Enikoe Seres

X-ray absorption spectroscopy in the keV range with laser generated high harmonic radiation

By irradiating He and Ne atoms with 3mJ, 12fs, near infrared laser pulses from a tabletop laser system, the authors generated spatially and temporally coherent x rays up to a photon energy of 3.5keV. With this source it is possible to use high-harmonic radiation for x-ray absorption spectroscopy in the keV range. They were able to clearly resolve the L absorption edges of titanium and copper and the K edges of aluminum and silicon. From the fine structure of the x-ray absorption they estimated the interatomic distances.

Ultrafast soft x-ray absorption spectroscopy with sub-20-fs resolution

The most challenging application of time-resolved spectroscopy is to watch the motion of atoms directly. In the current experiment, we were able to follow the displacement of atoms in silicon after excitation with an intense near infrared femtosecond laser pulse with time-resolved x-ray absorption spectroscopy. The sub-20-fs x-ray probe pulses in an energy range up to several 100eV were generated by high harmonic generation of intense femtosecond laser pulses at a repetition rate of 1kHz. In a proof of principle experiment, we found an oscillatory motion of the atoms which can be explained by the excitation of coherent phonons. Our setup can be easily extended for studying structural dynamics in many important systems not only in solids but also in gases and liquids.

Monitoring the He + ion channel formation by high-order harmonic generation

The macroscopic build-up of the high-order harmonic signal depends on the free electron density in the generation medium. The free electrons affect the harmonic yield and spectral shape through modifying the refractive index and the phase matching conditions. These dependences allow studying the He(+) ion channel formation in a He gas jet. The evolution of an ion channel created by an ultrashort laser pulse via optical field ionization was monitored using the harmonic signal generated by a collinear propagating second laser pulse. From the measured high harmonic signal as function of the delay we are able to gain information about the free electron density. Under our experimental condition, the ion channel has been fully formed 300 fs after the first laser pulse, resulting in an enhancement of harmonic yield of the second laser pulse by two orders of magnitude.

Avalanche of stimulated forward scattering in high harmonic generation.

Optical amplifiers in all ranges of the electromagnetic spectrum exhibit an essential characteristic, namely the input signal during the propagation in the amplifier medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth. We perform a theoretical study motivated and supported by experimental data on a He gas amplifier driven by intense 30-fs-long laser pulses and seeded with attosecond pulse trains generated in a separated Ne gas jet. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of the avalanche effect in the amplification of extreme ultraviolet attosecond pulse trains. We theoretically separate and identify different physical processes taking part in the interaction and we demonstrate that X-ray parametric amplification dominates over others. In particular, we identify strong-field mediated intrapulse X-ray parametric processes as decisive for amplification at the single-atom level. We confirm that the amplification takes place at photon energies where the amplifier is seeded and when the seed pulses are perfectly synchronized with the driving strong field in the amplifier. Furthermore, propagation effects, phase matching and seed synchronization can be exploited to tune the amplified spectral range within the seed bandwidth.

Attosecond sublevel beating and nonlinear dressing on the 3d-to-5p and 3p-to-5s core-transitions at 91.3 eV and 210.4 eV in krypton

Applying extreme ultraviolet (XUV) transient absorption spectroscopy, the dynamics of the two laser dressed transitions 3d5/2-to-5p3/2 and 3p3/2-to-5s1/2 at photon energies of 91.3 eV and 210.4 eV were examined with attosecond temporal resolution. The dressing process was modeled with density matrix equations which are found to describe very accurately both the experimentally observed transmission dynamics and the linear and nonlinear dressing oscillations at 0.75 PHz and 1.5 PHz frequencies. Furthermore, using Fourier transform XUV spectroscopy, quantum beats from the 3d5/2-3d3/2 and 3p3/2-3p1/2 sublevels at 0.3 PHz and 2.0 PHz were experimentally identified and resolved.

Non-planar femtosecond enhancement cavity for VUV frequency comb applications

External passive femtosecond enhancement cavities (fsECs) are widely used to increase the efficiency of non-linear conversion processes like high harmonic generation (HHG) at high repetition rates. Their performance is often limited by elliptical beam profiles, caused by oblique incidence on spherical focusing mirrors. We introduce a novel three-dimensionally folded variant of the typical planar bow-tie resonator geometry that guarantees circular beam profiles, maintains linear polarization, and allows for a significantly tighter focus as well as a larger beam cross-section on the cavity mirrors. The scheme is applied to improve focusing in a Ti:Sapphire based VUV frequency comb system, targeting the 5th harmonic around 160 nm (7.8 eV) towards high-precision spectroscopy of the low-energy isomer state of Thorium-229. It will also be beneficial in fsEC-applications with even higher seeding and intracavity power where the damage threshold of the mirrors becomes a major concern.

High harmonics from a radio-frequency pre-excited medium

We report preliminary results of the enhancement and generation of 6-8th harmonics using radio-frequency discharge pre-ionization in Xe gas for the realization of a VUV frequency comb.

Sub-20-fs Optical Pump-X-ray Probe Spectroscopy beyond the Si K Edge

Time resolved x-ray absorption spectroscopy is a powerful tool to follow the fast atomic, molecular changes on the atomic time and spatial scale. Here we demonstrate timeresolved XANES and EXAFS relying on a laser driven ultrafast XUV source. We have examined the electronic and structural changes in amorphous Si with a temporal resolution of 20 fs.

Ultrafast Soft X-Ray Absorption Spectroscopy

With a solid-state-laser driven soft-X-ray source we investigated the structural dynamic of a-Si, following the variation of the conduction and valence band state density, the inter-atomic distance with a temporal resolution of less than 20fs.