The timescale for electrons to break interatomic bonds during photoinduced physical or biochemical processes such as vision or photosynthesis is femtoseconds or less. For this reason, electron dynamics in a chemical bond must be viewed with attosecond pulses, but until now, laboratory attosecond sources have been too slow or weak to capture this intrabond motion. Tunable, powerful, attosecond X-rays from free-electron lasers now fill this gap and enable the first studies of site-specific electron motion within molecules, to the best of our knowledge. Here we explain how these sources work and how to use them to explore the attosecond frontier of physics, chemistry, and biology.

The development of integrated and programmable photonic devices has significantly affected modern communications and signal processing in both the classical and quantum domains. However, achieving the required performance for new smart applications presents challenges in terms of design, fabrication, and control over multiple parameters. Optimization methods that leverage metaheuristic algorithms, machine learning, and artificial neural networks offer efficient solutions for the complex design of photonic devices, enabling new and desired functionalities. This comprehensive review explores the use of these methods to enhance the fabrication of innovative devices for smart photonic applications in next-generation communication and signal processing. We begin by introducing the mathematical frameworks of these optimization methods. We then investigate how they enable customization, optimization, and new device functionalities. Ultimately, we present our conclusions and discuss future prospects, emphasizing the potential of optimization methods in promoting revolutionary advancements in photonics.

Confining electromagnetic fields inside an optical cavity can enhance the light–matter coupling between quantum materials embedded inside the cavity and the confined photon fields. When the interaction between the matter and the photon fields is strong enough, even the quantum vacuum field fluctuations of the photons confined in the cavity can alter the properties of the cavity-embedded solid-state materials at equilibrium and room temperature. This approach to engineering materials with light avoids fundamental issues of laser-induced transient matter states. To clearly differentiate this field from phenomena in driven systems, we call this emerging field cavity materials engineering. In this review, we first present theoretical frameworks, in particular, ab initio methods, for describing light–matter interactions in solid-state materials embedded inside a realistic optical cavity. Next, we overview a few experimental breakthroughs in this domain, detailing how the ground state properties of materials can be altered within such confined photonic environments. Moreover, we discuss state-of-the-art theoretical proposals for tailoring material properties within cavities. Finally, we outline the key challenges and promising avenues for future research in this exciting field.

High-precision optical time and frequency transfer is accomplished by a collection of laser-based techniques that achieve time dissemination with subpicosecond instabilities and frequency dissemination with instabilities below one part in 1016. The ability to distribute and compare time and frequency at these precisions enables current optical timing networks such as interconnected optical atomic clocks for the redefinition of the second, relativistic geodesy, and fundamental physics tests as well as time and frequency dissemination systems for large-scale scientific instruments. Future optical timing networks promise to expand these applications and enable new advances in distributed coherent sensing, precise navigation, and more. The field of high-precision optical time and frequency transfer has made significant advances over the last 20 years and has begun to transition from technique development to deployment in applications. Here, we present a review of approaches to high-precision optical time and frequency transfer. We first present a brief overview of the metrics used to assess time and frequency transfer. We then provide a discussion of the difference between time transfer and frequency transfer and review the various technical noise sources. We also provide a background on the optical frequency comb and its role in optical time and frequency transfer for additional context. The next sections of the paper cover specific time–frequency transfer techniques and demonstrations beginning with time and frequency transfer over fiberoptic links including continuous-wave (CW) laser-based frequency transfer, CW-laser-based time transfer, and frequency-comb-based time transfer. We then discuss approaches for time and frequency transfer over free-space including pulsed-source time transfer, CW-laser-based frequency transfer, and frequency-comb-based time transfer. Since no known existing review article covers frequency-comb time transfer over free-space, we provide additional details on the technique. Finally, we provide an outlook that outlines outstanding challenges in the field as well as possible future applications.

Topological textures are well-established topics in condensed matter systems and nonlinear field theories. A typical example is the magnetic spin texture, which promises high-density data storage and information processing applications. With the recent development of nanophotonics and structured light, the topological optical textures, which are analogous to magnetic spin textures, can be created in linear electromagnetic fields with connections to solid-state physics but relying on radically different mechanisms. The emerging field of free-space topological optical textures has begun to show its ability to emulate diversified topologies in higher-dimensional light fields and open new directions of topologically protected information transfer. This article reviews the background of such topological textures, introduces a tutorial of fundamental theories for diverse topological textures in free space, and then provides perspective on the future potential applications to revolutionize our information society.

