  High-harmonic generation (HHG), which is generation of multiple optical
harmonic light, is an unconventional nonlinear optical phenomenon beyond
perturbation regime. HHG, which was initially observed in gaseous media, has
recently been demonstrated in solid state materials. How to control the extreme
nonlinear optical phenomena is a challenging subject. Compared to atomic gases,
solid state materials have advantages in controlling electronic structures and
carrier injection. Here, we demonstrate control of HHG by tuning electronic
structure and carrier injection using single-walled carbon nanotubes (SWCNTs).
We reveal systematic changes in the high-harmonic spectra of SWCNTs with a
series of electronic structures from a metal to a semiconductor. We demonstrate
enhancement or reduction of harmonic generation by more than one order of
magnitude by tuning electron and hole injection into the semiconductor SWCNTs
through electrolyte gating. These results open a way to control HHG within the
concept of field effect transistor devices.
