Disorder-assisted Robustness of Ultrafast Cooling in High Doped CVD-Graphene

Dirac Fermion, which is the low energy collective excitation near the Dirac cone in monolayer graphene, have gained great attention by low energy Terahertz probe. In the case of undoped graphene, it has been generally understood that the ultrafast terahertz thermal relaxation is mostly driven by the electron-phonon coupling (EOP), which can be prolonged to tens and hundreds of picoseconds. However, for the high doped graphene, which manifests the negative photoinduced terahertz conductivity, there is still no consensus on the dominant aspects of the cooling process on a time scale of a few picoseconds. Here, the competition between the disorders assisted defect scattering and the electron-phonon coupling process in the cooling process of the graphene terahertz dynamics is systematically studied and disentangled. We verify experimentally that the ultrafast disorder assisted lattice-phonon interaction, rather than the electron-phonon coupling process, would play the key role in the ultrafast thermal relaxation of the terahertz dynamics. Furthermore, the cooling process features robustness which is independent on the pump wavelength and external temperature. Our finding is expected to propose a considerable possible cooling channel in CVD-graphene and to increase the hot electron extracting efficiency for the design of graphene-based photoconversion devices

Role of the Optical–Acoustic Phonon Interaction in the Ultrafast Cooling Process of CVD Graphene

The Dirac fermion, a high-mobility electron in the Dirac cone of monolayer graphene, has significant potential for use in the terahertz probing technique. For undoped graphene, ultrafast terahertz conductivity relaxation is mostly driven by electron–acoustic phonon supercollision coupling. The decay time of this coupling can be increased to tens or hundreds of picoseconds by decreasing the temperature. However, for chemical vapor deposition (CVD)-grown graphene, which exhibits negative photoinduced terahertz conductivity, there is currently no consensus on the dominant aspects of the terahertz conductivity relaxation process on time scales of less than 10 ps. In this study, the competition between electron–acoustic and optical–acoustic phonon coupling processes during the cooling of CVD graphene was systematically investigated. We experimentally verified that the ultrafast disorder-assisted optical–acoustic phonon interaction plays a key role in ultrafast terahertz conductivity relaxation. Furthermore, the ultrafast cooling process was found to be robust under different pump wavelengths and external temperatures, and it could be modulated by substrate doping. These findings are expected to contribute to a possible cooling channel in CVD graphene and to increase hot electron extraction efficiency for the design of graphene-based photoconversion devices

Hot Carrier Transfer in PtSe₂/Graphene Enabled by the Hot Phonon Bottleneck

The charge transfer (CT) process of two-dimensional (2D) graphene/transition metal dichalcogenides (TMDs) heterostructures makes the photoelectric conversion ability of TMDs into a wider spectral range for the light harvester and photoelectric detector applications. However, the direct in situ investigation of the hot carrier transport in graphene/TMDs heterostructures has been rarely reported. Herein, using the optical pump and a terahertz (THz) probe (OPTP) spectroscopy, the CT process from graphene to five-layer PtSe2 in the PtSe2/graphene (P/G) heterostructure is demonstrated to be related to the pump fluence, which is enabled by the hot phonon bottleneck (HPB) effect in graphene. Furthermore, the frequency dispersion conductivity and the THz emission spectroscopy of the P/G heterostructure confirmed the existence of interlayer CT and its pump fluence-dependent behavior. Our results provide in-depth physical insights into the CT mechanism at the P/G van der Waals interface, which is crucial for further exploration of optoelectronic devices based on P/G heterostructures

Ultrafast Drift Current Terahertz Emission Amplification in the Monolayer WSe₂/Si Heterostructure

Two-dimensional transition metal dichalcogenides (TMDs) have great potential application for seamless on-chip integration due to their strong photon–electron–spin–valley coupling. However, the contact-free measurements of the valley-coupled photocurrent in TMDs is still challenging. Here, ultrafast terahertz emission spectroscopy is employed to investigate the photocurrent dynamics in monolayer WSe2, and an interface-induced drift current amplification is found in the WSe2/Si heterostructure. The amplification of terahertz emission comes from the photocurrent enlarged by band bending in the WSe2 and Si junction, and the amplification ratio increase further near the valley resonant transition of WSe2. In addition, the valley-momentum locked photocurrent in the WSe2/Si heterostructure reserves the same chirality with monolayer WSe2 at room temperature. These findings could provide a new method for manipulating valley-momentum locked photocurrent by photon helicity and open new avenues for TMD-based valley-polarized terahertz emission devices