Substrate Interference\nand Strain in the Second-Harmonic\nGeneration from MoSe<sub>2</sub> Monolayers

Nonlinear optical materials of atomic thickness, such\nas non-centrosymmetric\n2H transition metal dichalcogenide monolayers, have a second-order\nnonlinear susceptibility (χ<sup>(2)</sup>) whose intensity can\nbe tuned by strain. However, whether χ<sup>(2)</sup> is enhanced\nor reduced by tensile strain is a subject of conflicting reports.\nHere, we grow high-quality MoSe<sub>2</sub> monolayers under controlled\nbiaxial strain created by two different substrates and study their\nlinear and nonlinear optical responses with a combination of experimental\nand theoretical approaches. Up to a 15-fold overall enhancement in\nsecond-harmonic generation (SHG) intensity is observed from MoSe<sub>2</sub> monolayers grown on SiO<sub>2</sub> when compared to its\nvalue on a Si<sub>3</sub>N<sub>4</sub> substrate. By considering an\ninterference contribution from different dielectrics and their thicknesses,\na factor of 2 enhancement of χ<sup>(2)</sup> was attributed\nto the biaxial strain: substrate interference and strain are independent\nhandles to engineer the SHG strength of non-centrosymmetric 2D materials.