  Light-matter interaction with two-dimensional materials gained significant
attention in recent years leading to the reporting of weak and strong coupling
regimes, and effective nano-laser operation with various structures.
Particularly, future applications involving monolayer materials in
waveguide-coupled on-chip integrated circuitry and valleytronic nanophotonics
require controlling, directing and optimizing photoluminescence. In this
context, photoluminescence enhancement from monolayer transition-metal
dichalcogenides on patterned semiconducting substrates becomes attractive. It
is demonstrated in our work using focussed-ion-beam-etched GaP and monolayer
WS2 suspended on hexagonal-BN buffer sheets. We present a unique optical
microcavity approach capable of both efficient in-plane and out-of-plane
confinement of light, which results in a WS2 photoluminescence enhancement by a
factor of 10 compared to the unstructured substrate at room temperature. The
key concept is the combination of interference effects in both the horizontal
direction using a bull's-eye-shaped circular Bragg grating and in vertical
direction by means of a multiple reflection model with optimized etch depth of
circular air-GaP structures for maximum constructive interference effects of
the applied pump and expected emission light.
