  Irradiating intercalated nanorings by optical vortices ignites a charge flow
that emits coherent trains of high harmonic bursts with frequencies and time
structures that are controllable by the topological charge of the driving
vortex beam. Similar to synchrotron radiation, the polarization of emitted
harmonics is also selectable by tuning to the appropriate emission angle with
respect to the ring plane. The nonequilibrium orbital magnetic moment triggered
in a ring tunnels quantum mechanically to smaller and larger rings leading
respectively to high and low-frequency harmonic generation. The frequencies of
the emitted harmonics are tunable by simply changing the waist and/or the
winding number of the optical vortex, without the need to increase the pulse
intensity which can lead to material damage. These findings follow from
full-fledged quantum dynamic simulations for realistic material and laser
parameters. The proposed setup is non-destructive as only short vortex pulses
of moderate intensities are needed, and it offers a versatile tool for
nanoscale optical and spectroscopic applications such as local, single beam
pump-probe experiments.
