Janus Metasurfaces: Looking Both Ways at Once

Researchers at the Korea Advanced Institute of Science and Technology have developed an innovative metasurface that exhibits distinct optical responses based on the direction of incoming light1. Named after the two-faced Roman god Janus, these metasurfaces can manipulate light differently depending on the direction it comes from. This unique ability could help to authenticate secure communications.

What makes Janus metasurfaces special? Unlike traditional optical devices, which are typically static in their functionality, Janus metasurfaces provide asymmetric optical responses. For instance, the same thin film can act as a magnifying lens when light shines from one side and as a polarisation- sensitive camera when illuminated from the other. This dual functionality is achieved at the pixel level, within a structure thinner than a human hair.

Schematics of a device featuring asymmetric transmission (© Advanced Materials 2024)

The secret lies in their design. Janus metasurfaces are composed of nanoscale structures that precisely control the properties of light – such as its amplitude, phase, and polarisation. These metasurfaces break the usual symmetry of optical devices, creating ‘directional asymmetry’. This means light behaves differently depending on whether it enters from the front or back, a property that defies traditional expectations for reciprocal systems.