  Ultrafast all-optical logic devices based on nonlinear light-matter
interactions hold the promise to overcome the speed limitations of conventional
electronic devices. Strong coupling of excitons and photons inside an optical
resonator enhances such interactions and generates new polariton states which
give access to unique nonlinear phenomena, such as Bose-Einstein condensation,
used for all-optical ultrafast polariton transistors. However, the pulse
energies required to pump such devices range from tens to hundreds of pJ,
making them not competitive with electronic transistors. Here we introduce a
new paradigm for all-optical switching based on the ultrafast transition from
the strong to the weak coupling regime in microcavities embedding atomically
thin transition metal dichalcogenides. Employing single and double stacks of
hBN-encapsulated MoS$_2$ homobilayers with high optical nonlinearities and fast
exciton relaxation times, we observe a collapse of the 55-meV polariton gap and
its revival in less than one picosecond, lowering the threshold for optical
switching below 4 pJ per pulse, while retaining ultrahigh switching
frequencies. As an additional degree of freedom, the switching can be triggered
pumping either the intra- or the interlayer excitons of the bilayers at
different wavelengths, speeding up the polariton dynamics, owing to unique
interspecies excitonic interactions. Our approach will enable the development
of compact ultrafast all-optical logical circuits and neural networks,
showcasing a new platform for polaritonic information processing based on
manipulating the light-matter coupling.
