Long range epitaxial growth of prismatic heterostructures on the facets of catalyst-free GaAs nanowires

Molecular beam epitaxy is used for the synthesis of catalyst-free GaAs nanowires and related quantum heterostructures. After growth of the nanowire GaAs core, the conditions are changed in situ towards standard MBE planar growth in order to obtain quantum heterostructures on the facets of the nanowires. Depending on the nanowire orientation, different geometries of the quantum heterostructures are obtained. This growth method is fully characterized by high resolution and scanning transmission electron microscopy and Z-contrast electron tomography. The growth conditions are also tuned for the optimization and homogeneity of the optical properties. The feedback of these analyses allows the tuning of the growth conditions according to the required optical properties. This work is the basis for obtaining a new generation of devices based on the heterostructures existing on the nanowire facets

Ultralow emittance electron beams from a laser-wakefield accelerator

Using quadrupole scan measurements we show laser-wakefield accelerated electrons to have a normalized transverse emittance of 0.21 +0.01-0.02 π mm mrad at 245 MeV. We demonstrate a multishot and a single-shot method, the mean emittance values for both methods agree well. A simple model of the beam dynamics in the plasma density downramp at the accelerator exit matches the source size and divergence values inferred from the measurement. In the emerge range of 245 to 300 MeV the normalized emittance remains constant

Imaging laser-wakefield-accelerated electrons using miniature magnetic quadrupole lenses

The improvement of the energy spread, beam divergence, and pointing fluctuations are some of the main challenges currently facing the field of laser-wakefield acceleration of electrons. We address these issues by manipulating the electron beams after their generation using miniature magnetic quadrupole lenses with field gradients of ~500 T/M. By imaging electron beams the spectral resolution of dipole magnet spectrometers can be significantly increased, resulting in measured energy spreads down to 1.0% rms at 190 MeV. The focusing of different electron energies demonstrates the tenability of the lens system and could be used to filter out off-target energies in order to reduce the energy spread even further. By collimating the beam, the shot-to-shot spatial stability of the beam is improved by a factor of 5 measured at a distance of 1 m from the source. Additionally, by deliberating transversely offsetting a quadrupole lens, the electron beam can be steered in any direction by several mrad. These methods can be implemented while still maintaining the ultrashort bunch duration and low emittance of the beam and, except for undesired electron energies in the energy filter, without any loss of charge. This reliable and compact control of laser-wakefield accelerated electron beams is independent of the accelerator, itself, allowing immediate application of currently available beams