Spatial Modulation of Radiative Decay Dynamics of Exciton Complexes in WSe₂ Monolayer

Excitons in transition metal dichalcogenides (TMDs) monolayer possess strong binding energy, valley selectivity, and high quantum efficiency, which are useful for various applications in excitonics, quantum many-body physics, and optoelectronics. TMDs support not only excitons but also a variety of exciton complexes such as biexcitons, charged excitons, and dark excitons that have distinct features and capabilities from excitons: Biexcitons in TMDs have high valley selectivity. Charged excitons can be effectively controlled by the application of electric fields. Dark excitons have a longer intrinsic lifetime and thus can diffuse a longer distance. It has therefore been essential to examine and understand the radiative decay dynamics of exciton complexes in various environmental conditions. The Purcell effect in terms of vacuum field interference has been widely used to control the radiative decay rate of the exciton complexes. However, the radiative decays of the exciton complexes occur at different emission wavelengths and their detailed properties vary from specimen to specimen. The conventional planar mirrors or resonators are not practical to provide the target TMD material with various optical environments depending on the position and emission wavelength at the same time, which is required for investigating the decay dynamics of the exciton complexes systematically.<br/>In this research, we present the spatial modulation of radiative decay dynamics of exciton complexes in the WSe₂ monolayer employing a gradient-thickness mirror (GTM). Our GTM structure consists of the Si/Al (80 nm)/SU8 multilayer. The Al layer acts as a bottom mirror, and the thickness of the SU8 layer gradually changes from 80 to 500 nm over a horizontal distance of 20 micrometers with a slope of ~1°. The flake of hBN (150 nm)/WSe₂ monolayer/hBN (70 nm) was transferred onto the surface of the GTM structure. At a given wavelength, the vacuum field interference and the Purcell effect depend on the thickness of the dielectric SU8 layer. The GTM structure supports a white light interference pattern, in which the constructive/destructive interference conditions for the different emission wavelengths of the exciton (720 nm), biexciton (730 nm), and dark exciton (740 nm) in the WSe₂ monolayer appear at different spatial positions and repeat with different spatial frequencies. As a result, the radiative decay dynamics of each exciton complexes can be modulated in different manners along the direction of the thickness gradient, which enables us to examine the decay dynamics of exciton complexes in the identical WSe₂ monolayer under a variety of conditions.<br/>We measured the photoluminescence (PL) spectrum from the exciton complexes in the WSe₂ monolayer pumped with He-Ne laser via a confocal scanning microscopy. The PL intensity ratio between the exciton complexes, related to their relative radiative decay rates, change depending on the position along the thickness gradient of the GTM structure. We demonstrated that the relative PL intensity ratio of biexcitons to bright excitons spatially changes from ~0.28 to 2.1 with one order of magnitude. We also demonstrated an optical situation where the emission from dark excitons dominates in those of other exciton complexes, which have been difficult to realize due to the weak radiative recombination of the dark exciton. In addition, we were able to systematically investigate the Lamb shift of the bright exciton emission spectrum depending on the vacuum field interference strength.