Artist’s impression of the world’s first electrically-driven topological quantum cascade laser, created by physicists and engineers at NTU and the University of Leeds.
Scientists and engineers from Nanyang Technological University, Singapore (NTU) and the University of Leeds in the UK have created the first electrically-driven ‘topological’ laser — a device that can route light waves around corners and cope with defects introduced in the manufacture of the device.
Electrically driven semiconductor lasers are the most common type of laser used today. Their numerous applications include optical fibre communications, laser printing, laser spectroscopy, and barcode readers, owing to their small footprint, high efficiency, and easy operation. However, the fabrication of these devices is an exacting process, and current laser designs do not work well if any defects are introduced into the structure of the device during manufacturing.
The Singapore-UK advance, which was reported in the journal Nature in February 2020, overcomes this problem through the development of a new type of laser that is insensitive to defects. This was accomplished by harnessing a concept from theoretical physics known as ‘topological states’, in order to make a ‘topological laser’.
The concept of topological states originated in condensed matter physics as a way to describe certain types of exotic matter. In the 1980s, physicists discovered that electrons in some materials can flow around obstacles without scattering or leaking. This phenomenon was later explained theoretically using a branch of mathematics known as topology, an achievement honored by the award of the 2016 Nobel Prize in Physics.
By applying the concept of topological states to the flow of light in semiconductor microstructures, the Singapore-UK team created the first electrically-driven laser with topological states of light that are resistant to the effects of imperfections.
“Every batch of manufactured laser devices has some fraction that fails to emit laser light due to defects introduced during fabrication and packaging,” explains Professor Qi Jie Wang, one of the lead investigators and a faculty member at NTU’s School of Electrical and Electronic Engineering (EEE). “This was one of our motivations for exploring topological states of light, which are much more robust than ordinary light waves.”
Photograph of the topological laser (center), with a Singapore 5-cent coin and ruler also shown for comparison. Photo credit: L. Kok.
To develop the topological laser, the team used advanced semiconductor wafers developed at the University of Leeds. They etched the wafer with a regular pattern called a photonic crystal, specially designed to support topological states. Within the microstructure, the topological states of light circulate within a tiny triangular loop of 1.2 millimetre circumference, which acts as an optical resonator to accumulate the light energy required to form a laser beam.
“The fact that light circulates in this loop, past the sharp corners of the triangle, is due to the special features of topological states,” says Associate Professor Yidong Chong, a theoretical physicist in NTU’s School of Physical and Mathematical Sciences (SPMS) and co-lead investigator of the project. “Ordinary light waves would be disrupted by the sharp corners, preventing them from circulating smoothly.”
In 2018, a team at the Technion – Israel Institute of Technology and the University of Central Florida in the USA had developed a topological laser made from an array of connected optical resonators. However, that prototype laser had the drawback of being much larger than most semiconductor lasers, as well as being ‘optically driven’, meaning that it was powered by another laser.
By contrast, the topological laser developed by NTU and Leeds is highly compact, and is driven electrically rather than optically.
Another interesting feature of the new topological quantum cascade laser is that it emits light at terahertz frequencies, between the microwave and infrared regions of the electromagnetic spectrum. Terahertz radiation is a developing frontier for technological applications in sensing, illumination, and wireless communications.
Four members of the NTU team. From left: Dr Yongquan Zeng, Prof. Qijie Wang, Assoc. Prof. Baile Zhang, and Assoc. Prof. Yidong Chong. Photo credit: L. Kok.
This research project spanned two years, and involved an interdisciplinary team of twelve researchers. Other team members include Associate Professor Baile Zhang, a co-lead investigator and faculty member at NTU SPMS; Dr Yongquan Zeng, a postdoctoral research fellow at NTU EEE and first author of the paper; as well as Professor Edmund Linfield, Professor of Terahertz Electronics, and Dr Lianhe Li, Senior Research Fellow, both at Leeds.
Looking ahead, the joint team is working on lasers that make use of other types of topological states.
“The design we used in this project, called a valley photonic crystal, is not the only way to create topological states,” says Professor Wang. “There are many different types of topological states, imparting protection against different kinds of imperfections. We think it will be possible to tailor the design to the needs of different devices and applications.”
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