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

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 10⁷ 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 × 10¹⁰ 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.