To spotlight advances in thin film lithium niobate technology, we present a special collection.

Lithium niobate (LN) has experienced significant developments during past decades due to its versatile properties, especially its large electro-optic (EO) coefficient. For example, bulk LN-based modulators with high speeds and a superior linearity are widely used in typical fiber-optic communication systems. However, with ever-increasing demands for signal transmission capacity, the high power and large size of bulk LN-based devices pose great challenges, especially when one of its counterparts, integrated silicon photonics, has experienced dramatic developments in recent decades. Not long ago, high-quality thin-film LN on insulator (LNOI) became commercially available, which has paved the way for integrated LN photonics and opened a hot research area of LN photonics devices. LNOI allows a large refractive index contrast, thus light can be confined within a more compact structure. Together with other properties of LN, such as nonlinear/acousto-optic/pyroelectric effects, various kinds of high-performance integrated LN devices can be demonstrated. A comprehensive summary of advances in LN photonics is provided. As LN photonics has experienced several decades of development, our review includes some of the typical bulk LN devices as well as recently developed thin film LN devices. In this way, readers may be inspired by a complete picture of the evolution of this technology. We first introduce the basic material properties of LN and several key processing technologies for fabricating photonics devices. After that, various kinds of functional devices based on different effects are summarized. Finally, we give a short summary and perspective of LN photonics. We hope this review can give readers more insight into recent advances in LN photonics and contribute to the further development of LN related research.

Materials that exhibit visible luminescence upon X-ray irradiation show great potential in the medical and industrial fields. Pure organic materials have recently emerged as promising scintillators for X-ray detection and radiography, due to their diversified design, low cost, and facile preparation. However, recent progress in efficient radioluminescence has mainly focused on small molecules, which are inevitably associated with processability and repeatability issues. Here, a concise strategy is proposed to prepare radioluminescent polymers that exhibit multiple emission colors from blue to yellow with high brightness in an amorphous state by the radical copolymerization of negatively charged polyacrylic acid and different positively charged quaternary phosphonium salts. One of the obtained polymers exhibits excellent photostability under a high X-ray irradiation dosage of 27.35 Gy and has a detection limit of 149 nGy s ^{- 1}. This performance is superior to that of conventional anthracene-based scintillators. Furthermore, by simply drop-casting a polymer methanol solution on a quartz plate, a transparent scintillator screen was successfully fabricated for X-ray imaging with a resolution of 8.7 line pairs mm ^{- 1}. The pure organic phosphorescent polymers with a highly efficient radioluminescence were demonstrated for the first time, and the strategy reported herein offers a promising pathway to expand the application range of amorphous organic scintillators.

Constructions of synthetic lattices in modulated ring resonators attract growing attention to interesting physics beyond the geometric dimensionality, where complicated connectivities between resonant frequency modes are explored in many theoretical proposals. We implement experimental demonstration of generating a stub lattice along the frequency axis of light, in two coupled ring resonators of different lengths, with the longer one dynamically modulated. Such a synthetic photonic structure intrinsically exhibits the physics of flat band. We show that the time-resolved band structure read-out from the drop-port output of the excited ring is the intensity projection of the band structure onto a specific resonant mode in the synthetic momentum space, where gapped flat band, mode localization effect, and flat-to-nonflat band transition are observed in experiments and verified by simulations. This work provides evidence for constructing a synthetic stub lattice using two different rings, which, hence, makes a solid step toward experimentally constructing complicated lattices in multiple rings associated with synthetic frequency dimensions.