Efficient High-order Harmonic Generation Research Articles

GUIDED-WAVE PHASE-MATCHING OF ULTRASHORT-PULSE LIGHT

We review the use of hollow waveguides for frequency conversion of ultrafast laser pulses the ultraviolet and extreme ultraviolet. Phase-matching of these processes is reached through a balance of gas and waveguide dispersion. By mixing 400 nm with 800 nm light, ultrashort (8 fs) pulses are generated near 270 nm with high efficiency > 20%. Tuning of the longer-wavelength component in the mixing process allows tuning of the output from 215–308 nm. In the XUV, this guided-wave phase-matching has allowed an increase of conversion efficiency of high-order harmonic generation of 100–1000x over that obtained with a gas jet, in an experimentally-convenient geometry.

Influence of the density of a medium on high-order harmonic generation by atoms in strong laser fields

An investigation is reported of the maximum possible increase in the density of a medium needed to increase the efficiency of high-order harmonic generation by atoms ionised with high-intensity radiation. This investigation is based on a classical description of the motion of a quasi-free photoelectron in a continuum and of its perturbation by the surrounding ions. A strong dependence of the optimal concentration of atoms on the pump radiation wavelength is revealed. The spectral characteristics of high-order harmonic generation resulting from an increase in the density of the medium are predicted.

Harmonic generation in a plasma with laser-heated electron velocity distributions

The theory of high-order harmonic generation taking into account plasma electron heating due to inverse bremsstrahlung absorption of an intense, high-frequency electromagnetic wave is developed. We consider physical situations in which the electron quiver energy is generally much larger than the electron thermal velocity. High-order harmonic spectra are calculated for two cases: that in which the heating mechanism forms an isotropic electron distribution function, and that in which, instead, an anisotropic electron distribution function is formed having a prolate shape, i.e., elongated perpendicular to the radiation electric-field direction. This kind of anisotropy is found to inhibit considerably the generation of high-order harmonics, because the energy from the radiation field is preferentially transferred, at least in the first stages of interaction, to the electron degrees of freedom perpendicular to the electric-field direction. The interplay between the efficiency of high-order harmonic generation and the kind of anisotropy of the electron distribution is discussed.

HIGH-ORDER HARMONIC GENERATION WITH HIGH-INTENSITY FEMTOSECOND LASER PULSES

High-order harmonic generation experiments using intense, femtosecond dye (616 nm), XeCl (308 nm) and Ti:sapphire (782 nm) laser pulses are reviewed. Results obtained at focused intensities of 1013 ~ 1015 W/cm 2 have shown characteristic behaviors of the high-order harmonic and its distribution which depend on the laser wavelength, the laser intensity and the rare gas used as the nonlinear medium. Interaction processes for the efficient generation of high-order harmonics or short-wavelength coherent radiation in the multiphoton and tunneling ionization regimes are discussed based on the results obtained.

Optimal quasi-phase-matching for high-order harmonic generation in gases and plasma.

We theoretically demonstrate the feasibility of optimal quasi-phase-matching (QPM) of high-order harmonic generation in gases and plasma with modulated density. QPM optimization, being possible for both tight and loose focusing of the fundamental beam, may increase the conversion efficiency of high-order harmonic generation by several orders of magnitude as compared to the efficiency attainable now.