Abstract
Waves propagating in a negative-index material have wave-front propagation (wavevector, k) opposite in direction to that of energy flow (Poynting vector, S). Here we present an experimental realisation at microwave frequencies of an analogous surface wave phenomenon whereby a metasurface supports a surface mode that has two possible wavevector eigenstates within a narrow band of frequencies: one that supports surface waves with positive mode index, and another that supports surface waves with negative mode index. Phase sensitive measurements of the near-field of surface waves across the metasurface show the contrasting spatial evolution of the two eigenstates, providing a unique opportunity to directly observe the negative-index phenomenon.
Highlights
Waves propagating in a negative-index material have wave-front propagation opposite in direction to that of energy flow (Poynting vector, S)
Waves that propagate in any medium characterised by a negative refractive index are defined by the propagation of wave-fronts (k) that move in the opposite direction to the energy flow (S)
In a theoretical study in 1968, Veselago proposed[2] that a similar result may be observed in a bulk material that possesses simultaneously negative electric permittivity ε and magnetic permeability μ. Such negative-index materials have not been found to exist in nature, so this idea lay dormant until the year 2000 when it was revived by Pendry[3]
Summary
Our experimental methods allow for the measurement of the instantaneous electric field profile of the surface waves that are excited from point source located at the sample’s centre The exact experimental arrangement involves one probe (that is designated as the source antenna) being threaded through a small hole that is drilled through the centre of the sample, at a point precisely between four patches The corners of those four patches are slightly modified in order to prevent any electrical contact with the antenna. An animation of the surface wave fields on resonance provides a beautiful demonstration of the negative mode index phenomenon This animation (Movie S1) is generated by systematically advancing the phase (across a full temporal cycle), with the instantaneous electric field recalculated for every increment in phase. This allows the dynamics of the available surface-wave eigenstates to be directly visualised
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