Enhanced high-harmonic generation from chromium-doped magnesium oxide

High-order harmonic generation (HHG) from crystals offers a new source of coherent extreme ultraviolet (XUV) attosecond radiation. The process is extremely sensitive to the band structure and symmetries. Here, we tailor the high-order harmonic radiation by engineering the band structure of the bulk material using dopant-induced vacancy defects. We provide a comparison of the HHG signal in the XUV domain from undoped bulk magnesium oxide (MgO) to the HHG signal from MgO doped with chromium atoms at sub-percent concentration. We experimentally demonstrate an increase in the HHG efficiency as well as an extension of the highest detectable harmonic order from chromium doped MgO with Mg vacancies below the laser-induced damage threshold. An anisotropy measurement of the harmonic emission as a function of the laser polarization shows that the crystal symmetry is preserved for the case of doped MgO. Using a reasoning based on tunneling theory, we provide an explanation about the HHG efficiency increase in MgO with dopant-induced vacancy defects. Our study paves the way towards the control of the HHG properties in solids with complex defects caused by transition metals doping. As a promising example, the energetic cutoff extension can be applied to control the attosecond emission.