  We propose and demonstrate hybrid all-optical switching devices that combine
silicon nanocavities and two-dimensional semiconductor material. By exploiting
the refractive index modulation caused by photo-induced carriers in the
two-dimensional material instead of the silicon substrate, we overcome the
switching performance limitation imposed by the substrate material. Air-mode
photonic crystal nanobeam cavities capable of efficient interaction with
two-dimensional materials are fabricated, and molybdenum ditelluride, a
two-dimensional material with rapid carrier recombination, is transferred onto
the cavities. The molybdenum ditelluride flake is excited by an optical pump
pulse to shift the resonant wavelength of the cavity for switching operation.
We have successfully achieved all-optical switching operations on the time
scale of tens of picoseconds while requiring low switching energies of a few
hundred femtojoules.
