Spatiotemporal light control with active metasurfaces

BACKGROUND Metasurfaces have opened up a number of remarkable new approaches to manipulate light. These flat optical elements are constructed from a dense array of strongly scattering metallic or semiconductor nanostructures that can impart local changes to the amplitude, phase, and polarization state of light waves. They have facilitated a relaxation of the fundamental Snell’s law for light refraction and enabled the creation of small form factor optical systems capable of performing many tasks that currently can only be achieved with bulky optical components. New photon management capabilities have also emerged, including the achievement of multiple optical functions within a single metasurface element, the realization of very high-numerical apertures, and dispersion engineering with metasurface building blocks. Despite this impressive progress, most metasurfaces we see today are static in nature, and their optical properties are set in stone during their fabrication. However, we are currently witnessing an evolution from passive metasurfaces to active metasurface devices. This natural progression stems from the notion that space and time play complementary roles in Maxwell’s equations. It suggests that structuring materials in both space and time can bring forth new physical phenomena and further broaden the range of possible applications. This Review discusses what is required to create high-performance spatiotemporal metasurfaces and analyzes what new applications and physics they have to offer. ADVANCES To realize the dream of dynamically controlled metasurfaces, we need to achieve strong and tunable light-matter interactions in ultrathin layers of material. In doing so, we cannot rely on the long interactions of lengths and times provided by bulk optical crystals or waveguides. This has stimulated much research aimed at identifying new materials and nanostructures capable of providing dramatically enhanced light-matter interaction and highly tunable optical responses. There are already well-established ways to boost light-matter interaction through the engineering of plasmonic and Mie-style resonances in metallic and semiconductor nanostructures. However, the best approaches to dynamically alter their optical response are the topic of current study. We highlight different approaches that involve electrical gating, optical pumping, mechanical actuation, stimulating phase transitions, magneto-optical effects, electrochemical metallization, liquid-crystal control, and nanostructured nonlinearities. We also discuss how metasurfaces can be used to realize reconfigurable devices, such as tunable lenses and holograms, optical phase modulators, and polarization converters. In addition to these emerging applications, it has become apparent that temporal control of metasurfaces at an ultrafast speed can unlock entirely new physical effects that are not accessible in their static counterparts. Photons interacting with spatiotemporally modulated metasurfaces can display changes in their frequency as well as their linear momentum, angular momentum, and spin. This opens the door to new operating regimes for metasurfaces in which light can experience Doppler shifts, break Lorentz reciprocity, or produce time-reversed optical beams. OUTLOOK The emergence of active metasurfaces is very timely given the many applications that would benefit from having tunable optical components that are flat and easy to integrate. These include a variety of wearables, autonomous vehicles, robotics, augmented and virtual reality, sensing, imaging, and display technologies. However, a massive challenge lies ahead toward realizing the full technological potential of these new elements. The ultimate unit cell of an active metasurface should be subwavelength in size and facilitate large, dynamic amplitude and phase tuning. For larger metasurfaces, the need to individually address and activate the massive number of tiny unit cells will also pose integration and power-consumption challenges that rival those that are currently faced by the semiconductor industry in the creation of the next generation of integrated circuits. If realized, such elements may radically outperform conventional systems that are based on bulky optical and mechanical parts. As new physical effects appear in spatiotemporal metasurfaces, new fundamental questions are also bound to arise. It is already clear that the very basic processes of light absorption, modulation, fluorescent and thermal emission, frequency conversion, and polarization conversion can be manipulated in new ways. As a result, the typically assumed limits for time-invariant or reciprocal systems will need to be reconsidered in these dynamic systems. A concerted, highly interdisciplinary effort is thus required to uncover and push the bounds by which these sheets of spatiotemporally structured materials can manipulate light.