Enhancement of bichromatic high-harmonic generation with a high-frequency field

Using a high-frequency field superposed to a linearly polarized bichromatic laser field composed by a wave with frequency $\omega$ and a wave with frequency $2\omega$, we show it is possible to enhance the intensity of a group of high harmonics in orders of magnitude. These harmonics have frequencies about 30% higher than the monochromatic-cutoff frequency, and, within the three-step-model framework, correspond to a set of electron trajectories for which tunneling ionization is strongly suppressed. Particular features in the observed enhancement suggest that the high-frequency field provides an additional mechanism for the electron to reach the continuum. This interpretation is supported by a time-frequency analysis of the harmonic yield. The additional high frequency field permits the control of this group of harmonics leaving all other sets of harmonics practically unchanged, which is an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures, references and equations include

Demonstration of aluminum

Targets consisting of 25\un{nm} \chem{Al} layers buried in solid carbon at depths of up to 400\un{nm} have been irradiated with frequency doubled \chem{Ti}:sapphire laser pulses of 150\un{fs} duration at an intensity of a few 1017 \un{W/cm^2}. The analysis of the emitted \chem{Al} K-shell spectra yields densities close to that of solid \chem{Al} and temperatures around 500\un{eV}. By using the cold \chem{Si} $K_{\alpha}$ line for wavelength calibration, the presence of a red dense-plasma line shift for several K-shell resonance and associated satellite lines is clearly demonstrated

Demonstration of aluminum K-shell line shifts in isochorically heated targets driven by ultrashort laser pulses

Targets consisting of 25 nm Al layers buried in solid carbon at depths of up to 400 nm have been irradiated with frequency doubled Ti:sapphire laser pulses of 150 fs duration at an intensity of a few 1017 W/cm2. The analysis of the emitted Al K-shell spectra yields densities close to that of solid Al and temperatures around 500 eV. By using the cold Si Kα line for wavelength calibration, the presence of a red dense- plasma line shift for several K-shell resonance and associated satellite lines is clearly demonstrated

Role of the plasma scale length in the harmonic generation from solid targets

We have investigated the generation of high harmonics from the interaction of 150 fsec, 790 nm, and 395 nm laser pulses with solid targets. Experiments are presented that demonstrate a strong dependence of the conversion efficiency on the temporal pulse shape and the resulting density scale length (L/λ) of the preformed plasma. The highest conversion efficiencies are achieved for short density scale lengths (L/λ ≤ 0.4), which result from high contrast ratio pulse interactions