Abstract
A method and system for utilizing the laser defocusing effect for phase matching of high;order harmonic generation in a tight focusing geometry are provided. The most suitable;focusing geometry, especially those five parameters including: (1) the aperture size of the; adjustable iris, (2) the focus position with respect to the gas cell, (3) the backing pressure of;the gas cell, (4) the focal length and (5) the length of the gas cell, for achieving phase;matching of the target harmonic order and maximal yield of the target harmonic order are;disclosed.