Out-of-plane trion emission in monolayer WSe2 revealed by whispering gallery modes of dielectric microresonators

Out-of-plane photon emission in 2D semiconductors is rare but crucial for efficient light manipulation in planar optoelectronic devices and photonic chips. Here, an out-of-plane dipolar component of trions in monolayer WSe2 is revealed by exciting the whispering gallery modes of SiO2 microspherical resonators. The manipulation of light emitted by two-dimensional semiconductors grounds forthcoming technologies in the field of on-chip communications. However, these technologies require from the so elusive out-of-plane photon sources to achieve an efficient coupling of radiated light into planar devices. Here we propose a versatile spectroscopic method that enables the identification of the out-of-plane component of dipoles. The method is based on the selective coupling of light emitted by in-plane and out-of-plane dipoles to the whispering gallery modes of spherical dielectric microresonators, in close contact to them. We have applied this method to demonstrate the existence of dipoles with an out-of-plane orientation in monolayer WSe2 at room temperature. Micro-photoluminescent measurements, numerical simulations based on finite element methods, and ab-initio calculations have identified trions as the source responsible for this out-of-plane emission, opening new routes for realizing on-chip integrated systems with applications in information processing and quantum communications.