Two-dimensional (2D) antiferromagnets hold great promise for application in next-generation computational devices. A critical challenge of the application of antiferromagnets lies in the detection of the Néel vector to enable an antiferromagnetic (AFM) state readout. Recently, a time-reversal-odd ($\mathcal{T}$-odd) second-harmonics generation (SHG) signal was experimentally discovered in 2D antiferromagnets. However, opposite Néel vectors generate identical SHG intensities without phase information, hindering magnetic domain discrimination. To address this problem, we propose to use a nonmagnetic substrate together with the antiferromagnet to form a heterojunction. This introduces $\mathcal{T}$-even SHG that interferes with the intrinsic $\mathcal{T}$-odd SHG of AFM materials, generating giant nonlinear magneto-optical (NLMO) effects that enable the detection of the Néel vector. Taking heterojunctions formed by antiferromagnetic bilayer ${\mathrm{CrBr}}_3$ and monolayer and bilayer ${\mathrm{MoS}}_2$ as examples, through symmetry analysis and first-principles calculations we reveal the mechanism of SHG responses and derive conditions to achieve a most notable NLMO effect under linearly and circularly polarized light. Theoretically, we find that the strict equality between the $\mathcal{T}$-even and $\mathcal{T}$-odd SHG is not necessary. Practically, we discover that the $\mathcal{T}$-even SHG originated from interlayer coupling interferes strongly with the $\mathcal{T}$-odd SHG from 2D antiferromagnets, demonstrating exceptional advantages for generating giant NLMO effects. Our work provides a unique strategy for probing AFM order and advances the development of ultrathin magnetic storage devices.

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• Magnetic order
• Magneto-optics
• Nonlinear optical susceptibility
• Second order nonlinear optical processes
• Antiferromagnets
• Transition metal dichalcogenides
• Transition metal trichalcogenides
• First-principles calculations