Mathematics used to make powerful lasers

Nature – Scientists have used the mathematics 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.

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

Above – A photograph of topological chip; the laser beam emerges at top-left.Credit: M.A. Bandres & S. Wittek

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.

The team etched an array of circular channels into the surface of a chip of semiconductor material, and cast infrared light onto the structure from above. The circles — each a few microns in diameter — caught light waves only of precise wavelengths, which then moved from one loop to the next.

In Segev’s system, some loops were asymmetrically shaped, which made the light flow preferentially in one direction. As the system received increasing energy from the infrared source, the circulating light pulse was reinforced, or amplified. Eventually, the light waves bounced out of an exit channel — pulsating in step as a focused laser beam.

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.