2018, Kluge, Thomas, Rödel, Melanie, Metzkes-Ng, Josefine, Pelka, Alexander, Laso Garcia, Alejandro, Prencipe, Irene, Rehwald, Martin, Nakatsutsumi, Motoaki, McBride, Emma E., Schönherr, Tommy, Garten, Marco, Hartley, Nicholas J., Zacharias, Malte, Grenzer, Jörg, Erbe, Artur, Georgiev, Yordan M., Galtier, Eric, Nam, Inhyuk, Lee, Hae Ja, Glenzer, Siegfried, Bussmann, Michael, Gutt, Christian, Zeil, Karl, Rödel, Christian, Hübner, Uwe, Schramm, Ulrich, Cowan, Thomas E.

The complex physics of the interaction between short-pulse ultrahigh-intensity lasers and solids is so far difficult to access experimentally, and the development of compact laser-based next-generation secondary radiation sources, e.g., for tumor therapy, laboratory astrophysics, and fusion, is hindered by the lack of diagnostic capabilities to probe the complex electron dynamics and competing instabilities. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small-angle x-ray scattering of femtosecond x-ray free-electron laser pulses facilitates new capabilities for direct in situ characterization of intense short-pulse laser-plasma interactions at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short-pulse high-intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the preinscribed grating, due to plasma expansion. The density maxima are interleaved, forming a double frequency grating in x-ray free-electron laser projection for a short time, which is a hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets.

2010, Chèze, Caroline

This work focuses on the nucleation and growth mechanisms of GaN nanowires (NWs) by molecular beam epitaxy (MBE). The main novelties of this study are the intensive employment of in-situ techniques and the direct comparison of self-induced and catalyst-induced NWs. On silicon substrates, GaN NWs form in MBE without the use of any external catalyst seed. On sapphire, in contrast, NWs grow under identical conditions only in the presence of Ni seeds. The processes leading to NW nucleation are fundamentally different for both approaches. In the catalyst-assisted approach, Ga strongly reacts with the catalyst Ni particles whose crystal structure and phases are decisive for the NW growth, while in the catalyst-free approach, N forms an interfacial layer with Si before the intense nucleation of GaN starts. Both approaches yield monocrystalline wurtzite GaN NWs, which grow in the Ga-polar direction. However, the catalyst-assisted NWs are longer than the catalyst-free ones after growth under identical conditions, and they contain many stacking faults. By comparison the catalyst-free NWs are largely free of defects and their photoluminescence is much more intense than the one of the catalyst-assisted NWs. All of these differences can be explained as effects of the catalyst. The seed captures Ga atoms arriving at the NW tip more efficiently than the bare top facet in the catalyst-free approach. In addition, stacking faults could result from both the presence of the additional solid phase constituted by the catalyst-particles and the contamination of the NWs by the catalyst material. Finally, such contamination would generate non-radiative recombination centers. Thus, the use of catalyst seeds may offer an additional way to control the growth of NWs, but both the structural and the optical material quality of catalyst-free NWs are superior.