We experimentally demonstrate an optically switchable gallium-based metasurface, in which a reversible light-induced transition between solid and liquid phases occurring in a confined nanoscale surface layer of the metal drives significant changes in reflectivity and absorption. The metasurface architecture resonantly enhances the metal's “active plasmonic” phase-change nonlinearity by an order of magnitude, offering high contrast all-optical switching in the near-infrared range at low, μW μm−2, excitation intensities.

We report on directed self-assembly of ordered, vapor-deposited gallium nanoparticles on surface-relief-structured polymer substrates. Grating templates impose periodic order in one dimension, most effectively when the grating half-period is of the order of the mean unperturbed center-to-center particle spacing for a given mass-thickness of Ga. Self-organized order also emerges in the perpendicular direction as a consequence of the liquid-phase particles' nucleation, growth, and coalescence on the ridges of the grating pattern in relative isolation from the adjacent slots, and vice versa.

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Changes in the arrangement of atoms in matter, known as structural phase transitions or phase changes, offer a remarkable range of opportunities in photonics. They are exploited in optical data storage laser-based manufacturing, and have been explored as underpinning mechanisms for controlling laser dynamics, optical and plasmonic modulation, and low-energy switching in single nanoparticle devices and metamaterials. Comprehensive modelling of phase change processes in photonics is however extremely challenging as it involves a number of entangled processes including atomic/molecular structural change, domain and crystallization dynamics, change of optical properties in inhomogeneous composite media, and the transport and dissipation of heat and light, which happen on time and length scales spanning several orders of magnitude. Here, for the first time, we show that the description of such complex nonlinear optical processes in phase change materials can be reduced to a cellular automata model. Using the important example of a polymorphic gallium film, we show that a cellular model based upon only a few independent and physically-interpretable parameters can reproduce the experimentally measured behaviors of gallium all-optical switches over a wide range of optical excitation regimes. In an era of otherwise largely opaque computational modelling techniques, the cellular automata methodology has considerable heuristic value for the study of complex nonlinear optical processes without the need to understand details of atomic dynamics, band structure and energy conservation at the nanoscale.

Surface-driven metallization in a nanoscale layer of elemental gallium forming the backplane of a photonic metamaterial absorber provides a mechanism for reversible all-optical and thermo-optical tuning of resonant response.OCIS codes: (160.3918) Metamaterials; (240.6700) SurfacesWe report on the development and first experimental demonstration of a gallium-based photonic metamaterial, in which a reversible transition between solid and liquid phases occurring in a confined nanoscale surface layer of the metal drives significant changes in resonant response, offering high contrast visible/near-infrared reflectivity switching in the visible/near-infrared range that may be controlled by low intensity optical excitation.The generic form of a resonant metamaterial absorber comprises a planar array of sub-wavelength plasmonic metal resonators and a continuous metallic (mirror) backplane, separated by a thin dielectric spacer -the resonant frequency being set by the design of the nanostructured metal layer and thickness of the spacer. Mechanisms to achieve dynamic tuning/switching of metamaterial response, i.e. frequency and/or amplitude modulation, typically rely on functional media (e.g. silicon, chalcogenide phase-change glasses, liquid crystals) placed in contact/proximity to the metal framework. Here instead we harness an interfacial structural phase transition in the metallic framework itself (the mirror plane of a resonant absorber) to drive reversible changes in spectral response (Fig. 1). Gallium is known as a remarkably polymorphic element, existing in up to nine different phases with properties ranging from those of the 'semi-metallic' α-phase (the stable bulk solid form) through to those of the liquid, which is highly essentially a free-electron metal. A significant change in optical properties is thus associated with its solidliquid transition, which occurs in the bulk metal at T m = 29.8°C. But gallium shows strong 'surface melting' behavior whereby a layer (only a few nanometers thick) of the highly metallic liquid phase forms at an interface between the solid α-phase and a dielectric even at temperatures several degrees below T m . The thickness of this