Uniaxial Strain Tuning of Upconversion Photoluminescence in Monolayer WSe₂

Strain engineering is one of the leading mechanical ways to tune the optical properties of monolayer transition metal dichalcogenides among different techniques. Here, uniaxial strain is applied on exfoliated 1L-WSe2 flakes transferred on flexible polycarbonate substrates to study the strain tuning of upconversion photoluminescence. It is demonstrated that the peak position of upconversion photoluminescence is redshifted by around 20 nm as the applied uniaxial strain increases from 0% to 1.17%, while the intensity of upconversion photoluminescence increases exponentially for the upconversion energy difference ranging from -155 to -32 meV. The linear and sublinear power dependence of upconversion photoluminescence is observed for different excitation wavelengths with and without uniaxial strain, suggesting the multiphonon-assisted mechanism in one-photon regime for the upconversion process. These results offer the potential to advance 2D material-based optical upconversion applications in night vision, strain-tunable infrared detection, and flexible optoelectronics.