Reconfigurable photonic devices capable of dynamically tuning their optical characteristics to adapt to different application demands on-the-fly have become a focus of intensive research and development efforts in recent years. Compared to classical tuning mechanisms relying on thermo-optic or electro-optic effects, the giant nonvolatile refractive index contrast provided by phase change materials (PCMs) upon solid-state structural transitions enables reconfigurable devices with an unprecedented compact footprint, zero static power consumption, and superior optical performance. These unique features have catalyzed phase change photonics, an emerging field characterized by a rapidly evolving research landscape. This review presents a deep dive into this dynamic field, spanning fundamental material design principles and processing techniques to PCM integration into various state-of-the-art device platforms and industry-standard foundry manufacturing processes. We also identify challenges that PCM photonics need to address and point out directions where exciting innovations will likely come to fruition in the near future.
Stimulated Raman scattering (SRS) microscopy has shown enormous potential in revealing molecular structures, dynamics, and couplings in complex systems. For most biomolecules, the detection sensitivity of SRS is fundamentally limited to the milli-molar level due to the shot noise and the small modulation depth. Additionally, the operation of SRS imaging is complicated by cross phase modulation. We recently revisited SRS from the perspective of energy deposition. Via intensity gain in the Stokes beam and loss in the pump beam, the SRS process pumps molecules to their vibrationally excited states. The thereafter relaxation heats up the surroundings and induces refractive index changes. By probing the refractive index changes with a laser beam, stimulated Raman photothermal (SRP) microscopy is developed, where a >500-fold boost of modulation depth is achieved. Moreover, SRP imaging can be operated with a noisy fiber laser for excitation and a long working distance air condenser for signal collection. Two implementations and broad biological applications are reviewed. In summary, SRP microscopy opens a new way to perform chemical imaging with ultrahigh sensitivity and long working distance optics toward clinical translation.
Spin-glass theory emerged in the 1980s as a merger between theoretical physics and condensed matter. Soon, physicists realized that spin glasses serve as a paradigm for complex systems, as underscored by the 2021 Nobel Prize in Physics, and for applications in machine learning and neuroscience, with a profound connection with the Hopfield model and Boltzmann machines, subjects of the 2024 Nobel Prize in Physics. However, the connection with optics and photonics is even more profound and fundamental; this connection was identified as early as 1982, with the first realizations of optical neural networks. Thirty years later, the first experimental demonstration of a pillar of spin-glass theory, the replica symmetry breaking, was reported in photonics. Nowadays, many scientists consider photonics as an effective solution for new hardware in artificial intelligence, capable of reducing energy consumption in training large machine-learning modules, and also more suitable for realizing fully connected models that underpin modern data-driven analysis. The substantial equivalence between linear optical propagation and a system of interacting binary spins is now well recognized, triggering the development of a new family of devices for both classical and quantum computing. This review is intended to detail the work of the past twenty years concerning the link between spin-glass theory and optics. After a simple introduction to the main ideas of spin glasses, we start from the first works aimed at finding a direct experimental proof of ideas such as the landscape and ultrametricity; then we report on “linear optical spin glasses,” which refer to the photonic simulation of various Ising models for combinatorial optimization and interlinked with quantum computers; finally, we discuss the emerging field of “nonlinear optical spin glasses,” driven by the impressive progress in the realization of coherent Ising machines with parametric oscillators, that opened an new research direction driven by the cross-fertilization of advanced theoretical physics, artificial intelligence, classical and quantum nonlinear optics.
