  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.
