VCSELs made to act together as one in ‘topological laser’ array

04 Oct 2021

Israeli and German breakthrough paves way towards medical and communications devices, and many “real-world applications”.

Artist’s rendering of a topological array of VCSELs.

Israeli and German researchers have developed a way to force an array of vertical cavity lasers (VCSELs) to act together as a single laser. They say that this “highly effective laser network” is the size of a grain of sand. The work is described in Science.

Scientists have long sought to enhance the power emitted by VCSELs by combining many such units and trying to make them act together as a single coherent laser, but so far have had limited success.

The latest achievement uses a different scheme: it employs a special geometrical arrangement of VCSELs on the chip that forces the flight to flow in a specific path – a so-called photonic topological insulator platform.

Topological lasers

Topological insulators are revolutionary quantum materials that insulate on the inside but conduct electricity on their surface – without loss.

Several years ago, the Technion Group led by Prof. Mordechai Segev demonstrated the first photonic topological insulator, in which light travels around the edges of a two-dimensional array of waveguides without being affected by defects or disorder. This opened a new field, now known as Topological Photonics, where hundreds of groups currently have active research.

In 2018, the same group also found a way to use the properties of photonic topological insulators to force many micro-ring lasers to lock together and act as a single laser. But that system still had a major bottleneck: the light was circulating in the photonic chip confined to the same plane used for extracting the light out.

The current breakthrough uses a different scheme: the lasers are forced to lock within the planar chip, but the light is now emitted through the surface of the chip from each tiny laser and can be easily collected.

This German-Israeli research project originated primarily during the Covid pandemic. The research was conducted by Alex Dikopoltsev from Prof. Segev’s team at Technion, Israel, Tristan H. Harder at the University of Würzburg, Germany, and the Cluster of Excellence ct.qmat – Complexity and Topology in Quantum Matter. Device fabrication took advantage of the clean room facilities at the University of Würzburg, Germany.

‘Once considered impossible’

Prof. Segev commented, “Back in 2015, when we started to work on topological insulator lasers, nobody believed it was possible, because the topological concepts known at that time were limited to systems that do not, in fact cannot, have gain. But all lasers require gain. So topological insulator lasers stood against everything known at that time. We were searching for something that was considered impossible. And now we have made a significant step towards real technology that has many applications.”

“The topological principle of this laser can generally work for all wavelengths and thus a range of materials,” commented German project leader Prof. Sebastian Klembt of the University of Würzburg, working on light-matter interaction and topological photonics within the ct.qmat Cluster of Excellence.

“Exactly how many microlasers need to be arranged and connected would always depend entirely on the application. We can expand the size of the laser network to a large size, and in principle it will remain coherent also for large numbers. It is great to see that topology, originally a branch of mathematics, has emerged as a revolutionary new toolbox for controlling, steering and improving laser properties.”

The team says that the research has demonstrated that it is possible to combine VCSELs to achieve a more robust and highly efficient laser. The results of the study pave the way towards applications of numerous future technologies such as medical devices, communications, and a variety of real-world applications.

The Cluster of Excellence ct.qmat–Complexity and Topology in Quantum Matter has been a joint research collaboration by Julius-Maximilians-Universität Würzburg and Technische Universität (TU) Dresden since 2019.