This tutorial provides an overview of free-space optical (FSO) communications operating in the mid-infrared (mid-IR) wavelength bands. In the mid-IR region, there are two atmosphere transmission windows: (mid-wave infrared (MWIR) at 3–5 µm and long-wave infrared (LWIR) at 8–12 µm) with relatively low atmospheric absorption. Compared to near-infrared (near-IR) wavelengths (e.g., telecom C-band at ∼1.55 µm), mid-IR tends to be more resilient to atmospheric degradation effects (e.g., fog-induced scattering and atmospheric turbulence). Thus, using mid-IR wavelengths can enable more robust FSO links through challenging atmospheric conditions. This tutorial aims to review recent developments and advances in mid-IR FSO communications. Various devices and approaches for mid-IR data transmitters and receivers are discussed. Advanced demonstrations for single-channel and multi-channel-multiplexed high-capacity mid-IR FSO links are reviewed. This tutorial also discusses the challenges and limitations of current techniques, as well as the outlook for future research.
Kerr frequency combs have recently emerged as an exciting new photonic technology, with applications across science and engineering. Their formation within driven optical resonators that possess a Kerr nonlinearity is enabled through the rich landscape of localized nonlinear dissipative structures intrinsic to these systems. This article offers a comprehensive review of the physics that underpins these nonlinear comb-generating structures. Particular attention is placed on bright temporal cavity solitons and nonlinear switching waves – the canonical stable comb-generating states in the anomalous and normal dispersion regimes, respectively. Written as both a review and tutorial, the article also includes an in-depth treatment of the numerical methods required to simulate driven Kerr resonators, alongside a comprehensive discussion of the laboratory techniques used to experimentally realize and characterize Kerr combs.
Thermal photonics has attracted considerable interest due to its ability to artificially manipulate thermal radiation. However, many thermal photonic devices are constrained by Kirchhoff's law of thermal radiation, with identical directional emissivity and absorptivity. The rise of non-reciprocal thermal photonics offers the potential to overcome this constraint, enabling independent control of emission and absorption. This tutorial summarizes recent progress in non-reciprocal thermal photonics and the generalized Kirchhoff's law of thermal radiation, establishing a comprehensive relationship between directional absorptivity and emissivity, irrespective of its reciprocity. It also highlights challenges in non-reciprocal thermal radiation experiments and discusses potential solutions. Providing a thorough overview of the fundamentals, experimental approaches, and applications of non-reciprocal thermal photonics, this tutorial aims to enhance understanding of its principles and facilitate advancements in energy harvesting and heat flux control. (c) 2026 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
Optical skyrmions are topological structures formed by the distribution of light's vectorial properties, including polarization, spin, and electromagnetic fields. This tutorial provides a comprehensive overview of the theoretical foundations, configurations, generation mechanisms, and applications of optical skyrmions. Beginning with the historical development from Kelvin's vortex theory to Skyrme's soliton model, the article establishes the topological framework using homotopy groups to classify vortices, skyrmions, hopfions, and other related topological structures. The distinct types of optical skyrmions-such as N & eacute;el-type, Bloch-type, and high-order variants-are characterized using topological invariants like vorticity, polarity, and helicity. Connections between optical skyrmions and other topological entities are explored, showing how structures like hopfions and knots arise from field mappings between compactified manifolds. Experimental realizations through interference of evanescent fields, metasurfaces, spatial light modulators, and spatiotemporal modulation are described, along with methods for dynamic control and topological transitions. Applications in optical information processing, sensing, and quantum technologies are discussed, highlighting the robustness and subwavelength precision enabled by topological protection. Although prior reviews and tutorials exist, this tutorial is necessary because at this pivotal transition from fundamental research to applications, a comprehensive tutorial is significant for navigating new understanding and applications. This tutorial aims to equip readers with both foundational and practical knowledge, positioning optical skyrmions as versatile tools for advancing topological photonics and next-generation photonic technologies. (c) 2026 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
The analysis of symmetries is extremely useful across science. In Physics, symmetries are used to derive conservation laws and selection rules for transitions in interacting systems. In the early days of nonlinear optics (NLO), symmetries were used to formulate a set of rules for photonic processes according to the medium's symmetries that are reflected in the NLO coefficient tensor. While this approach was believed to be complete and closed, the field has recently reignited as multi-color ultrashort laser pulses with tailored polarization and spatiotemporal structures become standard in NLO. A more complete theory has been recently emerging, which aims to incorporate all possible dynamical degrees of freedom of light: spin and orbital angular momentum, spatial structure, time-dependent polarizations, temporal envelopes, etc., in addition to the symmetries of the medium. This theoretical development is also accompanied by experimental advances that rely on tailored light beams that can now be generated with ever-increasing complexity, including topologies in real and a variety of synthetic dimensions, carrying poly-chromatic carrier waves, time-dependent varying angular momenta, local-chirality, and more. The nonlinear interactions between light fields with unique symmetries (or asymmetries) and matter is especially appealing, since that holds the key for developing new ultrafast spectroscopies with sub-femtosecond resolution, for exerting exact control over matter, and improving our fundamental understanding of how light and matter interact. We review these recent advances in this expanding field, focusing on the theory, its implications, and seminal experiments. We aim to establish a comprehensive database of symmetries and selection rules governing NLO light-matter interactions within the emerging new formalism, and invite the scientific community to contribute to this effort.
