  Diamond is emerging as an attractive third-generation wide-bandgap
semiconductor for future on-chip nonlinear photonics and quantum optics due to
its unique thermal, optical, and mechanical properties. However, the
light-driven current under below-bandgap excitation from the second-order
nonlinear optical effect in diamond is still challenging. Herein, a giant
second-order nonlinear photocurrent is observed in the chemical vapor
deposition (CVD) diamond by utilizing terahertz (THz) emission spectroscopy.
This ultrafast photocurrent originates from the photon drag effect (PDE),
during which the momentum transfer from the incident photons to the charge
carriers at the rich grain boundaries of the CVD diamond after the exclusive
subgap {\pi}-{\pi}* transition upon femtosecond laser excitation. Especially,
the interplay between circular and linear PDE to the THz generation has been
clarified and distinguished under elliptically polarized light excitation.
Furthermore, the picosecond ultrafast dynamics of these charge carriers are
also verified by the infrared spectroscopy. Owing to the giant
photon-drag-induced ultrafast photocurrent, the CVD diamond presents the
highest THz emission efficiency compared with the reported carbon allotropes,
which expands the new functionality of diamond nonlinear photonics into on-chip
THz devices.
