The aim of this study was to clarify the initiation process and the propagation mechanism of positive underwater streamers under the application of pulsed voltage with a duration of 10 μs, focusing on two different theories of electrical discharges in liquids: the bubble theory and the direct ionization theory. The initiation process, which is the time lag from the beginning of voltage application to streamer inception, was found to be related to the bubble theory. In this process, Joule heating resulted in the formation of a bubble cluster at the tip of a needle electrode. Streamer inception was observed from the tip of a protrusion on the surface of this bubble cluster, which acted as a virtual sharp electrode to enhance the local electric field to a level greater than 10 MV/cm. Streak imaging of secondary streamer propagation showed that luminescence preceded gas channel generation, suggesting a mechanism of direct ionization in water. Streak imaging of primary streamer propagation revealed intermittent propagation, synchronized with repetitive pulsed currents. Shadowgraph imaging of streamers synchronized with the light emission signal indicated the possibility of direct ionization in water for primary streamer propagation as well as for secondary streamer propagation.

The collapse of a cavitation bubble is always associated with the radiation of intense shock waves, which are highly relevant in a variety of applications. To radiate a strong shock wave, it is necessary to converge energy at the collapse, and understanding generation processes of multiple shock waves at the collapse is a key issue. In the present study, we investigated the formation of multiple shock waves generated by the collapse of a laser-induced bubble. We used a high-speed imaging system with unprecedented spatiotemporal resolution. We developed a triggering procedure of high precision and reproducibility based on the deflection of a laser beam by the shockwave passage. The high-speed videos clearly show that: (A) a first shockwave is emitted as the micro-jet hits the bottom of the bubble interface, followed by a second shock wave due to the collapse of the remaining toroidal bubble; (B) a sequential collapse of elongated bubbles, where the top part of the bubble collapses slightly before the bottom of the bubble; and (C) the formation of compression shock waves from multiple sites on a toroidal bubble.

We present experimental observations of microjets formed by cavitation microbubbles. An underwater electric discharge, applied beneath a flat free surface, produces a primary compression wave, which undergoes several phase inversions upon reflections from the free surface and spark-bubble interface. The first reflection yields a tension wave, which produces a cloud of secondary cavitation bubbles in the liquid, some of which form microjets upon collapse. The tuning of these reflections enables an effective control of the microjet direction in the bubble cloud. All of the jets of the microbubbles between the spark bubble and free surface are directed radially away from the spark bubble. The mechanical response of an alumina plate placed between the electrodes and free surface generates a quasi-planar compression wave, which, following its multiple reflections from the free surface and plate, orients the microjets in the same direction toward the plate. These observations imply that the jet direction is determined mainly by the secondary compression wave, which is the first and thus most energetic compression wave acting on a sufficiently grown cavitation bubble.

Well-thickness dependence of intersubband absorption energies is investigated in $n$ -doped InAs/AlSb multiple-quantum-well structures grown by molecular-beam epitaxy. Decreasing the InAs well thickness from 18 to 9 monolayers, the absorption peak shifts from 3.31 μm (375 meV) to 1.98 μm (627 meV). These absorption energies are found to be larger than those of the corresponding spatially indirect band gap between the electron ground state in the InAs well and the heavy-hole ground state in the AlSb barrier. Intersubband transition energies are calculated self-consistently using multiband ${k\cdot p}$ calculation combined with Poisson equation, and compared with the experimental results.