Fractals commonly show up in nature, from spiral-shaped seashells to heads of cauliflower. Now physicists have found these complex, self-repeating patterns in a very unnatural spot: laser light.
Peer closely at a small section of a fractal, and it looks just like the whole. About 20 years ago, researchers predicted that this type of pattern could appear in light from certain types of lasers. But no one had seen those patterns until now, scientists report in a study published January 25 in Physical Review A.
The inside of a typical laser consists of a cavity with mirrors at both ends, in which light bounces back and forth, through a crystal that amplifies the light. It’s not obvious how something that simple could make something as complex as a fractal. “In lasers, it is really very surprising” to find fractals, says physicist Johannes Courtial of the University of Glasgow in Scotland, a coauthor of the study.
Depending on the shape of an opening called an aperture inside a laser (indicated at top), scientists saw various fractal patterns (red).
In the laser the researchers studied, a pattern of bright and dark spots of light is imprinted by an aperture, a hole of a particular shape within the mirrored cavity that the light passes through as it bounces back and forth. That dappled pattern gets magnified with each bounce, creating the same shapes on small and large scales — a fractal.
Scientists revealed the fractal pattern by extracting some of the light from the cavity, and measuring its brightness in a 2-D slice. The fractal’s appearance varied depending on the size and shape of the aperture the scientists used, which included hexagon and snowflake shapes.
Theoretical calculations hint that, in addition to these 2-D patterns, laser light should also display 3-D fractals. Finding those patterns will be scientists’ next challenge.