Coherent Phonons in van der Waals MoSe₂/WSe₂ Heterobilayers

Abstract

The increasing role of two-dimensional (2D) devices requires the development of new techniques for ultrafast control of physical properties in 2D van der Waals (vdW) nanolayers. A special feature of heterobilayers assembled from vdW monolayers is femtosecond separation of photoexcited electrons and holes between the neighboring layers, resulting in the formation of Coulomb force. Using laser pulses, we generate a 0.8 THz coherent breathing mode in MoSe 2 /WSe 2 heterobilayers, which modulates the thickness of the heterobilayer and should modulate the photogenerated electric field in the vdW gap. While the phonon frequency and decay time are independent of the stacking angle between the MoSe 2 and WSe 2 monolayers, the amplitude decreases at intermediate angles, which is explained by a decrease in the photogenerated electric field between the layers. The modulation of the vdW gap by coherent phonons enables a new technology for the generation of THz radiation in 2D nanodevices with vdW heterobilayers.

Enhanced Photon–Phonon Interaction in WSe2 Acoustic Nanocavities

“…Electronic triggering provides the temporal resolution ∼1 ps, which allows us to define accurately the oscillation frequencies up to 200 GHz. To measure the signals with low amplitude and better temporal resolutions (but a smaller temporal interval), it is required to use other systems (e.g., with fast delay lines). It should be taken into account that ASOPS systems may give parasite oscillations due to the interference of laser beams.…”

“…Examples of high-frequency ANCs include superlattices, which localize phonons with frequencies lying in the acoustic stop-bands, , and membranes fabricated from various semiconductors (e.g., Si, , GaAs, GaN). Unique sub-THz and THz acoustic properties have been revealed in nanolayer-related heterostructures fabricated from van der Waals (vdW) materials like graphene, transition metal dichalcogenides (TMDs), − and others. , VdW nanolayers, which are easily fabricated by exfoliation techniques, enable ANCs to reach f ∼ 1 THz and quality factor Q > 10 3 so that f × Q reaches the record value of 10 14 Hz , (for lower frequency nanomechanical properties, see review). Such high finesse at extremely high acoustic frequencies leads to a new paradigm in the engineering of high-frequency communication and quantum devices.…”

“…In the previous studies − of vdW-based ANCs, the main task was to increase the frequency and decay time of the generated coherent LA acoustic phonons while the efficiency of their generation and detection did not get much attention. Nevertheless, the amplitude of the signal induced by coherent phonon oscillations generated and detected in ANCs is the most important parameter for exploiting vdW ANCs in practical devices.…”