Polarised sunglasses that shield the eyes from damaging rays have a chemical coating to selectively block certain light orientations, reduce glare and enhance visual clarity. Such manipulation of light, pivotal in optical technologies, is also central to applications from medical imaging and telecommunications to quantum computing and virtual reality.
Miniaturising components such as waveplates, crucial components for manipulating light polarisation in optics, is a pressing challenge—conventional methods involve precisely cutting and shaping large quartz crystals, a process that is often expensive, inefficient and imprecise.
“Rather than relying on the conventional top-down approach of working with large chunks of crystals, our team focused on a ‘bottom-up’ synthesis method,” said Qian Wang, a Senior Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE). “We chose to work with ferrocene, a compound containing iron from the metallocene family, due to its unique properties conducive to our objectives.”
Together with researchers from La Trobe University and RMIT University in Australia; Institute of Biointelligence Technology, China; Université Montpellier, France and the National University of Singapore, the team selected ferrocene for its optical anisotropy—its unequal refraction of light in different directions—which allows waveplates to modulate the polarisation state of light. Secondly, ferrocene crystals are transparent, allowing light to pass through with minimal loss. Lastly, ferrocene can self-assemble under controlled conditions, aligning the growth direction with the crystal’s principal refractive axis.
“This ability to self-assemble is especially important, as it means that once the ferrocene crystals achieve the desired thickness during preparation, they are immediately usable as miniaturised true zero-order waveplates, eliminating the need for further processing,” explained Wang. “This makes our bottom-up approach particularly effective for producing high-quality, micron-scale waveplates.”
A schematic illustration of how self-assembled ferrocene crystals with different thicknesses convert light to different polarisations, which include linear and circular forms. Each crystal has a fast axis that runs along a natural edge, making it readily useable as a miniature true zero-order waveplate. Top right: an optical microscope image of a ferrocene crystal.
Wang and colleagues discovered that their self-assembled ferrocene crystals can change the phase between two perpendicular polarisation components of the light wave (high birefringence) without absorbing light (low dichroism), making the crystals prime candidates for use in miniaturised waveplates.
These advancements have the potential to change how optical devices are made, particularly in the field of nanophotonics, which requires ultra-thin, compact components for optical communication and neural computing applications.
“Our current focus is on harnessing surface tension at liquid-liquid interfaces to drive the self-assembly and crystallisation of ferrocene, a technique we plan to extend to other materials,” Wang elaborated.
In addition, the team is investigating the optical traits of other metallocene compounds, opening exciting avenues in next-generation optical technologies.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
