This concept of controlling light with nanostructured interfaces is still at its infancy, which therefore offers many opportunities to innovate beyond the state of the art.
The first progress realized beyond the state of the art and that helped us for design of metasurfaces used in all WPs is the utilization of numerical optimization tools to design efficient metasurfaces. Collaborative efforts with experienced researchers in optimization methods led to some of the first optimized metasurface design to improve optical efficiency of the components.
Beyond the state of the art in designing optical metasurfaces, we have implemented a new set of boundary conditions at interfaces of arbitrary shapes. This concept of conformal boundary optics is necessary to design any free-form optical devices. After understanding the underlying theoretical physics, we have elaborated a model and implemented numerically new boundary conditions of light at interfaces to design various sort of free-form metasurfaces. We have written a new simulation software, based on modified FDTD, for testing the conventional generalized sheet boundary conditions in some simple cases and propose new devices such as free-form metalenses and aberration-reducing in curved metalenses.
We also reported an unexpected fabrication process for metasurfaces relying on material selective sublimation. With respect to traditional nanofabrication techniques, the semiconducting material is removed without using conventional reactive ion etching using selective evaporation process of the crystal along well defined crystalline-axis. This method works only for crystalline materials.
We also created achromatic optical devices by compensating the dispersion from conventional optical components using the dispersion properties of metasurfaces. With this work, we demonstrated how metasurfaces can be combined with refractive materials to achieve achromatic behaviour at multiple wavelengths, ruling out some of the limitations of conventional refractive and diffractive optics.
Our last discovery beyond the state of the art is a new phase addressing mechanism for metasurface design. We used concepts from topological physics and the extinction of the light beam corresponding to the presence of a singularity in the space of parameters defining the nanostructure final geometry. The amplitude of the light wave being zero, its phase is no longer defined. By staying in the vicinity of the singularity, it is then possible to draw antennas whose characteristics will give the wave the desired phase, between 0 and 2 pi. This total control of the wavefront makes it possible to develop a nanostructured interface reflecting a light ray at a well-determined angle, but also to obtain exceptional scattering effects, such as non-reflection behavior, perfect absorption on certain transmission or reflection channels.
