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

The strain control of higher-order nonlinear optical (NLO) processes in crystals with in-plane anisotropy remains mostly unexplored. Here, we show that 2D ternary Nb₂SiTe₄ crystals provide an excellent platform for tuning NLO properties with uniaxial strain due to their anisotropic band structure around the fundamental band gap in the near-infrared region. By realizing resonant conditions between interband excitations and third harmonic signals, we observed a record high third-order susceptibility of 2.43×10⁻¹⁸ m²/V², exceeding by over an order of magnitude the values reported for standard NLO crystals. The third harmonic intensity as a function of linear polarization revealed that the anisotropic characteristics of Nb₂SiTe₄ can be tuned into different regimes by applying uniaxial strain along the principal crystallographic axes. Notably, with increasing strain, the anisotropic direction can be reoriented. Our findings offer unique perspectives for applying broadband NLO modulation in optoelectronics and all-optical devices.