  Resonances in the photo-absorption spectrum of the generating medium can
modify the spectrum of high order harmonics. In particular, window-type Fano
resonances can reduce photo-absorption within a narrow spectral region and,
consequently, lead to an enhanced emission of high-order harmonics in
absorption-limited generation conditions. For high harmonic generation in argon
it is shown that the 3s3p6 np 1P1 window resonances (n=4,5,6) give rise to
enhanced photon yield. In particular, the 3s3p6 4p 1P1 resonance at 26.6 eV
allows a relative enhancement up to a factor of 30 compared to the
characteristic photon emission of the neighboring harmonic order. This
enhanced, spectrally isolated and coherent photon emission line has a relative
energy bandwidth of only {\Delta}E/E=3*10-3. Therefore, it might be directly
applied for precision spectroscopy or coherent diffractive imaging without the
need of additional spectral filtering. The presented mechanism can be employed
for tailoring and controlling the high harmonic emission of manifold target
materials.
