  Second-harmonic generation is of paramount importance in several fields of
science and technology, including frequency conversion, self-referencing of
frequency combs, nonlinear spectroscopy and pulse characterization. Advanced
functionalities are enabled by modulation of the harmonic generation
efficiency, which can be achieved with electrical or all-optical triggers.
Electrical control of the harmonic generation efficiency offers large
modulation depth at the cost of low switching speed, by contrast to all-optical
nonlinear devices, which provide high speed and low modulation depth. Here we
demonstrate all-optical modulation of second-harmonic generation in MoS2 with a
modulation depth of close to 100% and speed limited only by the fundamental
pulse duration. This result arises from a combination of D3h crystal symmetry
and the deep subwavelength thickness of the sample, it can therefore be
extended to the whole family of transition metal dichalcogenides to provide
great flexibility in the design of advanced nonlinear optical devices such as
high-speed integrated frequency converters, broadband autocorrelators for
ultrashort pulse characterization, and tunable nanoscale holograms.
