We demonstrate a novel technique for coupling XUV harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the 3rd through 21st odd harmonics with a dramatic increase in usable power over previous results of high harmonic generation at high repetition rates.

3 pages, 2 figures