Transient SHG Imaging on Ultrafast Carrier Dynamics of MoS₂ Nanosheets

Abstract

Understanding the collaborative behaviors of the excitons and phonons that result from light-matter interactions is important for interpreting and optimizing the underlying fundamental physics at work in devices made from atomically thin materials. In this study, the generation of exciton-coupled phonon vibration from molybdenum disulfide (MoS ) nanosheets in a pre-excitonic resonance condition is reported. A strong rise-to-decay profile for the transient second-harmonic generation (TSHG) of the probe pulse is achieved by applying substantial (20%) beam polarization normal to the nanosheet plane, and tuning the wavelength of the pump beam to the absorption of the A-exciton. The time-dependent TSHG signals clearly exhibit acoustic phonon generation at vibration modes below 10 cm (close to the Γ point) after the photoinduced energy is transferred from exciton to phonon in a nonradiative fashion. Interestingly, by observing the TSHG signal oscillation period from MoS samples of varying thicknesses, the speed of the supersonic waves generated in the out-of-plane direction (Mach 8.6) is generated. Additionally, TSHG microscopy reveals critical information about the phase and amplitude of the acoustic phonons from different edge chiralities (armchair and zigzag) of the MoS monolayers. This suggests that the technique could be used more broadly to study ultrafast physics and chemistry in low-dimensional materials and their hybrids with ultrahigh fidelity.

Probing the upper band gap of atomic rhenium disulfide layers

“…1a illustrates the microscopic setup to conduct the SHG study of the ReS 2 crystal. More details about the TSHG optical microscopy setup were reported previously (refer to the Section 1, Supporting Information ) 18 . A dual-mode Er-doped fiber laser system (Spectra-Physics, Insight Deepsee Dual) was used as a laser pumping oscillator (80 MHz) for a confocal galvanometric scanning (Olympus, Fluoview 1000) microscope (Olympus, IX 83) to create the SHG images of the exfoliated ReS 2 flakes.…”

“…2b ). The TSHG signal (Δ I ) and its phase information can be simply defined as the difference between the SHG signals obtained with and without the pump beams and can be given as follows 18 . See the Supporting Information for the detailed derivation, Section 4b .…”

“…Thus, the polarization-dependent SHG signals of ReS 2 crystals are not caused by the dependency of A u on the increment in the layer number but rather by the stacking alignment-induced broken inversion center, which is similar to that observed in 3R-like MoS 2 crystals 12 , 17 . Furthermore, the aforementioned SHG “probe” could be utilized to create a time-resolved SHG (TSHG) spectroscopic condition characterized by the synchronized illumination of the pump pulse beam, which features the ultrafast carrier dynamics of atomically thin MoS 2 even without an SHG contrast (i.e., for even-number MoS 2 layers) 18 , 23 . In TSHG spectroscopy, the probing action of SHG is analogous to single-photon transient absorption, which is observed at the energy of the SHG of the probe photon 22 , 23 .…”

“…In TSHG spectroscopy, the probing action of SHG is analogous to single-photon transient absorption, which is observed at the energy of the SHG of the probe photon 22 , 23 . More specifically, Jang et al 18 employed TSHG microspectroscopy to visualize the structure-dependent electron and phonon behavior with a spatial resolution of ~300 nm. In the excited state, absorption occurred at ~4.20 eV (by combining the 1.82 eV pump and the 2.38 eV SHG probe) from the valence band of MoS 2 ; this was not observed for WSe 2 above 4.03 eV (by combining the 1.65 eV pump and the 2.38 eV SHG probe).…”

MoS2-Based Photodetectors Powered by Asymmetric Contact Structure with Large Work Function Difference

“…Self-powered and high-sensitivity miniature photodetectors are very attractive for a broad range of applications, spanning the imaging, environmental monitoring, and biomedical diagnostics fields [1][2][3][4][5][6][7]. Two-dimensional (2D) materials offer the opportunity to produce the performance of self-powered and high-sensitivity miniature photodetectors [8][9][10][11][12][13][14].…”