Integrated photonics will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying optical quantum technologies can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration have enabled table-top experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. These advances have enabled integrated quantum photonic technologies combining up to 650 optical and electrical components onto a single chip that are capable of programmable quantum information processing, chip-to-chip networking, hybrid quantum system integration, and high-speed communications. In this roadmap article, we highlight the status, current and future challenges, and emerging technologies in several key research areas in integrated quantum photonics, including photonic platforms, quantum and classical light sources, quantum frequency conversion, integrated detectors, and applications in computing, communications, and sensing. With advances in materials, photonic design architectures, fabrication and integration processes, packaging, and testing and benchmarking, in the next decade we can expect a transition from single- and few-function prototypes to large-scale integration of multi-functional and reconfigurable devices that will have a transformative impact on quantum information science and engineering.

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects, ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in the life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nano-particle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration.

Virtual reality (VR) and augmented reality (AR) are revolutionizing the ways we perceive and interact with various types of digital information. These near-eye displays have attracted significant attention and efforts due to their ability to reconstruct the interactions between computer-generated images and the real world. With rapid advances in optical elements, display technologies, and digital processing, some VR and AR products are emerging. In this review paper, we start with a brief development history and then define the system requirements based on visual and wearable comfort. Afterward, various VR and AR display architectures are analyzed and evaluated case by case, including some of the latest research progress and future perspectives.

Machine learning provides a promising platform for both forward modeling and the inverse design of photonic structures. Relying on a data-driven approach, machine learning is especially appealing for situations when it is not feasible to derive an analytical solution for a complex problem. There has been a great amount of recent interest in constructing machine learning models suitable for different electromagnetic problems. In this work, we adapt a region-specified design approach for the inverse design of multilayered nanoparticles. Given the high computational cost of dataset generation for electromagnetic problems, we specifically investigate the case of a small training dataset, enhanced via random region specification in an inverse convolutional neural network. The trained model is used to design nanoparticles with high absorption levels and different ratios of absorption over scattering. The central design wavelength is shifted across 350–700 nm without re-training. We discuss the implications of wavelength, particle size, and the training dataset size on the performance of the model. Our approach may find interesting applications in the design of multilayer nanoparticles for biological, chemical, and optical applications as well as the design of low-scattering absorbers and antennas.

Silicon carbide has recently surged as an alternative material for scalable and integrated quantum photonics, as it is a host for naturally occurring color centers within its bandgap, emitting from the UV to the IR even at telecom wavelength. Some of these color centers have been proved to be characterized by quantum properties associated with their single-photon emission and their coherent spin state control, which make them ideal for quantum technology, such as quantum communication, computation, quantum sensing, metrology and can constitute the elements of future quantum networks. Due to its outstanding electrical, mechanical, and optical properties which extend to optical nonlinear properties, silicon carbide can also supply a more amenable platform for photonics devices with respect to other wide bandgap semiconductors, being already an unsurpassed material for high power microelectronics. In this review, we will summarize the current findings on this material color centers quantum properties such as quantum emission via optical and electrical excitation, optical spin polarization and coherent spin control and manipulation. Their fabrication methods are also summarized, showing the need for on-demand and nanometric control of the color centers fabrication location in the material. Their current applications in single-photon sources, quantum sensing of strain, magnetic and electric fields, spin-photon interface are also described. Finally, the efforts in the integration of these color centers in photonics devices and their fabrication challenges are described.

The last decade has seen the development of a wide set of tools, such as wavefront shaping, computational or fundamental methods, that allow us to understand and control light propagation in a complex medium, such as biological tissues or multimode fibers. A vibrant and diverse community is now working in this field, which has revolutionized the prospect of diffraction-limited imaging at depth in tissues. This roadmap highlights several key aspects of this fast developing field, and some of the challenges and opportunities ahead.

In its 60 years of existence, the field of nonlinear optics has gained momentum especially over the past two decades thanks to major breakthroughs in material science and technology. In this article, we present a new set of data tables listing nonlinear-optical properties for different material categories as reported in the literature since 2000. The papers included in the data tables are representative experimental works on bulk materials, solvents, 0D–1D–2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, hybrid waveguiding systems, and materials suitable for nonlinear optics at THz frequencies. In addition to the data tables, we also provide best practices for performing and reporting nonlinear-optical experiments. These best practices underpin the selection process that was used for including papers in the tables. While the tables indeed show strong advancements in the field over the past two decades, we encourage the nonlinear-optics community to implement the identified best practices in future works. This will allow a more adequate comparison, interpretation and use of the published parameters, and as such further stimulate the overall progress in nonlinear-optical science and applications.

