Control of Ultrafast Many-Body Physics in Monolayer Transition Metal Dichalcogenides by Means of Applied Gate Bias Voltage

Monolayer group-VI transition metal dichalcogenides (ML TMDs) exhibit large quantum confinement effects and reduced dielectric screening [1], leading to many-body features such as neutral and charged excitons [2] with binding energies of several hundred of meV. The manipulation of these features via charge injection enables numerous optoelectronic and photonic functionalities such as single-photon generation from light-emitting diodes (LEDs) [3], or valleytronic-based devices for encryption and processing of quantum information [4], among others. However, it has so far been challenging to control excitons and trions in the two-dimensional limit, and a deeper physical comprehension is highly required for developing a feasible tool that enables to tune and enhance the optoelectronic behaviour of these promising novel materials. In this work we study the ultrafast generation and decay of both exciton and trion states in ML TMDs upon the variation of carrier density, introduced by means of the application of gate bias voltages in a FET configuration. We analyse the pump-probe spectra, obtained with a narrow pump above the bandgap and a broadband white probe, originated on a Ti:Sapphire laser with a 2 kHz repetition rate. The setup is coupled with a microscope to focus the pump and probe beams onto the few-micron transistor channel, and with a closed-cycle cryostat for cooling down to temperatures where the exciton and trion features are spectrally well separated. With this ultrafast spectroscopic technique we are able of spectrally resolving the many-body features and disentangle their different dynamics. We demonstrate the gate tunability of the optical response of monolayer TMDs, showing energy renormalization due to many-body effects, shifts on the excitonic peaks, and the sudden formation of trions by the interaction of excitons and injected free charge carriers, when the doping level of the monolayers is not in the neutral point due to the applied backgate voltages. The exciton and trion features form instantaneously within the instrument response function around 500 fs. Our results therefore demonstrate the possibility of electrically tuning the optical response of ML TMDs, and provides a deeper understanding of the ultrafast physical processes that take part in these materials. We also show the generation of stable positively and negatively charged excitons with binding energies of some tens of meV and tunable emission energies by means of applied voltage, and give some insight about their formation lifetimes and recombination mechanisms. This may impact the development of new optoelectronic and photonic devices, based on the control of the light-matter interaction in nanosized volumes of matter.<br/>References<br/>[1] G. Wang, A. Chernikov, M. M. Glazov, T. F. Heinz, X. Marie, T. Amand, and B. Urbaszek, Rev. Mod. Phys. 2018, 90, 021001.<br/>[2] Mak, K., He, K., Lee, C. et al. Tightly bound trions in monolayer MoS2. Nature Mater 2013, 12, 207–211.<br/>[3] X. Lin, X. Dai, C. Pu, Y. Deng, Y. Niu, L. Tong, W. Fang, Y. Jin, X. Peng, Nat. Commun. 2017, 8, 1132.<br/>[4] Morozov, S., Wolff, C., Mortensen, N. A., <i>Adv. Optical Mater.</i> 2021, 2101305.