Femtosecond spectroscopy with vacuum ultraviolet pulse pairs

VUV and XUV reflectance of optically coated mirrors for selection of high harmonics

We report the reflectance, ~1° from normal incidence, of six different mirrors as a function of photon energy, using monochromatic vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation with energies between 7.5 eV and 24.5 eV. The mirrors examined included both single and multilayer optical coatings, as well as an uncoated substrate. We discuss the performance of each mirror, paying particular attention to the potential application of suppression and selection of high-order harmonics of a Ti:sapphire laser.

Time resolved 3D momentum imaging of ultrafast dynamics by coherent VUV-XUV radiation

We present a new experimental setup for measuring ultrafast nuclear and electron dynamics of molecules after photo-excitation and ionization. We combine a high flux femtosecond vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) source with an internally cold molecular beam and a 3D momentum imaging particle spectrometer to measure electrons and ions in coincidence. We describe a variety of tools developed to perform pump-probe studies in the VUV-XUV spectrum and to modify and characterize the photon beam. First benchmark experiments are presented to demonstrate the capabilities of the system.

Chirped seeded free-electron lasers: self-standing light sources for two-color pump-probe experiments

We demonstrate the possibility of running a single-pass free electron laser (FEL) in a dynamical regime, which can be exploited to perform two-color pump-probe experiments in the vacuum ultraviolet or x-ray domain, using the free-electron laser emission both as a pump and as a probe. The studied regime is induced by triggering the free-electron laser process with a powerful laser pulse, carrying a significant and adjustable frequency chirp. As a result, the output FEL radiation is split in two pulses, separated in time (as previously observed by different authors), and having different central wavelengths. We show that both the spectral and temporal distances between FEL pulses can be independently controlled. We also provide a theoretical description of this phenomenon, which is found in good agreement with experiments performed on the FERMI@Elettra free-electron laser.

Resolving vibrational wave-packet dynamics of D2(+) using multicolor probe pulses

We demonstrate the generation and real-time observation of the vibrational wave packet of D(2)(+) by using a sub-10-fs extreme UV high-harmonic pump pulse and a three-color probe laser pulse whose wavelength ranges from near-IR to vacuum UV. This multicolor pump-probe scheme can provide us with a powerful experimental tool for investigating a variety of wave packets evolving with a time scale of ~20 fs.

Power optimization of XUV frequency combs for spectroscopy applications [Invited]

We address technical impediments to the generation of high-photon flux XUV frequency combs through cavity-enhanced high harmonic generation. These difficulties arise from mirror damage, cavity nonlinearity, the intracavity plasma generated during the HHG process, and imperfect phase-matching. By eliminating or minimizing each of these effects we have developed a system capable of generating > 200 μW and delivering ~20 μW of average power for each spectrally separated harmonic (wavelengths ranging from 50 nm - 120 nm), to actual comb-based spectroscopy experiments.

Generation of intense sub-20-fs vacuum ultraviolet pulses compressed by material dispersion

We present our latest results on the generation of ultrashort vacuum UV (VUV) pulses by nonresonant four-wave mixing of chirped broadband pulses generated by filamentation of the fundamental of a Ti:sapphire laser with relatively narrowband pulses at the third harmonic. Positive chirp at the broadband idler yields negatively chirped VUV pulses necessary to compensate for material dispersion of a MgF(2) window in the VUV beam path. Pulse energies exceeding 400 nJ are available for time-resolved experiments. Pulse duration is measured by pump-probe ionization of Xe gas, providing the cross correlation between the fifth harmonic and the fundamental.