Characteristics of high-harmonic generation(HHG) from noble gases driven by a femtosecond terawatt Ti:sapphire laser pulse was investigated. The HHG experiment was performed with a femtosecond chirped-pulse amplification Ti:sapphire laser operating at 10 Hz. The pulse duration used for experiments was between 23 and 30 fs. The laser pulse was focused into a gas jet with a small nozzle of 200-㎛ diameter. Characteristic behaviors of the HHG from noble gases such as He, Ne, and Ar were observed by changing laser intensity, laser pulse duration, gas density, and target position. The intensity of HHG was proportional to the square of the gas density. In the case of Ar which has a relatively low ionization potential compared with He or Ne, the harmonic intensity became saturated above a certain gas density. It was also observed that the structure of the HHG was sensitive to the target position because the phase matching condition was dependent on the geometrical condition. Especially, when the target position was moved behind the focusing position of the laser pulse, harmonics from short trajectories were selected. From the strong-field approximation(SFA) model, it was shown that if we could select high harmonics from short trajectories, high harmonics could be used for the attosecond pulse generation.
The strong blueshift of high harmonics was generated when intense laser pulses were applied to noble gases. A large blueshift enough to cover the frequency interval between odd harmnics was observed from Ar and Ne. Intensity, Density, and order dependence of harmonic blueshift was characterized. The density dependence of harmonic blueshift showed that the self-phase modulation of the fundamental laser pulse in an ionizing gas could not fully explain the observed blueshift.
A semiclassical calculation based on the SFA model showed that the observed large blueshift resulted from a rapidly increasing electric field, existing much earlier in time than the peak of the laser pulse, i.e., a nonadiabatic effect. The origin of characteristic behavior of harmonic blueshift such as intensity and order dependence was explained by the semiclassical model. The calculated blueshift could well match the observed result. The time-dependent Schrodinger equation(TDSE) model and the SFA model were utilized to check the validity of the semiclassical model. The large blueshift of high harmonics is expected to open a way to a tunable femtosecond coherent x-ray source, i.e. a table-top synchrotron.
