  In the present study, we demonstrate a vacuum thermal switch based on
near-field thermal radiation between phase transition materials, i.e., vanadium
dioxide (VO2), whose phase changes from insulator to metal at 341 K. Similar
modulation effect has already been demonstrated and it will be extended to
thin-film structure with substrate in this paper. Strong coupling of surface
phonon polaritons between two insulating VO2 plates significantly enhances the
near-field heat flux, which on the other hand is greatly reduced when the VO2
emitter becomes metallic, resulting strong thermal switching effect.
Fluctuational electrodynamics predicts more than 80% heat transfer reduction at
sub-30-nm vacuum gaps and 50% at vacuum gap of 1 micron. By replacing the bulk
VO2 receiver with a thin film of several tens of nanometers, the switching
effect can be further improved over a broad range of vacuum gaps from 10 nm to
1 um. In addition, for the purpose of more practical setup in experiments and
applications, the SiO2 substrate effect is also considered for the structure
with thin-film emitter or receiver.
