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\chapter{General conclusions and future work} \label{Chapter:Conclusions}

In this thesis, three main, experimental projects were undertaken aiming to enhance the functionality of metasurfaces and exploring new effects using nanoscale metasurfaces. 

In the first project, the reconfigurable metasurfaces with mechanical switching were investigated. The use of liquid crystals to reverse adhesion between the metasurface movable nanobridges was attempted with the aim to repair such metasurfaces by exploiting the elastic forces of liquid crystals. While the full reversal of stiction was not achieved, the results obtained indicated that liquid crystals affected the resonances of the liquid crystal loaded metasurface beyond just the change of the refractive index in the vicinity of the nano-bridges. The supply of good quality samples with such nanoscale metasurfaces proved increasingly difficult to obtain due to short circuit issues, possibly stemming from contamination during gold evaporation or gallium contamination during focused ion beam milling. Future work on this project, with good quality samples, could include using different fabrication techniques, such as e-beam. Other options include exploring different liquid crystals, namely those with higher elastic coefficients to engage stronger elastic forces to separate the stuck nanobridges, while also considering their viscosity and the conditions to promote their infiltration of the gaps between the nanobridges. Furthermore, with successful fabrication, the nanobridge wires of different length could be produced, thus exploring the strength of the restoring force for the liquid crystal. 

One additional challenge with this project is that the resonances and the response of the nanobridge wires cannot be easily modelled. The detection of the stiction effect taking place relies on capturing the SEM images before and after voltage is applied. Such imaging cannot be done with a layer of liquid crystal integrated with the nanobridges. Hence, the effect of liquid crystal and the potential repair needs to be deduced from the light transmission measurement, while ramping the voltage up and down. However, the liquid crystal alignment, and thus their effect on transmission will also change with applied voltage hence if they even partially remove the adhesion of the nanobridges, they may not be easily extracted from the experimental results. 

The zigzag design was used in the second project, where zigzag metasurfaces fabricated for this thesis were used to explore the spatial coherence of light. The intriguing effect, where the resonance detected for the incoherent light was found not to match the theoretical simulation, with the model that assumed a coherent illuminating beam. The fabrication and the experiments confirmed this effect and included experiments on varying the spatial coherence of the illuminating sources by changing the size of the aperture near the sample. The dependence of the measured resonance of such variation of spatial coherence was observed. The effect was later explored and confirmed in a separate experiment (led by Dr T. Cookson) using OPO. Future work on this system could include a more comprehensive investigation of different tilt angles to explore the direction of the re-radiated, scattered light and whether beam steering could be achieved.

The third project, which again involved fabrication on the nanoscale, explored the effect of specular optical activity using samples with asymmetric slits, such as $50/150 nm$. The challenging experimental work, as well as nanofabrication, provided evidence on the presence of such effect in the optical regime. In spite of the effect being quite subtle, the features observed for the Kerr signal reflected from such non-magnetic samples with respect to the angle of rotation showed potential specular optical activity. One of the main challenges concerned the use of MOKE instruments for such small and non-magnetic samples. MOKE magnetometers are intended for analysing much larger samples of ferromagnetic materials, inspecting for example, the magnetic grains of such bulk materials. Detecting specular optical activity with MOKE was carried out at the edge of this instrument resolution and proved challenging in understanding how the Kerr signal is processed, controlling the measurement of the signal and as well as achieving a precise alignment. This included the correct collimation of the beam to prevent the specular optical activity features being averaged out. Future work on this project, to achieve better resolution of the observed features and modulation would require building a dedicated optical set up to go beyond the indication of the effect collected from the use of a commercial MOKE magnetometer.