  We demonstrate a novel technique for producing high-order harmonics with
designer spectral combs in the extreme ultraviolet-soft X-ray range for
resonance applications using spectrally controlled visible lasers. Our approach
enables continuous tunability of the harmonic peaks while maintaining superb
laser-like features such as coherence, narrow bandwidth, and brightness. The
harmonics are conveniently shifted towards lower or higher energies by varying
the infrared pulse parameters, second harmonic generation phase-matching
conditions, and gas density inside a spectral-broadening waveguide. In the time
domain, the X-rays are estimated to emerge as a train of sub-300 attosecond
pulses, making this source ideal for studying dynamic processes in
ferromagnetic nanostructures and other materials through resonant
multidimensional coherent diffractive imaging or other X-ray absorption
spectroscopy techniques. Moreover, the visible driving laser beams exhibit an
ultrashort sub-10 fs pulse dues to nonlinear self-compression with a more than
30-fold enhancement in peak intensity that also extends the tunability of the
linewidth of the harmonic combs.
