Approaching the absorption limit with monolayer semiconductor superlattices

Abstract: Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. There exists, however, a fundamental limit of 50% absorptance for any ultrathin film in a symmetric environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption approaching the fundamental limit by artificially stacking WS$_2$ monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the record peak absorptance of WS2 monolayers, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach a transmittance contrast of 30% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer, reaching 28% transmittance contrast while increasing photoluminescence thanks to doping. Finally, we exploit atomic layer deposition of alumina spacers to boost the transmittance contrast to 36% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve exciton-polariton phenomena such as strong light-matter coupling and nanophotonic devices such as modulators and photodetectors.