  We propose a method for generating only even-order high harmonics. We
formulate a condition for a shift in the relative-phase between bi-chromatic
drivers that leads to a field sign-flip of only even-order harmonics. Induction
of this sign-flip periodically during propagation gives rise to quasi phase
matching of only even harmonics. We demonstrate this technique numerically and
also show that it leads to attosecond pulse trains with constant carrier
envelop phase with high repetition rate. This work opens the door for
quasi-phase matching of high harmonics with a designed selective enhancement
function.

Optics