P. Hough

Time-resolved pump-probe experiments beyond the jitter limitations at FLASH

Using a noninvasive, electro-optically based electron bunch arrival time measurement at FLASH (free electron laser in Hamburg) the temporal resolution of two-color pump-probe experiments has been significantly improved. The system determines the relative arrival time of the extended ultraviolet pulse of FLASH and an amplified Ti:sapphire femtosecond-laser pulse at the interaction region better than 90 fs rms. In a benchmarking pump-probe experiment using two-color above threshold ionization of noble gases, an enhancement in the timing resolution by a factor of 4 compared to the uncorrected data is obtained.

Emission characteristics and dynamics of the stagnation layer in colliding laser produced plasmas

The expansion dynamics of ion and neutral species in laterally colliding laser produced aluminum plasmas have been investigated using time and space resolved optical emission spectroscopies and spectrally and angularly resolved fast imaging. The emission results highlight a difference in neutral atom and ion distributions in the stagnation layer where, at a time delay of 80 ns, the neutral atoms are localized in the vicinity of the target surface (<1 mm from the target surface) while singly and doubly charged ions lie predominantly at larger distances, <1.5 and <2 mm, respectively. The imaging results show that the ions were found to form a well defined, but compressed, stagnation layer at the collision front between the two seed plasmas at early times (Δt<80 ns). On the other hand, the excited neutrals were observed to form a V-shaped emission feature at the outer regions of the collision front with enhanced neutral emission in the less dense, cooler regions of the stagnation layer.

Electron and ion stagnation at the collision front between two laser produced plasmas

We report results from a combined optical interferometric and spectrally resolved imaging study on colliding laser produced aluminium plasmas. A Nomarski interferometer was used to probe the spatio-temporal distribution of electron densities at the collision front. Analysis of the resulting interferograms reveals the formation and evolution of a localized electron density feature with a well-defined profile reminiscent of a stagnation layer. Electron stagnation begins at a time delay of 10 ns after the peak of the plasma generating laser pulse. The peak electron density was found to exceed 1019 cm−3 and the layer remained well defined up to a time delay of ca 100 ns. Temporally and spectrally resolved optical imaging was also undertaken, to compare the Al+ ion distribution with that of the 2D electron density profile. This revealed nascent stagnation of singly charged ions at a delay time of 20 ns. We attribute these results to the effects of space charge separation in the seed plasma plumes.

Atomic photoionization in combined intense XUV free-electron and infrared laser fields

We present a systematic study of the photoionization of noble gas atoms exposed simultaneously to ultrashort (20 fs) monochromatic (1–2% spectral width) extreme ultraviolet (XUV) radiation from the Free-electron Laser in Hamburg (FLASH) and to intense synchronized near-infrared (NIR) laser pulses with intensities up to about 1013 W cm−2. Already at modest intensities of the NIR dressing field, the XUV-induced photoionization lines are split into a sequence of peaks due to the emission or absorption of several additional infrared photons. We observed a plateau-shaped envelope of the resulting sequence of sidebands that broadens with increasing intensity of the NIR dressing field. All individual lines of the nonlinear two-color ionization process are Stark-shifted, reflecting the effective intensity of the NIR field. The intensity-dependent cut-off energies of the sideband plateau are in good agreement with a classical model. The detailed structure of the two-color spectra, including the formation of individual sidebands, the Stark shifts and the contributions beyond the classical cut-off, however, requires a fully quantum mechanical description, as is demonstrated with time-dependent quantum calculations in single-active electron approximation.

Growth and field emission properties of ZnO nanostructures deposited by a novel pulsed laser ablation source on silicon substrates.

Zinc oxide (ZnO) nanostructures were produced using a novel pulsed laser ablation apparatus comprising in-situ analysis of the plume by reflection time-of-flight mass spectrometry. Various morphologies of nano and microstructures were obtained for laser wavelengths of 1064 and 355nm, and oxygen ambient pressures of 10(-6) and 10(-2)mbar, respectively. None of the produced structures exhibited a particular type of self-organisation whereas all of them showed low aspect ratios and good field emission properties. Optimum values of 5.2Vmicrom(-1) and 2060 were obtained for the turn-on field and Fowler-Nordheim enhancement factor, respectively, for deposited nano-tipped microstructures presenting a high coverage of the substrate. The experimental data showed that for a given laser wavelength, higher field enhancement factors were obtained for the samples grown at the lower pressure of 10(-6)mbar. In these conditions, the deposited materials showed distinct nanostructuring and comparison with existing data showed the corresponding ablation plumes to contain (ZnO)(n) clusters, up to n=13. This work also shows that the electronic properties of the nanostructured ZnO produced in our conditions, as determined by the oxygen concentration during deposition, have an influence on the field emission properties in addition to the nanostructure morphology.

The Laser-assisted photoelectric effect of He, Ne, Ar and Xe in intense extreme ultraviolet and infrared laser fields

In this paper, we report results on two-colour above-threshold ionisation, where extreme ultraviolet pulses of femtosecond duration were synchronised to intense infrared laser pulses of picosecond duration, in order to study the laser-assisted photoelectric effect of atomic helium, neon, krypton and xenon which leads to the appearance of characteristic sidebands in the photoelectron spectra. The observed trends are found to be well described by a simple model based on the soft-photon approximation, at least for the relatively low optical intensities of up to employed in these early experiments.

Colliding laser produced plasmas as novel sources: Optical diagnostics

We have developed a new laboratory facility to investigate and explore the potential applications of colliding laser produced plasmas. Specifically we have employed optical diagnostics such as laser interferometry, spectrally resolved fast photography and optical emission spectroscopy to investigate the dynamics of the collisions between electrons, atoms and ions. Very different collisional behaviors are observed for the different plasma constituents. We present first results on work aimed at exploiting the stagnation layer created at the collision front between the two plasmas as a new optimised source of nano-particles/clusters. A substantial increase in nano-particle deposition is observed with the presence of the stagnation layer.

Atomic photoionization in weak and strong two-color radiation fields

Two-color photoionization processes in rare gases have been studied using the combination of XUV pulses from the Free Electron Laser in Hamburg (FLASH) and intense femtosecond pulses from an external synchronized near infrared laser. In the low field regime of the NIR dressing laser ( 1111 W/cm2), multi-photon processes are dominant and theoretical descriptions beyond time-dependent second order perturbation formalism have to be applied.