Near-field thermal radiation of gradient refractive index slab: Internal polaritons

We investigate near-field thermal radiation of gradient refractive index slabs (GRISs). It is demonstrated that internal polaritons (IPs) can be supported in the interior of GRISs. IPs provide more evanescent electromagnetic states especially in the lower frequency region and cause red-shift in the near-field heat flux spectrum, as well as enhance local absorption inside the medium. This work sheds light on the mechanism of near-field thermal radiation between GRISs and provides additional design freedom for near-field energy devices.