Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

Abstract

Two-dimensional transition metal dichalcogenides (TMD) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMD monolayers and heterostructures in consideration of the material’s structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS2 flakes grown on a SiO2/ Si(001) substrate are presented.

Ultrafast Charge-Transfer Dynamics in Twisted MoS2/WSe2 Heterostructures

“…By careful selection of the polarization angle of the 800 nm (1.55 eV) probe light, we are able to extract changes in the nonlinear susceptibility of individual monolayers inside the TMD heterostructures. For TMD monolayers, the pump-induced changes of the SH response upon resonant optical excitation of intralayer excitons have been shown to be closely correlated to the observed exciton dynamics in linear optical spectroscopy . In the case of TMD heterostructures, our pump–probe experiments schematically sketched in Figure (e) allow us to distinguish the transient SH response from differently oriented layers by polarization-dependent measurements.…”

“…For TMD monolayers, the pump-induced changes of the SH response upon resonant optical excitation of intralayer excitons have been shown to be closely correlated to the observed exciton dynamics in linear optical spectroscopy. 35 In the case of TMD heterostructures, our pump−probe experiments schematically sketched in Figure 1(e) allow us to distinguish the transient SH response from differently oriented layers by polarization-dependent measurements. By changing the pumpphoton energy in addition, our technique can reveal a very clear picture of the ultrafast charge-transfer process.…”

“…Long-term measurements with these fluences applied did not exhibit any multishot damage. A more detailed description of the setup can be found elsewhere . Sample preparation and characterization is described in detail in ref .…”

Ultrafast charge-transfer dynamics in twisted MoS$_2$/WSe$_2$ heterostructures

“…By careful selection of the polarization angle of the 800nm (1.55 eV) probe light, we are able to extract changes in the nonlinear susceptibility of individual monolayers inside the TMD heterostructures. For TMD monolayers, the pump-induced changes of the SH response upon resonant optical excitation of intralayer excitons have been shown to be closely correlated to the observed exciton dynamics in linear optical spectroscopy 28 . In case of TMD heterostructures, our pump-probe experiments schematically sketched in Fig.…”

“…Long term measurements with these fluences applied did not exhibit any multishot damage. A more detailed description of the setup can be found elsewhere 28 . Sample preparation and characterization is described in detail in Ref.…”