Los obstáculos como prolongación de la Dama 26.

The high vacuum attosecond experimental chamber hosts the setup by which isolated at- tosecond laser pulses are generated from high-order harmonic radiation, are delayed with respect to the NIR driving laser pulse and are characterized. Finally they are used in attosecond transient absorption experiments.

At the entrance of the attosecond end station, the two collinear beams pass a variable filter setup that permits separation of the beams into the central, low divergence XUV part and the NIR driving laser pulse as shown in Fig. 3.1 a)-c). The uncoated laser grade pellicles have an effective optical diameter of 30 mm and therefore support the complete NIR laser beam. Small rings made of stainless steel or super invar are glued in the middle of the pellicle and host a free-standing metallic filter (_∼150 nm thickness) to block the NIR radi- ation in the central part of the beam. The filter material is chosen such that it blocks the NIR light, has a high transmission in the spectral range of the generated XUV cut-off and also acts as a high-pass filter for the high-order harmonics. In the conducted experiments zirconium (Zr) has been chosen since its transmittance characteristics match the spectral bandwidth required for the experiment. After the filtering, both beams impinge on a two component delay mirror assembly shown in Fig. 3.1 a). The mirror assembly consists of a core and an outer mirror which has a hole in its center that is marginally larger than the inner mirror to allow for a slight displacement of the inner and outer one along the beam axis. This displacement along the normal of the mirror surface introduces the delay between the XUV and NIR by means of a closed loop controlled piezo stage that allows translation of the core mirror with a resolution better than 2 nm setting the ultimate limit for the smallest possible delay step to the equivalent of_∼14 as. The inner mirror is coated by a multilayer (molybdenum/silicon) structure designed for high reflectivity in the XUV range. Adjusting the relative layer thickness, the sequence of materials and the number of layers, the spectral response as well as the reflectance of the XUV optic can be tuned [99, 38, 100]. Recently even chirped mirrors for soft X-ray radiation have been demon- strated [101]. Appropriate selection of thin foils along with multilayer mirrors can isolate a desired spectral band for experiments and are therefore the prerequisite for the generation of isolated attosecond laser pulses. The outer mirror is coated by silver. Both mirrors are produced out of the same one inch substrate which has a ROC of -250 mm. Both beams get spatially and temporally focused under a narrow angle (<5°) into the interaction vol- ume, such that the focus is located several millimeter to the side of the incoming beams (Fig. 3.1 a). The astigmatism and the waveform distortion resulting from the aberration are small enough not to influence the experiments. For pump-probe experiments a spatial overlap of the XUV and NIR focus is very essential. In order to overlap both foci, the outer mirror features two independently adjustable angles to precisely position the focus of the NIR on top of the focus of the XUV. By removing the pellicle filter, both inner and outer mirror are illuminated with NIR radiation and a subsequent imaging of the two foci by means of an imaging system based on a biconvex lens (f=15 cm) magnifies (factor 3.35) and images both foci onto a beam profiler. Fine-adjusting the outer mirror such that both beam profiles overlap on the CCD camera also ensures spatial overlap of the XUV and

NIR beam in the interaction volume. Due to the shorter wavelength of the XUV radiation, the focus of the XUV pulse is safely smaller than the laser focus. A typical XUV flux attainable in an isolated pulse on target was estimated to yield 5_·108 _{photons per second.}

A single pulse therefore contains on average 105 _{photons [102].}

For the optimization of the XUV source, the direct observation of the direction and spec- tral distribution of the beam is essential. Therefore an imaging spectrometer based on a grazing-incidence flat field grating and a back-illuminated CCD camera are located in the home-built XUV spectrometer after the exit of the experimental chamber. By moving a translation stage - which carries the double-mirror assembly - out of the beam path, the XUV beam is released and leaves the chamber to impinge on one of the two plane gold/platinum coated bending mirrors. By means of a translation stage, the bending mir- rors are moved into the beam, where the one steers the XUV radiation such that it passes by the grating and directly impinges onto CCD chip. The number of counts recorded with the camera serve as a quantity for the daily optimization procedure of the harmonic yield in the cutoff range. An XUV beam profile is displayed in Fig. 3.1 e) which is limited to the spectral range of 60 to 100 eV since a Zr filter is used in front of the XUV camera to prevent saturation and damage of the sensitive CCD chip. The other XUV bending mir- ror redirects the beam under almost gracing incidence onto the flat field corrected grating which disperses the photons onto the CCD camera. Monitoring the spectrum for instance supports the adjustment of the high-order harmonic cut-off. A 500 µm thick zirconium foil in front of the camera avoids damage of the chip.

