For many people, the words ‘ultraviolet light’ or ‘UV light’ might bring to mind sunburn or skin cancer. You might be surprised to find UV lights in a range of helpful applications today, ranging from medical equipment sterilisers to indoor farm lighting.
One form of UV light, known as coherent deep-UV (DUV), already plays key roles in making nanoelectronics and purifying water. Researchers such as Omar Abdelraouf, a Research Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), think coherent DUV could do a lot more if it could be generated on compact chips, rather than via bulky and power-hungry laser equipment.
“We’re aiming for on-chip, energy efficient, ultracompact light sources for DUV nanophotonics applications,” said Abdelraouf, referring to a new generation of light-based computer circuits that could run faster than today’s electronics.
Abdelraouf and A*STAR IMRE colleagues including Hong Liu, Group Leader of A*STAR IMRE’s Intelligent Nano-optics Group, teamed up with Nanyang Technological University, Singapore, to develop a crystalline silicon (c-Si) metasurface to produce coherent DUV from a low-power laser light source. Comprising a sapphire sheet dotted with rows of fin-shaped c-Si structures—each a hundred times thinner than a human hair—their metasurface relies on third harmonic generation (THG), an optical process which combines a trio of low-energy photons into a single high-energy photon.
The team had to negotiate a delicate balancing act with their metasurface, which Abdelraouf compared to a magnifying glass for focusing sunlight. “Tiny imperfections in c-Si structures, like round edges, reduce their ability to trap and amplify light,” said Abdelraouf. “On the other hand, if you turn up the laser’s power to increase DUV output, the system breaks.”
Central to their metasurface’s design was a deliberate breaking of its structural symmetry, causing it to activate specialised light-trapping phenomena called bound states in the continuum (BIC). Abdelraouf noted that this approach not only boosted the metasurface’s capacity to confine and convert light energy, but allowed more fine-tuning than existing methods of enhancing BIC resonance, which rely on changing materials or geometric shapes.
Through careful fabrication and tuning, the team achieved a 14-fold increase in THG power and a conversion efficiency to DUV of 5.2 × 10⁻⁶ percent, outperforming other metasurface designs in current literature.
To address light ‘leaks’ and other performance issues from minor structural flaws, the team used advanced nanofabrication techniques and robust, error-tolerant designs. This aligned their experimental results with simulation data, bringing their silicon platform one step closer to being a part of practical, chip-based DUV light sources.
“Our next steps include exploring novel optical materials with strong nonlinearity for DUV THG enhancement; advanced nanofabrication techniques for higher accuracy and conversion efficiency; as well as integration technology for on-chip devices,” Abdelraouf said.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
