Dr Karim Achouri obtained an Ambizione fellowship from the Swiss National Science Foundation, to lead an independent research effort at the NAM. His research focuses on the development of metasurfaces for optical analog processing. This page presents some of the recent results obtained within this the scope of this ongoing project.
The general idea behind this project heavily relies on the concept of Fourier optics, which provides an elegant explanation of how light propagates and how optical images are formed. This is studied in the context of Fourier analysis, which provides a mathematical foundation for expressing any optical image as a superposition of light fields with various sinusoidal properties. This is illustrated above as the image of Joseph Fourier is decomposed into a sum sinusoidal functions. It turns out that each of these functions physically correspond to light fields propagating in specific directions of space. In other words, an image corresponds to beam of lights propagating at different angles.
This concept of Fourier optics is often used in practice for optical analog processing, such as, for instance, in the context of phase-contrast imaging or schlieren imaging. Specifically, it may be used for filtering out some undesired parts of the image light field so as to highlight other parts of interest. A conventional optical filtering setup is shown in the image above, where the filtering process requires the presence of two lenses and a mask leading to a relatively bulky system. The large physical size of such a setup makes it inadequate for embedded optical systems thus limiting its application.
One of the main objectives of this Ambizione project is to overcome this limitation by realizing compact and light weight metasurface based optical filtering systems. This goal is illustrated in the image below, where the same filtering operation as above is performed but with a single metasurface.
Among the most challenging aspects of this project is the fabrication of metasurface optical filters with asymmetric transmission functions, which is required for some practical applications such as schlieren imaging. In such a case, the asymmetry of the transmission function necessarily requires an asymmetry of the metasurface not only in its plane but also in its vertical dimension, thus requiring 3D nano-fabrication techniques. An example of such a metasurface is shown below, which is composed of vertical L-shaped gold structures. The asymmetry of the structure in the xz-plane leads to an asymmetric transmission function with respect to the angle of propagation θ.
