We study, both theoretically and experimentally, tunable metasurfaces supporting sharp Fano-resonances inspired by optical bound states in the continuum. We explore the use of arsenic trisulfide (a photosensitive chalcogenide glass) having optical properties which can be finely tuned by light absorption at the post-fabrication stage. We select the resonant wavelength of the metasurface corresponding to the energy below the arsenic trisulfide bandgap, and experimentally control the resonance spectral position via exposure to the light of energies above the bandgap.

In this Perspective, we discuss the different opportunities offered by time-modulated metasurfaces for dynamic wavefront engineering and space-time photonics. Efforts in codesigning a photonic response while taking into careful consideration the switching/tuning mechanisms, including thermal, electronic, optical, chemical, and mechanical actuation, are essential for achieving sufficient amplitude, phase, and polarization modulation. Here, we examine in detail how the key enabling photonic technologies currently available and relying on similar tuning mechanisms can be applied for the conception of tunable metasurfaces. We review the latest developments and discuss the advantages and limitations of each approach, providing the reader with a clear vision of the current state of the art in active metasurfaces. We also address the readiness of each technological approach to drawing short- and long-term application perspectives. Finally, we discuss perspectives for spatiotemporal metasurface modulation opening new horizons toward unlimited wavefront engineering capabilities.

Metasurfaces and, in particular, metalenses have attracted large interest and enabled various applications in the near-infrared and THz regions of the spectrum. However, the metalens design in the visible range stays quite challenging due to the smaller nanostructuring scale and the limited choice of lossless CMOS-compatible materials. We develop a simple yet efficient design of a polarization-independent, broadband metalens suitable for many CMOS-compatible fabrication techniques and materials and implement it for the visible spectral range using niobium pentoxide (Nb2O5). The produced metalens demonstrates high transmittance and focusing ability as well as a large depth of focus, which makes it a promising solution for a new generation of silicon photomultiplier photodetectors with reduced fill factor impact on the performance and reduced electron–hole generation regions, which altogether potentially leads to improved photodetection efficiency and other characteristics.

Full wavefront control by photonic components requires that the spatial phase modulation on an incoming optical beam ranges from 0 to 2𝝅. Because of their radiative coupling to the environment, all optical components are intrinsically non-Hermitian systems, often described by reflection and transmission matrices with complex eigenfrequencies. Here, it is shown that parity or time symmetry breaking-either explicit or spontaneous-moves the position of zero singularities of the reflection or transmission matrices from the real axis to the upper part of the complex frequency plane. A universal 0 to 2𝝅-phase gradient of an output channel as a function of the real frequency excitation is thus realized whenever the discontinuity branch bridging a zero and a pole, that is, a pair of singularities, is crossing the real axis. This basic understanding is applied to engineer electromagnetic fields at interfaces, including, but not limited to, metasurfaces. Non-Hermitian topological features associated with exceptional degeneracies or branch cut crossing are shown to play a surprisingly pivotal role in the design of resonant photonic systems.

Full wavefront control by photonic components requires that the spatial phase modulation on an incoming optical beam ranges from 0 to 2π. Because of their radiative coupling to the environment, all optical components are intrinsically non-Hermitian systems, often described by reflection and transmission matrices with complex eigenfrequencies. Here, we show that Parity-Time symmetry breaking -either explicit or spontaneous-moves the position of Zero singularities of the reflection or transmission matrices from the real axis to the upper part of the complex frequency plane. A universal 0 to 2π-phase gradient of an output channel as a function of the real frequency excitation is thus realized whenever the discontinuity branch bridging a Zero and a Pole, i.e. a pair of singularities, is crossing the real axis. This basic understanding is applied to engineer electromagnetic fields at interfaces, including, but not limited to, metasurfaces. Non-Hermitian topological features associated with exceptional degeneracies or branch cut crossing are shown to play a surprisingly pivotal role in the design of resonant photonic systems.

Topological Origin of a Universal 2π‐Phase Retardation in Non‐Hermitian Metasurfaces In article number 2200976, Rémi Colom, Patrice Genevet, and colleagues demonstrate the role of topology in the phase response of reflected or transmitted light from metasurfaces. Their phase behavior depends on the relative positions in the complex frequency of zeros and poles which are topological singularities with opposite charges. Their positions can be controlled using either explicit or spontaneous symmetry breaking. This article provides new mechanisms for the design of metasurfaces.

Optical metasurfaces are becoming ubiquitous optical components to control light properties. However, most of these devices are passive and cannot be arbitrarily reconfigured according to the change in the surrounding environment. Here the authors propose an innovative design strategy relying on the position of topological singularities to address full phase modulation of light with almost unity efficiency. The active metasurface unit cells consist of asymmetric Gires-Tournois resonators filled with either silicon or hetero-structured materials to leverage on the thermo-optical or electro-optical effects, respectively. In both cases, a full phase modulation associated with 100% reflection amplitude is observed even when dealing with extremely low refractive index change, on the order of 0.01. Improving the deflection efficiencies for each deflection angle is performed by optimizing the refractive index modulation profile in the extended unit cell using an advanced statistical learning optimization methodology. Consequently, active beam steering designs for active thermo-optical effect with ultimate performance exceeding 90% have been optimized. Furthermore, active wavefront splitting using electro-optics materials is optimized to reach ultimate modulation performances with nearly 92% efficiency. The realization of highly efficient active beam-forming operating at high frequencies would open important applications in imaging microscopy, and 3D light detection and ranging (LiDAR).

The design of wavefront-shaping devices is conventionally approached using real-frequency modeling. However, since these devices interact with light through radiative channels, they are by default non-Hermitian objects having complex eigenvalues (poles and zeros) that are marked by phase singularities in a complex frequency plane. Here, by using temporal coupled mode theory, we derive analytical expressions allowing to predict the location of these phase singularities in a complex plane and as a result, allowing to control the induced phase modulation of light. In particular, we show that spatial inversion symmetry breaking—implemented herein by controlling the coupling efficiency between input and output radiative channels of two-port components called metasurfaces—lifts the degeneracy of reflection zeros in forward and backward directions, and introduces a complex singularity with a positive imaginary part necessary for a full 2π-phase gradient. Our work establishes a general framework to predict and study the response of resonant systems in photonics and metaoptics.