First Electrically-Driven Topological Laser

Scientists and engineers from Nanyang Techno­logical University, Singapore and the University of Leeds in the UK have created the first electrically-driven topological laser, which has the ability to route light particles around corners and to cope with defects in the manufacture of the device. 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.

The electrically-driven topological laser prototype in the lab measures a tiny 4 mm in length. A Singapore 5 cent coin was placed above it for size comparison. (Source: NTU)

However, their manufacture is an exacting process and current laser designs do not work well if any defects are introduced into the structure of the laser during these processes. The Singapore-UK advance 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, physicists and material scientists have applied this topological approach to photons. “Every batch of manu­factured laser devices has some fraction that fails to emit laser light due to imperfections introduced during fabrication and packaging,” said Qi Jie Wang. “This was one of our motivations for exploring topo­logical states of light, which are much more robust than ordinary light waves.”

The researchers worked with a quantum cascade laser, based on advanced semi­conductor wafers developed at the University of Leeds. Giles Davies Freng, Pro-Dean for Research and Innovation in the Faculty of Engi­neering and Physical Sciences at the University of 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.”

To achieve topo­logical states on a laser platform, the 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.

“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,” says Yi Dong Chong, a theoretical physicist in NTU Singapore’s School of Physical and Mathe­matical Sciences. “Ordinary light waves would be disrupted by the sharp corners, preventing them from circulating smoothly.” 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.

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, a valley photonic crystal, is not the only way to create topo­logical states,” Wang said. “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.”

In 2018, a team at the Technion – Israel Institute of Techno­logy and the University of Central Florida in the USA developed a topo­logical laser made from an array of connected optical resonators. The researchers showed that the topo­logical states of light could travel effi­ciently around corners and defects in the laser array. However, this proto­type laser had the drawback of being much larger than most semi­conductor lasers, as well as being opti­cally driven. (Source: NTU)

Reference: Y. Zeng et al.: Electrically pumped topological laser with valley edge modes, Nature 578, 246 (2020); DOI: 10.1038/s41586-020-1981-x

Link: Centre for OptoElectronics and Biophotonics, School of Electrical and Electronic Engineering & The Photonics Institute, Nanyang Technological University, Singapore, Singapore

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