Observing Colossal and Tunable Near-field Thermal Radiation with A Highly Sensitive Annular Micro-thermocouple Junction
Near-field radiative heat transfer between two objects has been theoretically predicted and experimentally demonstrated to exceed far-field blackbody limit across nanoscale vacuum gap distance enabled by evanescent surface waves coupling, while significant enhancement by more than 100 times usually requires sub-50-nm vacuum gaps around room temperature. This is challenging for parallel-plate configuration with millimeter sample sizes due to intrinsic wafer bow and contaminant particles. Sphere-plate configuration with a microsphere attached to bimaterial cantilevers or micro-thermocouple tips has been used to experimentally demonstrate near-field radiative heat transfer down to 30-nm gaps, but it is much less developed because of the challenges in the sophisticated sensor fabrication, low sensitivity and weak signals. In this work, we overcome these challenges by an annular micro-thermocouple junction fabricated at the end of a glass fiber with straightforward thin-film deposition to achieve high measurement accuracy with large Seebeck coefficient 25 uV/K and thermal resistance 8.7e6 K/W. With a silica microsphere attached underneath the micro-thermocouple junction, we report experimental observation of colossal near-field radiation heat transfer over blackbody limit down to 10-nm gap up to 2600 times with quartz and 900 times with doped silicon. Upon phase transition of VO2 thin film emitter, tunable near-field heat transfer up to 430-fold enhancement is experimentally demonstrated at 15-nm gap with 64% reduction. Experimental data agrees well with rigorous modeling based on fluctuational electrodynamics and Derjaguin approximation, and underlying mechanism is understood by energy transmission calculations. The results will advance the experimental study and fundamental understanding of energy transport at nanoscale gaps.