Fig. 2: The coordinate system based on monolayer MoS₂ sample.

MoS₂ crystal structure is illustrated by the ball and stick model. Cyan and yellow spheres represent Molybdenum (Mo) and Sulfur (S) atoms, respectively. The crystal coordinate system is based on this crystal structure, where the black solid axes denoted [100] and [010] correspond to the ZZ and AC crystal directions of MoS₂, respectively. e′_m is the initial polarization vector, who has an arbitrary initial angle ϕ₀ relative to [010] direction. Along the direction of e′_m, an experimental coordinate system X′-Y′ is established (viz. black dashed axes). Therefore, the angle between the X′ axis and the [010] axis is also ϕ₀. e_m, the blue dashed lines with arrows, denotes the polarization vector of the incident laser. The angle between e_m and X′ axis is defined as ϕ. The black square in the upper right sub-figure represents the unit body of in-plane strain state, where the normal strain ε_xx and ε_yy, and shear strain γ_xy (γ_yx) are shown as black solid arrows, while the principal strain ε₁ and ε₂, are shown as blue dashed arrows, and θ denotes the principal strain angle.