Classical theory of thermal radiation can be used to describe radiative transfer between two objects when the characteristic length scales, such as linear dimensions of the objects and inter-object separation, are much longer than the characteristic thermal wave length given by Wien’s displacement law. In situations where the separation is comparable or smaller than the characteristic wave length, near field effects need to be taken into account. Near field radiative transfer can be enhanced by several orders of magnitude if the materials of the objects support surface phonon polaritons. Although near field radiation between two parallel surfaces have been well studied theoretically, the difficulties in probing radiative transfer between parallel surfaces with nanoscale gaps have prevented meaningful experimental validation. In recent years several experiments have measured near field radiation beyond blackbody limitation between a microsphere and a substrate. In those experiments employing optical beam deflection technique, the sphere has to be placed near the edge of the substrate in order to prevent unintentional chopping of the laser beam by the edge of the substrate. The actual measurements are performed between a sphere and a semi-infinite plane instead of an infinite plane. We report here an improved optical beam deflection system to measure the near field radiative heat transfer between a sphere and a substrate. With the new setup, the sphere can be placed sufficiently far away from the substrate edge, rendering it a better approximation of a sphere and an infinite plane. The experimental results and the numerical prediction using modified proximity approximation will be discussed.

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