Strain Control of Third Harmonic Generation in Nb₂SiTe₄ Driven by Tuneable Anisotropic Characteristics

Abstract Strain engineering constitutes one of the main control knobs for material properties. The material's crystal and band structure can be strongly modified by mechanical strain, thus tailoring the electronic and photonic properties toward specific applications. However, the strain control of the higher‐order nonlinear optical (NLO) processes in crystals characterized by in‐plane anisotropy remains mostly unexplored. It is demonstrated that 2D ternary Nb 2 SiTe 4 crystals provide an excellent platform for tuning NLO properties with uniaxial strain due to the inherently anisotropic band structure around the fundamental bandgap in the near‐infrared region. Resonant conditions between the interband excitations and third harmonic signals are realized, a record‐high third‐order susceptibility of 2.43 × 10 −18 m 2 /V 2 is observed, exceeding by over an order of magnitude the values reported for commonly used NLO crystals. The measurements of the third harmonic intensity as a function of the linear polarization revealed that the anisotropic characteristics of Nb 2 SiTe 4 can be tuned into qualitatively different regimes by the application of a uniaxial strain along the principal crystallographic axes. Notably, with increasing strain, the characteristic anisotropic direction can be reoriented. The findings provide unique perspectives for applying the modulation of the broadband NLO properties enabled by anisotropic materials in optoelectronics and all‐optical devices.