through them. Some of the pixels were also partially covered with metal, which controlled the intensity of the light waves passing through them.

An illustration of the laser beam passing through the integrated optics and becoming an accelerating beam.
Image Credit: Yuanhui Wen, Yujie Chen, and Siyuan Yu
By passing a laser beam through an integrated optics plate, the team created a beam that traveled along a curved, non-convex path in one direction, as well as a beam that wound around in two directions. The results were a great match to their predictions. This bodes well for the team’s new method of designing accelerating beams that can curve and twist in a variety of ways, and their use of integrated optics to produce them. The work could help researchers send light beams winding along paths that weren’t possible until now, which would open the door to even more potential applications.

—Kendra Redmond

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