Surface nonlinear optics lays at the heart of integrated photonics and micro-nano optoelectronics, whose efficiency is restricted by the finite nonlinear susceptibility of matter and the intrinsic atomic-layered interaction length between light and matter. Herein, we originally demonstrate that the centrosymmetric topological semimetal HfGe0.92Te crystal possesses a giant and anisotropic surface second-order nonlinear susceptibility up to 5535 ± 308 pm·V− 1 and manifests efficient and unprecedented second-harmonic generation (SHG) based on the angular engineering strategy. The maximum optical conversion efficiency is up to 3.75‰, a value that is 1015 orders of magnitude larger than the conventional surface SHG. Benefiting from the linear dispersion in a large energy range around the Dirac points, we find that this high conversion efficiency can be maintained with the SHG wavelengths ranging from the visible region to the deep ultraviolet one (515 nm-257.5 nm). Our work may open the door for the development of topological photonics and integrated nonlinear photonics based on topological semimetals.

A new series of compounds, ANi 5 Bi 5.6+δ (where A = K, Rb, and Cs) are discovered with a quasi-one-dimensional (Q1D) [Ni 5 Bi 5.6+δ ] − double-walled column and a coaxial inner one-dimensional Bi atomic chain. The columns are linked to each other by intercolumn Bi−Bi bonds and separated by an A + cation. Typical metallic behaviors with strong correlation of itinerant electrons and the Sommerfeld coefficient enhanced with the increasing cationic radius were experimentally observed and supported by first-principles calculations. Compared to AMn 6 Bi 5 (where A = K, Rb, and Cs), the enhanced intercolumn distances and the substitution of Ni for Mn give rise to strong diamagnetic susceptibilities in ANi 5 Bi 5.6+δ . First-principles calculations reveal possible uncharged Ni atoms with even number of electrons in ANi 5 Bi 5.6+δ , which may explain the emergence of diamagnetism. ANi 5 Bi 5.6+δ , as Q1D diamagnetic metals with strong electron correlation, provide a unique platform to understand exotic magnetism and explore novel quantum effects.

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