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Tiny lasers acting together as one

New type of a topological vertical cavity laser array developed

25.10.2021 - Israeli and German researchers have developed a way to force an array of vertical cavity lasers to act together as a single laser – a highly effective laser network the size of a grain of sand.

Cell phones, car sensors or data trans­mission in fiber optic networks are all using Vertical-Cavity Surface-Emitting Lasers (VCSELs) – semi­conductor lasers that are firmly anchored in our everyday technology. Though widely used, the VCSEL device has miniscule size of only a few microns, which sets a stringent limit on the output power it can generate.  For years, scientists have sought to enhance the power emitted by such devices through combining many tiny VCSELs and forcing them to act as a single coherent laser, but had limited success. The current break­through uses a different scheme: it employs a unique geo­metrical arrangement of VCSELs on the chip that forces the flight to flow in a specific path – a photonic topo­logical insulator platform.

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 Mordechai Segev has introduced these innovative ideas into photonics, and demonstrated the first photonic topo­logical insulator, where light travels around the edges of a two-dimensional array of waveguides without being affected by defects or disorder. This opened a new field of 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. That meant that the whole system was again subject to a power limit, imposed by the device used to get the light out, similar to having a single socket for a whole power plant. 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.

The research was conducted by PhD student Alex Diko­poltsev from the team of Mordechai Segev, of the Physics Department and the Electrical & Computer Engi­neering Department at the Technion, Israel Institute of Technology, and PhD student Tristan H. Harder from the team of Sebastian Klembt and Sven Höfling at the Chair of Applied Physics at the University of Würzburg, and the Cluster of Excellence ct.qmat - Complexity and Topology in Quantum Matter, in colla­boration with researchers from Jena and Oldenburg. The device fabrication took advantage of the excellent clean room faci­lities at the University of Würzburg. “It is fascinating to see how science evolves”, said Segev of the Technion. “We went from funda­mental physics concepts to foundational changes therein, and now to real technology that is now being pursued by companies. Back in 2015, when we started to work on topological insulator lasers, nobody believed it's 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 like a bunch of lunatics searching for something that was considered impossible. And now we have made a large step towards real technology that has many appli­cations.”

The team utilized the concepts of topo­logical photonics with VCSELs that emit the light vertically, while the topo­logical process responsible for the mutual coherence and locking of the VCSELs occurs in the plane of the chip. The end result is a powerful but very compact and efficient laser, not limited by a number of VCSEL emitters, and undisturbed by defects or altering tempera­tures. “The topological principle of this laser can generally work for all wavelengths and thus a range of materials,” explains Sebastian Klembt, 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 very 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 ground­breaking research has demonstrated that it is in fact theo­retically and experi­mentally possible to combine VCSELs to achieve a more robust and highly efficient laser. As such, 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. (Source: U. Wuerzburg)

Reference: A. Dikopoltsev et al.: Topological insulator vertical-cavity laser array, Science 373, 1514 (2021); DOI: 10.1126/science.abj2232 

Link: Würzburg-Dresden Cluster of Excellence ct.qmat, Universität Würzburg, Würzburg, Germany • Electrical & Computer Engi­neering Dept., Technion – Israel Institute of Technology, Haifa, Israel

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