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

Nonlinear optical materials of atomic thickness, such as non-centrosymmetric 2H transition metal dichalcogenide monolayers, have a second-order nonlinear susceptibility (χ) whose intensity can be tuned by strain. However, whether χ is enhanced or reduced by tensile strain is a subject of conflicting reports. Here, we grow high-quality MoSe 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.