Topological photonics seeks to control the behaviour of the light through the design of protected topological modes in photonic structures. While this approach originated from studying the behaviour of electrons in solid-state materials, it has since blossomed into a field that is at the very forefront of the search for new topological types of matter. This can have real implications for future technologies by harnessing the robustness of topological photonics for applications in photonics devices. This roadmap surveys some of the main emerging areas of research within topological photonics, with a special attention to questions in fundamental science, which photonics is in an ideal position to address. Each section provides an overview of the current and future challenges within a part of the field, highlighting the most exciting opportunities for future research and developments.

Structured illumination microscopy (SIM), is a wide-field, minimally-invasive super-resolution optical imaging approach with optical sectioning capability, and it has been extensively applied to many different fields. During the past decades, SIM has been drawing great attention for both the technique development and applications. In this review, firstly, the basic conception, instrumentation, and functionalities of SIM are introduced concisely. Secondly, recent advances in SIM which enhance SIM in different aspects are reviewed. Finally, the variants of SIM are summarized and the outlooks and perspectives of SIM are presented.

The demand is growing for new nanoscience-based technologies with unique properties that are different from traditional wet-chemical techniques. In recent years, laser ablation in liquid (LAL) has attracted increasing attention for nanomaterial synthesis, which has rapidly advanced both fundamental research and applications. Compared to other techniques, LAL is easy to set up and simple to perform. A large diversity of bulk and powder targets can be employed for LAL, which combined with an enormous variety of liquids, greatly diversify the nanomaterials that can be synthesized by LAL in terms of size, composition, shape, and structure. Although many reviews related to LAL have been published, a comprehensively thorough introduction that deals with the diversity of the targets and liquids used for LAL is still missing. To fill this gap, this review gives a comprehensive summary of the nanomaterials synthesized by LAL using different types of target and liquid, with an emphasis on the effects of liquids on the final nanoproducts. In order to provide a better understanding of the liquids' effects, this review also discusses liquid additives such as salts, polymers, support materials, and their mixtures. Since many reactions occur during LAL, the scope of reactive laser ablation in liquid (RLAL) is redefined, and the representative reactions for each type of liquid used for LAL are summarized and highlighted. Consequently, this review will be a useful guide for researchers developing desirable nanomaterials via LAL.

Finding appropriate strategies to increase the robustness through turbulence with extended depth of focus (DOF) is a common requirement in developing high-resolution imaging through air or water media. However, conventional lenses with a specially designed structure require high manufacturing costs and are limited by a lack of dynamic modulation characteristics. Spatial light modulators (SLMs) are unique flat-panel optical devices which can overcome the distance limitation of beam propagation for the dynamic modulation property. In this work, we address the dynamic generation of a steady optical beam (STOB) based on the mechanism of transverse wave vector elimination. STOBs generated by the SLM have significant advantages over Gaussian beams for the characteristics of peak intensity, robust propagation, extended-DOF beam profile, and dynamic wavefront modulation over a long distance under strong turbulent media. Our versatile, extensible, and flexible method has promising application scenarios for the realization of turbulence-resistant circumstances.

In its 60 years of existence, the field of nonlinear optics has gained momentum especially over the past two decades thanks to major breakthroughs in material science and technology. In this article, we present a new set of data tables listing nonlinear-optical properties for different material categories as reported in the literature since 2000. The papers included in the data tables are representative experimental works on bulk materials, solvents, 0D–1D–2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, hybrid waveguiding systems, and materials suitable for nonlinear optics at THz frequencies. In addition to the data tables, we also provide best practices for performing and reporting nonlinear-optical experiments. These best practices underpin the selection process that was used for including papers in the tables. While the tables indeed show strong advancements in the field over the past two decades, we encourage the nonlinear-optics community to implement the identified best practices in future works. This will allow a more adequate comparison, interpretation and use of the published parameters, and as such further stimulate the overall progress in nonlinear-optical science and applications.

Collective interaction of emitter arrays has lately attracted significant attention due to its role in controlling directionality of radiation, spontaneous emission and coherence. We focus on light interactions with engineered arrays of solid-state emitters in photonic resonators. We theoretically study light interaction with an array of emitters or optical centers embedded inside a microring resonator and discuss its application in the context of solid-state photonic systems. We discuss how such arrays can be experimentally realized and how the inhomogeneous broadening of mesoscopic atomic arrays can be leveraged to study broadband collective excitations in the array.

The market for Internet-of-things (IoT) with integrated wireless sensor networks is expanding at a rate never seen before. The thriving of IoT also brings an unprecedented demand for sustainable micro-Watt-scale power supplies. Radiative cooling (RC) can provide a continuous temperature difference which can be converted by a thermoelectric generator (TEG) into electrical power. This novel combination of RC with TEG expands the category of sustainable energy sources for energy harvesting. However, the further application of RC-TEG requires a holistic investigation of its RC-TEG performance which is dependent on many different parameters. Using 3D finite element method simulation, this works provides a comprehensive analysis of the concept of RC-TEG by investigating the impact of radiative cooler properties, TEG parameters, and environmental conditions, to provide a full picture of the performance of RC-TEG devices. The capability of RC-TEG to provide continuous power supply is tested using real-time environmental data from both Singapore and London on two different days of the year, demonstrating continuous power supply sufficient for a wide range of physical devices.

