The ideas of topology have found tremendous success in Hermitian physical systems, but even richer properties exist in the more general non-Hermitian framework. Here, we theoretically propose and experimentally demonstrate a new topologically-protected bulk Fermi arc which-unlike the well-known surface Fermi arcs arising from Weyl points in Hermitian systems-develops from non-Hermitian radiative losses in photonic crystal slabs. Moreover, we discover half-integer topological charges in the polarization of far-field radiation around the Fermi arc. We show that both phenomena are direct consequences of the non-Hermitian topological properties of exceptional points, where resonances coincide in their frequencies and linewidths. Our work connects the fields of topological photonics, non-Hermitian physics and singular optics, and paves the way for future exploration of non-Hermitian topological systems.In recent years, topological physics has been widely explored in closed and lossless Hermitian systems, revealing novel phenomena such as topologically non-trivial band structures [1][2][3][4][5][6][7][8][9] and promising applications including backscattering-immune transport [10][11][12][13][14][15][16][17][18][19][20][21]. However, most systems, particularly in photonics, are generically non-Hermitian due to radiation into open space or material gain/loss. NonHermiticity enables even richer topological properties, often with no counterpart in Hermitian frameworks [22][23][24][25]. One such example is the emergence of a new class of degeneracies, commonly referred to as exceptional points (EPs), where two or more resonances of a system coalesce in both eigenvalues and eigenfunctions [26][27][28]. So far, isolated EPs in parameter space [29][30][31][32][33][34][35] and continuous rings of EPs in momentum space [36][37][38] have been studied across different wave systems due to their intriguing properties, such as unconventional transmission/reflection [39][40][41], relations to parity-time symmetry [42][43][44][45][46][47][48], as well as their unique applications in sensing [49,50] and single-mode lasing [51][52][53].Here, we theoretically design and experimentally realize a new configuration of isolated EP pairs in momentum space, which allows us to reveal the unique topological signatures of EPs in the band structure and far-field polarization, and to extend topological band theory into the realm of non-Hermitian systems. Specifically, we demonstrate that a Dirac point (DP) with nontrivial Berry phase can split into a pair of EPs [54][55][56] when radiation loss-a form of non-Hermiticity-is added to a 2D-periodic photonic crystal (PhC) structure. The EPpair generates a distinct double-Riemann-sheet topology in the complex band structure, which leads to two novel consequences: bulk Fermi arcs and polarization half charges. First, we discover and experimentally demonstrate that this pair of EPs is connected by an open-ended isofrequency contourwe refer to it as a bulk Fermi arc-in direct contrast to the common intuiti...

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Due to their ability to confine light, optical resonators 1-3 are of great importance to science and technology, yet their performances are often limited by out-of-plane scattering losses from inevitable fabrication imperfections 4, 5 . Here, we theoretically propose and experimentally demonstrate a class of guided resonances in photonic crystal slabs, where out-of-plane scattering losses are strongly suppressed due to their topological nature. Specifically, these resonances arise when multiple bound states in the continuum -each carrying a topological charge 6 -merge in the momentum space and enhance the quality factors of all resonances nearby. We experimentally achieve quality factors as high as 4.9 × 10 5 based on these resonances in the telecommunication regime, which is 12-times higher than ordinary designs.We further show this enhancement is robust across the samples we fabricated. Our work paves the way for future explorations of topological photonics in systems with open boundary condition and their applications in improving optoelectronic devices in photonic integrated circuits.

We investigate the formation of photonic bound states in the continuum (BICs) in photonic crystal slabs from an analytical perspective. Unlike the stationary at-Γ BICs which originate from the geometric symmetry, the tunable off-Γ BICs are due to the weighted destructive via the continuum interference in the vicinity of accidental symmetry when the majority of the radiation is precanceled. The symmetric compatible nature of the off-Γ BICs leads to a trapping of light that can be tuned through continuously varying the wave vector. With the analytical approach, we explain a reported experiment and predict the existence of a new BIC at an unrevealed symmetry.

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Unidirectional radiation is important for a variety of optoelectronic applications. Many unidirectional emitters exist, but they all rely on the use of materials or structures that forbid outgoing waves, i.e. mirrors. Here, we theoretically propose and experimentally demonstrate a class of resonances in photonic crystal slabs, which only radiate towards a single side with no mirror placed on the other side-we call them "unidirectional bound states in the continuum". These resonances are found to emerge when a pair of half-integer topological charges in the polarization field bounce into each other in the momentum space. We experimentally demonstrate such resonances in the telecommunication regime, where we achieve single-sided quality factor as high as 1.6 × 10 5 , equivalent to a radiation asymmetry ratio of 27.7 dB. Our work represents a vivid example of applying topological principles to improve optoelectronic devices. Possible applications of our work include grating couplers, photoniccrystal surface-emitting lasers, and antennas for light detection and ranging. Topological defects 1 , characterized by quantized invariants, offer a general picture to un-1

A method to enhance the data transfer rate of eutectic Sb-Te phase-change recording media J. Appl. Phys. 98, 023102 (2005); 10.1063/1.1957132Laser-induced generation of micrometer-sized luminescent patterns on rare-earth-doped amorphous films

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