Moiré photonics and optoelectronics

Moiré superlattices, the artificial quantum materials, have provided a wide range of possibilities for the exploration of completely new physics and device architectures. In this Review, we focus on the recent progress on emerging moiré photonics and optoelectronics, including but not limited to moiré excitons, trions, and polaritons; resonantly hybridized excitons; reconstructed collective excitations; strong mid- and far-infrared photoresponses; terahertz single-photon detection; and symmetry-breaking optoelectronics. We also discuss the future opportunities and research directions in this field, such as developing advanced techniques to probe the emergent photonics and optoelectronics in an individual moiré supercell; exploring new ferroelectric, magnetic, and multiferroic moiré systems; and using external degrees of freedom to engineer moiré properties for exciting physics and potential technological innovations. Description Patterning optoelectronics Moiré patterns develop when two or more periodic lattices are overlaid over each other. These long-range patterns can also be seen in two-dimensional materials and give rise to rich physics within these quantum materials systems. Du et al. reviewed recent developments in moiré-patterned two-dimensional materials with a focus on the photonic and optoelectronic aspects and how the properties can be controlled by engineering the stacking order and twist angle between the layers. The wealth of properties seen indicates that these systems may be a powerful platform to develop an exotic optoelectronic device technology. —ISO A review describes recent developments in moiré photonics and optoelectronics. BACKGROUND Moiré pattern refers to a new, long-length scale of structure arising from the interference between two or more periodic templates. The scientific and engineering value of moiré patterns was firstly appreciated by the physicist Lord Rayleigh in 1874. Thereafter, the study of moiré patterns enabled the birth of moiré techniques for a wide variety of practical applications, e.g., precision measurement and positioning, automatization, and strain measurements. In condensed-matter physics, moiré patterns (also referred to as moiré superlattices) can be formed by vertically stacking two or more two-dimensional (2D) layered materials with a tiny twist angle and/or a slight lattice mismatch. Over the past decade, moiré superlattices have made a spectacular appearance with a cornucopia of previously unachieved new phenomena and distinct functionalities, transforming the landscape of fundamental solid-state physics, materials science, and engineering. ADVANCES Moiré superlattices introduce a new length-scale potential much larger than the crystal periodicity of the constituent 2D layers, providing a powerful strategy for engineering electronic band structures and hence resulting in a plethora of emergent quantum phenomena. For example, the long-wavelength moiré periodic potential can fold the electronic band structure into a mini-Brillouin zone, giving rise to the formation of flat bands and a rich phase diagram of correlated states, such as unconventional superconductivity, orbital magnetism, Mott insulator states, and topological multiferroic orders. When moiré superlattices meet light, they provide a powerful platform for discovering emergent photonic and optoelectronic phenomena and device architectures. In early 2019, four research groups independently demonstrated the pronounced effects of moiré potential on excitons of transition-metal dichalcogenide heterostructures from the perspective of both real and momentum spaces. The new degree of freedom afforded by using moiré superlattices provides possibilities for engineering the exciton band structures and light-matter interactions for numerous applications, such as versatile quantum light sources, new quantum many-body physics, and long-sought boson crystals. These breakthroughs in the study of moiré excitons have excited tremendous interest in moiré photonics and optoelectronics. Indeed, a rich diversity of emergent moiré photonic and optoelectronic properties have been witnessed with unprecedented speed over the past few years—properties covering collective moiré excitations, moiré polaritons, photonics of moiré correlated electronic states, giant mid- and far-infrared photoresponses, tunable bulk photovoltaic effect, and so forth. Notably, the moiré bulk photovoltaic effects are ultrastrong and much larger than previous demonstrations, enabling the demonstration of intelligent infrared sensors with merely a subwavelength footprint potentially for next-generation nonlinear photonics and optoelectronics. OUTLOOK Currently, all measurements of moiré photonics and optoelectronics are performed in the far-field limit with a collection of signals from more than 10,000 moiré cells. Therefore, developing advanced techniques that can simultaneously probe the photonic and optoelectronic properties in an individual moiré supercell and evaluate the moiré potential landscape will be valuable for advancing our understanding. Meanwhile, current research efforts on moiré photonics and optoelectronics mainly focus on twisted graphene and twisted transition-metal dichalcogenides. Exploring new moiré systems, such as moiré superlattices composed of 2D ferroelectric, magnetic, or multiferroic crystals, and moiré devices involving two or multiple single moiré superlattices, will initiate research directions of moiré photonics and optoelectronics. Furthermore, engineering the moiré photonics and optoelectronics through external means (e.g., electric or magnetic field, strain, and ultrafast optical excitations) has the potential to spark the next “gold rush” for technical innovations (e.g., quantum nonlinear optics, ultracompact optical modulators, and terahertz single-photon devices). We are just starting down the path of exploring moiré photonics and optoelectronics, and there will be many more surprises to come. Moiré photonics and optoelectronics. Moiré superlattices introduce a paradigm to engineer the band structures of collective excitations and light-matter interaction for a plethora of emergent photonic and optoelectronic phenomena, such as moiré excitons, moiré polaritons, photonics of moiré correlated electronic states, intelligent infrared sensing, quantum light sources, single-photon detection, and symmetry-breaking optoelectronics.