Anisotropic second harmonic generation (SHG) is of central importance in nonlinear photonics because it enables highly directional light manipulation and advanced polarization-sensitive functionalities. Here, taking two-dimensional (2D) transition metal dichalcogenides as a prototypical platform, we realize extremely anisotropic SHG by progressively lowering structural symmetry through Janus engineering and lateral heterostructuring. This symmetry reduction pathway is exemplified by pristine ${\mathrm{MoS}}_2$, Janus MoSSe to in-plane heterostructures ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$, and MoSSe/WSSe. Along this evolution, the successive breaking of out-of-plane mirror symmetry and in-plane rotational symmetry activates an expanded set of independent susceptibility components, thereby inducing SHG anisotropy both along the same axis and across different axes. Their coexistence in MoSSe/WSSe ultimately drives the SHG response into an extremely anisotropic regime; i.e., the SHG along different incident and polarization angles of light is thoroughly different. Meanwhile, the SHG is strong only along a certain direction while very weak along other directions. We further derive analytical expressions that explicitly reveal how symmetry removal reshapes the polarization-resolved SHG patterns and governs the evolution of SHG anisotropy. By establishing the relationship between the anisotropy parameters and the susceptibility tensor components, the excitation conditions required for realizing extreme anisotropy are identified. Our work establishes Janus lateral heterostructuring as an effective strategy for enhancing SHG anisotropy, providing a promising platform for the rational design of highly directional and polarization-sensitive nonlinear photonic devices.

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• 2-dimensional systems
• First-principles calculations
• Optical second-harmonic generation