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 non-linear optical responses with a combination of experimental and theoretical approaches. A 15-fold overall enhancement in second harmonic generation (SHG) intensity is observed on MoSe₂ monolayers grown on SiO₂ when compared to its value when on a Si₃N₄ substrate. A seven-fold enhancement was ascertained to substrate interference, and a factor of two to the enhancement of χ⁽²⁾ arising from biaxial strain: substrate interference and strain are independent handles to engineer the SHG strength of non-centrosymmetric 2D materials.