Graphene, a two-dimensional honeycomb lattice of carbon atoms, has been attracting much interest in recent years. Electrons therein behave as massless relativistic particles, giving rise to strikingly unconventional phenomena. Graphene edge states are essential for understanding the electronic properties of this material. However, the coarse or impure nature of the graphene edges hampers the ability to directly probe the edge states. Perhaps the best example is given by the edge states on the bearded edge that have never been observed-because such an edge is unstable in graphene. Here, we use the optical equivalent of graphene-a photonic honeycomb lattice-to study the edge states and their properties. We directly image the edge states on both the zigzag and bearded edges of this photonic graphene, measure their dispersion properties, and most importantly, find a new type of edge state: one residing on the bearded edge that has never been predicted or observed. This edge state lies near the Van Hove singularity in the edge band structure and can be classified as a Tamm-like state lacking any surface defect. The mechanism underlying its formation may counterintuitively appear in other crystalline systems.

Pseudospin, an additional degree of freedom inherent in graphene, plays a key role in understanding many fundamental phenomena such as the anomalous quantum Hall effect, electron chirality and Klein paradox. Unlike the electron spin, the pseudospin was traditionally considered as an unmeasurable quantity, immune to Stern-Gerlach-type experiments. Recently, however, it has been suggested that graphene pseudospin is a real angular momentum that might manifest itself as an observable quantity, but so far direct tests of such a momentum remained unfruitful. Here, by selective excitation of two sublattices of an artificial photonic graphene, we demonstrate pseudospin-mediated vortex generation and topological charge flipping in otherwise uniform optical beams with Bloch momentum traversing through the Dirac points. Corroborated by numerical solutions of the linear massless Dirac-Weyl equation, we show that pseudospin can turn into orbital angular momentum completely, thus upholding the belief that pseudospin is not merely for theoretical elegance but rather physically measurable.

Flatband systems typically host "compact localized states" (CLS) due to destructive interference and macroscopic degeneracy of Bloch wave functions associated with a dispersionless energy band. Using a photonic Lieb lattice (LL), we show that conventional localized flatband states are inherently incomplete, with the missing modes manifested as extended line states which form non-contractible loops winding around the entire lattice. Experimentally, we develop a continuous-wave laser writing technique to establish a finite-sized photonic LL with speciallytailored boundaries, thereby directly observe the unusually extended flatband line states. Such unconventional line states cannot be expressed as a linear combination of the previously observed CLS but rather arise from the nontrivial real-space topology. The robustness of the line states to imperfect excitation conditions is discussed, and their potential applications are illustrated.Flatband systems, first proposed for the study of ferromagnetic ground states in multiband Hubbard models, have proven to be conceptually effective and important in condensed matter physics [1][2][3]. They are characterized by a band structure with one band being completely flat, signaling macroscopic degeneracy. One can construct CLS which remain intact during evolution due to destructive interference. Over the years, a variety of approaches have been developed to design and characterize different flatband systems [4][5][6][7][8], with lattice geometries ranging from sawtooth, stub, diamond, dice, kagome, to Lieb and perovskite lattices in general [7][8][9][10][11][12]. This is largely due to the flatband systems providing a platform for probing various fundamental phenomena that have intrigued scientists for decades, including Anderson localization [6,13, 14], nontrivial topological phases and quantum Hall states [15][16][17][18][19], and flatband superfluidity [20,21].The Lieb lattice (LL) -a face-centered square depleted lattice [ Fig. 1(a)] -is geometrically different from other two-dimensional lattices such as square and honeycomb lattices. This peculiar system possesses a single conical intersection point in its Brillouin zone (BZ), where the flatband is sandwiched between two conical Bloch bands [ Fig. 1(b)]. The flatband in the LL is protected by a chiral symmetry, and its intersection with the dispersive bands is protected by real-space topology [12,22,23]. Recently, LLs have been realized in several different settings, including Bose-Einstein condensates [4,24], surface state electrons [25,26], exciton-polaritons in micropillars [27], and waveguide arrays in photonic structures [28][29][30][31][32]. However, so far most of previous experimental studies have focused on the demonstration of the LL structures and their associated CLS, overlooking unusual features that arise in infinitely extended lattices [ Fig. 1(c)] or finite (truncated) lattices with different cutting boundaries [Figs. 1(d, e)].In this Letter, we demonstrate the CLS previously investigated in the LL are lin...

Crystalline lithium niobate (LN) is an important optical material because of its broad transmission window that spans from ultraviolet to mid-infrared and its large nonlinear and electro-optic coefficients. Furthermore, the recent development and commercialization of LN-on-insulator (LNOI) technology has opened an avenue for the realization of integrated on-chip photonic devices with unprecedented performances in terms of propagation loss, optical nonlinearity, and electro-optic tunability. This review begins with a brief introduction of the history and current status of LNOI photonics. We then discuss the fabrication techniques of LNOI-based photonic structures and devices. The recent revolution in the LN photonic industry has been sparked and is still being powered by innovations of the nanofabrication technology of LNOI, which enables the production of building block structures, such as optical microresonators and waveguides of unprecedented optical qualities. The following sections present various on-chip LNOI devices categorized into nonlinear photonic and electro-optic tunable devices and photonic-integrated circuits. Some conclusions and future perspectives are provided.