Scientists and engineers have created the first electrically-driven ‘topological’ laser, which has the ability to route light particles around corners.
Electrically-driven semiconductor lasers are the most common type of laser device today. They are used in products such as barcode readers and laser printers, for fibre optic communications, and in emerging applications such as laser-ranging sensors for self-driving cars.
But their manufacture is a complex process and current laser designs do not work well if any defects are introduced into the structure of the laser.
The breakthrough reported in Nature overcomes this long-standing problem and promises to lead to more efficient and less wasteful manufacturing using existing semiconductor technologies. This is accomplished by harnessing a concept from theoretical physics known as the topological states, in order to make a "topological laser".
In the 1980s, scientists found that electrons flowing in certain materials have topological features, meaning that they can flow around corners or imperfections without scattering or leaking. The 2016 Nobel Prize in Physics was awarded to three theoretical physicists who pioneered the study of such topological states of electrons.
Now, an interdisciplinary team of engineers and physicists from the University of Leeds and Nanyang Technological University in Singapore (NTU), have applied this topological approach to light particles, known as photons.
One of the senior authors of the study, Professor Giles Davies FREng, Pro-Dean for Research and Innovation in the Faculty of Engineering and Physical Sciences at Leeds, said: The topological laser is a great example of a fascinating fundamental scientific phenomenon being applied to a practical electronic device, and as our study shows, it has the potential to improve the performance of laser systems.
In the present study, the researchers worked with a type of electrically-driven laser called a quantum cascade laser, based on advanced semiconductor wafers developed at the University of Leeds.
Professor Qi Jie Wang, the lead scientist in the study, from NTUs School of Electrical and Electronic Engineering, said: Every batch of manufactured laser devices has some fraction that fails to emit laser light due to imperfections introduced during fabrication and packaging.
This was one of our motivations for exploring topological states of light, which are much more robust than ordinary light waves.
The researchers note that an interesting feature of the new topological quantum cascade laser is that the light it emits is at terahertz frequencies between the microwave and infrared regions of the electromagnetic spectrum. Terahertz light has been identified as one of the principal realms from which future technological applications in sensing, illumination, and wireless communications may emerge.
This research project spanned two years, and involved an interdisciplinary team of 12 researchers. Team members also include NTU physicists: Associate Professor Baile Zhang, postdoctoral research fellow and first author of the paper, Dr Yongquan Zeng; 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 research at the University of Leeds was funded by the UKs Engineering and Physical Sciences Research Council.
Exploiting complex physics
To achieve topological states on a laser platform, the NTU and Leeds team developed a new design containing a valley photonic crystal, which was inspired by electronic topological materials known as two-dimensional valleytronic insulators.
The design consists of hexagonal holes arranged in a triangular lattice, etched into a semiconductor wafer, making it extremely compact.
Within the microstructure, the topological states of light circulate within a triangular loop of 1.2 millimetre circumference, acting as an optical resonator to accumulate the light energy required to form a laser beam.
Associate Professor Yi Dong Chong, a theoretical physicist at NTU and a senior author of the paper, said: The fact that light circulates in this loop, including going around the sharp corners of the triangle, is due to the special features of topological states.
Ordinary light waves would be disrupted by the sharp corners, preventing them from circulating smoothly.
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