  The control of heat flow is a formidable challenge due to lack of good
thermal insulators. Promising new oppor-tunities for heat flow control were
recently theoretically discovered for radiative heat flow in near field, where
large heat flow contrasts may be achieved by tuning electronic excitations on
surfaces. Here we show experi-mentally that the phase transition of VO2 entails
a change of surface polariton states that significantly affects radiative heat
transfer in near field. In all cases the Derjaguin approximation correctly
predicted radiative heat transfer in near field, but it underestimated the
farfield limit. Our results indicate that heat flow contrasts can be realized
in near field that can be larger than those obtained in farfield.
