Drop a rock in a still pond, and it creates ripples that gradually flatten as they travel outward. But what if those waves—and the energy they carry—could be held in place? Researchers such as Son Tung Ha and Thu Ha Do, respectively a Principal Investigator and a Senior Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), are exploring how to achieve this with light.
“Many optical systems, such as lasers or sensors, rely on confining light in one place, as the longer it stays, the more effectively it can be used,” said Ha.
These systems rely on optical cavities: components designed to trap, amplify and release light in a controlled manner. A key challenge in designing such optical cavities lies in balancing a high quality (Q) factor—a strong ability to trap light—with the ability to release light efficiently under practical operating conditions.
Now, a multidisciplinary research team has designed a new optical cavity for nanoscale lasers that effectively ‘holds’ light waves in place within engineered nanostructures by leveraging an optical effect known as bound states in the continuum (BIC).
The work was the product of a close international collaboration between the research group led by Ha at A*STAR IMRE; the National Semiconductor Translation and Innovation Centre, Singapore; Nanyang Technological University (NTU), Singapore; the Université de Lyon and École Centrale de Lyon, France; and joint French-Singaporean research laboratory CINTRA at NTU.
“A BIC is a special situation where light remains trapped, even though it exists in a setting where it should be free to escape,” explained Do. “It’s like a ripple on water that can’t spread because different wave components cancel any outward motion.”
Many existing BIC-based optical cavities only achieve extremely high Q-factors under very precise conditions, which limits their practical applications. To make BIC a more practicable approach, the team engineered a system that ‘flattens’ a BIC’s band, preserving its light mode over a wider range of operating conditions.
The team’s device consisted of a carefully designed array of titanium dioxide nanopillars with a slight stretch introduced to its periodic pattern. This brought together different light modes in the structure—which would otherwise spread in various directions—causing them to cancel out each other’s behaviour and form a flatband BIC state that effectively traps light in all directions.
“This approach balances two opposing tendencies so that overall, light waves in the cavity have no strong preference to move or spread,” said Do. “As a result, the trapping effect is far less sensitive to small changes in ambient conditions. It also increases the number of available optical states, enabling stronger interactions between light and device materials.”
Experiments confirmed that the device successfully produced laser emissions from the flatband BIC mode with a Q-factor of around 9,100 at room temperature conditions. The team also found that the device required four times less energy to initiate lasing compared to conventional BIC-based devices.
The team is currently scaling up and integrating their technology into existing semiconductor platforms. “We’re translating these flat optics concepts into manufacturable devices using industry-compatible fabrication methods,” shared Ha “This will enable mass production of compact, high-performance optical components for on-chip lasers, optical communications, sensing, advanced displays and other applications.”
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
