Tunable All-dielectric Metasurfaces: Fundamentals and Applications

Abstract

All-dielectric metasurfaces have received significant attention in the past years, and have been established as a platform for efficient manipulation of optical beams. Advances in the design and fabrication of such dielectric metasurfaces have led to the development of several ultra-thin optical metadevices, including flat lenses, beam converters, deflectors and holograms. Composed of periodic or aperiodic lattices of dielectric nanoparticles, metasurfaces exhibit low absorption in the infrared and visible spectral ranges. Low losses allow nanoparticles to exhibit Mie-type resonances with a higher quality-factor in comparison to their plasmonic counterparts. Furthermore, Mie-type resonances in dielectric nanoparticles offer two independent families of resonant modes - electric and magnetic. The far-field interference of these two types of resonant modes leads to fundamentally new effects, such as unidirectional scattering, unconventional reflection behaviour associated with the generalised Brewster effect and near-unity transmission in the so-called Huygens' regime. Operating in the Huygens' regime of the dielectric metasurfaces enables the combination of near-unity transmission together with a full range of phase modulation, thus being the key to enabling functional dielectric metasurfaces with nearly 100% efficiency.

Most functional dielectric metasurfaces to date are based on static designs, defined through geometrical parameters, such as nanoparticle shape, size, and array layout. However, in many applications, it is crucial to enable dynamic tunability of the device functionality with time. For example, the focal distance of a camera lens needs to be changed when taking pictures of objects at different distances; the position of the ranging beam in a LIDAR (light imaging, detection, and ranging) for driverless vehicles needs to scan different directions. Therefore, implementing dynamic control} over the response of the metasurfaces is of paramount importance for their practical implementation.

In this thesis, I discuss possible ways to achieve tunability from dielectric metasurfaces. At first, I consider existing methods, their pros and cons, as well as possible applications. Further, I offer several methods that I developed and investigated, both theoretically and experimentally. Namely, I describe tuning by changing the properties of resonator material, where I utilise a thermo-optic effect to control the refractive index of resonator particles and consequently the optical response of metasurfaces. Next, I consider tuning by changing the properties of surrounding material, first theoretically, and later demonstrating experimental realisation of this concept using a liquid crystal as a tuning medium. Finally, I describe two tunable metadevices: one, for switching a beam deflection, and second, for active tuning of spontaneous emission.