Strongly Coupled Coherent Phonons in Single-Layer MoS₂

Abstract

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS2, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A ′ 1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in 1L-MoS2 band structure induced by the A ′ 1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. This demonstrates that CP excitation in 1L-MoS2 can be described as a Raman-like scattering process. These results explain the CP generation process in 1L-TMDs, paving the way for coherent all-optical control of excitons in layered materials in the THz frequency range.Coherent modulation of the optical properties of a material, following impulsive photo-excitation of the lattice, is fundamentally interesting and technologically relevant because it can be used for applications in sensors[1], actuators and transducers [2][3][4], that can be operated at extremely high frequencies (up to several THz[5]). In order to exploit this effect, it is necessary to understand the mechanism underlying the coherent phonon (CP) generation process and to identify the physical parameters (such as the pump pulse photon energy) that allow their efficient excitation. In view of possible device applications, it is of paramount importance to detect the spectral dependence of the CP amplitude, in order to identify in which photon energy window the optical response of the material can be efficiently modulated.We present a novel transient absorption (TA) setup, combining broadband detection from 1.8 to 3eV, with extremely high temporal resolution (∼20fs). This is ideally suited to trigger and detect vibrational coherences in different classes of materials. We use it to generate and detect CPs in 1L-MoS 2 , as a representative semiconducting 1L-transition metal dichalcogenide (TMD). We focus on TMDs because they support strongly bound excitons with unique physical properties, enabling novel applica-tions in optoelectronics and photonics [6][7][8]. However, in TMDs, the dynamical interaction between excitons and phonons, when constrained to 1L, is unexplored.When TMDs are exfoliated down to 1L, they undergo a transition from indirect to direct band gap[9], accompanied by a strong enhancement of the photoluminescence (PL) quantum yie...

Exciton–phonon coupling strength in single-layer MoSe2 at room temperature

“…Transitions between |g 0 ⟩ and |e ≥2 ⟩ states are not observed in the 2D maps. This agrees with resonance Raman scattering 19,20 and their time-domain analogues 21,22 , where the A'1 overtone was not detected. This may imply an efficient nonradiative decay channel for the |e 2 ⟩ state, which results in a fast dephasing time for the hot vibronic band transitions.…”

“…Vibrational wave packets were reported at RT in Ref. 21,22, with a dephasing time ~4.5 ps for 1L-and few-layer WSe2 21 and ~1.7 ps for 1L-MoS2 22 . Therefore, EXPC can explain the oscillations in our 2D maps.…”

“…Yet the spectral signature of this coupling is obscured by inhomogeneous broadening 18 . The presence of EXPC was inferred from resonance Raman scattering 19,20 as well as time-resolved transmission measurements 21,22 , where the A'1 optical phonon mode was observed to couple with the A excitonic resonance.…”

“…This displacement is the EXPC strength, as further detailed below, that determines how strongly phonons will be excited upon an optical transition to the exciton state. To the best of our knowledge, the EXPC strength has never been determined for any 1L-TMD at room temperature, because overtone bands of the optical phonon mode were not detectable [19][20][21][22] , and thus the optical characterization of this (and similar) material(s) is incomplete. We determine the missing quantity in the present work.…”

A Reciprocal-Space Formulation of Surface Hopping

“…The recent years have seen a surge in interest in crystalline materials where band-like electronic conductance coexists with strong electron-phonon interactions, examples of which include hybrid metal-halide perovskites [1][2][3] and monolayer transition-metal dichalcogenides. [4][5][6][7][8] Few of the theoretical methods currently available are able to efficiently describe lattice-based electron-phonon couplings beyond the perturbative regime, [9][10][11] hampering our ability to unravel the nonequilibrium behavior of such materials.…”