Wave propagation and focusing of soft X-rays by spherical bent microchannel plates.

Synchrotron radiation sources have been used to study the focusing properties and angular distribution of X-ray radiation at the exit of spherically bent microchannel plates (MCPs). In this contribution it is shown how soft X-ray radiation at energies up to 1.5 keV can be focused by spherically bent MCPs with curvature radii R of 30 mm and 50 mm. For these devices, a focus spot is detectable at a distance between the detector and the MCP of less than R/2, with a maximum focusing efficiency up to 23% of the flux illuminating the MCP. The soft X-ray radiation collected at the exit of microchannels of spherically bent MCPs are analyzed in the framework of a wave approximation. A theoretical model for the wave propagation of radiation through MCPs has been successfully introduced to explain the experimental results. Experimental data and simulations of propagating radiation represent a clear confirmation of the wave channeling phenomenon for the radiation in spherically bent MCPs.

Broadband optical ultrafast reflectivity of Si, Ge and GaAs.

Ultrafast optical reflectivity measurements of silicon, germanium, and gallium arsenide have been carried out using an advanced set-up providing intense subpicosecond pulses (35 fs FWHM, [Formula: see text] = 400 nm) as a pump and broadband 340-780 nm ultrafast pulses as a white supercontinuum probe. Measurements have been performed for selected pump fluence conditions below the damage thresholds, that were carefully characterized. The obtained fluence damage thresholds are 30, 20.8, 9.6 mJ/[Formula: see text] for Si, Ge and GaAs respectively. Ultrafast reflectivity patterns show clear differences in the Si, Ge, and GaAs trends both for the wavelength and time dependences. Important changes were observed near the wavelength regions corresponding to the [Formula: see text], [Formula: see text] singularities in the joint density of states, so related to the peculiar band structure of the three systems. For Ge, ultrafast reflectivity spectra were also collected at low temperature (down to 80 K) showing a shift of the characteristic doublet peak around 2.23 eV and a reduction of the recovery times.

A cryogenic magneto-optical device for long wavelength radiation.

We present here a small-scale liquid helium immersion cryostat with an innovative optical setup suitable to work in long wavelength radiation ranges and under an applied magnetic field. The cryostat is a multi-stage device with several shielding in addition to several optical stages. The system has been designed with an external liquid nitrogen boiler to reduce liquid bubbling. The optical and mechanical properties of the optical elements were calculated and optimized for the designed configuration, while the optical layout has been simulated and optimized among different configurations based on the geometry of the device. The final design has been optimized for low-noise radiation measurements of proximity junction arrays under an applied magnetic field in the wavelength range λ = 250 µm-2500 µm.

Electrical Contacts on Silicon Nanowires Produced by Metal-Assisted Etching: a Comparative Approach.

Silicon nanowires fabricated by metal-assisted chemical etching can present low porosity and a rough surface depending on the doping level of the original silicon wafer. In this case, wiring of silicon nanowires may represent a challenging task. We investigated two different approaches to realize the electrical contacts in order to enable electrical measurement on a rough silicon nanowire device: we compared FIB-assisted platinum deposition for the fabrication of electrical contact with EBL technique.

Effect of carrier tunneling on the structure of Si nanowires fabricated by metal assisted etching.

The metal assisted etching mechanism for Si nanowire fabrication, triggered by doping type and level and coupled with choice of metal catalyst, is still very poorly understood. We explain the different etching rates and porosities of wires we observe based on extensive experimental data, using a new empirical model we have developed. We establish as a key parameter, the tunneling through the space charge region (SCR) which is the result of the reduction of the SCR width by level of the Si wafer doping in the presence of the opposite biases of the p- and n-type wafers. This improved understanding should permit the fabrication of high quality wires with predesigned structural characteristics, which hitherto has not been possible.

Diffusion induced effects on geometry of Ge nanowires.

We report diffusion induced germanium nanowire growth and its dependence on the Ge evaporation flux. The wires show a growth rate (dL/dt) in agreement with the previously reported models, but detection of anomalies in the grown wires may indicate the prevalence of the direct Ge impinging effect on large diameter wires. Additionally, we demonstrate that change in deposition flux could directly affect the diffusion length of the Ge adatoms on the wire sidewalls. This in turn modifies the geometry of the grown wires by introducing a lateral growth starting from the base of the wire. A detailed understanding of the deposition flux effect on the growth and geometry of wires will result in improved knowledge of physical properties of wires.