Metasurface Based Mid-infrared Devices

Abstract

The development of compact, efficient, and powerful mid-infrared devices is mainly restrained by the limited choice of materials due to the high loss of conventional optical materials in the mid-infrared range. The aim of this work was to find alternative novel materials which would enable the realization of devices with smaller size while maintaining its functionality. Metasurface and graphene have emerged as promising materials which can help us to manipulate the infrared light within nano-meter scale thickness. In this thesis, three different mid-infrared devices, thermal emitter, wave trapping sensor and phase modulator were designed based on either metasurface or both metasurface and graphene. Devices were all fabricated with modern semiconductor fabrication processes and their performances were also fully investigated, both experimentally and through simulations. A metasurface was first designed as a frequency selective layer on a graphene thermal emitter to tailor the graybody emission spectrum from a graphene filament into two discrete narrow bands for applications such as gas sensing or molecule detection. The emission and reflectance spectra of the devices were characterised using (FTIR) Fourier transform infrared spectroscopy and showed good agreement with simulations based on the Finite-difference time-domain (FDTD). method. The use of a metasurface to enhance the interaction between molecular vibrations and the evanescent waves, in a total attenuated reflectance system, was also explored. A complementary ring-resonator structure was patterned onto both silicon and SiO2/Si substrates, and the spectral properties of both devices were characterised using an FTIR-ATR system. Experiments were undertaken using 5µL mixtures containing trace amounts of butyl acetate diluted with oleic acid. Without the use of a metasurface, the minimum concentration of butyl acetate that could be clearly detected was 10%, whereas the use of the metasurface on the SiO2/Si substrate allowed the detection of 1% butyl acetate. Finally, graphene was integrated into a metasurface structure to achieve tunability of the design. The third device investigated was a phase modulator which shows the capability to change the amplitude and phase of the reflected wave by electrostatically gating the graphene from -90V to 90V. A dynamic beam steering lens model which is made up of a unit cell consisting of four phase modulator with different phase shift was also proposed to control the angle for the reflected wave from specular to 30°.