J. Seres

Laser technology: source of coherent kiloelectronvolt X-rays.

Generating X-rays that have the properties of laser light has been a long-standing goal for experimental science. Here we describe the emission of highly collimated, spatially coherent X-rays, at a wavelength of about 1 nanometre and at photon energies extending to 1.3 kiloelectronvolts, from atoms that have been ionized by a 5-femtosecond laser pulse. This means that a laboratory source of laser-like, kiloelectronvolt X-rays, which will operate on timescales relevant to many chemical, biological and materials problems, is now within reach.

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.

Generation of coherent keV x-rays with intense femtosecond laser pulses

The realization of a compact laboratory x-ray source delivering spatially and temporally coherent ultrashort pulses is of great interest for time-resolved x-ray spectroscopy. Here, we describe the design and the parameters of a short wavelength source based on high-harmonic generation (HHG) delivering radiation up to 1.3 keV. The extension of the cutoff is attributed to intense few-cycle driving laser pulses delivered via a tabletop near-infrared laser system.

Generation of intense 8 fs laser pulses

Sub-25 fs pulses from a multipass amplifier system have been spectrally broadened in a hollow fiber up to 250 nm. Using a combination of a prism compressor and an improved acousto-optic programmable dispersive filter (AOPDF), we were able to compress the pulses close to their transform limit. Under optimized conditions we achieved pulses with a duration of 8 fs and a peak power up to 9 GW.

High-harmonic generation and parametric amplification in the soft X-rays from extended electron trajectories

We report, for the first time, the generation of high-order harmonics in a spectral range between 200 eV and 1 keV with an unusual spectral property: only every 4th (4i + 1, i∈ℵ) harmonic line appears, whereas the usual high-harmonic spectra consist of every odd (2i + 1) harmonic. We attribute this unique property to the quantum path interference of two extended electron trajectories that experience multiple re-scattering. In the well-established theory, electrons emitted via tunnel ionisation are accelerated by a laser field, return to the ion and recombine. The acceleration typically lasts for less than one optical cycle, and the electrons radiate in the extreme ultraviolet range at recombination. In contrast, for extended trajectories, electrons are accelerated over two or more optical cycles. Here, we demonstrate that two sets of trajectories dominate and provide substantial contributions to the generated soft X-ray radiation because they fulfil the resonance condition for X-ray parametric amplification.

Parametric amplification of attosecond pulse trains at 11 nm

We report the first experimental demonstration of the parametric amplification of attosecond pulse trains at around 11 nm. The helium amplifier is driven by intense laser pulses and seeded by high-order harmonics pulses generated in a neon gas jet. Our measurements suggest that amplification takes place only if the seed pulse-trains are perfectly synchronized in time with the driving laser field in the amplifier. Varying the delay, we estimate the durations of the individual extreme ultraviolet pulses within the train to be on the order of 0.2 fs. Our results demonstrate that strong-field parametric amplification can be a suitable tool to amplify weak attosecond pulses from non-destructive pump-probe experiments and it is an important step towards designing amplifiers for realization of energetic XUV pulses with sub-femtosecond duration using compact lasers fitting in university laboratories.

Coherent amplification of attosecond light pulses in the water-window spectral region

We present a theoretical study on coherent extreme ultraviolet (XUV) attosecond pulse amplification mediated by nonlinear parametric enhanced forward scattering occurring in the interaction of a strong femtosecond infrared (IR) laser pulse combined with a weak attosecond XUV pulse train with an atom. We predict large amplification of XUV radiation when the IR strong pulse and the XUV weak pulse are optimally phased. We study high-order harmonic processes (HHG) in He, He(+) and Ne(++), and show how although the HHG yield is largely affected by the particular atom used as target, nonlinear parametric XUV amplification is only weakly affected. We conclude that XUV nonlinear parametric attosecond pulse amplification can be most efficiently observed by using atoms with a high ionization potential and that the nonlinear amplification is robust at high photon energies where HHG is not efficient, such as in the water-window spectral region.

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.