Metasurfaces offer unprecedented freedom in the flexible and efficient manipulation of light across multiple dimensions such as amplitude, phase, and polarization, enabling the realization of low-cost, high-performance, ultra-lightweight, and ultra-thin optical elements with novel functionalities to overcome traditional optical limitations in volume, weight, functionality, and efficiency. In recent years, this potential has attracted growing attention from both academic and industrial communities. After more than a decade of development, on the one hand, the design of a wide range of metasurfaces has gradually become mature, encompassing physical principles, design methodologies, and devices for diverse applications; on the other hand, metasurfaces with novel physical concepts and functionalities have emerged through integration with other disciplines, including nonlinearity, chirality, optical resonance, non-Hermitian physics, Fourier optics, and topological photonics. In this tutorial, we propose a unified and forward-looking theoretical framework that connects traditional metasurfaces with these emerging meta-surfaces, aiming to clarify their underlying relationships and development trends. We introduce the mainstream design methodologies, highlighting the most recent advancements. We further examine five representative application areas: metalenses, metasurface holograms, quantum photonic metasurfaces, optical computing metasurfaces, and active metasurfaces, in terms of their principles, evaluation criteria, theoretical analyses, and design methods. By bridging fundamentals and emerging concepts under a unified perspective, this tutorial aims to provide a holistic physical understanding and critical insight into the global metasurface landscape, from fundamentals to emerging concepts and applications. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
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. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
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?
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.
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. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
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. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
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, especially, 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.
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. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
Compact laboratory-scale x-ray sources still rely on the same fundamental principles as did the first x-ray tubes developed more than a century ago. In recent years, significant research and development has focused on large-scale x-ray sources such as synchrotrons and free-electron lasers, leading to the generation of high-brightness coherent x-rays. However, the large size and high costs of such sources prevent their widespread use. The quest for a compact and coherent x-ray source has long been a critical objective in modern physics, gaining further importance in recent years for industrial applications and fundamental scientific research. Here, we review the physical mechanisms governing compact coherent x-ray generation. Of current interest are coherent periodic interactions of free electrons in crystalline materials, creating hard x-rays via a mechanism known as parametric x-ray radiation (PXR). Over the past decade, x-ray sources leveraging this mechanism have demonstrated state-of-the-art tunability, directionality, and broad spatial coherence, enabling x-ray phase-contrast imaging on a compact scale. The coming years are expected to show substantial miniaturization of compact x-ray sources, facilitated by progress in electron beam technologies. This review compares the most promising mechanisms used for hard x-ray generation, contrasting parametric x-ray radiation with inverse Compton scattering and characteristic radiation from a liquid-jet anode. We cover the most recent advancements, including the development of new materials, innovative geometrical designs, and specialized optimization techniques, aiming toward x-ray flux levels suitable for medical imaging and x-ray spectroscopy at compact scales. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
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.
