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Spin-dependent Wave Propagation in Waveguides, Metasurfaces and 3D Photonic Crystals


Photon spin has received great interest in the recent decades for many applications such as encoding quantum information and spin-filtering. However, very little is known about controlling the direction and properties of the spin. It was recently found that surface waves with evanescent tails possess an inherent in-plane transverse spin which is dependent on the propagation direction.

In this dissertation, we investigate different 1D, 2D and 3D designs that support strong spin-dependent propagation. Starting with a 1D C-shaped waveguide, we show that the spin-density can be enhanced through dipole-to-dipole coupling resulting in highly directional wave propagation. We then show spin-dependent wave splitting in 2D metasurface by engineering the equifrequency contours. We demonstrate the possibility of steering the surface wave along curved paths. We also introduce a new type of surface wave called a chiral surface wave which has two transverse spins, an in-plane one that is inherent to any surface wave and an out-of-plane spin which is enforced by the design due to strong $x$-to-$y$ coupling and broken rotational symmetry. We show that the two transverse spins are locked to the momentum providing a highly confined spin-dependent propagation. Similar chiral modes can be induced in 3D structures by introducing screw dislocation defect in a diamond photonic crystal.

Our study opens a new direction for enhancing and controlling the spin properties of electromagnetic waves through engineering the symmetry of shapes in 1D, 2D and 3D. This provides an additional degree of freedom to control the propagation direction as well as the transverse spin of electromagnetic waves.

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