Enhanced Second‐Harmonic Generation in Quadratically Nonlinear Weyl Semimetal NbAs for Broadband Photodetection Applications

Abstract Quadratically nonlinear photodetectors (QNPDs) typically focus on 2D materials with high second‐order nonlinear polarizability, thereby severely disregarding bulk nonlinear optical (NLO) crystals as these rely on phase‐matching technology and achieving efficient bulk QNPDs remains a significant challenge. Weyl semimetal crystals have some signatures of inversion symmetry breaking, most notably second‐order NLO polarizability, while the inability to balance the low transmittance limits frequency conversion of the zero‐band gap absorption‐induced crystal. Herein, this study investigates an efficient QNPD based on bulk NbAs crystals designed with a strong second‐harmonic effect due to its large refractive index (≈5.0), resulting in an intense laser reflectivity of 50% on its surface, which creates a favorable environment for achieving second‐harmonic generation (SHG) without phase matching. The QNPD has a rectification ratio exceeding 10 7 with a dark current of 164 pA and an enhanced photoresponse in the 355‒1900 nm range, exhibiting a maximum responsivity of 4.1 mA W −1 with a detectivity of 0.8 × 10 10 Jones at 355 nm. The responsivity improvement rate is 88% higher than that of linear NbAs (001) photodetector. This study opens new avenues for designing QNPDs by utilizing the second harmonic effect in bulk Weyl semimetal crystals.