You have probably used a laser pointer, but have you ever wondered how such a small device produces a powerful, focused beam of light? The secret lies in the optical cavity, a space enclosed by two or more mirrors that trap light and amplify it through constructive interference. Inside, light waves bounce back and forth between the mirrors, reinforcing each other until they form a concentrated beam.
These cavities are rising stars in the field of optics; even simple designs, such as the Fabry-Pérot cavity consisting of just two mirrors separated by a gap, could become critical components of the biosensing and telecommunications devices of tomorrow.
Seeking to enhance their performance, Gandhi Alagappan, a Principal Scientist II at the A*STAR Skin Research Labs (A*STAR SRL), who was formerly at the A*STAR Institute of High Performance Computing (A*STAR IHPC), explored a fresh approach: replacing conventional mirrors with resonant metasurfaces.
Unlike traditional mirrors that merely reflect light, resonant metasurfaces can also ‘store’ and delay it, much like a trampoline bending under a bounce. “Each metasurface ‘pauses’ the light, shifting its phase before release. When two such mirrors face each other, their combined delays align the waves perfectly, producing stronger constructive interference—even within a smaller gap,” explained Alagappan.
Working with Francisco J. GarcíaVidal of the Universidad Autónoma de Madrid in Spain, Alagappan and colleagues at A*STAR IHPC designed a Fabry-Pérot cavity featuring two metasurface layers. Previous studies typically examined a single metasurface or used it merely as a coating layer. In contrast, the researchers treated the bilayer system as a full optical cavity on its own.
“This approach allowed us to predict how light behaves—where it concentrates, how sharp the resonances become—and to demonstrate these behaviours through detailed computer simulations,” Alagappan said. The team also derived mathematical equations describing the behaviour of light within these novel cavities.
The researchers found that Fabry-Pérot cavities made with two resonant metasurfaces enable the existence of bound states in the continuum, where the near-perfect trapping of light gives rise to ultra-sharp resonances. The length of the cavity also influences light behaviour, with three distinct regimes of cavity length generating different types of resonance.
Achieving sharper resonances is promising for developing ultra-sensitive medical devices, such as for detecting tiny amounts of blood biomarkers or non-invasive imaging of tissues. Their compatibility with semiconductors may also facilitate the integration of meta-mirror cavities into lab-on-chip systems, wearables and smartphones.
“Ultimately, we hope to bridge nanophotonics with healthcare, creating compact diagnostic and imaging platforms that improve early disease detection and personalised medicine,” said Alagappan.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Skin Research Labs (A*STAR SRL) and A*STAR Institute of High Performance Computing (A*STAR IHPC).
