Substrate Interference and Strain in the Second-Harmonic Generation from MoSe₂ Monolayers

Nonlinear optical materials of atomic thickness, such as non-centrosymmetric 2H transition metal dichalcogenide monolayers, have a second-order nonlinear susceptibility (χ⁽²⁾) whose intensity can be tuned by strain. However, whether χ⁽²⁾ is enhanced or reduced by tensile strain is a subject of conflicting reports. Here, we grow high-quality MoSe₂ monolayers under controlled biaxial strain created by two different substrates and study their linear and nonlinear optical responses with a combination of experimental and theoretical approaches. Up to a 15-fold overall enhancement in second-harmonic generation (SHG) intensity is observed from MoSe₂ monolayers grown on SiO₂ when compared to its value on a Si₃N₄ substrate. By considering an interference contribution from different dielectrics and their thicknesses, a factor of 2 enhancement of χ⁽²⁾ was attributed to the biaxial strain: substrate interference and strain are independent handles to engineer the SHG strength of non-centrosymmetric 2D materials.