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The control of a lens’s numerical aperture has potential applications in areas such as photography and imaging, displays, sensing, laser processing and even laser-implosion fusion. In such fields, the ability to control lens properties dynamically is of much interest, and active meta-lenses of various kinds are under investigation due to their modulation speed and compactness. However, as of yet, meta-lenses that explicitly offer dynamic control of a lens’s numerical aperture have received little attention. Here, we design and simulate active meta-lenses (specifically, focusing meta-mirrors) using chalcogenide phase-change materials to provide such control. We show that, operating at a wavelength of 3000 nm, our devices can change the numerical aperture by up to a factor of 1.85 and operate at optical intensities of the order of 1.2 × 109 Wm−2. Furthermore, we show the scalability of our design towards shorter wavelengths (visible spectrum), where we demonstrate a change in NA by a factor of 1.92.

7 pags., 9 figs., 1 tab., Periodic arrays of sub-wavelength-scale holes in plasmonic metal films are known to provide resonant transmission peaks via the extraordinary optical transmission (EOT) effect. Active control of the spectral position of such transmission peaks can be obtained by adding a layer of phase-change material (PCM) to the EOT device. Switching the PCM layer between its amorphous and crystalline states can shift the spectral position of the resonance, enabling potential applications in the fields of active filtering and sensing (e.g., multispectral sensing), and for signal modulation. Here, the design, fabrication, and characterization of active EOT devices are targeted at various important regions of the optical spectrum., Funding support is gratefully acknowledged from the EPSRC-QinetiQ TEAM-A Prosperity Partnership grant (EP/R004781/1), and from the EPSRC CDT in Metamaterials (EP/L015331/1). Thanks are also due to the various University of Exeter colleagues, in particular Dr. Liam Trimby, Dr. Santiago Garcia-Cuevas Carrillo, Dr. Yat-Yin Au, Dr. Hong Chang, Dr. Prarthana Vadegadde Dakappa, Mr. Joe Shields, Mr. Joe Pady, Ms. Hannah Barnard, and Prof. Geoff Nash, for contributing time, resources, and expertise to various aspects of this work. The authors also thank Dr. Geoff West of Warwick University, for providing cross-sectional TEM imaging

Materials of which the refractive indices can be thermally tuned or switched, such as in chalcogenide phase-change alloys, offer a promising path towards the development of active optical metasurfaces for the control of the amplitude, phase, and polarization of light. However, for phase-change metasurfaces to be able to provide viable technology for active light control, in situ electrical switching via resistive heaters integral to or embedded in the metasurface itself is highly desirable. In this context, good electrical conductors (metals) with high melting points (i.e., significantly above the melting point of commonly used phase-change alloys) are required. In addition, such metals should ideally have low plasmonic losses, so as to not degrade metasurface optical performance. This essentially limits the choice to a few noble metals, namely, gold and silver, but these tend to diffuse quite readily into phase-change materials (particularly the archetypal Ge2Sb2Te5 alloy used here), and into dielectric resonators such as Si or Ge. In this work, we introduce a novel hybrid dielectric/plasmonic metasurface architecture, where we incorporated a thin Ge2Sb2Te5 layer into the body of a cubic silicon nanoresonator lying on metallic planes that simultaneously acted as high-efficiency reflectors and resistive heaters. Through systematic studies based on changing the configuration of the bottom metal plane between high-melting-point diffusive and low-melting-point nondiffusive metals (Au and Al, respectively), we explicitly show how thermally activated diffusion can catastrophically and irreversibly degrade the optical performance of chalcogenide phase-change metasurface devices, and how such degradation can be successfully overcome at the design stage via the incorporation of ultrathin Si3N4 barrier layers between the gold plane and the hybrid Si/Ge2Sb2Te5 resonators. Our work clarifies the importance of diffusion of noble metals in thermally tunable metasurfaces and how to overcome it, thus helping phase-change-based metasurface technology move a step closer towards the realization of real-world applications.

The active control of artificially structured metasurfaces is a promising route for solving the operation bandwidth limitation of metasurfaces due to their resonant nature. Herein, the active tunability of the toroidal response in a terahertz hybrid metasurface is proposed and experimentally demonstrated. The top metallic layer of the metasurface has a toroidal configuration and is coupled to an electrically biased phase‐change silicon layer, whose conductive thickness and conductivity can be changed significantly when applying increased external current. The electrically biased hybrid metasurface shows high efficiency and complete electrical switching on the toroidal response in a broadband manner. Also, the optoelectronic metasurfaces modulated by biased currents are much easier to integrate in on‐chip optical devices. The hybrid metasurface taking advantage of the silicon layer with insulating‐state to conductive‐state transition in optical conductivity may facilitate the development of high‐performance active photonic applications in, for example, smart sensing in the terahertz regime.