(Invited) Plasmonic Metasurfaces Based on Epsilon-Near-Zero Materials
Authors: Berini Pierre 
Published: December 2024

We report work on tunable plasmonic metasurfaces exploiting epsilon-near-zero effects in metal-oxide-semiconductor structures fabricated using conductive oxides. The tunable metasurfaces comprise subwavelength pixels that produce no grating diffraction and are used in reflection to control the magnitude and phase of the reflected beam. Applications include optical phased arrays, spatial light modulators, beam steering devices and wavefront shaping devices.
Resonant nanometallic structures, such as plasmonic nanoantennas, are essential to convert light to surface plasmon-polaritons (SPPs) localized to ultra-small volumes. Such structures can provide highly enhanced fields, strong confinement, high surface sensitivity, and can double as a device electrode for applying voltages or passing currents to active regions in optoelectronic devices. In optoelectronics, plasmon enhancement can be exploited for high-performance electro-optic modulators and beam-steering devices [1-7].
Fig. 1 shows an electrically-tunable plasmonic metasurface on fused silica [7]. The metasurface consists of an array of Au dipole nanoantennas contacted perpendicularly by electrical contact lines (Fig. 1(b)). The contact lines are centred on the nanoantennas (dipoles), which ensures that they are minimally invasive optically, due to their orthogonal alignment relative to the incident polarisation (along the nanoantenna axes). The nanoantennas are connected to an electrical fan-out structure (Fig. 1(a)) and coated with a HfO2 layer then an ITO layer (Fig. 1c)), and finally by a metal mirror structure (Fig. 1(d)). Electrically the structure operates as a MOS capacitor (Au-HfO2-ITO) with the top Au mirror structure also acting as the Ohmic contact to the ITO.
When driven into strong accumulation, the ITO enters the epsilon-near-zero regime [6] resulting in large changes in the resonant behaviour of the nanoantennas. The structure produces a reflection coefficient at telecom wavelengths (l

0
~ 1550 nm) that varies significantly in phase with applied voltage but maintains a constant reflectance [7]. The individual pixels (meta-atoms) in this structure are subwavelength such that grating diffraction upon reflection is completely avoided. The structure forms the basis of a solid-state optical phased array or a spatial light modulator which find application in beam steering and wavefront shaping devices.
References

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Figure 1