There is an astonishing simplicity in some processes of intense laser-matter interactions. At high field amplitudes, the atomic electrons acquire energy from the driving laser, which can be released in the form of high frequency coherent radiation, in a process known as high-order harmonic generation (HHG). The lack of conventional lasers at high frequencies drives the technological interest of HHG as a basis for coherent short wavelength sources. Until very recently, intense laser technology was limited to near-infrared wavelengths (around 800 nm), consequently, the HHG conversion was...