The first-ever nonmagnetic topological insulator laser has been demonstrated by a team of optics researchers from the University of Central Florida. This latest discovery could considerably enhance the resilience, beam quality, and efficiency of semiconductor laser arrays.
The results of the study have been featured in two research papers – one elucidating the concept of topological lasers and the other the experiments. The papers were reported in Science.
The project is headed by Professors Demetrios Christodoulides and Mercedeh Khajavikhan of the College of Optics and Photonics (CREOL) and their graduate students Midya Parto, Steffen Wittek, and Jinhan Ren. It was performed in association with a research team from Technion - Israel that includes Miguel Bandres, Moti Segev, and Gal Harari. The Technion team initiated the theoretical component of the work, while the experimental part was performed at CREOL.
The teams were keen to solve an age-old problem in laser physics which puzzled researchers for the past four decades: how to produce a high-power, single frequency and ultimately focusable semiconductor laser array that maintains efficiency even when there is a malfunction or failure in its sub-elements. This type of laser could be used in various fields of technology and science. A somewhat unexpected place provided the solution to this problem.
We were inspired by the developments in topological insulator materials. It was less than two years ago that the Nobel Prize in Physics was awarded to the theoretical physicists whose work established the role of topology in understanding these exotic forms of matter.
Professor Demetrios Christodoulides
Over the past few years, topological insulators have developed into one of the most novel and promising areas of physics, providing a novel insight into the fundamental understanding of protected transport. These materials are extraordinary and conduct a "super-current" on their surface while being insulators in their interior.
Defects, disorder, or sharp corners do not have an effect on the current on the surface; the current continues unidirectionally and does not get scattered. A couple of years ago, the research team wondered if there is a means to use concepts from topological science into laser physics. This could ultimately lead to a whole new range of lasers with better performance characteristics.
The researchers decided to create a topological insulator for photons, but this is easier said than done because photons do not have a charge, unlike electrons. Additionally, magnetic fields do not considerably affect semiconductor light emitting materials.
"To solve these problems, we came up with clever designs to trick photons to feel as if they are experiencing a magnetic field and having a spin," said Khajavikhan, the lead experimentalist.
The researchers utilized an array of microring resonators which were organized in such a way to imitate the presence of a magnetic field. When they pumped only the rings on the array’s periphery, they triggered the laser to emit in the topological edge mode. By efficiently using all the available pump power, this mode, which travels along the edge, generates a coherent, single-mode, high power beam.
The study results in a new range of active topological photonic devices that could be easily incorporated into antennas, sensors, and other photonic devices. The study showed that topological insulator lasers are theoretically possible and experimentally viable, and they also marked the first-ever practical application utilizing such topological principles in optics.
"There is a great pleasure to see how a line of fundamental research can address such tangible and practical problems," Christodoulides said.