Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission

Electron dynamics at interfaces is a subject of great scientific interest and technological importance. Detailed understanding of such dynamics requires access to the angstrom length scale defining interfaces and the femtosecond time scale characterizing interfacial motion of electrons. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following ultrafast laser excitation. We apply this method to interlayer charge transfer in heterostructures of two transition metal dichalcogenide monolayers less than 0.7 nm apart. We find that charge relaxation and separation occur in less than 100 fs. This approach allows us to unambiguously determine the direction of current flow, to demonstrate a charge transfer efficiency of order unity, and to characterize saturation effects.

Stick-Slip Dynamics in Ultrasonic Additive Manufacturing

Ultrasonic Additive Manufacturing is a solid state manufacturing process that combines ultrasonic welding of layers of thin metal foil with contour milling. Bonding between two foils is accomplished by holding the foils together under pressure and applying high-frequency excitations normal to the pressure direction. The accepted explanation for bonding is that stresses due to both compression and friction stemming from the interfacial motion between the foils result in plasticity and ultimately produce a metallurgical bond. The process however, has been shown to have a critical shortcoming in its operation; namely, the presence of a range of build heights within which bonding cannot be initiated. To better understand the reasons for this anomaly, this paper simplifies the process into a lumped parameter dry friction oscillator and shows that complex stick-slip motions of the build feature near or above its resonance frequency may explain bond degradation. Specifically, it is shown through bifurcation maps obtained for different process parameters that, at the critical build heights, the feature exhibits pure stick motions due to primary resonant interactions between the external excitation and the feature. Furthermore, complex aperiodic responses are observed at build heights above resonance (short features). In such scenarios, bonding cannot be initiated because no or non-uniform interfacial motions occur between the tape and the feature. It is also observed that, once the height of the build feature increases beyond the critical value corresponding to resonance, periodic uniform responses essential for bonding, are recovered. These results corroborates previous experimental findings which demonstrate that bonding can be hard to initiate near or slightly above resonance (at or slightly below a critical height) but can be reinitiated below resonance (above the critical height).