Semimetal-Based Active Perfect Absorbers for Photonics and Optical Computing

Semimetallic elements like bismuth (Bi) and antimony (Sb) are extremely versatile materials that display great potential for nanophotonics. They are suitable for a wide range of applications, based on their remarkable properties that include ultra-high values of the refractive index in the IR (n∼10), as well as plasmonic-like behaviour in the UV-VIS range, along with their potential as single-element phase-change materials (PCM). Regarding their performance as PCMs, they display both volatile (crystalline-liquid) and non-volative (amorphous-crystalline) transitions for Bi and Sb, respectively. These transitions have been demonstrated either via hotplate [1,2] or via fs laser [3,4]. In our recent work we have shown the successful fabrication of plasmonic metasurfaces and Fabry-Perot cavities for structural color generation using these materials [5].

In this work we go one step further and we present lithography-free active metasurfaces on silicon (without a metallic back-mirror) that shows a markedly different behavior in the VIS and IR upon activation. In the VIS range we have designed it to perform as a broadband perfect absorber, featuring percolated Bi nanoparticles as the active PCM. This nanoparticles experience very strong absorption, which facilitatea the phase changing process. Upon phase change, since the optical contrast between phases for Bi in the VIS is smaller than most PCMs (Δn∼0.5) the visible optical response remains constant. While in the IR the refractive index contrast is very high (Δn∼3) and therefore significant changes in the optical response occur. These IR changes can be tuned only changing the fraction of molten Bi, resulting in an analogic response that can be easily controlled. We have assessed and followed these changes by in-situ ellipsometry measurements. Assessing the potential applications of this structure, note that since silicon is employed as the back mirror, this could be easily integrated in photonic or photovoltaic systems, as well as any other CMOS technology. This type of structure can have relevant filtering and encoding applications in the field of optical and neuromorphic computing, as it is robust enough to withstand indefinite melting cycles.[4]


[1] M. Garcia-Pardo et al, Nanophotonics 9 (2020) 885–896.
[2] D.T. Yimam, B.J. Kooi, ACS Appl Mater Interfaces 14 (2022) 13593–13600.
[3] S. Aggarwal et al, Nano Lett 22 (2022) 3532–3538.
[4] M. Alvarez-Alegria et al, Adv Opt Mater 10 (2022).
[5] F. Chacon-Sanchez et al, Adv Opt Mater 12 (2024) 2302130.