Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets.

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions

All-optical spatio-temporal control of electron emission from SiO2 nanospheres with femtosecond two-color laser fields

Field localization by nanostructures illuminated with laser pulses of well-defined waveform enables spatio-temporal tailoring of the near-fields for sub-cycle control of electron dynamics at the nanoscale. Here, we apply intense linearly-polarized two-color laser pulses for all-optical control of the highest energy electron emission from SiO2 nanoparticles. For the size regime where light propagation effects become important, we demonstrate the possibility to control the preferential emission angle of a considerable fraction of the fastest electrons by varying the relative phase of the two-color field. Trajectory based semi-classical simulations show that for the investigated nanoparticle size range the directional steering can be attributed to the two-color effect on the electron trajectories, while the accompanied modification of the spatial distribution of the ionization rate on the nanoparticle surface has only a minor effect. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaf

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions

Static Polarizabilities of Charged Silver Metal Clusters Extracted from the Optical Spectra

The use of sum rules appears to be a powerful tool for obtaining experimental data on static properties of charged metal clusters which are currently not available. For this purpose the optical spectra of the clusters must be known. The measured absorption profiles of sputtered, i.e. hot, and which have been measured in our group give information about the desired properties. The extracted static polarizabilities show a distinct shell structure which reflects the overall lectronic shape of the cluster. A comparison is made with AgN and positive alkali metal clusters. The polarizabilities of closed-shell and are in reasonable agreement with SIC-LDA corrected RPA polarizabilities

Bound-Free Collective Electron Excitation in Negatively Charged Metal Clusters

We report measurements of the photodestruction cross sections of small sputtered negatively charged silver clusters (AgN−, 5 ≤ N ≤ 19). Resonance-like peaks are observed in the optical spectra which can be attributed to collective electron excitations. In contrast to neutral and positively charged clusters, the resonance energies are far above the electron emission threshold. With decreasing cluster size the resonance positions distinctly shift towards lower energies. This red-shift is explained by an extension of a semi-empirical model by Liebsch which we previously used to interpret the blue-shift in AgN+ [J. Tiggesbäumker, L. Köller, K.H. Meiwes-Broer and A. Liebsch, Phys. Rev. A 48 (1993) R1749], now including the polarizabilities of small metal clusters

Bound-Free Collective Electron Excitation in Negatively Charged Metal Clusters

We report measurements of the photodestruction cross sections of small sputtered negatively charged silver clusters (AgN−, 5 ≤ N ≤ 19). Resonance-like peaks are observed in the optical spectra which can be attributed to collective electron excitations. In contrast to neutral and positively charged clusters, the resonance energies are far above the electron emission threshold. With decreasing cluster size the resonance positions distinctly shift towards lower energies. This red-shift is explained by an extension of a semi-empirical model by Liebsch which we previously used to interpret the blue-shift in AgN+ [J. Tiggesbäumker, L. Köller, K.H. Meiwes-Broer and A. Liebsch, Phys. Rev. A 48 (1993) R1749], now including the polarizabilities of small metal clusters

Blue Shift of the Mie Plasma Frequency in Ag Clusters and Particles

Photodepletion spectroscopy is used to measure the optical activity of sputtered Ag ionic clusters with up to 70 atoms. With decreasing cluster size the giant resonance caused by collective excitation of the valence electrons shifts to higher frequencies. This blue shift is qualitatively explained in terms of a reduced s-d screening interaction in the surface region of the particles