Interface-Phase-Shift-Induced Interference and Enhanced Absorption of MoS₂ Monolayers

The extremely thin nm-thick MoS₂ layers limit optical absorption in spite of their very large absorption coefficients in visible wavelength range. To enhance the light-matter interaction in MoS₂ layers, researchers have tried several approaches, using plasmonic metal nanostructures, Fabry-Perot-type cavities, and photonic nanostructures. In this work, we investigated how reflection and transmission phase-shift at the highly absorbing MoS₂ interface could affect the absorption spectra of the MoS₂ monolayers on SiO₂/Si substrates (SiO₂ thickness: 40 ~ 130 nm). Such interface-phase-shift gave rise to interference in MoS₂ layer, although the layer thickness was only 0.7 nm, much smaller than the wavelength. We compared measured and calculated optical reflection spectra, which showed that the interference enhanced optical absorption in MoS₂ monolayers in whole visible wavelength range. Raman intensity of MoS₂ monolayers largely varied depending on the SiO₂ thickness, which could be well explained by the interference-enhanced absorption in MoS₂ layers. This work showed that proper choice of the SiO₂ thickness could provide us a simple and useful means to improve broadband optical absorption in MoS₂ monolayers.