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.

This tutorial offers a comprehensive overview of photonic time crystals: artificial materials whose electromagnetic properties are periodically modulated in time at scales comparable to the oscillation period of light while remaining spatially uniform. Being the temporal analogs to traditional photonic crystals, photonic time crystals differ in that they exhibit momentum bandgaps instead of energy bandgaps. The energy is not conserved within momentum bandgaps, and eigenmodes with exponentially growing amplitudes exist in the momentum bandgap. Such properties make photonic time crystals a fascinating novel class of artificial materials from a basic science and applied perspective. This tutorial gives an overview of the fundamental electromagnetic equations governing photonic time crystals and explores the ground-breaking physical phenomena they support. Based on these properties, we also oversee the diverse range of applications they unlock. Different material platforms suitable for creating photonic time crystals are discussed and compared. Furthermore, we elaborate on the connections between wave amplification in photonic time crystals and parametric amplification mechanisms in electrical circuits and nonlinear optics. Numerical codes for calculating the band structures of photonic time crystals using two approaches, the plane wave expansion method and the transfer matrix method, are provided. This tutorial will be helpful for readers with physics or engineering backgrounds. It is designed to serve as an introductory guide for beginners and to establish a reference baseline reflecting the current understanding for researchers in the field.

Halide perovskites have emerged as a new class of materials for photoelectric conversion, attracting an ever-increasing level of attention within the scientific community. These materials are characterized by expansive compositional choices, ease of synthesis, an impressively high light absorption coefficient, and extended carrier recombination lifetimes. These attributes make halide perovskites an ideal candidate for future optoelectronic and photonic applications, including solar energy conversion, photodetection, electroluminescence, coherent light generation, and nonlinear optical interactions. In this review, we first introduce fundamental concepts of perovskites and categorize perovskite photonic devices by the nature of their fundamental mechanisms, i.e., photon-to-electron conversion devices, electron-to-photon conversion devices, and photon-to-photon devices. We then review the significant progress in each type of perovskite device, focusing on working principles and device performances. Finally, future challenges and outlook in halide perovskite photonics will be provided.

Recent decades have seen significant advancements in integrated photonics, driven by improvements in nanofabrication technology. This field has been developed from integrated semiconductor lasers and low-loss waveguides to optical modulators, enabling the creation of sophisticated optical systems on a chip-scale capable of performing complex functions such as optical sensing, signal processing, and metrology. The tight confinement of optical modes in photonic waveguides further enhances the optical nonlinearity, leading to a variety of nonlinear optical phenomena such as optical frequency combs, second-harmonic generation, and supercontinuum generation. Active tuning of photonic circuits not only is crucial for offsetting variations caused by fabrication in large-scale integration but also serves as a fundamental component in programmable photonic circuits. Piezoelectric actuation in photonic devices offers a low-power, high-speed solution and is essential in the design of future photonic circuits due to its compatibility with materials such as Si and Si3N4, which do not exhibit electro-optic effects. Here, we provide a detailed review of the latest developments in piezoelectric tuning and modulation by examining various piezoelectric materials, actuator designs tailored to specific applications, and the capabilities and limitations of current technologies. In addition, we explore the extensive applications enabled by piezoelectric actuators, including tunable lasers, frequency combs, quantum transducers, and optical isolators. These innovative ways of managing photon propagation and frequency on-chip are expected to be highly sought after in the future advancements of advanced photonic chips for both classical and quantum optical information processing and computing.

Non-Hermitian band structures have gained considerable attention due to the novel phenomena not present in their Hermitian counterparts and their connection to various branches of mathematics such as topology and complex analysis. The study of such band structures may also find applications in laser design and in sensing. The spectra and eigenmode characteristics of extended non-Hermitian systems depend strongly on the boundary conditions. With periodic boundary conditions, the spectra can become complex, leading to band winding on the complex frequency plane. With open boundary conditions, the eigenmodes have spatial profiles that are localized at the boundary, an effect known as the non-Hermitian skin effect. Here we provide an overview of the band winding and skin effects in non-Hermitian photonics bands, focusing on one-dimensional cases and photonic applications. We aim to provide a detailed, consistent, and unifying treatment of various phenomena associated with non-Hermitian band structures.

