Theophilos Maltezopoulos

Single-shot timing measurement of extreme-ultraviolet free-electron laser pulses

Arrival time fluctuations of extreme-ultraviolet (EUV) pulses from the free-electron laser in Hamburg (FLASH) are measured single-pulse resolved at the experimental end-station. To this end, they are non-collinearly superimposed in space and time with visible femtosecond laser pulses on a GaAs substrate. The EUV irradiation induces changes of the reflectivity for the visible pulse. The temporal delay between the two light pulses is directly encoded in the spatial position of the reflectivity change which is captured with a CCD camera. For each single shot, the relative EUV/visible arrival-time can be measured with about 40 fs rms accuracy. The method constitutes a novel route for an improvement of future pump–probe experiments at short-wavelength free-electron lasers (FELs) by a pulse-wise correction with simultaneously measured arrival times of individual EUV pulses.

Direct observation of confined states in metallic single-walled carbon nanotubes

We investigated the local density of states (LDOS) of extended individual metallic single-walled carbon nanotubes using low-temperature scanning tunneling spectroscopy. We observed that the LDOS oscillates with energy close to the Fermi level. The oscillation period of about 50 meV varies with position on the nanotube. Maps of the LDOS reveal that the peaks in the oscillation are related to confined states. The widths of the peaks increase with increasing distance from the Fermi level.

Time-resolved ion spectrometry on xenon with the jitter-compensated soft x-ray pulses of a free-electron laser

Atomic inner-shell relaxation dynamics were measured at the free-electron laser in Hamburg, FLASH, delivering 92 eV pulses. The decay of 4d core holes created in xenon was followed by detection of ion charge states after illumination with delayed 400 nm laser pulses. A timing jitter of the order of several hundred femtoseconds between laser- and accelerator-pulses was compensated for by a simultaneous delay measurement in a single-shot x-ray/laser cross-correlator. After sorting of the tagged spectra according to the measured delays, a temporal resolution equivalent to the pulse duration of the optical laser could be established. While results on ion charge states up to Xe4+ are compatible with a previous study using a high-harmonic soft x-ray source, a new relaxation pathway is opened by the nonlinear excitation of xenon atoms in the intense free-electron laser light field, leading to the formation of Xe5+.

Ultrafast photofragmentation dynamics of molecular iodine driven with timed XUV and near-infrared light pulses.

Photofragmentation dynamics of molecular iodine was studied as a response to the joint illumination with femtosecond 800 nm near-infrared and 13 nm extreme ultraviolet (XUV) pulses delivered by the free-electron laser facility FLASH. The interaction of the molecular target with two light pulses of different wavelengths but comparable pulse energy elucidates a complex intertwined electronic and nuclear dynamics. To follow distinct pathways out of a multitude of reaction channels, the recoil of created ionic fragments is analyzed. The delayed XUV pulse provides a way of following molecular photodissociation of I(2) with a characteristic time-constant of (55 ± 10) fs after the laser-induced formation of antibonding states. A preceding XUV pulse, on the other hand, preferably creates a 4d(-1) inner-shell vacancy followed by the fast Auger cascade with a revealed characteristic time constant τ(A2)=(23±11) fs for the second Auger decay transition. Some fraction of molecular cationic states undergoes subsequent Coulomb explosion, and the evolution of the launched molecular wave packet on the repulsive Coulomb potential was accessed by the laser-induced postionization. A further unexpected photofragmentation channel, which relies on the collective action of XUV and laser fields, is attributed to a laser-promoted charge transfer transition in the exploding molecule.

Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide

Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.

Time-diagnostics for improved dynamics experiments at XUV FELs

Significantly structured and fluctuating temporal profiles of pulses from self-amplified spontaneous emission free electron lasers as well as their unstable timing require time diagnostics on a single-shot basis. The duration and structure of extreme-ultraviolet (XUV) pulses from the Free Electron Laser (FEL) in Hamburg (FLASH) are becoming accessible using a variation of the streak camera principle, where photoemitted electrons are energetically streaked in the electric field component of a terahertz electromagnetic wave. The timing with respect to an independently generated laser pulse can be measured in an XUV/laser cross-correlator, based on a non-collinear superposition of both pulses on a solid state surface and detection of XUV-induced modulations of its reflectivity for visible light. Sorting of data according to the measured timing dramatically improves the temporal resolution of an experiment sampling the relaxation of transient electronic states in xenon after linear- as well as nonlinear excitation with intense XUV pulses from FLASH.

Mapping few-femtosecond slices of ultra-relativistic electron bunches

Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds.

Free-electron laser multiplex driven by a superconducting linear accelerator.

Free-electron lasers (FELs) generate femtosecond XUV and X-ray pulses at peak powers in the gigawatt range. The FEL user facility FLASH at DESY (Hamburg, Germany) is driven by a superconducting linear accelerator with up to 8000 pulses per second. Since 2014, two parallel undulator beamlines, FLASH1 and FLASH2, have been in operation. In addition to the main undulator, the FLASH1 beamline is equipped with an undulator section, sFLASH, dedicated to research and development of fully coherent extreme ultraviolet photon pulses using external seed lasers. In this contribution, the first simultaneous lasing of the three FELs at 13.4 nm, 20 nm and 38.8 nm is presented.