Theory of high-harmonic generation by low-frequency laser fields

Extreme-ultraviolet harmonic generation near 13 nm with a two-color elliptically polarized laser field

Theoretical results for high-harmonic generation by a two-color elliptically polarized laser field are presented. Special emphasis is placed on coherent radiation near 13 nm in connection with the development of extreme-ultraviolet lithography. Linearly polarized radiation at this wavelength can be obtained with a linearly polarized bichromatic laser field, whereas circularly polarized radiation can be generated with a bichromatic circularly polarized field with counterrotating coplanar components. In both cases the harmonic emission efficiencies at the saturation intensity are more than 1 order of magnitude larger than for harmonic generation with a monochromatic linearly polarized field.

Spectral features and modeling of high-order harmonics generated by sub-10-fs pulses

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments.

Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays

When an intense laser pulse is focused into a gas, the light-atom interaction that occurs as atoms are ionized results in an extremely nonlinear optical process--the generation of high harmonics of the driving laser frequency. Harmonics that extend up to orders of about 300 have been reported, some corresponding to photon energies in excess of 500 eV. Because this technique is simple to implement and generates coherent, laser-like, soft X-ray beams, it is currently being developed for applications in science and technology; these include probing the dynamics in chemical and materials systems and imaging. Here we report that by carefully tailoring the shapes of intense light pulses, we can control the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the interaction between different orders of nonlinear processes.

Self-phase-matched high harmonic generation

Our theoretical analysis reveals that tunnel ionization significantly modifies the electric field of few-cycle laser pulses within a single oscillation period. This subcycle self-modulation is predicted to result in phase matching, making high harmonic generation in the x-ray regime possible for the first time. Such a radiation source opens novel possibilities in the investigation of matter with x-ray techniques, such as time resolved x-ray diffraction and absorption.

Rescattering processes for elliptical polarization: A quantum trajectory analysis

High-harmonic generation and high-order above-threshold ionization spectra calculated in the strong-field approximation are analyzed in terms of the complex space-time orbits that result from a saddle point analysis of the underlying integrals. For elliptical polarization, the plateaus of the spectra of high-harmonic generation and high-order above-threshold ionization each turn into a staircase of very similar appearance. Each step of the stair can be traced to a particular pair of orbits which are almost identical in both cases.

Interpretation of momentum distribution of recoil ions from laser induced nonsequential double ionization

We show that the recently reported momentum distribution of recoil ions from laser induced nonsequential double ionization can be interpreted as a combined effect of interelectron correlation and final state field interaction ("Volkov dressing") of the two outgoing electrons. We also find a cutoff formula for the recoil momentum that gives the maximum momenta observed in the experiments.

Relativistic resonances in combined magnetic and laser fields

We propose a mechanism for experimental investigation of relativistic effects in the laser-atom interaction with moderate (nonrelativistic) laser intensities that involves placing the system in a static magnetic field parallel to the laser's magnetic-field component. The resonantly induced relativistic motion of the atomic electron leads to a variety of novel phenomena: a relativistic dephasing leading to a ringlike spatial probability density, a counterintuitive window of relativistically enhanced motion, and a sequence of sawtooth-shaped resonances that may increase the harmonic generation.

Determining the absolute carrier phase of a few-cycle laser pulse

In a strong laser field, electrons tunnel from an atom at a rate determined by the instantaneous field. If the pulse is only a few cycles in duration, the highly nonlinear nature of tunnel ionization ensures that the resultant electron wave packet is primarily formed in less than one period. Measuring the direction of above-threshold-ionization electrons produced by circularly polarized light provides a direct method of measuring the absolute carrier phase of a single pulse. The method is robust, surviving spatial and temporal integration as well as intensity fluctuations.

Phase-dependent loss due to nonadiabatic ionization by sub-10-fs pulses

For the first time to the author's knowledge, the effect of phase-dependent loss that can be used to stabilize the phase of a sub-10-fs laser pulse with respect to the envelope is shown. This loss is caused by the nonadiabatic response of an atom illuminated by a short pulse, which leads to the dependence of the tunneling ionization on the absolute phase of the pulse. The phase selectivity can be employed as a phase-dependent filter within the laser cavity with important applications in nonlinear optics in the single-cycle regime.

Photoemission with laser-generated harmonics tunable to 80 eV

I describe the generation of broadly tunable harmonic photons with energies to 80 eV from a laser system that produces 200-fs pulses of 0.5-mJ 610-nm light at a repetition rate of 540 Hz. The harmonics are produced by focusing of the 610-nm light into high densities of rare gas atoms produced at the output of a pulsed valve. Harmonic photons are wavelength selected by a grazing incidence toroidal grating and refocused onto samples that reside in a vacuum analysis chamber. Valence and atomic core-level photoemission spectra generated with this light are described.

Femtosecond measurement of high-order harmonic pulse width and electron recombination time by field ionization

We measured the ultrashort XUV pulse width by the pump-probe method, using femtosecond dynamics of field ionization. We measured the pulse width of the ninth harmonic (88.3 nm) of a Ti:sapphire laser, using the difference of the absorption cross section between Kr and Kr(+). The change of transmission of the ninth harmonic for the delay between the pump and the probe pulses gives both the pulse width of the ninth harmonic and the recombination time of Kr(+). The measured pulse width of the ninth harmonic was 260 fs. The recombination time of Kr(+) was ~2 ps, which does not contradict the estimated time of the threebody recombination.