Mie-resonant metaphotonics is a rapidly developing field that employs the physics of Mie resonances to control light at the nanoscale. Mie resonances are excited in high-refractive-index transparent nanoparticles and voids created in dielectric media, and they can be used to achieve a wide range of optical effects, including enhanced light–matter interaction, nonlinear optical effects, and topological photonics. Here, we review the recent advances in Mie-resonant metaphotonics, with a focus on the physics of Mie resonances and their applications in metaphotonics and metasurfaces. Through a comprehensive multipolar analysis, we demonstrate the complex interplay of electric and magnetic multipoles that govern their interaction with light. Recent advances have unveiled a diverse spectrum of scattering phenomena that can be achieved within precisely engineered structures. Within this framework, we review the underlying mechanics of the first and second Kerker conditions and describe the intricate mechanisms guiding these nanostructures’ light-scattering properties. Moreover, we cover intriguing phenomena such as the anapole and bound or quasi-bound states in the continuum. Of profound interest are the numerous practical applications that result from these revelations. Ultrafast processes, the emergence of nanolasers, and advancements in magneto-optic devices represent just a fraction of the transformative applications.

The editors of Advances in Optics and Photonics and Optica introduce a collaborative publishing effort that highlights emerging fields in ways that will benefit both new and seasoned researchers. In this first example, a tutorial and a mini-review cover the physics of second-order nonlinear interactions in dispersion-engineered nonlinear photonic devices.

Photonic integrated circuits with second-order (χ(2)) nonlinearities are rapidly scaling to remarkably low powers. At this time, state-of-the-art devices achieve saturated nonlinear interactions with thousands of photons when driven by continuous-wave lasers, and further reductions in these energy requirements enabled by the use of ultrafast pulses may soon push nonlinear optics into the realm of single-photon nonlinearities. This tutorial reviews these recent developments in ultrafast nonlinear photonics, discusses design strategies for realizing few-photon nonlinear interactions, and presents a unified treatment of ultrafast quantum nonlinear optics using a framework that smoothly interpolates from classical behaviors to the few-photon scale. These emerging platforms for quantum optics fundamentally differ from typical realizations in cavity quantum electrodynamics due to the large number of coupled optical modes. Classically, multimode behaviors have been well studied in nonlinear optics, with famous examples including soliton formation and supercontinuum generation. In contrast, multimode quantum systems exhibit a far greater variety of behaviors, and yet closed-form solutions are even sparser than their classical counterparts. In developing a framework for ultrafast quantum optics, we identify what behaviors carry over from classical to quantum devices, what intuition must be abandoned, and what new opportunities exist at the intersection of ultrafast and quantum nonlinear optics. Although this article focuses on establishing connections between the classical and quantum behaviors of devices with χ(2) nonlinearities, the frameworks developed here are general and are readily extended to the description of dynamical processes based on third-order χ(3) nonlinearities.

Hexagonal boron nitride (hBN), also known as white graphite, is a transparent layered crystal with a wide bandgap. Its crystal structure resembles graphite, featuring layers composed of honeycomb lattices held together through van der Waals forces. The layered crystal structure of hBN facilitates exfoliation into thinner flakes and makes it highly anisotropic in in-plane and out-of-plane directions. Unlike graphite, hBN is both insulating and transparent, making it an ideal material for isolating devices from the environment and acting as a waveguide. As a result, hBN has found extensive applications in optical devices, electronic devices, and quantum photonic devices. This comprehensive tutorial aims to provide readers with a thorough understanding of hBN, covering its synthesis, lattice and spectroscopic characterization, and various applications in optoelectronic and quantum photonic devices. This tutorial is designed for both readers without prior experience in hBN and those with expertise in specific fields seeking to understand its relevance and connections to others.

Spatiotemporal sculpturing of light pulses with sophisticated structures on demand is one major goal of the everlasting pursuit of ultrafast information transmission and processing as well as ultraintense energy concentration and extraction using light. It may hold the key to unlocking new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are treated as spatiotemporally separable wave packets as a solution to Maxwell’s equations. In the past decade, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This tutorial intends to provide the necessary basics on how to sculpture light in the spatiotemporal domain to realize spatiotemporal structures on demand and highlight some of the recent advances in the creation and characterization of increasingly complex spatiotemporal wave packets. These spatiotemporally separable to complex nonseparable states with diverse geometric and topological structures exhibit unique physical properties during propagation, focusing, and interaction with matter. The broad potential applications as well as outlook and future trends and open challenges in this field are presented.