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The mechanism of shock-induced dynamic friction has been explored through an integrated programme of experiments and numerical simulations. A novel experimental technique has been developed for observing the sub-surface deformation in aluminium when sliding against a steel anvil at high velocity and pressure. The experimental observations suggest that slight differences in conditions at the interface between the metals affect frictional behaviour even at the very high-velocity, high-pressure regime studied here. However, a clear finding from the experimental work is the presence of two distinct modes of deformation termed deep and shallow. The deep deformation is observed in a region of the aluminium specimen where the interfacial velocity is relatively low and the shallow deformation is observed in a region where the interfacial velocity is higher. A 1D numerical treatment is presented which predicts the existence of two mechanisms for dynamic friction termed ‘asymptotic melting’ and ‘slide-then-lock’. In both modes there is a warm-up phase in which the interface temperature is increased by frictional heating. For high initial sliding velocity, this is followed by the onset of the asymptotic melting state, in which the temperature is almost constant and melting is approached asymptotically. This mechanism produces low late-time frictional stress and shallow deformation. For lower initial sliding velocity, the warm-up terminates in a violent work hardening event that locks the interface and launches a strong plastic shear wave into the weaker material. This slide-then-lock mechanism is characterized by sustained high frictional stress and deep plastic deformation. These predicted mechanisms offer a plausible and consistent explanation for the abrupt transitions in the depth of sub-surface deformation observed in the experiments. A key conclusion arising from the current work is that the frictional stress does not vary smoothly with pressure or sliding velocity. Instead the pressure and sliding velocity determine whether the impulse will be very high or very low. The next generation of friction models for hydrocodes will need to account for these factors.

A series of GaAs/AlGaAs quantum-cascade lasers has been studied in which the confinement of the upper lasing level is systematically varied. This is achieved by modifying the aluminum composition, and hence the height, of a single barrier in each active region. Increasing the height of the barrier increases the upper laser level lifetime, while decreasing the optical transition matrix element. We find an optimum barrier composition (Al0.4Ga0.6As), with the sample containing this barrier exhibiting a significantly improved low-temperature threshold current density (3.8 kA/cm2) and peak power output (∼800 mW) relative to previously reported GaAs-based quantum-cascade lasers. The temperature performance of all the samples is very similar, indicating that thermal activation of electrons from the upper laser level is not the dominant factor limiting high-temperature operation.

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Complementary intersubband and interband optical measurements have been employed in order to study the bias dependence of the carrier distribution and energy level alignment within a GaAs–AlGaAs quantum cascade structure. Using these techniques, we have measured the redistribution of electrons throughout the bridging regions and upper states in the active regions with increasing bias. The high quality of the sample gives very narrow linewidths in the optical spectra, permitting the resolution of transitions involving closely spaced energy levels. This has allowed the direct observation of level alignment at the onset of current flow through the device.