Efficient, self-phase matched, narrow-band, engineered shape, tunable high-order harmonic generation based on femtosecond dynamic grating

Abstract: The high-order harmonic generation (HHG), if phase matched, opens a unique way to produce coherent ultrafast extreme-UV (XUV) or soft x-ray light sources. Here we describe a robust and tunable method for scaling and narrowing the bandwidth of high-order harmonics in different transversal modes based on femtosecond dynamic grating in a gas jet. This technique has potential of self-phase controlled XUV generation by adjusting the intersect angle and the intensity ratio of interfering fundamental beams. The limitations of using high intensity laser fundamental pulses in gas, such as non-negligible the magnetic field which prevents recombination with parent and self-focusing of laser and plasma creation in the generated harmonics beam path which prevents phase matching, can be solved by using dynamic grating generated by the fundamental pulse and/or its harmonic with small intersect angle. The grating pattern introduces a phase modulation in the dipole emission imprint such pattern in the gas medium. As the spatial and temporal behaviors of the grating are closely connected in harmonic generation, by shaping of the spatial profile of the interfering beams it is possible to create a more complex tunable beating pattern as harmonic sources to transfer desired optical information to XUV pulses. In this case, the phase mismatch can be completely compensate by tuning the gas pressure, adjusting the intersect angle as well as intensity ratio of the interfering beams to produce a bright XUV with desired transversal mode.