Hidden in everyday devices are invisible materials with light-bending superpowers. Nanophotonic optical coatings shape beams of light for applications such as harvesting energy from the sun and powering display and communication technologies.
Jinghua Teng and K.V. Sreekanth, researchers from A*STAR’s Institute of Materials Research and Engineering (IMRE), said that despite the promise, conventional thin film coatings fall short in the face of next-generation technology demands.
The problem with current optical coatings is their inability to consistently absorb light when it strikes from different angles in an issue known as impedance mismatch. This discrepancy in the way light is transmitted from one medium, like air, to another, like the film, lessens at acute angles, paradoxically improving absorption only at those angles.
Teng and Sreekanth theorised that for strong and broad spectral light absorption, the refractive index of a coating must be tuneable, while its extinction coefficient (a measure of how much light it can absorb without reflecting) should remain largely stable. Finding materials that tick both boxes has, until now, been incredibly challenging.
Progressing from their previous research, the team discovered that enhanced light absorption at near-normal angles is possible with a distinct cavity design. To broaden this effect across all angles, they identified the need for a material with unique characteristics.
Their search led them to stibnite (Sb2S3), a versatile chalcogenide phase change material. Remarkably, Sb2S3 can rearrange its atoms to greatly vary its refractive index while maintaining its light absorption capacity.
Using silver coated with Sb2S3, the team achieved greater than 99 percent light absorption and created a phase modulator that can adjust up to 140 degrees. “We achieved near-perfect absorption even in the crystalline phase of Sb2S3, from normal incidence to a wide range of angles,” Teng and Sreekanth commented. This is contrary to previous findings where the crystalline form showed weaker absorption.
The researchers said that this pioneering work can lead to vibrant, adjustable colour filters for more dynamic and energy-efficient displays, or optical switches, crucial for rerouting light in high-speed data networks.
Moreover, the exceptional light absorption properties of Sb2S3 may lead to thinner, high efficiency solar panels, more responsive photodetectors, and compact lasers being developed, all of which are foundational elements for the impending optoelectronic revolution.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
