Optical second-harmonic generation (SHG) is a nonlinear parametric process that doubles the frequency of incoming light. Only allowed in non-centrosymmetric materials 1 , it has been widely used in frequency modulation of lasers 2 , surface scientific investigation 3 , and label-free imaging in biological and medical sciences 4 . Two-dimensional crystals are ideal SHG-materials not only for their strong light-matter interaction 5 and atomic thickness defying the phase-matching requirement but also for their stackability into customized hetero-crystals with high angular precision and material diversity 6 . Here we directly show that SHG in hetero-bilayers of transition metal dichalcogenides (TMDs) is governed by optical interference between two coherent SH fields with material-dependent phase delays using spectral phase interferometry. We also quantify the frequencydependent phase difference between MoS2 and WS2, which also agrees with polarizationresolved data and first-principles calculations on complex susceptibility. The secondharmonic analogue of Young's double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.Two-dimensional (2D) materials have emerged as promising platforms for various photonic applications such as ultrafast photodetectors of gapless graphene 7 , valleytronics of

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