Manipulating the coupling and scattering between surface waves and plane waves on metasurfaces, and at their discontinuities

Joint supervisors: Prof Alastair P Hibbins, Prof J Roy Sambles
External supervisor: Prof Richard Craster (Imperial College London)
External partner: MBDA

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This project will explore the coupling of incident microwave radiation into surface waves, and also the reverse process: the reradiation of surface waves into free-space radiation. We will employ experimental, analytical and numerical (e.g. finite element method) techniques.

Surfaces patterned with a sub-wavelength scale elements (metasurfaces) can often be described using reactive boundary condition (either inductive or capacitive) that defines an effective skin depth that supports bound waves.

The impedance is determined by the geometry of the elements comprising the surface, and therefore frequency-dependent. The modes supported are inherently broad-band in nature, typically existing from DC up to a limit dictated by a geometric resonance of the elements that form the pattern.

Once excited, they are non-radiative on a planar surface, propagating over many tens or hundreds of wavelengths, only decaying via joule-heating or loss in the surrounding dielectric, or reradiating by diffraction at discontinuities or through surface curvature. Careful design of the shape, spacing and size of the surface elements allows manipulation of the flow of energy across the array, in terms of the direction, speed, loss and localisation of the mode.

Similarly the careful design of defects in the surface can yield strong coupling between free space radiation and the surface-bound energy. In this project, the student will study the scattering (i.e. radiation into free space) and the reflection of surface waves at and from defects and discontinuities.

We will study how grading of the surface impedance can reduce this effect, e.g., through variation of the geometry of patterning, or the addition of tapered overlayers. Outcomes may include efficient conversion of surface waves into plane wave radiation, or perfect absorption of surface wave energy. In parallel, we shall consider how to design surface structures to enable efficient excitation of surface waves.

Together, these two methodologies will yield a surface that converts incident radiation into surface waves over a broad bandwidth, which then decay to heat without further reradiation. We will explore theoretical ideas such as those involving the mathematics of topology [1] to design surfaces that constrain wave propagation to only one direction.

We will consider ’dispersion engineering’ by exploiting non-local (spatially dependent) boundary conditions [2] to optimise the bandwidth over which efficient coupling can be achieved.

We will also investigate how reinterpretation of the Kramers-Kronig relations in the spatial domain can supress reflection of surface waves [3].

[1] Yang et al., ‘Direct observation of topological surface-state arcs in photonic metamaterials’ Nature Communications 8:7 (2017).

[2] Chasnitsky et al., ’Broadband surface plasmon wave excitation using dispersion engineering’ Optics Express 23, 30570 (2015).

[3] Horsley et al., ’Spatial Kramers–Kronig relations and the reflection of waves’ Nature Photonics 9, 436 (2015).