  Electronic states and their dynamics are of critical importance for
electronic and optoelectronic applications. Here, we probe various relevant
electronic states in monolayer MoS2, such as multiple excitonic Rydberg states
and free-particle energy bands, with a high relative contrast of up to >200 via
broadband (from ~1.79 to 3.10 eV) static third-harmonic spectroscopy, which is
further supported by theoretical calculations. Moreover, we introduce transient
third-harmonic spectroscopy to demonstrate that third-harmonic generation can
be all-optically modulated with a modulation depth exceeding ~94% at ~2.18 eV,
providing direct evidence of dominant carrier relaxation processes, associated
with carrier-exciton and carrier-phonon interactions. Our results indicate that
static and transient third-harmonic spectroscopies are not only promising
techniques for the characterization of monolayer semiconductors and their
heterostructures, but also a potential platform for disruptive photonic and
optoelectronic applications, including all-optical modulation and imaging.
