Speaker
Description
The low dimensional nature of transition metal dichalcogenides (TMDCs) and the resulting reduced screening significantly influence their non-equilibrium optical properties, as dynamic screening by photoexcited quasiparticles governs the transient response. In this work, we investigate the role of different photoexcited quasiparticles on the dynamic response of WS2 monolayers. Firstly, we provide a formalism that allows the quantitative comparison of exciton dynamics in TMDCs for different samples, optical sampling techniques, and substrates, valid within the accuracy of the excited particle density determination. We then unveil the role of the presence of different quasiparticles upon resonant/above resonance photoexcitation by fitting a phenomenological model describing the microscopic time evolution of the photo-excited quasiparticle population to the pump energy and fluence dependence of the exciton dynamics. In particular, when photo-excited excitons are present in the system, a blueshift that can be modelled with a binding energy reduction is observed, and exciton-exciton scattering is the primary source of linewidth broadening. Instead, the presence of quasi-free carriers induces a redshift of the excitonic resonance that can be described with a bandgap renormalization and, through exciton-QFC scattering, yields to a more pronounced broadening of the excitonic resonance. We shine light on the complex recovery pathways that govern the system's recovery back to equilibrium, ultimately unravelling the timescales and interplay of electronic and thermal processes contributing to the exciton dynamics.
The combination of TMDCs with organic dye molecules is promising for the next generation of optoelectronic and light harvesting devices, provided the chromophores exhibit appropriate energy levels and sufficient excited state lifetimes to allow for charge or energy transfer processes. Towards the investigation of hybrid interfaces, we studied terrylene molecules in solution and thin films. Time-resolved photoluminescence and broadband transient absorption experiments showed a lifetime of 3.7 ns lifetime for the singlet state S1. Thin terrylene film shows absorption bands strongly (> 0.5 eV) blue-shifted due to H-aggregation. Photoexcitation of the aggregate leads to the formation of an induced monomer-like absorption band at 2.3 eV, which arises from the interference between Frenkel exciton state and charge-transfer (CT) state. A phenomenological fit model helps to extract a time constant of 150 ps for the ground state bleach recovery while the CT state relaxes into the dark state of the H-band within 230 ps. Preliminary analysis of hybrid terrylene/WS2 sample indicates the appearance of hybridized states around 2.6 eV.
Beyond their sensitivity to dynamic screening, TMDC monolayers are also highly sensitive to local effects arising from edges, defects, strain, and variations of the dielectric constant in the template and/or adsorbate. To study femtosecond dynamics on optical energy scales and at the nanoscale, we are developing time-resolved interferometric optical nanoscopy (TRION). This will yield unprecedented observations of the spatial dependence of electron dynamics in TMDC monolayers and TMDC/organic heterojunctions with nm spatial resolution and fs temporal resolution. The poster will show first near-field images acquired using our femtosecond laser system.
