  Circularly polarized extreme ultraviolet (XUV) radiation is highly
interesting for investigation of chirality-sensitive light-matter interactions.
Recent breakthroughs have enabled generation of such light sources via high
harmonic generation (HHG) from rare gases. There is a growing interest in
extending HHG medium from gases to solids, especially to 2D materials, as they
hold great promise to develop ultra-compact solid-state photonic devices and
provide insights into electronic properties of the materials themselves.
However, HHG in graphene driven by terahertz to mid-infrared fields reported so
far only generate low harmonic orders, and furthermore no harmonics driven by
circularly polarized lasers. Here, using first-principles simulations within a
time-dependent density-functional theory framework, we show that it is possible
to generate HHG extending to the XUV spectral region in monolayer extended
graphene excited by near-infrared lasers. Moreover, we demonstrate that a
single circularly polarized driver is enough to ensure HHG in graphene with
circular polarization. The corresponding spectra reflect the six-fold
rotational symmetry of the graphene crystal. Extending HHG in graphene to the
XUV spectral regime and realizing circular polarization represent an important
step towards the development of novel nanoscale attosecond photonic devices and
numerous applications such as spectroscopic investigation and nanoscale imaging
of ultrafast chiral and spin dynamics in graphene and other 2D materials.
