Miniaturization is a strong demand of modern scientific technology. However, conventional optical components based on refraction suffer from functional degradation as the device size decreases. Metasurfaces consisting of subwavelength optical antenna arrays have emerged as planar optical devices that enable many promising applications in lenses, holograms, and optical cloaks. During recent decades, metasurfaces have been developed for their specific functionalities by exploiting new materials and design algorithms. In this issue of MRS Bulletin, progress in metasurfaces is discussed to provide a comprehensive understanding of metasurfaces and their novel applications in optics and photonics.

To address critical energy issues in civic structures, we have developed a novel concept of optical thermal insulation (OTI) without relying on a conventional thermal intervention medium, such as air or argon, as often used in conventional window systems. We have synthesized the photothermal (PT) materials, such as the Fe3O4 and Fe3O4@Cu2−xS nanoparticles, that exhibit strong UV and near-infrared (NIR) absorptions but with good visible transparency. Upon coating the inner surface of the window glass with a PT film, under solar irradiation, the inner surface temperature rises due to the PT effect. Subsequently, the temperature difference, ΔT, is reduced between the single pane and room interior. This leads to lower the thermal loss through a window, reflected by the U-factor, resulting in considerable energy saving without double- or triple-glazing. Comparing with the Fe3O4 coatings, Fe3O4@Cu2−xS is spectrally characterized with a much stronger NIR absorbance, contributing to an increased PT efficiency under simulated solar irradiation (0.1 W/cm2). PT experiments are carried out via both white light and monochromic NIR irradiations (785 nm). The parameters associated with the thermal performance of the PT films are calculated, including PT conversion efficiency, specific absorption rate (SAR), and U-factor. Based on the concept of OTI, we have reached an optimum U-factor of 1.46 W/m2 K for a single pane, which is satisfactory to the DOE requirement (<1.7 W/m2 K).

The past decade has witnessed the advent of nanophotonics, where light–matter interaction is shaped, almost at will, with human-made designed nanostructures. However, the design process for these nanostructures has remained complex, often relying on the intuition and expertise of the designer, ultimately limiting the reach and penetration of this groundbreaking approach. Recently, there has been an increasing number of studies in applying machine learning techniques for the design of nanostructures. Most of these studies engage deep learning techniques, which entail training a deep neural network (DNN) to approximate the highly nonlinear function of the underlying physical process of the interaction between light and the nanostructures. At the end of the training, the DNN allows for on-demand design of nanostructures (i.e., the model can infer nanostructure geometries for desired light spectra). In this article, we review previous studies for designing nanostructures, including recent advances where a DNN is trained to generate a two-dimensional image of the designed nanostructure, which is not limited to a closed set of nanostructure shapes, and can be trained for the design of any geometry. This allows for better generalization, with higher applicability for real-world design problems.

Metasurfaces are thin-film electromagnetic devices with subwavelength-scale geometric structuring. They can be tailored to produce a broad range of optical functions due to the strong relationship between electromagnetic response and geometric shape. An open challenge has been understanding how to produce an ideal metasurface design when presented with a desired electromagnetic response. This article discusses the use of topology optimization as a design platform for high-performance, freeform metasurfaces. Two types of topology optimizers are covered—local gradient-based optimizers that leverage the adjoint variables method, and global population-based optimizers that reframe the optimization process as the training of a generative neural network. It is anticipated that these inverse design concepts will push metasurface performance to the physical limits of structured media and enable new functionalities in electromagnetic systems.

The development of optical elements has seen tremendous advances over the last few decades for a variety of applications, including displays, cameras, and microscopes. Various optical elements have been developed, ranging from refractive elements to diffractive elements. In recent years, metasurfaces have been in the spotlight to develop next-generation optical elements beyond existing refractive or diffractive optics. A metasurface is a two-dimensional metamaterial composed of subwavelength artificial structures and has been studied for the development of optical elements with the major advantage that the properties of light can be freely adjusted by a thin flat structure. Optical lenses using metasurfaces can be hundreds of times thinner than conventional lenses, while at the same time, provide excellent focusing performance. This suggests that they can be applied to mobile and high-performance imaging applications in the future. Here, we discuss developments of optical elements from refractive or diffractive optics to metasurface optics, including basic principles and properties, current issues, and future perspectives.

Dielectrics are electrical insulator materials, polarizable by opposite displacement of positive and negative ionized atoms via electric fields across the material’s thickness. Dielectrics are used in energy-storage capacitors, as key components in modern micro-/nanoelectronics, high-frequency and mobile communication devices, and life-saving microchips and other devices such as defibrillators and pacemakers implantable in humans. A key dielectric parameter is the dielectric constant (k), which largely controls the capacitance in capacitors with nanoscale area and dielectric layer thickness. Extremely high dielectric constants (k ≥1000) were observed in oxides (e.g., La1.8Sr0.12NiO4) with relaxor/ferroelectric materials and in combined semiconducting bulk properties with highly resistive grain boundaries. Giant dielectric constant films have also been demonstrated, based on integrating relatively low-dielectric-constant oxides into nanolaminate structures (e.g., TiOx/Al2O3; TiO2/HfO2) with tailored sublayer thicknesses, interfaces, and oxygen atom distributions. This overview article addresses the science and technology of high-dielectric-constant oxide materials with different compositions and structures.