Addressable nanomechanical photonic metamaterials : towards optical properties on demand

• Main Author: Cencillo Abad, Pablo Manuel
• Other Authors: Zheludev, Nikolai
• Published: University of Southampton 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.729752

While metamaterials offer engineered static optical properties, future artificial media with dynamic random-access control over shape and position of meta-molecules will provide arbitrary control of light propagation. In this thesis I report: • The experimental realization of the first addressable nanomechanical photonic metasurfaces allowing selective actuation of individual metamaterial strips in a single spatial dimension. The devices are constructed from individually addressable plasmonic chevron nanowires with a 230 nm × 100 nm cross-section, which consist of gold and silicon nitride. The active structure of the metadevice consists of 15 nanowires each 18 μm long and is fabricated by a combination of electron beam lithography and ion beam milling. It is packaged as a microchip device where the nanowires can be individually actuated by control currents via differential thermal expansion. • The novel concept and numerical characterization of a realistic metadevice that dynamically controls the optical phase of reflected light with sub-wavelength pixelation in one dimension. Based on nanomembrane technology, it consists of individually moveable metallic nanowire actuators that control the phase of reflected light by modulating the optical path length. The metadevice can provide on-demand optical wavefront shaping functionalities of diffraction gratings, beam splitters, phasegradient metasurfaces, cylindrical mirrors and mirror arrays with variable focal distance and numerical aperture without unwanted diffraction. • The novel concept and numerical characterization of a spatial intensity modulator with sub-wavelength resolution in one dimension that combines recent advances in reconfigurable nanomembrane metamaterials and coherent all-optical control of metasurfaces. The metadevice uses nanomechanical actuation of metasurface absorber strips placed near a mirror in order to control their interaction with light from perfect absorption to negligible loss, promising a path towards dynamic beam diffraction, light focusing and holography without unwanted diffraction artefacts. • The experimental demonstration of a reflective light modulator, a dynamic Salisbury screen where modulation of light is achieved by moving a thin metamaterial absorber to control its interaction with the standing wave formed by the incident wave and its reflection on a mirror. Electrostatic actuation of the plasmonic metamaterial absorber’s position leads to a dynamic change of the Salisbury screen’s spectral response and 50% modulation of the reflected light intensity in the near-infrared part of the spectrum. The demonstrated approach can also be used with other metasurfaces to control the changes they impose on the polarization, intensity, phase, spectrum and directional distribution of reflected light. In summary, dynamic control over optical properties both in time and space through addressable metamaterials enables focusing, diffraction and redirection of light on demand without unwanted diffracted beams present in commercial spatial light modulators. This work paves the way towards optical properties on demand.