Directional Manipulation of Guided Light by Anisotropic Chiral Particles


Student thesis: Doctoral Thesis

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Award date4 Oct 2023


Manipulating guided light with minimal optical structures promises broad applications in photonic integrated circuits, optical computing, quantum optics, etc. Recent research demonstrates the rich functionalities of meta-atoms and metasurfaces for light manipulations. In this thesis, we investigate anisotropic chiral particles as meta-atoms for achieving directional manipulation of guided light. We find that the synergy of chirality and anisotropy in a simple helix particle can enable the unified realization of the complete set of directional dipoles, including circular dipole, Huygens dipole, and Janus dipole. It can construct a complete directionality space and allow for the free switching of all types of optical directionality via activating different directional dipoles on different faces of the helix particle. The underlying physical mechanism can be well explained with a semi-analytical theory of mode expansions. We apply the helix particle to realize the face-multiplexed routing of guided light in three orthogonal directions based on three degrees of freedom: spin, power flow, and reactive power. The multiple directionalities can be integrated into one compact system to achieve unprecedented high-dimensional control of guided light’s propagation.

We then extend the mechanism to bi-helix-waveguide coupling system, where two helix particles couple with a dielectric waveguide. We show that the structure can give rise to multiplexed spectral line shape of the waveguide transmissions. This is achieved by employing the circular dipole deriving from the frequency-dependent chiral state of the helix particle. The variation of the dipole polarization induces asymmetric Pancharatnam-Berry phase shifts for the guided light propagating in opposite directions, based on the transverse spin-orbit interaction. The interference of the guided lights excited by the two helices can give rise to transmissions with various line shapes, including Lorentzian-like, antiresonance-like, Fano-like, and electromagnetically induced transparency (EIT)-like line shapes, which carry a remarkable property of line shape-momentum locking.

We also explore the application of the helix-particle meta-atoms in multiplexed wavefront shaping by constructing on-chip Janus metasurfaces. The metasurfaces can enable two-face wavefront manipulation of the guided light in a dielectric slab waveguide. This functionality originates from the asymmetric interference of electric and magnetic dipoles on the Huygens-dipole face of the helix. We design two Janus metasurfaces, a Janus meta-deflector and a Janus metalens, to asymmetrically tailor the spatial phase profile of the guided light and achieve one-side deflection and focusing.

The findings in the thesis reveal the fascinating light-manipulation ability of the anisotropic chiral particles for multiplexed and multifunctional nanophotonic designs, which can find broad applications in on-chip optical switching, sensing, computing, etc. The mechanisms can also be extended to other frequency regimes or other types of classical waves such as sound waves.