Exploring the multiparameter nature of EUV-visible wave mixing at the FERMI FEL.

The rapid development of extreme ultraviolet (EUV) and x-ray ultrafast coherent light sources such as free electron lasers (FELs) has triggered the extension of wave-mixing techniques to short wavelengths. This class of experiments, based on the interaction of matter with multiple light pulses through the Nth order susceptibility, holds the promise of combining intrinsic ultrafast time resolution and background-free signal detection with nanometer spatial resolution and chemical specificity. A successful approach in this direction has been the combination of the unique characteristics of the seeded FEL FERMI with dedicated four-wave-mixing (FWM) setups, which leads to the demonstration of EUV-based transient grating (TG) spectroscopy. In this perspective paper, we discuss how the TG approach can be extended toward more general FWM spectroscopies by exploring the intrinsic multiparameter nature of nonlinear processes, which derives from the ability of controlling the properties of each field independently.

New design of a multi-jet target for quasi phase matching.

An improved dual-gas quasi-phase matching (QPM) foil target for high harmonic generation (HHG) is presented. The target can be setup with 12 individual gas inlets each feeding multiple nozzles separated by a minimum distance of 10 µm. Three-dimensional gas density profiles of these jets were measured using a Mach-Zehnder Interferometer. These measurements reveal how the jets influence the density of gas in adjacent jets and how this leads to increased local gas densities. The analysis shows that the gas profiles of the jets are well defined up to a distance of about 300 µm from the orifice. This target design offers experimental flexibility, not only for HHG/QPM investigations, but also for a wide range of experiments due to the large number of possible jet configurations. We demonstrate the application to controlled phase tuning in the extreme ultraviolet using a 1 kHz-10 mJ-30 fs-laser system where interference between two jets in the spectral range from 17 to 30 nm was observed.

Thermal properties of borate crystals for high power optical parametric chirped-pulse amplification.

The potential of borate crystals, BBO, LBO and BiBO, for high average power scaling of optical parametric chirped-pulse amplifiers is investigated. Up-to-date measurements of the absorption coefficients at 515 nm and the thermal conductivities are presented. The measured absorption coefficients are a factor of 10-100 lower than reported by the literature for BBO and LBO. For BBO, a large variation of the absorption coefficients was found between crystals from different manufacturers. The linear and nonlinear absorption coefficients at 515 nm as well as thermal conductivities were determined for the first time for BiBO. Further, different crystal cooling methods are presented. In addition, the limits to power scaling of OPCPAs are discussed.

Design considerations for a high power, ultrabroadband optical parametric chirped-pulse amplifier.

A conceptual design of a high power, ultrabroadband optical parametric chirped-pulse amplifier (OPCPA) was carried out comparing nonlinear crystals (LBO and BBO) for 810 nm centered, sub-7.0 fs pulses with energies above 1 mJ. These amplifiers are only possible with a parallel development of kilowatt-level OPCPA-pump amplifiers. It is therefore important to know good strategies to use the available OPCPA-pump energy efficiently. Numerical simulations, including self- and cross-phase modulation, were used to investigate the critical parameters to achieve sufficient spectral and spatial quality. At high output powers, thermal absorption in the nonlinear crystals starts to degrade the output beam quality. Strategies to minimize thermal effects and limits to the maximum average power are discussed.

Long-term stabilization of high power optical parametric chirped-pulse amplifiers.

The long-term stability of optical parametric chirped-pulse amplifiers is hindered by thermal path length drifts affecting the temporal pump-to-signal overlap. A kilowatt-pumped burst amplifier is presented delivering broadband 1.4 mJ pulses with a spectral bandwidth supporting sub-7 fs pulse duration. Active temporal overlap control can be achieved by feedback of optical timing signals from cross-correlation or spectral measurements. Using a balanced optical cross-correlator, we achieve a pump-to-signal synchronization with a residual jitter of only (46 ± 2) fs rms. Additionally, we propose passive pump-to-signal stabilization with an intrinsic jitter of (7.0 ± 0.5) fs rms using white-light continuum generation.

Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation.

Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.

Regulation of cholesterol synthesis by oleic and palmitic acid in keratinocytes.

Cholesterol synthesis is essential for homeostasis of the epidermis, being required for both cell division and differentiation, as well as maintenance of the epidermal permeability barrier. Cholesterol synthesis in keratinocytes has been demonstrated to be regulated by sterol levels and the barrier function of the stratum corneum. Cholesterol synthesis in the epidermis is correlated with changes in mRNA levels for key enzymes, such as HMG-CoA synthase and HMG-CoA reductase, which have been previously demonstrated to be coordinately regulated by the sterol regulatory element binding proteins (SREBPs). In this study we demonstrate that a functional sterol regulatory element is required for sterol regulation of HMG-CoA synthase in keratinocytes. We also investigate the regulation of cholesterol synthesis by fatty acids, which are another important constituent of the stratum corneum lipids. Palmitic and oleic acid inhibit 14C-labelled acetate incorporation into sterols in a similar manner to sterols. However, unlike sterols, 50 microM oleic acid increase the steady state mRNA levels of HMG-CoA synthase and the activity of the HMG-CoA synthase promoter. The addition of 50 microM oleic acid to 25-hydroxycholesterol results in an enhancement of the inhibitory effect of the sterol on promoter activity. The inhibition of acetate incorporation into sterols in human keratinocytes by 50 microM palmitic and 50 microM oleic acid is not due to regulation of HMG-CoA synthase at the level of transcription.

Regulation of HMG-CoA synthase and HMG-CoA reductase by insulin and epidermal growth factor in HaCaT keratinocytes.

Synthesis of cholesterol, via the isoprenoid/mevalonate pathway, is required for keratinocyte growth and differentiation, and maintenance of the stratum corneum lipid lamellae. 3-hydroxy-3-methylglutaryl coenzyme A synthase catalyzes the first step in isoprenoid/mevalonate synthesis and under some conditions controls the flux into the pathway. We have investigated whether selected growth factors and hormones could increase 3-hydroxy-3-methylglutaryl coenzyme A synthase mRNA in keratinocytes. Northern blotting was used to demonstrate that 10 microg per ml insulin and 0.1 microg per ml epidermal growth factor both increased steady-state levels of 3-hydroxy-3-methylglutaryl coenzyme A synthase mRNA by 2.5 and 6-fold, respectively. Epidermal growth factor and insulin also increased 3-hydroxy-3-methylglutaryl coenzyme A reductase enzyme activity. 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter activity in a luciferase reporter construct was increased 2-fold by insulin and 2.9-fold by epidermal growth factor. When a mutation in the sterol regulatory element was introduced into the 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter, activity was not increased by insulin, but was increased by epidermal growth factor. Mutation of an AP-1 site in the 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter did not affect the increase in activity following treatment with insulin or epidermal growth factor. Therefore, 3-hydroxy-3-methylglutaryl coenzyme A synthase expression in keratinocytes is regulated by insulin and epidermal growth factor by different mechanisms. These results suggest a role for hormones and growth factors in the control of epidermal cholesterol synthesis.