The process of high‐order harmonic generation (HHG) can be used to generate bright, coherent beams of light in the extreme‐ultraviolet and soft x‐ray region of the spectrum by upconverting intense femtosecond pulses to very short wavelengths. These high‐order harmonics result from ionization of the gas used as a nonlinear medium; thus, a full understanding of the process involves incorporating atomic physics, quantum dynamics, and plasma physics into an already non‐trivial nonlinear optics problem. In the past several years, we have developed a new technology of “extreme” nonlinear optics that uses the rich, attosecond time‐scale physics of the process in novel ways to manipulate the characteristics of this source, improving both the flux and the spectral characteristics. Most recently, we have (1) demonstrated that quasi phase matching of the high‐order harmonic conversion process can be accomplished by the use of weak counterpropagating pulse trains that modulate the conversion process, constituting a nonlinear‐optical “crystal” made of light; and (2) we have demonstrated that high‐order harmonics can be generated by ionization of ions in a guided‐wave geometry, using a discharge‐created plasma waveguide to pre‐ionize the gas and form a guiding electron density profile.

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