  Achieving ultrafast all-optical switching in a silicon waveguide geometry is
a key milestone on the way to an integrated platform capable of handling the
increasing demands for higher speed and higher capacity for information
transfer. Given the weak electro-optic and thermo-optic effects in silicon,
there has been intense interest in hybrid structures in which that switching
could be accomplished by integrating another material into the waveguide,
including the phase-changing material, vanadium dioxide (VO2). It has long been
known that the phase transition in VO2 can be triggered by ultrafast laser
pulses, and that pump-laser fluence is a critical parameter governing the
recovery time of thin films irradiated by femtosecond laser pulses near 800 nm.
However, thin-film experiments are not a priori reliable guides to using VO2
for all-optical switching in on-chip silicon photonics because of the large
changes in VO2 optical constants in the telecommunications band, the
requirement of low insertion loss, and the limits on switching energy
permissible in integrated photonic systems. Here we report the first
measurements to show that the reversible, ultrafast photo-induced phase
transition in VO2 can be harnessed to achieve sub-picosecond switching when
small VO2 volumes are integrated in a silicon waveguide as a modulating
element. Switching energies above threshold are of order 600 fJ/switch. These
results suggest that VO2 can now be pursued as a strong candidate for
all-optical switching with sub-picosecond on-off times.
