We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe differential transmission spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers. The spectra also provide strong evidence for the multilayer structure and doping profile of thermally grown epitaxial graphene on SiC.

The combination of its high electron mobility, broadband absorption and ultrafast luminescence make graphene attractive for optoelectronic and photonic applications, including transparent electrodes, mode-locked lasers and high-speed optical modulators. Photo-excited carriers that have not cooled to the temperature of the graphene lattice are known as hot carriers, and may limit device speed and energy efficiency. However, their roles in charge and energy transport are not fully understood. Here, we use time-resolved scanning photocurrent microscopy to demonstrate that hot carriers, rather than phonons, dominate energy transport across a tunable graphene p-n junction excited by ultrafast laser pulses. The photocurrent response time varies from 1.5 ps at room temperature to 4 ps at 20 K, implying a fundamental bandwidth of ∼500 GHz (refs 12, 13, 21). Gate-dependent pump-probe measurements demonstrate that both thermoelectric and built-in electric field effects contribute to the photocurrent, with the contribution from each depending on the junction configuration. The photocurrent produced by a single pulsed laser also displays multiple polarity reversals as a function of carrier density, which is a possible signature of impact ionization.

We investigate the valley-related carrier dynamics in monolayer molybdenum disulfide using helicity-resolved nondegenerate ultrafast pump-probe spectroscopy at the vicinity of the high-symmetry K point under the temperature down to 78 K. Monolayer molybdenum disulfide shows remarkable transient reflection signals, in stark contrast to bilayer and bulk molybdenum disulfide due to the enhancement of many-body effect at reduced dimensionality. The helicity-resolved ultrafast time-resolved result shows that the valley polarization is preserved for only several picoseconds before the scattering process makes it undistinguishable. We suggest that the dynamical degradation of valley polarization is attributable primarily to the exciton trapping by defect states in the exfoliated molybdenum disulfide samples. Our experiment and a tight-binding model analysis also show that the perfect valley circular dichroism selectivity is fairly robust against disorder at the K point but quickly decays from the high-symmetry point in the momentum space in the presence of disorder.

The experimental manifestation of topological effects in bulk materials under ambient conditions, especially those with practical applications, has attracted enormous research interest. Recent discovery of Weyl semimetal provides an ideal material platform for such endeavors. The Berry curvature in a Weyl semimetal becomes singular at the Weyl node, creating an effective magnetic monopole in the k-space. A pair of Weyl nodes carry quantized effective magnetic charges with opposite signs, and therefore, opposite chirality. Although Weyl-point-related signatures such as chiral anomaly and non-closing surface Fermi arcs have been detected through transport and ARPES measurements, direct experimental evidence of the effective k-space monopole of the Weyl nodes has so far been lacking. In this work, signatures of the singular topology in a type-II Weyl semimetal TaIrTe4 is revealed in the photo responses, which are shown to be directly related to the divergence of Berry curvature. As a result of the divergence of Berry curvature at the Weyl nodes, TaIrTe4 exhibits unusually large photo responsivity of 130.2 mA/W with 4-m excitation in an unbiased field effect transistor at room temperature arising from the third-order nonlinear optical response.The room temperature mid-IR responsivity is approaching the performance of commercial HgCdTe detector operating at low temperature, making Type-II Weyl semimetal TaIrTe4 of practical importance in terms of photo sensing and solar energy harvesting. Furthermore, the circularly polarized galvanic response is also enhanced at 4-m, possibly due to the same Berry curvature singularity enhancement as the shift current. Considering the optical selection rule of Weyl cones with opposite chirality, it may open new experimental possibilities for studying and controlling the chiral polarization of Weyl Fermions through an in-plane DC electric field in addition to the optical helicities.

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Black phosphorus has recently emerged as a promising material for high-performance electronic and optoelectronic device for its high mobility, tunable mid-infrared bandgap, and anisotropic electronic properties. Dynamical evolution of photoexcited carriers and the induced transient change of electronic properties are critical for materials' high-field performance but remain to be explored for black phosphorus. In this work, we perform angle-resolved transient reflection spectroscopy to study the dynamical evolution of anisotropic properties of black phosphorus under photoexcitation. We find that the anisotropy of reflectivity is enhanced in the pump-induced quasi-equilibrium state, suggesting an extraordinary enhancement of the anisotropy in dynamical conductivity in hot carrier dominated regime. These results raise attractive possibilities of creating high-field, angle-sensitive electronic, optoelectronic, and remote sensing devices exploiting the dynamical electronic anisotropy with black phosphorus.

2 Impact ionization, which supports carrier multiplication, is promising for applications in single photon detection 1 and sharp threshold swing field effect devices 2 . However, initiating impact ionization of avalanche breakdown requires a high applied electric field in a long active region, hampering carrier-multiplication with high gain, low bias and superior noise performance 3, 4 . Here we report the observation of ballistic avalanche phenomena in sub-mean free path (MFP) scaled vertical InSe 5 /black phosphorus (BP) 6-9 heterostructures 10 . We use these heterojunctions to fabricate avalanche photodetectors (APD) and impact ionization transistors with sensitive mid-IR light detection (4 μm wavelength) and steep subthreshold swing (SS) (<0.25 mV/dec). The devices show a low avalanche threshold (<1 volt), low noise figure and distinctive density spectral shape. Our transport measurements suggest that the breakdown originates from a ballistic avalanche phenomenon, where the sub-MFP BP channel support the lattice impact-ionization by electrons and holes and the abrupt current amplification without scattering from the obstacles in a deterministic nature. Our results provide new strategies for the development of advanced photodetectors 1, 11, 12 via efficient carrier manipulation at the nanoscale.The upper panel of Fig. 1a shows the schematic of our heterostructure device with electrical connections. It consists of a thin γ-rhombohedral InSe/BP heterostructure connected to bottom and top metal electrodes on substrate. The detailed fabrication processes are presented in the Methods Section. During our measurements, we define the biased electrode connected to BP (~10 nm) as the drain and the grounded electrode connected to InSe (also ~10 nm) as the source. Considering that the lateral resistance of unstacked InSe is much smaller than that of the junction (Supplementary Section 1a), carriers also mainly transport vertically along the nanoscale InSe/BP channel. As a result, n-type InSe and p-type BP 13-15 form a vertical vdW heterojunction. The bottom panel of Fig. 1a 3 schematically shows the lattice structure at the junction interface. Notably, we assembled the InSe/BP junction in a glove box, resulting in a nearly perfect interface. In Fig. 1b, the high-resolution transmission electron microscope image verifies that the atomic stack is clean without the presence of any contamination or amorphous oxide even after all the device fabrication processes.We first characterized the transport properties of our heterostructure devices. With proper gate voltage 16 (10 V here, to ensure a necessary low doping level of BP and InSe, see below for details), the vertical vdW junction presents a standard rectification behaviour as a regular pn diode under moderate bias. In contrast, the reverse-biased current abruptly increases approximately 5 orders above a certain threshold voltage (~-4.8 V here), as shown in Fig. 1c. This "hard-knee" rapid change in current signals a typical avalanche breakdown 16, 17 resulting from impact ...

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