Picture the electronics of tomorrow: paper-thin TVs, holographic displays and augmented reality (AR) glasses. Such devices can be made possible by metasurfaces: ultrathin, lightweight materials that can manipulate how light behaves in precise and customisable ways.
For researchers like Ramón Paniagua-Domínguez, Deputy Head of the Advanced Optical Technologies Department at A*STAR's Institute of Materials Research and Engineering (IMRE), metasurfaces can replace traditional optical components for applications ranging from smartphone cameras to light detection and ranging (LiDAR). “They can be 1,000 times thinner than a human hair, making them far more compact than glass lenses,” said Paniagua-Domínguez.
However, the first generation of all-dielectric metasurfaces—made from materials like silicon—typically have their light-bending properties set in stone once manufactured. Phase-change materials (PCMs) offer exciting new possibilities for a new generation of metasurface designs, because their optical properties can be modified by external stimuli like heat, light or electricity.
“In a camera, ‘zooming’ works by moving a bulky system of lenses back and forth, with each lens having fixed optical properties; they bend light in a certain way, and that’s it,” said Paniagua-Domínguez. “But what if you can dynamically tune a lens to change how it bends light? You can control a camera’s zooming using the lens itself, meaning you’d need less electronic parts to achieve the same optical functions.”
Still, materials scientists have found it challenging to develop transparent PCM-based metasurfaces that work well in the visible light range; well-known PCMs like germanium-antimony-tellurium (GST), widely used in DVDs, tend to reflect light rather than let it pass through.
In collaboration with Robert Simpson’s group from the Singapore University of Technology and Design, Paniagua-Domínguez and the IMRE team investigated a new potential PCM for next-generation programmable metasurfaces: antimony trisulfide (Sb2S3). Transparent in the visible spectrum, Sb2S3’s atoms can be fluidly rearranged between two states—amorphous and crystalline—when subjected to heating or laser light. This alters how they refract light, offering more optical flexibility over conventional silicon-based metasurfaces.
“What sets Sb2S3 apart is its swift phase-changing ability, clocking in at nanoseconds, all while being more energy-efficient than conventional devices,” said Paniagua-Domínguez. “Plus, since its phase changes don’t tamper with its overall structure, the material retains its mechanical robustness.”
The researchers reported that their prototype Sb2S3 metasurfaces can change the phase of light by an impressive 2π while still allowing most light wavelengths to pass through. Such capabilities would make the material a prime candidate to enhance AR experiences, improve collision sensors in autonomous vehicles, and enable advanced spatial light modulators for holographic projection.
Still, Sb2S3 metasurfaces need inherent kinks to be ironed out before they can be considered for use in device prototypes. “Our next steps involve refining the material and ensuring its optical properties stand up to intense long-term use,” said Paniagua-Domínguez. “We’re also exploring electrical switching mechanisms, aiming for a seamless integration into next-gen electronics.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
