Abstract
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi2Se3. Using transient second-harmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac states.
Highlights
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time
We show that transient second-harmonic generation (TSHG) and TSHG rotational anisotropy (TSHGRA) techniques allow for independent monitoring of temporal electron dynamics confined to the topmost Se atomic layer along the in-plane and out-of-plane directions of the films, providing evidence for the excitation of transverse magnetic (TM)- and transverse electric (TE)-guided coherent acoustic Dirac plasmon (CADP) modes, respectively
The TR and TSHG traces measured simultaneously for the 10 nm thick Bi2Se3 film at various pump/probe powers and in the Pin À Spump À Pout and Sin À Ppump À Sout polarization geometries are shown in Figs 1 and 2, respectively, where P and S denote the polarization of the incident probe and pump laser beams (‘in’ and ‘pump’) and the outgoing reflected fundamental/SHG probe beam (‘out’) in the plane of incidence and in the plane of the film, respectively
Summary
Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. A technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac states. Low-energy (GHz-range) coherent APs for the two component Dirac fermions (that is, coherent acoustic Dirac plasmons (CADPs)) may exist due to a very anisotropic mass tensor of Bi-related materials (such as Bi and Bi2Te3)[36] This tensor for Bi2Se3 is almost isotropic[37], causing the CADPs to be sharply Landau damped by the high-density single-particle continua resulting from natural n-doping. This kind of acousto-plasmonic control is known to occur in complex nanostructures[56], while the resonance-type acousticplasmon-to-acoustic-phonon coupling has been considered theoretically only for Bi36
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
