Stand between two mirrors that face each other, and you’ll see your reflection multiplied countless times. Engineers use a similar optics trick to create lasers. In every laser is an optical cavity: an enclosed structure with mirror-like internal surfaces. Filled with gain materials (or light amplifiers), such cavities trap light waves (photons) and bounce them back and forth, aligning them into similar spatial patterns (modes) and amplifying their power. At a certain level of amplification, known as a lasing threshold, these photons pierce through one end of the cavity, creating the focused light cone we call a laser beam.
However, designing a good laser takes a balancing act. The better an optical cavity is at trapping light, the less initial power it needs to emit a beam. However, a cavity that’s too good at its job might emit weaker beams, as photons fail to escape it.
“The main challenge in optical cavity design is creating a balance between confining light and extracting a useful amount of it,” said Matthew Chua, a Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
A*STAR IMRE Principal Scientist Arseniy Kuznetsov added that nanoscale lasers present added hurdles. While these tiny devices make attractive platforms for both free-space and on-chip optical communication—which might drive next-generation computers—some key components can be easily degraded or heat-damaged when power is pumped into the system. These components include quantum dots: nanocrystals that act as potent gain materials. Hence, low lasing thresholds are critical for such devices.
To address the challenge of nanoscale lasing, Chua and A*STAR IMRE colleagues, including Senior Scientist Lu Ding and Principal Scientist Arseniy Kuznetsov, teamed up with Hilmi Volkan Demir and colleagues from Nanyang Technological University, Singapore, as well as Dalian Polytechnic University, China. Together, they recently trialled a novel nanophotonic cavity design utilizing an effect in periodic structure known as “Brilluoin Zone Folding (BZF).
In such an optical cavity, trapped photons couple to guided modes, while still being emitted in the device’s light cone. The team fabricated a device consisting of a titanium oxide metasurface with a pegboard-like array of identical nanoscale cylinders embedded in a homogeneous medium of hydrogen silsesquioxane. They then spread a gain medium of quantum dots over the metasurface.
The team found that despite the array’s finite extent, the distributed feedback from their test structure’s periodic design was sufficient to promote lasing. Then, for every 2x2 group of cylinders, they enlarged one cylinder’s diameter.
“The initial structure was carefully spaced out to create photons with appropriate guided modes on the edges of the array’s Brillouin Zone,” said Ding. “The slightly larger cylinder then introduced a perturbation to the pattern of guided modes, which helped ‘fold’ them into the emitted light cone. Controlling the perturbation’s magnitude allowed us to control the balance between trapped and extracted light.”
Put to the test, the team’s device demonstrated very low effective lasing thresholds of approximately 4.08 μJ/cm2. “This demonstrates the high efficiency enabled by the BZF approach,” noted Ding.
Having validated BZF’s potential in low-threshold nanoscale lasing, the researchers are studying alternative designs with other BZF concepts and laser types. “With careful cavity design, this BZF-based system may offer an alternative route toward low-threshold, vertically emitting lasers compatible with sensitive gain materials,” said Chua.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
