Extreme ultraviolet high-harmonic spectroscopy of solids

Intense light interacting with a thin film of silicon dioxide is used to generate broadband extreme ultraviolet radiation; the spectra reveal detailed information on the energy dispersion of the conduction band of silicon dioxide, which is at present inaccessible by conventional photoemission spectroscopy. Solid-state EUV spectroscopy Laser-based generation of extreme ultraviolet (EUV) radiation in gases, leading to the generation of light with frequencies that are high-order harmonics of the fundamental laser frequency, is widely used as a spectroscopy and imaging tool. High harmonics are also generated in bulk crystals, although the underlying emission mechanisms are under debate. Here Eleftherios Goulielmakis and colleagues extend the technique to generate broadband EUV radiation in thin films of silicon dioxide. The spectra reveal detailed information on the energy dispersion of the conduction band of silicon dioxide, which is at present inaccessible by conventional photoemission spectroscopy. This work is an important step in the development of the emerging field of solid-state EUV photonics. Extreme ultraviolet (EUV) high-harmonic radiation 1 , 2 emerging from laser-driven atoms, molecules or plasmas underlies powerful attosecond spectroscopy techniques 3 , 4 , 5 and provides insight into fundamental structural and dynamic properties of matter 6 , 7 . The advancement of these spectroscopy techniques to study strong-field electron dynamics in condensed matter calls for the generation and manipulation of EUV radiation in bulk solids, but this capability has remained beyond the reach of optical sciences. Recent experiments 8 , 9 and theoretical predictions 10 , 11 , 12 paved the way to strong-field physics in solids by demonstrating the generation and optical control of deep ultraviolet radiation 8 in bulk semiconductors, driven by femtosecond mid-infrared fields or the coherent up-conversion of terahertz fields to multi-octave spectra in the mid-infrared and optical frequencies 9 . Here we demonstrate that thin films of SiO 2 exposed to intense, few-cycle to sub-cycle pulses give rise to wideband coherent EUV radiation extending in energy to about 40 electronvolts. Our study indicates the association of the emitted EUV radiation with intraband currents of multi-petahertz frequency, induced in the lowest conduction band of SiO 2 . To demonstrate the applicability of high-harmonic spectroscopy to solids, we exploit the EUV spectra to gain access to fine