Active analog tuning of the phase of light in the visible regime by bismuth-based metamaterials

Active and analog tuning of the phase of light is needed to boost the switching performance of photonic devices. However, demonstrations of this type of tuning in the pivotal visible spectral region are still scarce. Herein we report active analog tuning of the phase of visible light reflected by a bismuth-based metamaterial, enabled by a reversible solid-liquid transition. This metamaterial, fabricated by a lithography-free approach, consists of two-dimensional assemblies of polydisperse plasmonic bismuth nanostructures embedded in a refractory and transparent aluminum oxide matrix. Analog tuning of the phase is achieved by controlled heating of the metamaterial to melt a fraction of the nanostructures. A maximum tuning of 320 deg (1.8pi) is observed upon complete melting of the nanostructures at 230 degrees Celsius. This tuning is reversible by cooling to 25 degrees Celsius. In addition, it presents a wide hysteretic character due to liquid bismuth undercooling. This enables the phase achieved by this analog approach to remain stable over a broad temperature range upon cooling and until re-solidification occurs around 100 degrees Celsius. Therefore, bismuth-based metamaterials are appealing for applications including optical data storage with enhanced information density or bistable photonic switching with a tunable "on" state.