Photovoltaics, a mature technology, is set to play a vital role in achieving a carbon-free energy system. This article examines the pivotal role of optics in advancing photovoltaics. We identify key scientific research areas where the optics community can make significant contributions. We are guided by the central question: How can optics facilitate the large-scale deployment of photovoltaics necessary for decarbonizing our societies?

Structured light has gained prominence of late, offering a modern toolkit for controlling all of light’s degrees of freedom and facilitating many applications. A highly topical application is the long distance free-space delivery of structured light, essential in classical and quantum communication, remote sensing, and energy transport. Unfortunately atmospheric turbulence tends to distort the structure of light, negating many of the benefits. For this reason, laboratory studies of structured light in simulated atmospheric turbulence are highly desirable in order to study and mitigate these deleterious effects. Here, we outline how to get started with simulating atmospheric turbulence in the laboratory, from single-phase-screen approximations of weak turbulence to experimentally simulating long path strong turbulent conditions. Core to our approach is the use of modern digital tools in the form of digital micro-mirror devices and liquid crystal spatial light modulators, allowing fast, efficient, and realistic conditions to be realized in the laboratory. We show how to create and pass structured light through the simulated medium and outline the toolkit available for fast probing of the medium. We highlight all the potential pitfalls and common errors in this topical field, providing the code to circumvent them for immediate implementation. Finally, we show how the tutorial can be extended to the quantum regime, as well as general studies of complex light in complex media. This tutorial will be beneficial to both a beginner audience wishing to get started, as well as experienced researchers who wish to unravel the nuances of this approach.

Optical metasurfaces are conventionally viewed as organized flat arrays of photonic or plasmonic nanoresonators, also called metaatoms. These metasurfaces are typically highly ordered and fabricated with precision using expensive tools. However, the inherent imperfections in large-scale nanophotonic devices, along with recent advances in bottom-up nanofabrication techniques and design strategies, have highlighted the potential benefits of incorporating disorder to achieve specific optical functionalities. This review offers an overview of the key theoretical, numerical, and experimental aspects related to the exploration of disordered optical metasurfaces. It introduces fundamental concepts of light scattering by disordered metasurfaces and outlines theoretical and numerical methodologies for analyzing their optical behavior. Various fabrication techniques are discussed, highlighting the types of disorder they deliver and their achievable precision level. The review also explores critical applications of disordered optical metasurfaces, such as light manipulation in thin film materials and the design of structural colors and visual appearances. Finally, the article offers perspectives on the burgeoning future research in this field. Disordered optical metasurfaces offer a promising alternative to their ordered counterparts, often delivering unique functionalities or enhanced performance. They present a particularly exciting opportunity in applications demanding large-scale implementation, such as sustainable renewable energy systems, as well as aesthetically vibrant coatings for luxury goods and architectural designs.

Plasmonics offers a groundbreaking avenue for manipulating light beyond the diffraction limit, finding utility in diverse applications ranging from optical cloaking and chemical sensing to super-resolution imaging. Despite these promising applications, plasmonic devices are always born with significant energy dissipation, posing substantial challenges to their efficiency and practical implementation. In the realm of plasmonics, researchers in the field of plasmonics have spent decades exploring alternatives to noble metals. Recently, alkali metals have garnered revived attention as promising candidates due to their exceptional light-manipulation capabilities and low losses. We elucidate the fundamental physical mechanisms behind the optical low-loss nature in alkali metals, alongside methodologies for characterizing alkali metal losses. To discern the suitable applications for alkali metal materials, we compare their advantages and disadvantages with those of other plasmonic materials. Furthermore, we introduce experimental techniques for measuring plasmonic losses and fabrication techniques and highlight potential applications of low-loss alkali metals.

Since the discovery of two-dimensional transition metal dichalcogenide monolayers as direct bandgap semiconductors with pronounced room-temperature exciton transitions, research on excitons and polaritons in these materials has exploded worldwide. Here, we give an introductory tutorial on the basic properties of excitons and polaritons in these materials, emphasizing how they are different from those in conventional semiconductors, and discuss some of the most exciting new phenomena reported.