  External electric fields are highly attractive for dynamical manipulation of
excitons in two-dimensional materials. Here, we theoretically study the
ionization of excitons in monolayer transition metal dichalcogenides (TMDs) by
intense pulsed electric fields in the terahertz (THz) regime. We find that THz
pulses with realistic field strengths are capable of ionizing a significant
fraction of photogenerated excitons in TMDs into free charge carriers. Short
THz pulses are therefore an efficient, non-invasive method of dynamically
controlling the free carrier concentration in monolayer TMDs, which is useful
for applications such as THz modulators. We further demonstrate that exciton
ionization probabilities should be experimentally measurable by comparing free
carrier absorption before and after the THz pulse. Detailed results are
provided for different TMDs in various dielectric environments.
