The superposition of selected higher order harmonics and continuous background, produced in Ar gas through two time delayed 60 fs pulses at 800 nm, is measured as a function of their delay time. An isolated less than 100 as sharp feature, indicative for pulse production in the attosecond regime, followed by a subfemtosecond beating, indicative for the production of trains of sub-fs XUV pulses, is clearly observable in the resulting temporal trace. The trace Fourier transforms to a frequency spectrum composed of harmonics from the selected ensemble and a continuous background that correlates with the observed ultrashort feature. Numerical simulation spectra encompass similar temporal localization.

We present experimental results and theoretical analysis of two-photon ionization of He by a superposition of the 7th to the 13th harmonic of a Ti:sapphire laser. Solving the time-dependent Schrödinger equation for He in a coherent polychromatic field, the ${\mathrm{H}\mathrm{e}}^{+}$ yield is calculated. From this yield the number of ${\mathrm{H}\mathrm{e}}^{+}$ ions produced has been estimated and found in reasonable agreement with its measured value. The present results establish the feasibility of a second-order autocorrelation measurement of superposition of harmonics, and thus they represent the precursor towards the direct temporal characterization of attosecond pulse trains.

High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources ($>2$) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.

The coherent properties of the temporally and spectrally narrowed emission of laser-induced fluorescence of organic dyes hosted inside artificial scattering matrices (random lasers) were investigated. The excitation source was a frequency-doubled 200-fs pulsed laser emitting at 400 nm. Spectral and temporal features were simultaneously recorded with a spectrograph and a streak camera operating in photon-counting mode. Photon-number distributions were thus created. The temporal coherence of the laserlike emission above and below the excitation energy threshold was investigated from the photon-number distribution that was obtained.

Illuminating a gold surface by strong (5 GW/${\mathrm{cm}}^2$) picosecond neodymium-doped yttrium aluminum garnet laser pulses at grazing incidence, we observed a generation of coherent beams of both even and odd harmonics up to fifth order in the reflected direction with efficiencies ${10}^{\mathrm{-}10–}$ ${10}^{\mathrm{-}13}$. The observed decrease of the harmonic efficiencies with increasing harmonic order is much weaker than predicted by perturbative theories.