  We reveal stacking-induced strong coupling between atomic motion and
interlayer excitons through photocurrent measurements of WSe$_2$/MoSe$_2$
heterojunction photodiodes. Strong coupling manifests as pronounced periodic
sidebands in the photocurrent spectrum in frequency windows close to the
interlayer exciton resonances. The sidebands, which repeat over large swathes
of the interlayer exciton photocurrent spectrum, occur in energy increments
corresponding directly to a prominent vibrational mode of the heterojunction.
Such periodic patterns, together with interlayer photoconductance oscillations,
vividly demonstrate the emergence of extraordinarily strong exciton-phonon
coupling - and its impact on interlayer excitations - in stack-engineered van
der Waals heterostructure devices. Our results establish photocurrent
spectroscopy as a powerful tool for interrogating vibrational coupling to
interlayer excitons and suggest an emerging strategy to control vibronic
physics in the solid-state.
