2, The University of Utah, Salt Lake City, Utah, United States
The interaction of a probe with the thermal near field emitted by a surface is of importance in near-field thermal spectroscopy (De Wilde et al., Nature 444, 740, 2006), tip-based manufacturing (Hawes et al., Opt. Lett. 33, 1383, 2008) and localized radiative cooling (Guha et al., Nano Lett. 12, 4546, 2012). Experimental studies (Jones and Raschke, Nano Lett. 12, 1475, 2012; Babuty et al., Phys. Rev. Lett. 110, 146103, 2013; O’Callahan, et al., Phys. Rev. B 89, 245446, 2014; O’Callahan and Raschke, APL Photonics 2, 021301, 2017) reported that the resonance frequency of the far-field scattered signal is spectrally redshifted compared to the near-field spectrum of the surface as predicted via fluctuational electrodynamics. It is not clear if the resonance redshift is due to near-field coupling between the probe and the surface or if it is an experimental artifact. Simplified models are inadequate for explaining the resonance redshift. In this study, the thermal discrete dipole approximation (T-DDA) with surface interaction (Edalatpour and Francoeur, Phys. Rev. B 94, 045406, 2016) is applied for modeling probe-surface interactions. The heat rate between the probe and the surface as well as the scattered far-field signal is calculated and analyzed. The results reveal that the resonance redshift is a physical phenomenon caused by the multiple reflections of the thermally generated waves between the probe and the surface. The magnitude of the resonance redshift is greatly affected by the shape and material properties of the probe. The possibility of designing a probe which does not introduce a resonance redshift in the scattered signal is discussed. This work will pave the way to the development of spectroscopy techniques enabling measurements of the near-field thermal spectrum.
Acknowledgment: This work is supported by the US Army Research Office under Grant No. W911NF-14-1-0210.