Spectral redshift of the thermal near field scattered by a probe

Abstract

The physics underlying spectral redshift of thermally generated surface phonon-polaritons (SPhPs) observed in near-field thermal spectroscopy is investigated. Numerically exact fluctuational electrodynamics simulations of the thermal near field emitted by a silicon carbide surface scattered in the far zone by an intrinsic silicon probe show that SPhP resonance redshift is a physical phenomenon. A maximum SPhP redshift of 19 cm -1 is predicted for a 200-nmdiameter hemispherical probing tip and a vacuum gap of 10 nm. Resonance redshift is mediated by electromagnetic gap modes excited in the vacuum gap separating the probe and the surface when the probing tip is much larger than the gap size. The impact of gap modes on the scattered field can be mitigated with a probing tip size approximately equal to or smaller than the vacuum gap. However, sharp probing tips induce important spectral broadening of the scattered field. It is also demonstrated that a dipole approximation with multiple reflections cannot be used for explaining the physics and predicting the amount of redshift in near-field thermal spectroscopy.This work shows that the scattered field in the far zone is a combination of the thermal near field † Corresponding author (S. Edalatpour).

Thermal radiation in systems of many dipoles

“…Although most studies have focused on radiation between two objects or one-dimensional layered structures, a growing body of work focuses on many-body interactions. These have been used to predict interesting and potentially useful effects such as many-body amplification of thermal radiation [3], heat superdiffusion in systems of plasmonic particles [31], spectral redshifts in near-field thermal spectroscopy [32], and several magnetooptical phenomena [33][34][35][36] such as a thermal Hall effect [33].…”

Generalized coupled dipole method for thermal far-field radiation

“…On the other hand, the strong form of coupled dipole moments leads to the polarizability (34) which coincides in the limit k 0 R ≪ 1 with the weak form α w−cdm like it is used in Ref. [63].…”

“…without substrate, they are wellknown and long-established [59][60][61]. Nonetheless, throughout the literature different forms are used which lead to different expressions for the dressed polarizability [1,3,53,60,[62][63][64][65][66], especially for DDA. When considering spherical nanoparticles, for instance, one finds the expressions [3,53,59,62,66]…”

“…which includes the so-called "radiation correction" term and is also called "Draine's form" of coupled dipole moments (D-CDM) [59]. On the other hand, the expression [59,60,63]…”

Measurement of near-field thermal emission spectra using an internal reflection element

“…The measured peaks using this technique are redshifted (from 3 cm -1 to 63 cm -1 ) and broadened relative to the theoretical predictions using the fluctuational electrodynamics [48][49][50][51][52][53][54]. It is shown experimentally [51], and theoretically [54], that the spectral redshift and broadening of the peaks are strongly dependent on the geometry of the probe. For example, three different values of 898 3 cm -1 , 923 cm -1 , and 943 cm -1 were obtained for the SPhP resonance of SiC when different probes were used in the same experimental setup [51].…”