  Recently, moir\'{e} superlattices have attracted considerable attentions
because they are found to exhibit intriguing electronic phenomena of tunable
Mott insulators and unconventional superconductivity. These phenomena are
highly related to the physical mechanism of the interlayer coupling. However,
up to now, there has not existed any theory that can completely interpret the
experimental results of the interlayer conductance of moir\'{e} superlattice.
In order to solve this problem, the superposition of periods and the
corresponding coherence, which are the essential characteristics of moir\'{e}
superlattice, should be considered more sufficiently. Therefore, it is quite
necessary to introduce optical methods to study moir\'{e} superlattices. Here,
we develop a theory for moir\'{e} superlattices which are founded on
traditional optical scattering theory. The theory can interpret both the
continuously decreasing background and the peak of the interlayer conductance
observed in the experiments by a unified mechanism. We show that, the
decreasing background of the interlayer conductance arises from the increasing
strength of the interface potential, and the peak roots from the scattering
resonance of the interface potential. The present work is crucial for
understanding the interlayer coupling of the moir\'{e} superlattice, and
provide a solid theoretical foundation for the application of moir\'{e}
superlattice.
