Electrically Driven Topological Laser Bypasses Manufacturing Imperfections

Researchers from Nanyang Technological University (NTU Singapore) and the University of Leeds (UL) have created what they believe to be the first electrically driven topological laser. The use of topologically protected photonic modes enables this laser to efficiently bypass manufacturing defects as well as corners.

In the 1980s, scientists found that electrons flowing in certain materials had “topological features”; that is, they could flow around corners or imperfections without scattering or leaking. In collaboration with material scientists from UL, the NTU Singapore team applied this topological approach to photons. The researchers worked with a quantum cascade laser based on advanced semiconductor wafers developed by the UL team.

To achieve topological states on a laser platform, the researchers developed a design containing a valley photonic crystal. Their design was inspired by electronic topological materials known as two-dimensional (2D) valleytronic insulators.

The compact design consists of hexagonal holes arranged in a triangular lattice, etched into a semiconductor wafer. Within the microstructure, the topological states of light circulate within a triangular loop with a 1.2-mm circumference. The loop acts as an optical resonator to accumulate the light energy required to form a laser beam.

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. Courtesy of NTU Singapore.

“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,” professor Yi Dong Chong said. “Ordinary lightwaves would be disrupted by the sharp corners, preventing them from circulating smoothly.”

The topological quantum cascade laser emits light at terahertz (THz) frequencies. Previous demonstrations of topological lasers have required an external laser source for optical pumping and have operated in the conventional optical frequency regime. The NTU Singapore/UL team demonstrated an electrically pumped THz quantum cascade laser based on topologically protected valley edge states, whose compact design makes use of the valley degree of freedom in photonic crystals.

Electrically driven semiconductor lasers are the most common type of laser device today and are used for a range of products, from barcode readers to laser ranging sensors for autonomous vehicles. 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 manufacturing. The new topological laser overcomes this issue and could lead to more efficient manufacturing using existing semiconductor technologies.

From left: NTU Singapore scientists associate professor Baile Zhang, professor Qi Jie Wang, associate professor Yi Dong Chong, and researcher Yongquan Zeng, who worked with their collaborators at the University of Leeds to develop the first electrically driven topological laser. Courtesy of NTU Singapore.

“Every batch of manufactured laser devices has some fraction that fails to emit laser light due to imperfections introduced during fabrication and packaging,” professor Qi Jie Wang said. “This was one of our motivations for exploring topological states of light, which are much more robust than ordinary lightwaves.”

Looking ahead, the joint team plans to work 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,” 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.”

The research was published in Nature (www.doi.org/10.1038/s41586-020-1981-x).