Spectral near-field thermal emission extraction by optical waveguides

As far-field thermal emission is limited by blackbody radiation, the significant enhancement of thermal radiation in the near-field plays a vital role in a variety of applications such as infrared sensing, radiation cooling, and thermophotovoltaics. Yet the techniques of exporting the near-field signal to the far field are still not mature, typically relying on complex instrumentation capable of being applied only to the surfaces of model systems and structures. Here we develop an efficient method of extracting near-field thermal radiation to the far field by integrating a nanoscale thermal emitter with a high-index optical waveguide. By directly depositing the emitter onto the optical waveguide, it requires no vacuum gap and enables efficient coupling of near-field thermal radiation into propagating wave-guided modes. A single-mode planar waveguide incorporated with an indium-tin oxide (ITO) film as a thermal emitter is theoretically investigated to prove the feasibility of thermal extraction. The Wiener-chaos expansion method is applied to directly calculate the thermal radiation from the emitter-waveguide system. We experimentally demonstrate the fidelity of the optical-waveguide-assisted near-field thermal extraction by measuring the emission spectrum of an ITO-coated optical fiber and comparing with theoretical predictions. Our experimental demonstration in conjunction with the direct simulation paves the way for effectively extracting near-field thermal radiation with applications in chemical sensing, infrared imaging, and near-field thermal energy management.