Phase-matched High-order Harmonics Research Articles

Phase-Matched High-Order Harmonics from N2 Using kHz Femtosecond Laser Pulses

We report the phase-matched high-harmonic generation from N2 driven by kHz femtosecond laser pulses. By varying the position of the laser focus in the gas tube, we observe the radiation of different harmonics. Our results provide a potential source for studying ultrafast molecular dynamics.

High order harmonic generation for study of atomic and molecular dynamics

We present a pump-probe experiment based on the use of a second electrical field to modulate the intensity and the spatial profile of the phase-matched high-order harmonics radiation generated by the first strong driving field. The modification is caused by the variation of the phase-matching condition, e.g., change of electron trajectory and the effect of field gradient forces on the electron that reflect the dynamics of atomic and molecular gases. Atomic argon and molecular oxygen gases are used to demonstrate the feasibility of the technique.

Phase-matched emission of few high-harmonic orders from a helium gas cell

We report on femtosecond-laser-based generation of a few phase-matched high-order harmonics in a helium semi-infinite gas cell. The harmonic beam consists of effectively four to six intense orders with wavelengths around 9.5 nm. By varying the effective interaction length, we observe the effects of coherence length related to plasma formation and the Gouy phase shift on the output. The atomic scattering factors for photoabsorption, the effective propagation lengths (leff,He≈1 mm, leff,Ar≈7.2 mm), and the energy conversion efficiencies (εHe≈10−7, εAr≈5×10−7) are deduced for harmonic generation in helium (77th to 95th orders) and also argon (21st to 27th orders) for comparison.

Multiphoton ionization ofHeby using intense high-order harmonics in the soft-x-ray region

We report on the multiphoton ionization processes in the soft-x-ray region $(\ensuremath{\lambda}\ensuremath{\leqslant}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. On the basis of the measured time-of-flight spectra for both ions and electrons obtained using intense soft-x-ray pulses produced by high-order harmonics, the cross sections of the two-photon double ionization and above-threshold ionization of $\mathrm{He}$ are estimated. The high-intensity soft-x-ray radiation achieved by phase-matched high-order harmonics enables the investigation of these nonlinear optical processes, which were beyond the reach of conventional light sources.

Phase-matched high-order harmonics by inter-action of Ar atoms with high-repetition-rate low-energy femtosecond laser pulses

Phase-matched high-order harmonic generation in Ar gas-filled cell was investigated experimentally. We obtained phase-matched 27th order harmonic driven by a commercially available solid-state femtosecond laser system at 0.55 mJ/pulse energy level and 1 kHz repetition rate. To our knowledge, this is the lowest driving laser energy used to obtain phase-matched 27th order harmonic in a static gas cell. High-order harmonic generation at different gas density was studied systematically. Spectral blueshift and broadening of high harmonics under different pressure were analyzed. We found that the source size and spatial distribution of high-order harmonics are quite different under the phase-matching condition from those of the phase-mismatching case.

Intensity limits in phase-matched high-order harmonics generation

The phase-matching process of high-order harmonics generated in rare gases confined in a hollow-fiber wave-guide is investigated theoretically. A simple calculation is built to explore the effects of several important parameters in phase-matched high-order harmonics generation. A cutoff intensity for a certain order phase-matched harmonic is found, which depends on the maximum order of phase-matched harmonic. The calculations show that an ultrashort laser pulse and the medium with a higher ionization potential are necessary for generation of higher-order phase-matched harmonics.