Light has played a crucial role in the age of information technology and has facilitated the soaring development of information optics. The ever-increasing demand for high-capacity optical devices has prompted the use of physically orthogonal dimensions of light for optical multiplexing. Recent advances in nanotechnology, mainly stemming from functionalized nanomaterials and powerful nanofabrication tools, have propelled the fusion of optical multiplexing and nanophotonics (the study of light at nanoscale and of its interactions with nanostructures) by enabling ultrahigh-capacity information technology. This review aims to introduce the emerging concept of angular momentum (AM)-involved information optics and its implementation in nanophotonic devices. First, previous researches on the manipulation of spin angular momentum (SAM) and orbital angular momentum (OAM) by nanostructures will be reviewed. We then summarize the SAM multiplexing technology on the platform of metasurfaces. Particularly, we elaborately summarize our recent progress in the area of information optics, including OAM holography and on-chip AM multiplexing technology. Finally, a perspective in the combination of this emerging field with optical artificial intelligence (AI) will be given.

Fano resonance is a universal phenomenon observed in many areas where wave propagation and interference are possible. Fano resonance arises from the interference of broad and narrow spectra of radiation and becomes an important tool for many applications in the physical, chemical, and biological sciences. At the beginning of this paper, we consider Fano resonances in individual particles, primarily of spherical and cylindrical shapes, and discuss their connection with the physics of bound states in the continuum that determine the high quality factors of resonators. Further, we discuss two areas in which structures with Fano resonances have already found or will find real application in the nearest future—sensors and lasers. The penultimate section concerns our future, which will be associated with the complete replacement of electronic processing, transmission, and storage of information with optical devices as many hope. It is believed that this sophisticated goal can be achieved with devices that implement the slow-light regime associated with the phenomenon of electromagnetically induced transparency, which can be considered as a special case of Fano resonance. The review completes with one more promising topic related to quantum electrodynamics in structures with Fano cavities.

Nanophotonics is an important branch of modern optics dealing with light–matter interaction at the nanoscale. Nanoparticles can exhibit enhanced light absorption under illumination by light, and they become nanoscale sources of heat that can be precisely controlled and manipulated. For metal nanoparticles, such effects have been studied in the framework of thermoplasmonics, which, similar to plasmonics itself, has a number of limitations. Recently emerged all-dielectric resonant nanophotonics is associated with optically induced electric and magnetic Mie resonances, and this field hasdeveloped very rapidly over the past decade. As a result, thermoplasmonics is being complemented by all-dielectric thermonanophotonics with many important applications such as photothermal cancer therapy, drug and gene delivery, nanochemistry, and photothermal imaging. This review paper aims to introduce this new field of non-plasmonic nanophotonics and discuss associated thermally induced processes at the nanoscale.

Photonic integration has seen tremendous progress over the previous decade, and several integration platforms have reached industrial maturity. This evolution has prepared the ground for miniaturized photonic sensors that lend themselves to efficient analysis of gaseous and liquid media, exploiting large interactions lengths of guided light with surrounding analytes, possibly mediated by chemically functionalized waveguide surfaces. Among the various sensor concepts, phase-sensitive approaches are particularly attractive: offering a flexible choice of the operation wavelength, these schemes are amenable to large-scale integration on mature technology platforms such as silicon photonics or silicon nitride ( S i 3 N 4 ) that have been developed in the context of tele- and data-communication applications. This paves the path toward miniaturized and robust sensor systems that offer outstanding scalability and that are perfectly suited for high-volume applications in life sciences, industrial process analytics, or consumer products. However, as the maturity of the underlying photonic integrated circuits (PICs) increases, system-level aspects of mass-deployable sensors gain importance. These aspects include, e.g., robust system concepts that can be operated outside controlled laboratory environments as well as readout schemes that can be implemented based on low-cost light sources, without the need for benchtop-type tunable lasers as typically used in scientific demonstrations. It is, thus, the goal of this tutorial to provide a holistic system model that allows us to better understand and to quantitatively benchmark the viability and performance of different phase-sensitive photonic sensor concepts under the stringent limitations of mass-deployable miniaturized systems. Specifically, we explain and formulate a generally applicable theoretical framework that allows for a quantitatively reliable end-to-end analysis of the overall signal chain. Building upon this framework, we identify and explain the most important technical parameters of the system, comprising the photonic sensor circuit, the light source, and the detector, as well as the readout and control scheme. We quantify and compare the achievable performance and the limitations that are associated with specific sensor structures based on Mach–Zehnder interferometers (MZIs) or high-Q optical ring resonators (RRs), and we condense our findings by formulating design guidelines both for sensor concepts. As a particularly attractive example, we discuss an MZI-based sensor implementation, relying on a vertical-cavity surface-emitting laser (VCSEL) as a power-efficient low-cost light source in combination with a simple and robust readout and control scheme. In contrast to RR-based sensor implementations, MZIs can be resilient to laser frequency noise, at the cost of a slightly lower sensitivity and a moderately increased footprint. To facilitate the application of our model, we provide a MATLAB-based application that visualizes the underlying physical principles and that can be readily used to estimate the achievable performance of a specific sensor system. The system-level design considerations are complemented by an overview of additional aspects that are important for successful sensor system implementation such as the design of the underlying waveguides, photonic system assembly concepts, and schemes for analyte handling.

