High harmonic generation (HHG) is an extreme nonlinear frequency conversion process in which ultrafast laser light in the visible or infrared regions can be converted to much smaller wavelengths in the EUV or soft X-ray regions of the spectrum. The resulting light source lends itself to applications in imaging, spectroscopy, magnetics and thermal transport. The micro- and macroscopic dynamics during HHG govern the characteristics and limitations of the upconverted light. By driving HHG with mid-infrared light, the harmonic emission can extend to very high photon energies (up to 1.6 keV) confined within an isolated pulse with attosecond-scale durations. Experimental results demonstrating these characteristics will be presented. Further, driving with even longer wavelengths (about 10 μm) can theoretically extend harmonics to the hard X-ray region (about 10 keV), where many materials become transparent. However, second-order effects during the HHG process become non-negligible when driven with these longer wavelengths, possibly resulting in the complete shut-off of harmonic emission. A theoretical investigation into these limitations will also be presented.
