  Exciton polaritons in atomically-thin transition metal dichalcogenide
crystals (monolayer TMDCs) have emerged as a promising candidate to enable
topological transport, ultra-efficient laser technologies, and collective
quantum phenomena such as polariton condensation and superfluidity at room
temperature. However, integrating monolayer TMDCs into high-quality planar
microcavities to achieve the required strong coupling between the cavity
photons and the TMDC excitons (bound electron-hole pairs) has proven
challenging. Previous approaches to integration had to compromise between
various adverse effects on the strength of light-matter interactions in the
monolayer, the cavity photon lifetime, and the lateral size of the microcavity.
Here, we demonstrate a scalable approach to fabricating high-quality planar
microcavities with an integrated monolayer WS$_2$ layer-by-layer by using
polymethyl methacrylate/silicon oxide (PMMA/SiO$_x$) as a cavity spacer.
Because the exciton oscillator strength is well protected against the required
processing steps by the PMMA layer, the microcavities investigated in this
work, which have quality factors of above $10^3$, can operate in the strong
light-matter coupling regime at room temperature. This is an important step
towards fabricating wafer-scale and patterned microcavities for engineering the
exciton-polariton potential landscape, which is essential for enabling many
proposed technologies.
