Nanophotonic systems with dynamically tunable properties are of great interest due to their practical applications in optical devices, including power limiters. Among the different tuning mechanisms within the system, one of the most popular is to employ materials that change their refractive index as a result of phase transition. In that regard, vanadium dioxide (VO2) stands out due to the significant index modulation across its insulating-metallic phase transition, which can be triggered thermally or electrically.
The potential of VO2 tunable transmission, a concept illustrated in Fig. 1(a). has been explored in the visible and near-infrared spectral ranges, motivated by the industrial application market, including self-activated optical limiters, temperature-responsive smart windows, etc. However, due to the high optical loss of VO2, the possible switching of the transmission contrast was strongly limited. The maximum contrast for telecommunication wavelength range is around 40%, applicable to both thin-film multilayer stacks or metasurfaces. To date, it has yet to be known what the maximally achievable switching contrast is and the fundamental physical limits defining it.
To answer this question, the research group led by Prof. Dragomir Neshev from the Australian National University, in collaboration with the team of Prof. Costantino De Angelis from the University of Brescia, proposed a 2D effective medium model to capture the optical responses of an arbitrary metasurface with a deep subwavelength unit cell, aiming at utilizing the more complicated optical response to achieve a higher contrast. The maximum transmission contrast is predicted to reach 72%, about twice the previous achievements.
The relevant research results were published in Photonics Research, Volume, 11, 2023 (Bohan Li, Rocio Camacho-Morales, Neuton Li, Andrea Tognazzi, Marco Gandolfi, Domenico de Ceglia, Costantino De Angelis, Andrey A. Sukhorukov, Dragomir N. Neshev. Fundamental limits for transmission modulation in VO2 metasurfaces[J]. Photonics Research, 2023, 11(1): B40.)
The model used in the work is based on quasistatic electromagnetics and boundary condition matching. In particular, the electric field inside each component was assumed to be constant in space, and the electromagnetic boundary conditions interconnect the field amplitudes of different structural components. Numerical studies were conducted to verify the validity of the theoretical model. Specifically, local topology optimization based on adjoint calculation was implemented to search for the maximum transmission contrast from the numerical end. The numerical optimization results found free-form nanostructured VO2 films with transmission contrast approaching the theoretical model of over 70%.
Fig. 1. (a) Schematic of the working principle of transmission modulation using insulator-metal phase transition in VO2. (b) Enhancing the transmission contrast through nanostructuring. The hypothetical enhanced performance (orange curve) as compared to the transmission from an actual multilayer structure.
Other attractive designs can also be found within the resonant regime, where the periodicity of the system is comparable to the wavelength. In that case, a sparse grating structure has been shown to exhibit a similar transmission contrast level as in the deep subwavelength structures. Surprisingly, the contrast is inverted in this case: near-zero transmission is achieved by the low-loss insulating VO2 phase instead of the regular, highly lossy metallic phase. Electromagnetic simulations were done to study the mechanism behind this notable high contrast. It turned out that the low transmission is fulfilled by resonant absorption, which is enormously boosted by the interplay between the local resonance of the individual grating elements and a non-local leaky-mode resonance associated with the periodic grating.
Future work aims at the experimental realization of the results. The second design within the resonant regime is more promising regarding experiments compared to the first topology-optimized structure. Studying from the experimental side can help build an understanding of the direct structuring of VO2 material and lay the foundation of VO2-tunable nanophotonics. The team aims to explore such structures as efficient optical limiters.
