At the Lawrence Livermore National Laboratory (LLNL) we have engineered a silicon prototype sample that can be used to reflect focused hard x-ray photons at high intensities in back-scattering geometry.1 Our work is motivated by the need for an all-x-ray pump-and-probe capability at X-ray Free Electron Lasers (XFELs) such as the Linac Coherent Light Source (LCSL) at SLAC. In the first phase of our project, we exposed silicon single crystal to the LCLS beam, and quantitatively studied the x-ray induced damage as a function of x-ray fluence. The damage we observed is extensive at fluences typical of pump-and-probe experiments. The conclusions drawn from our data allowed us to design and manufacture a silicon mirror that can limit the local damage, and reflect the incident beam before its single crystal structure is destroyed. In the second phase of this project we tested this prototype back-reflector at the LCLS. Preliminary results suggest that the new mirror geometry yields reproducible Bragg reflectivity at high x-ray fluences, promising a path forward for silicon single crystals as x-ray back-reflectors.
The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing
incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot,
respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at
the surface of various solids (Si, Al, Ti, Ta, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate)
- PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, respectively. The optical emission of the
plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer
MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms
were observed at intensities around 1013 W/cm2. No lines of atomic ions have been identified in UV-vis spectra emitted
from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 1016 W/cm2, the OE
spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The
abundance ratio of Al/Al+ should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also
registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG
crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).
Femtosecond time-resolved small and wide-angle x-ray diffuse scattering techniques are applied to investigate the
ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense
optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is
observed and the build-up of these fluctuations is measured in real-time. Small-angle scattering measurements reveal
the first steps in the nucleation of nanoscale voids below the surface of the semiconductor and support MD simulations
of the ablation process.
The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements demonstrate that the initial stage of crystal disordering results from inertial motion on a laser softened potential energy surface. These inertial dynamics dominate for the first half picosecond following laser excitation, indicating that inter-atomic forces minimally influence atomic excursions from the equilibrium lattice positions, even for motions in excess of an Å. This also indicates that the atoms disorder initially without losing memory of their lattice reference.
In order to investigate the ultrafast dynamics of free carriers generated in bulk dielectrics by intense femtosecond laser pulses we have designed a setup for ultrafast time-resolved imaging Mach-Zehnder interferometry. The application of the 2D-Fourier-transform technique allows us to accurately reconstruct the actual laser-induced phase shifts and transmission changes for the probe pulses, which provide the properties of free carriers. Interferometric measurements in high-purity fused silica clearly demonstrate that the dominant ionization mechanism for intensities below 10 TW/cm2 is multiphoton ionization.
Ultrashort laser pulse interaction with material involves a number of specialities as compared to longer irradiations. Applying femtosecond laser pulses, the fundamental physical processes such as excitation, melting and ablation are temporally separated, allowing a separate investigation of each of them. The irradiated material passes through highly non-equilibrium states of different kinds on different timescales after irradiation. Thus, the theoretical description of the investigated processes may differ strongly from the classical descriptions valid for equilibrium or steady-state conditions. On a femtosecond timescale we investigate the non-equilibrium of the laser-excited electron gas. With the help of a detailed microscopic approach we study the applicability of simplified macroscopic descriptions of laser absorption and free-electron excitation. We study different melting processes occurring on different timescales in the picosecond regime. The nature of the melting process depends on the laser and material parameters, respectively. Material removal, i.e. ablation, occurs on a pico- to nanosecond time scale, depending on excitation strength. We show theoretical and experimental investigations of the expansion dynamics of the excited material.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).
The formation of well-defined craters is a general feature of laser ablation with ultrashort laser pulses, indicative of a sharp ablation threshold. Results of a microscopic characterization of ablation craters on semiconductors after irradiation with single intense ultrashort laser pulses are presented.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern. Optically sharp surface occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials.
Using ultrafast x-ray diffraction from a laser-plasma x-ray source, we have observed coherent photon generation and propagation in bulk(111)-GaAs, (111)-Ge, and thin(111)-Ge- on-Si films. At higher optical pump fluences, ultrafast melting of Ge films is observed.
Removal of material from the surface of metals and semiconductors following irradiation with pico- or femtosecond laser pulses is a thermal process involving states of matter having unusually thermodynamic, hydrodynamic and optical properties.
Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 1013 W/cm2 with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the laser generated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.
We have investigated femtosecond laser-induced ablation of gallium arsenide and silicon using time-of-flight mass spectroscopy. Below the ablation threshold we observe free flight desorption of atoms from the laster heated surface. The absence of collisions between particles leaving the solid allows to obtain the maximum surface temperature during laser irradiation of Gallium Arsenide. We estimated maximum surface temperatures of the order of 3500 K at the ablation threshold, where we observed a step-like increase in the number of detected particles. In the case of Silicon the existence of molecules of up to 6 atoms does not allow to measure the surface temperature. With increasing fluence free flight desorption transforms into a collisional expansion process. The behavior of Gallium particles can be quantitatively described through Knudsen-layer theory, indicating that Gallium particles expand as a non-ideal gas close to the ablation threshold ((gamma) equals Cv/Cp less than 5/3). Above fluences of approximately 2.5 Fth (gamma) approaches 5/3 indicating an ideal gas behavior for the expanding material. Dilution into the two phase regime of a superheated liquid characterizes ablation close to threshold.