  High-order harmonic generation (HHG) in isolated atoms and molecules has been
widely utilized in extreme ultraviolet (XUV) photonics and attosecond pulse
metrology. Recently, HHG has also been observed in solids, which could lead to
important applications such as all-optical methods to image valance charge
density and reconstruction of electronic band structures, as well as compact
XUV light sources. Previous HHG studies are confined on crystalline solids;
therefore decoupling the respective roles of long-range periodicity and high
density has been challenging. Here, we report the first observation of HHG from
amorphous fused silica. We decouple the role of long-range periodicity by
comparing with crystal quartz, which contains same atomic constituents but
exhibits long-range periodicity. Our results advance current understanding of
strong-field processes leading to high harmonic generation in solids with
implications in robust and compact coherent XUV light sources.
