Nonlinear Optics using Epsilon Near Zero Materials

Lead Research Organisation: University of St Andrews
Department Name: Physics and Astronomy

Abstract

This project will centre around designing metasurfaces which use an ENZ (Epsilon Near Zero) material, such as Indium Tin Oxide (ITO) as a thin film substrate which is coupled to an antenna array. ENZs display interesting properties within the spectral range where the permittivity of the material becomes less than one. ENZs are very useful in many optical applications both linear and nonlinear due to their unique properties. For example, In the ENZ spectral range phase propagation through the material is small removing the need for phase matching of nonlinear optical processes. Another example is how enhanced electric field intensity within the medium means that by coupling it with an antenna array a sample can be produced which requires relatively low irradiance to observe significant intensity dependent refraction.

Within this project FDTD simulations will be made using Lumerical. Allowing effective design of the geometries of metasurfaces such that it can be designed to give a tailored optical response such as beam steering for example. This will also allow for the thickness of the ITO to be tuned and the coupling between the antennas and the ITO to be optimal. Thin films of ITO will be deposited on glass using spluttering deposition. The thickness of the film which is deposited will determine the modes which can be supported by the film. ENZs support optical modes with a large density of states. The mode of concern is the Ferrel-Berreman mode, free space excitation of radiative volume plasmons, and how this mode couples with the antenna resonance. The optical linearity and optical nonlinearity of samples which are fabricated will be characterised through transmission measurements. The optical nonlinearity will be characterised using methods such as the z-scan technique.

Many have utilised materials with plasmonic resonances for their antennas. These are fabricated by using electron-beam lithography followed by metal evaporation and lift off. Usually using materials such as gold and silver. They are a popular choice because of their large negative permittivity which makes them very effective scatters. However, they suffer from limitations in how efficiently they do this due to their strong absorption. Early samples that are fabricated will use gold for the antenna arrays but later the aim is to fabricate samples which have dielectric antennas coupled to the ITO. Using dielectric antennas as opposed to metallic ones should reduce absorption leading to improved efficiency.

Another goal within this project is to fabricate a metasurface which is capable of beam steering. V shaped antennas will be made on the ITO such that the phase across incoming gaussian beam can be manipulated to steer the beam in a desired direction. The double resonance properties of the antenna shape allow for full phase control. The orientation of the antennas with respect to the electric field will dictate whether the resonance is symmetric or antisymmetric. The current through an arm approximates to current through different straight antenna lengths for the two different resonances. The antennas will be designed such that there is a phase gradient across the surface, due to the abrupt phase change each antenna introduces, while keeping the scattering amplitude at a constant.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R513337/1 01/10/2018 30/09/2023
2179928 Studentship EP/R513337/1 26/01/2019 31/07/2022 Laura Wynne