M. Jurek

Characteristics of focused soft X-ray free-electron laser beam determined by ablation of organic molecular solids

A linear accelerator based source of coherent radiation, FLASH (Free-electron LASer in Hamburg) provides ultra-intense femtosecond radiation pulses at wavelengths from the extreme ultraviolet (XUV; lambda<100nm) to the soft X-ray (SXR; lambda<30nm) spectral regions. 25-fs pulses of 32-nm FLASH radiation were used to determine the ablation parameters of PMMA - poly (methyl methacrylate). Under these irradiation conditions the attenuation length and ablation threshold were found to be (56.9+/-7.5) nm and approximately 2 mJ*cm(-2), respectively. For a second wavelength of 21.7 nm, the PMMA ablation was utilized to image the transverse intensity distribution within the focused beam at mum resolution by a method developed here.

Spot size characterization of focused non-Gaussian X-ray laser beams

We present a new technique for the characterization of non-Gaussian laser beams which cannot be described by an analytical formula. As a generalization of the beam spot area we apply and refine the definition of so called effective area (A(eff)) [1] in order to avoid using the full-width at half maximum (FWHM) parameter which is inappropriate for non-Gaussian beams. Furthermore, we demonstrate a practical utilization of our technique for a femtosecond soft X-ray free-electron laser. The ablative imprints in poly(methyl methacrylate) - PMMA and amorphous carbon (a-C) are used to characterize the spatial beam profile and to determine the effective area. Two procedures of the effective area determination are presented in this work. An F-scan method, newly developed in this paper, appears to be a good candidate for the spatial beam diagnostics applicable to lasers of various kinds.

Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses

We report the first observation of single-shot soft x-ray laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate)--PMMA. Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7 nm, the samples have been investigated by atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze single-shot imprints in PMMA. An intermediate regime of materials removal has been found, confirming model predictions. We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32 nm. Short-time exposure imprints provide sufficient information about transverse beam profile in HOHs tight focus whereas long-time exposed PMMA exhibits radiation-initiated surface ardening making the beam profile measurement infeasible.

Conductors, semiconductors, and insulators irradiated with short-wavelength free-electron laser

The results of a study of irreversible changes induced at surfaces of metals, semiconductors, and insulators by extreme ultraviolet (λ<100nm) ultrashort pulses provided by TESLA Test Facility Free-Electron Laser, Phase 1 (TTF1 FEL) are reported and discussed. The laser was tuned at 86, 89, and 98nm during the experiments reported here. Energy spectra of ions ejected from the irradiated surfaces are also reported. Special attention is paid to the difference in the ablation behavior of (semi)conductors and insulators that we have observed. The difference is dramatic, while the absorption coefficients are similar for all materials at the TTF1 FEL wavelength.

Ablation of various materials with intense XUV radiation

Ablation behavior of organic polymer (polymethylmethacrylate) and elemental solid (silicon) irradiated by single pulses of XUV radiation emitted from Z-pinch, plasma-focus, and laser-produced plasmas was investigated. The ablation characteristics measured for these plasma-based sources will be compared with those obtained for irradiation of samples with XUV radiation generated by a free-electron laser.

Soft x-ray free electron laser microfocus for exploring matter under extreme conditions

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Experimental station to study the interaction of intense femtosecond vacuum ultraviolet pulses with matter at TTF1 free electron laser

An experimental station to study the interaction of intense femtosecond vacuum ultraviolet pulses generated by the TTF1 free electron laser (FEL) (DESY, Germany) with solids was developed. The vacuum chamber, the sample holder and the detectors had been designed to fulfill strong constraints caused both by the unique properties of the interacting radiation and by TTF1 FEL innovative design. The applied mounting system allowed one to move and rotate the samples precisely with 4degrees of freedom and to heat them up to the maximal temperature of 1000K. In order to accomplish the in situ growth of thin metallic layers on the sample surfaces, evaporation cells had been installed in the vacuum chamber. A time-of-flight apparatus capable of recording both electrons and ions excited on the solid surfaces by the laser pulses had been included in the chamber design. A pulse energy monitor had been placed in the laser beam outside the experimental chamber. A second energy detector had been mounted inside the chambe...

Investigating the interaction of x-ray free electron laser radiation with grating structure.

The interaction of free electron laser pulses with grating structure is investigated using 4.6±0.1 nm radiation at the FLASH facility in Hamburg. For fluences above 63.7±8.7 mJ/cm2, the interaction triggers a damage process starting at the edge of the grating structure as evidenced by optical and atomic force microscopy. Simulations based on solution of the Helmholtz equation demonstrate an enhancement of the electric field intensity distribution at the edge of the grating structure. A procedure is finally deduced to evaluate damage threshold.

X-ray laser-induced ablation of lead compounds

The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first choice. In this contribution, single-shot ablation thresholds are reported for PbWO4 and PbI2 exposed to ultra-short pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm2 is expected according to the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e., 79 mJ/cm2 at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy. Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the radiation is in both materials quite the same. Lower thermal and radiation stability of PbI2 is responsible for this finding.

Ablation of Organic Polymers and Elemental Solids Induced by Intense XUV Radiation

The ablation efficiency of organic polymers (polymethylmethacrylate ‐ PMMA, polytetrafluoroethylene ‐ PTFE, polyethyleneterephtalate ‐ PET, and polyimide ‐ PI) and elemental solids (aluminum and silicon) by single pulses of extreme ultraviolet (XUV) radiation emitted from Z‐pinch, plasma‐focus, and laser‐produced plasmas was investigated. The ablation characteristics measured for these plasma‐based sources will be compared with those obtained for irradiation of samples with XUV radiation generated by a free‐electron laser (FEL). The Z‐pinch was driven by the S‐300 pulsed‐power machine (Kurchatov Institute, Moscow) and the plasma focus was realized in the PF‐1000 machine (Institute of Plasma Physics and Laser Microfusion, Warsaw). Higher temperature plasma than with the discharge plasmas was obtained by focusing the near‐infrared (fundamental frequency) beam from the PALS high‐power iodine laser system (Czech Academy of Sciences, Prague) on the surface of a metallic slab target or into single‐ and double‐g...