  Optical second-harmonic generation (SHG) is a nonlinear parametric process
that doubles the frequency of incoming light. Only allowed in
non-centrosymmetric materials, it has been widely used in frequency modulation
of lasers, surface scientific investigation, and label-free imaging in
biological and medical sciences. Two-dimensional crystals are ideal
SHG-materials not only for their strong light-matter interaction and atomic
thickness defying the phase-matching requirement but also for their
stackability into customized hetero-crystals with high angular precision and
material diversity. 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 frequency-dependent phase
difference between MoS2 and WS2, which also agrees with polarization-resolved
data and first-principles calculations on complex susceptibility. The
second-harmonic analogue of Young double-slit interference shown in this work
demonstrates the potential of custom-designed parametric generation by
atom-thick nonlinear optical materials.