Machine learning provides a promising platform for both forward modeling and the inverse design of photonic structures. Relying on a data-driven approach, machine learning is especially appealing for situations when it is not feasible to derive an analytical solution for a complex problem. There has been a great amount of recent interest in constructing machine learning models suitable for different electromagnetic problems. In this work, we adapt a region-specified design approach for the inverse design of multilayered nanoparticles. Given the high computational cost of dataset generation for electromagnetic problems, we specifically investigate the case of a small training dataset, enhanced via random region specification in an inverse convolutional neural network. The trained model is used to design nanoparticles with high absorption levels and different ratios of absorption over scattering. The central design wavelength is shifted across 350–700 nm without re-training. We discuss the implications of wavelength, particle size, and the training dataset size on the performance of the model. Our approach may find interesting applications in the design of multilayer nanoparticles for biological, chemical, and optical applications as well as the design of low-scattering absorbers and antennas.

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects, ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in the life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nano-particle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration.

Alloys of sulfur, selenium and tellurium, often referred to as chalcogenide semiconductors, offer a highly versatile, compositionally-controllable material platform for a variety of passive and active photonic applications. They are optically nonlinear, photoconductive materials with wide transmission windows that present various high- and low-index dielectric, low-epsilon and plasmonic properties across ultra-violet, visible and infrared frequencies, in addition to an, non-volatile, electrically/optically induced switching capability between phase states with markedly different electromagnetic properties. This roadmap collection presents an in-depth account of the critical role that chalcogenide semiconductors play within various traditional and emerging photonic technology platforms. The potential of this field going forward is demonstrated by presenting context and outlook on selected socio-economically important research streams utilizing chalcogenide semiconductors. To this end, this roadmap encompasses selected topics that range from systematic design of material properties and switching kinetics to device-level nanostructuring and integration within various photonic system architectures.

White light-emitting diodes (WLEDs) serve as a replacement for traditional incandescent light due to their excellent characteristics, such as high brightness, efficiency in energy consumption, and long lifetime. The high-efficiency and low-cost white-emitting materials and light-emitting diode devices has always been the goal pursued in the field of lighting technology. Recently, metal halide perovskites are emerging as one of the most promising luminescent materials for next-generation WLEDs due to their facile synthesis and excellent optoelectronic properties, such as high photoluminescence quantum yields, widely tunable bandgap, and high charge-carrier mobility. Although the luminescence efficiency of perovskite emitters and WLED devices has increased rapidly over the past several years, achieving high-efficiency and stable WLEDs remains great challenges. In this review, we focus on the recent progresses of WLEDs based on metal halide perovskites including color-conversion WLEDs, tandem structure of WLEDs, double-emissive-layer of WLEDs, and single-emissive-layer of WLEDs. Importantly, we highlight the WLEDs based on a single emissive layer that show white electroluminescence directly from the perovskite emitters. Finally, we will give an outlook of future research avenues on how to reach the goal of efficient and stable perovskite-based WLEDs.

The last decade has seen the development of a wide set of tools, such as wavefront shaping, computational or fundamental methods, that allow us to understand and control light propagation in a complex medium, such as biological tissues or multimode fibers. A vibrant and diverse community is now working in this field, which has revolutionized the prospect of diffraction-limited imaging at depth in tissues. This roadmap highlights several key aspects of this fast developing field, and some of the challenges and opportunities ahead.

Organic light emitting diode (OLED) technology continues to make strides, particularly in display technology, with costs decreasing and consumer demand growing. Advances are also seen in OLED solid state lighting (SSL) though broad utilization of this technology is lagging. This situation has prompted extensive R&D to achieve high-efficiency SSL devices at cost-effective fabrication. Here we review the advances and challenges in enhancing forward light outcoupling from OLEDs. Light outcoupling from conventional bottom-emitting OLEDs (through a transparent anode) is typically ∼20%, largely due to external losses, i.e., substrate waveguide modes, internal waveguide modes between the metal cathode and the anode/substrate interface, and surface plasmon-polariton modes at the metal cathode/organic interface. We address these major photon loss paths, presenting various extraction approaches. Some approaches are devoid of light extraction structures; they include replacing the commonly used ITO anode, manipulating the refractive index of the substrate and/or organic layers, and evaluating emitters with preferential horizontal transition dipoles. Other approaches include the use of enhancing structures such as microlens arrays, scattering layers and patterned substrates, as well as substrates with various buried structures that are planarized by high index layers. A maximal external quantum efficiency as high as 78% was reported for white planarized OLEDs with a hemispherical lens to extract the substrate mode. Light outcoupling from OLEDs on flexible substrates is also addressed, as the latter become of increasing interest in foldable displays and decorative lighting, with plastic substrates also being evaluated for biomedical, wearable, and automotive applications.