A scene from a sci-fi movie plays out—the hero dons their invisibility cloak just in time to escape the clutches of a fast-approaching villain. Believe it or not, materials with the invisibility cloak’s optical properties aren’t just the stuff of movies.
Metamaterials are specially engineered structures that defy the natural laws of physics, giving them unique optical properties such as “invisibility”. Their surfaces are made of nanostructures smaller than the wavelength of light, which allow them to bounce inbound light waves off much like a mirror’s reflection.
Metamaterials are being used in some real-world applications in the aerospace and defence industries, although experts say manufacturing challenges limit their potential in other spheres. “A wider range of applications can be realised if more design dimensions are provided from material and fabrication perspectives to allow dynamic control of light,” explained Hong Liu, Head of Nanofabrication Department at the Institute of Materials Research and Engineering (IMRE).
Liu co-led a collaborative effort with researchers from Nanyang Technological University to overcome these hurdles by creating a next-generation class of high-efficiency, tunable metasurfaces. The researchers combined metamaterials principles with a group of materials known as optical phase-change materials (PCM) that possess large changes in optical properties to create the hybrid silicon metasurface.
“We incorporated PCMs such as germanium-antimony-tellurium (GST), which can rapidly switch its state from amorphous to crystalline when heated , offering a precise approach to control its light emission,” added Liu. “Through the hybrid design of nanopatterning the PCMs on top of the silicon metasurfaces, we created a single device that is tunable and multi-functional.”
Liu added that silicon was specially selected as it offers strong light-matter interaction, which enables highly efficient third-harmonic generation, an optical phenomenon where light hitting the metasurface can be manipulated.
The researchers further refined their innovation by optimising the thickness of the GST layer, a key step in mitigating the material’s optical losses. Their optimal device design was found to be 32 times more efficient than reported metasurfaces, paving the way for a multitude of advanced applications in quantum photonics, spectrometry and lasers.
Liu’s team has plans to continue developing their hybrid metasurface by focusing on a reversible tuning strategy, a novel technique that could enable powerful nanophotonic devices with unprecedented light emission and control. “We are also seeking funding support to develop these hybrid metasurfaces for ultraviolet light applications, as the current state of the technology hinders its practical usage,” Liu said.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).