Researchers from USA and India have developed micro-metalenses to detect microscopic defects, addressing a need across industries for precise material testing.
A research team from Indian Institute of Technology (IIT) Madras, India and Intelligent Optical Systems, USA, have achieved a groundbreaking advancement in material diagnostics through high-resolution ultrasonic imaging. Traditional methods like X-ray imaging offer high resolution but lack effective penetration in solid materials and pose radiation risks, limiting their broad applicability. Ultrasonic technology, however, penetrates deeper with a non-ionizing, cost-effective approach, though it faces limitations in imaging tiny internal defects due to its longer wavelengths. By leveraging micro-metalenses, this new research aims to overcome such limitations, improving the capacity of ultrasound to reveal deeper microscopic structures.
The team tackled the “diffraction limit,” which restricts imaging resolution, by developing silicon-based metalenses featuring 10-micron square holes. Additionally, they utilized a micro-focal laser with sub-micron spot detection to capture precise measurements, significantly enhancing defect visibility. In experiments, these silicon-based lenses demonstrated the ability to achieve 50-micron resolution, a notable improvement for detecting slit-type defects within silicon samples. This breakthrough in resolution and depth detection appeals to a broad spectrum of sectors where accurate internal imaging is essential for safety and quality control.
A specialized setup was used for this study, with an ultrasonic transmitter on a computer-controlled scanning stage and a laser receiver that converted detected displacements into ultrasonic signals for high-resolution imaging. Silicon samples, equipped with metalens and immersed in a water tank, underwent deep reactive ion etching (DRIE) to create the micron-scale holes, while a gold coating improved signal reflectivity. To facilitate consistent detection, the channels in the metalens were oxidized for hydrophilic properties, ensuring stable water levels within the channels.
The results indicated clear separation of defects spaced as close as 50 microns. In addition, analysis of the B-scan profile revealed that while side lobes were present, the signal quality was sufficiently high, allowing for accurate defect identification. The finite element (FE) simulations confirmed the effective resolution achievable with this technique, albeit limited to the periodicity of the metalens.
This innovation holds promise for applications in non-destructive evaluation (NDE), biomedical imaging, and the diagnostics of electronic materials, with far-reaching implications in industries such as aviation, nuclear power, and quantum material research. Future enhancements to micro-metalens technology and setup optimisation will likely broaden its impact on precise, in-depth imaging for diverse scientific and industrial applications.
