Physicists harness twisted mathematics to make powerful laser

Researchers have exploited the twisty nature of topological physics to produce a high-quality beam of laser light — a step that could lead to the first practical application of this burgeoning field. A team of physicists describes its device, and the theory behind the technology, in two studies^1,2 published on 1 February in Science.

The demonstration “brings topological photonics substantially closer to real applications”, says Marin Soljačić, a physicist at the Massachusetts Institute of Technology in Cambridge.

Topology is a branch of mathematics that studies shapes and their possible arrangements in space — from simple knotted loops to the higher-dimensional universes of string theory. Since the 1980s, physicists have discovered a number of states of matter that derive odd properties from topological phenomena, such as the way that magnetization — pictured as a field of arrows — winds around a material. (Some of the founders of the field received the 2016 Nobel Prize in Physics.)

In particular, theorists have predicted — and experimentalists have confirmed — that certain insulating solids can, counterintuitively, conduct electricity thanks to topological properties. The currents arise from flows of electrons on the materials’ surfaces, and they have a unique advantage: the wavy patterns — like knotted loops — can be difficult to disrupt. This feature, known as topological protection, enables electrons to flow unimpeded. It prevents electron flows from changing direction or bouncing back off imperfections, which decrease efficiency.

Physicists have also observed topological protection in electromagnetic waves. In a solid, topological effects emerge when electrons manoeuvre their way around the arrangement of atoms. Similarly, photons of light can be made to negotiate a material that has a regular pattern of gaps. “There are many types of topological phenomena that can happen in almost any wave system in nature, when you make it periodic,” says Soljačić, whose team found some of the first experimental evidence of topological photonics about a decade ago.

A new way to channel light

In the latest studies, Mordechai Segev of the Technion Institute in Haifa, Israel, and his collaborators used topological photonics to create a laser beam, in which the light waves are in phase.