For attosecond streaking experiments, which characterize the NIR and the attosecond XUV laser pulse, a carbon-coated and grounded glass nozzle (for minimizing charging effects) with a tip diameter of _∼ 20µm (installed perpendicular to the laser beams), ap- proaches the focus very closely from the side. The effusive gas cone is well suitable for streaking experiments since the pressure in the focus close to the tip of the nozzle can be very high without deteriorating the vacuum conditions over an extended region. This ap- proach limits the interaction of the attosecond and laser pulses to within a fraction of the focal volume and minimizes spatial averaging effects that might be related to spatiotem- poral effects (Guoy phase shifts) or non-uniform intensity distributions. In close proximity to the interaction volume, the entrance of an electron time-of-flight (TOF) spectrometer is placed which spectrally disperses launched photoelectrons. The resolution of _∼1 % of the electron kinetic energy is satisfactory for streaking experiments, even though it sets a limit to the potentially observed fine-structure in the recorded streaking spectrograms. The accuracy for determining the central energy of a detected photoelectron distribution or induced asymmetries like the ones associated with the chirp is substantially better than 1 %. In the TOF spectrometer the electrons pass a magnetic shielded drift tube after which they undergo acceleration. The post acceleration ensures a uniform response of the microchannel plate, irrespective of the electrons kinetic energy. In order to enhance the count rate and reduce the necessary integration time for recording an electron TOF spec- trum, an electrostatic lens is incorporated at the entrance of the flight tube. It increases the acceptance angle of electrons within a certain range of kinetic energies and thus en-

hances the count rate up to around 2_·103 _{electron counts per second. A typical electron}

time-of-flight spectrum in neon was shown and analyzed in chapter 2. Streaking measure- ments are generally performed with an intensity of _∼ 1_·1012 W

cm2. Higher field strength

would lead to several complications. First, the electrostatic lens would not cover the very large momentum shift of the photoelectron wave-packet. Second, electrons originating from above-threshold ionization (ATI) would overlap energetically with electrons being released by the XUV photons. In addition to unavoidable space-charge effects, streaking traces obtained under those conditions would loose a substantial amount of their clear structure which originates from the launched electron wave-packet. This would exacerbate the re- trieval of pulse characteristics.

For attosecond transient absorption spectroscopy (ATAS) which is the forefront of this thesis, a second XUV spectrometer enables the observation of the spectrum of attosecond pulses after their transmission through an ensemble of atoms within a confined interaction volume as depicted in Fig. 3.1 b). In this class of experiments, the NIR pulse serves as the pump and the attosecond XUV pulse probes the atomic ensemble and can therefore explore ultrafast transient phenomena of the target atoms which are encoded as delay-dependent absorption features in its spectrum. For good signal to noise ratios, ATAS experiments require, besides even higher laser stabilities, also a gas target which can provide a higher interaction gas density than the one used for attosecond streaking experiments. A quasi static gas target, similar to the one used for high-order harmonic generation, is used instead of a diffusive gas nozzle. The inner diameter of the gas cell which amounts to 1 mm con- fines the interaction volume between the NIR pump and attosecond probe pulse since the gas pressure exponentially decays outside of the nickel tube. Depending on the position of a movable mirror (see Fig. 3.1 b, c) the beam is either redirected into the imaging system or onto a concave (ROC= -340 mm) multilayer broadband coated XUV optic whose reflec- tivity is optimized for an angle of incidence of 67°(reflectivity depicted in Fig. A.5). This concave optic creates a line focus in the plane of the entrance slit of the spectrometer. A suitable thin metal filter (here zirconium with a thickness of 150 nm) placed in front of the entrance slit suppresses a substantial amount of the main part of the co-propagating NIR radiation. During operation of the transient absorption quasi static gas cell, the pressure in the experimental chamber increases up to 3_·10−3 _{mbar. The zirconium filter which}

blocks the NIR radiation in front of the spectrometer further serves as a very essential pressure separator between the experimental chamber and the ultra high vacuum unit of the XUV spectrometer and supports a pressure of 2_·10−6 _{mbar in the separately pumped}

commercially available Rowland-circle based spectrometer. We use a concave grating with 2400 grooves per millimeter. The detection of XUV photons is realized by means of a microchannel plate in combination with a phosphor screen and fiber-coupled Peltier/water cooled CCD camera. Typical XUV spectra recorded with the XUV spectrometer in at- tosecond transient absorption experiments are shown in Fig. 4.1 b). The experimentally achieved resolution amounts to 0.35 eV (FWHM value of Gaussian spectrometer response function). Compared to an XUV sensitive CCD camera, this detection scheme is more superior for ATAS experiments because it inherits a heavily reduced sensitivity to leaking

and scattered parasitic NIR light which can not be completely blocked by the 150 nm thin zirconium filter.

More details about ATAS experiments are provided in the following chapter, especially in section 4.2. In sharp contrast to utilized intensities in streaking experiments, transient absorption experiments generally are aiming at the observation of multiply charged states. This requires preferably the focusing of a larger portion of the beam. In turn, this increases the thermal load on the crucial pellicle filter which leads to rapidly damaged pellicles even after exposure times of a couple of hours. We observe that damaged pellicles develop an almost opaque white area around the metal ring. In an earlier damage state, the beam profile of the NIR radiation changes, which is accompanied by a drop in the peak inten- sity. Since it is very time consuming to prepare pellicle filters, we were working on an expansion of the life time of the pellicle during laser irradiation. An increase of the pellicle thickness from 5 µm to 15 µm and approaching smallest possible ring sizes (do = 3.0 mm,

di = 2.5 mm) made of stainless steel to increase the transmitted NIR light power, helped

to improve the lifetime of those delicate pellicle filter stacks. Developed preparation tools, assisting the filter preparation, reduced preparation times necessary for the pellicle/filter assembly (Fig. 3.1 d). For completeness it shall be mentioned that our recently published technical paper which describes this attosecond beamline AS1 at the Max Planck Institute of Quantum Optics [80] provides in addition to an overview of other performed experiments at this beamline also information about commercial suppliers and item specifications on a more technical perspective which would exceed the scope of this work.