Second-harmonic generation

Abstract: Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity ($\chi^{(2)}$), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric down-conversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings, where quasi-phase matching is realized by field-assisted domain engineering. Meanwhile, dual-band operation is accessed by optimizing the coupling conditions in fundamental and second-harmonic bands via a single pulley waveguide. As a result, when pumping a periodically poled LN microring in the low power regime at around 1617nm, an on-chip SHG efficiency of 250,000%/W is achieved, a state-of-the-art value reported among current integrated photonics platforms. An absolute conversion efficiency of 15% is recorded with a low pump power of 115$\mu$W in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase matched $\chi^{(2)}$ nonlinear optical processes.

Abstract: If a bucket is to hold more water, its shortest plank must be made longer This guideline also applies to the exploration of ultraviolet (UV) and deep-UV (DUV) nonlinear optical (NLO) materials that are limited by multiple criteria Phosphates are one kind of promising candidate for new NLO materials Unfortunately, the small birefringence, as the shortest plank, severely restricts the phase-matching of second harmonic generation (SHG) in the UV/DUV region In this work, fluorophosphates are rationally proposed as substitutes for phosphates to break down the limitation of birefringence and simultaneously enhance SHG response and retain wide UV transmittance The (PO3F)2– and (PO2F2)− groups are confirmed as superior material genomes to achieve the discussed combination properties Accordingly, (NH4)2PO3F was screened out by density functional theory calculation, and single crystals with centimeter size have been grown It possesses a powder SHG efficiency of 1 × KH2PO4 (KDP) and is phase-matchable with ou

Abstract: A new nonlinear optical (NLO) material, Bi3TeBO9 (BTBO), is successfully grown from high temperature solution method. BTBO crystallizes in a polar space group of P63 with a framework structure composed of [Bi3O9] blocks, with TeO6 and BO3 interconnection. It is interesting that in the BTBO structure three types of NLO-active units, including stereochemically active lone pair cations (Bi3+ cations), second-order Jahn–Teller distorted octahedra (TeO6 octahedra) and π-orbital planar groups (BO3 groups), simultaneously exist. The additive contribution from these three types of groups results in an extremely large second harmonic generation (SHG) response in BTBO (about 20 times that of KDP), exhibiting the largest SHG effect among the known borate NLO materials. The enhancement of the nonlinear optical property is elucidated by the first-principles analysis.

Abstract: : We present a study of the second-order nonlinear optical properties of metal-based metamaterials. A hydrodynamic model for electronic response is used, in which nonlinear surface contributions are expressed in terms of the bulk polarization. The model is in good agreement with published experimental results, and clarifies the mechanisms contributing to the nonlinear response. In particular, we show that the reported enhancement of the second harmonic in split-ring resonator based media is driven by the electric rather than the magnetic properties of the structure.