Interference-Enhanced Broadband Absorption of Monolayer MoS₂ on Sub-100 nm Thick SiO₂/Si Substrates: Reflection and Transmission Phase Changes at Interfaces

The optical characteristics of MoS₂ monolayers on SiO₂/Si substrates with an SiO₂ thickness ranging from 40 to 130 nm are investigated. The measured Raman and optical reflection spectra of the MoS₂ monolayers vary considerably depending on the SiO₂ thickness. The Raman peak intensity of the MoS₂ monolayer on the substrate with an 80 nm thick SiO₂ layer is four times larger than those in the cases of 40- and 130 nm thick SiO₂ layers, indicating a significant difference in the absorption at the excitation wavelength. The incident light undergoes anomalous phase changes upon reflection and transmission at the highly absorbing MoS₂/nonabsorbing SiO₂ or air interfaces. The phase changes at these interfaces in conjunction with those induced by the propagation of light in the SiO₂ layer cause complex interference, which dramatically tunes the absorption spectrum of the MoS₂ layer with changing SiO₂ thickness. Neither wavelength nor the incident angle of light strongly affects the interface phase change. Thus, the MoS₂ monolayers on sub-100 nm thick SiO₂/Si substrates exhibit broadband omnidirectional absorption in the visible range. This work demonstrates that SiO₂/Si wafers, which are the most popular substrates, allow the optical responses of MoS₂ monolayers to be optimized for optoelectronic applications.