We discuss recent developments in measurement protocols that generate quantum entanglement between two remote qubits, focusing on the theory of joint continuous detection of their spontaneous emission. We consider a device geometry similar to that used in well-known Bell state measurements, which we analyze using a conceptually transparent model of stochastic quantum trajectories; we use this to review photodetection, the most straightforward case, and then generalize to the diffusive trajectories from homodyne and heterodyne detection as well. Such quadrature measurement schemes are a realistic two-qubit extension of existing circuit QED experiments, which obtain quantum trajectories by homodyning or heterodyning a superconducting qubit’s spontaneous emission, or an adaptation of existing optical measurement schemes to obtain jump trajectories from emitters. We mention key results, presented from within a single theoretical framework, and draw connections to concepts in the wider literature on entanglement generation by measurement (such as path information erasure and entanglement swapping). The photon which-path information acquisition, and therefore the two-qubit entanglement yield, is tunable under the homodyne detection scheme we discuss, at best generating equivalent average entanglement dynamics as in the comparable photodetection case. In addition to deriving this known equivalence, we extend past analyses in our characterization of the measurement dynamics: we include derivations of bounds on the fastest possible evolution toward a Bell state under joint homodyne measurement dynamics and characterize the maximal entanglement yield possible using inefficient (lossy) measurements.

We present an erratum to correct the x, z labels in Figs. 6 and 7 from Adv. Opt. Photon. 10, 484 (2018)10.1364/AOP.10.000484.

We present a general theory of optical coherence tomography (OCT), which synthesizes the fundamental concepts and implementations of OCT under a common 3D -space framework. At the heart of this analysis is the Fourier diffraction theorem, which relates the coherent interaction between a sample and plane wave to the Ewald sphere in the 3D -space representation of the sample. While only the axial dimension of OCT is typically analyzed in -space, we show that embracing a fully 3D -space formalism allows explanation of nearly every fundamental physical phenomenon or property of OCT, including contrast mechanism, resolution, dispersion, aberration, limited depth of focus, and speckle. The theory also unifies diffraction tomography, confocal microscopy, point-scanning OCT, line-field OCT, full-field OCT, Bessel beam OCT, transillumination OCT, interferometric synthetic aperture microscopy (ISAM), and optical coherence refraction tomography (OCRT), among others. Our unified theory not only enables clear understanding of existing techniques but also suggests new research directions to continue advancing the field of OCT.

Topological photonics is a new and rapidly growing field that deals with topological phases and topological insulators for light. Recently, the scope of these systems was expanded dramatically by incorporating non-spatial degrees of freedom. These synthetic dimensions can range from a discrete ladder of cavity modes or Bloch modes of an array of waveguides to a time-bin division (discrete time steps) in a pulsed system or even to parameters such as lattice constants. Combining spatial and synthetic dimensions offers the possibility to observe fundamental and exotic phenomena such as dynamics in four dimensions or higher, long-range interaction with disorder, high-dimensional nonlinear effects, and more. Here, we review the latest developments in using non-spatial dimensions as a means to enhance fundamental features of photonic topological systems, and we attempt to identify the next challenges.

In this paper, we review spatial light interference microscopy (SLIM), a common-path, phase-shifting interferometer, built onto a phase-contrast microscope, with white-light illumination. As one of the most sensitive quantitative phase imaging (QPI) methods, SLIM allows for speckle-free phase reconstruction with sub-nanometer path-length stability. We first review image formation in QPI, scattering, and full-field methods. Then, we outline SLIM imaging from theory and instrumentation to diffraction tomography. Zernike's phase-contrast microscopy, phase retrieval in SLIM, and halo removal algorithms are discussed. Next, we discuss the requirements for operation, with a focus on software developed in-house for SLIM that enables high-throughput acquisition, whole slide scanning, mosaic tile registration, and imaging with a color camera. We introduce two methods for solving the inverse problem using SLIM, white-light tomography, and Wolf phase tomography. Lastly, we review the applications of SLIM in basic science and clinical studies. SLIM can study cell dynamics, cell growth and proliferation, cell migration, mass transport, etc. In clinical settings, SLIM can assist with cancer studies, reproductive technology, blood testing, etc. Finally, we review an emerging trend, where SLIM imaging in conjunction with artificial intelligence brings computational specificity and, in turn, offers new solutions to outstanding challenges in cell biology and pathology.

Lithium niobate (LN), an outstanding and versatile material, has influenced our daily life for decades—from enabling high-speed optical communications that form the backbone of the Internet to realizing radio-frequency filtering used in our cell phones. This half-century-old material is currently embracing a revolution in thin-film LN integrated photonics. The successes of manufacturing wafer-scale, high-quality thin films of LN-on-insulator (LNOI) and breakthroughs in nanofabrication techniques have made high-performance integrated nanophotonic components possible. With rapid development in the past few years, some of these thin-film LN devices, such as optical modulators and nonlinear wavelength converters, have already outperformed their legacy counterparts realized in bulk LN crystals. Furthermore, the nanophotonic integration has enabled ultra-low-loss resonators in LN, which has unlocked many novel applications such as optical frequency combs and quantum transducers. In this review, we cover—from basic principles to the state of the art—the diverse aspects of integrated thin-film LN photonics, including the materials, basic passive components, and various active devices based on electro-optics, all-optical nonlinearities, and acousto-optics. We also identify challenges that this platform is currently facing and point out future opportunities. The field of integrated LNOI photonics is advancing rapidly and poised to make critical impacts on a broad range of applications in communication, signal processing, and quantum information.