Observation of the thermally excited electromagnetic evanescent field on ultrathin metallic films

This study explored thermally excited electromagnetic evanescent fields on gold ultrathin metallic films (UTMFs) using a passive scattering-type scanning near-field optical microscope in the long-wavelength infrared range (14.1 μm). Near-field (NF) signals were measured on gold UTMFs with thicknesses ranging from 5.6 to 69.4 nm. A sharp drop in the NF signal intensity was observed near the critical thickness (dc = 9.2 nm), marking the metal-to-insulator transition. This decline was linked to reduced free electron density N, caused by structural changes like island formation, cracks, and reduced film thickness. These constraints confined electron motion, lowering the plasma frequency (ωp), as described by a thickness-dependent Drude model. Additionally, measurements of NF signal decay length revealed the thermal interaction range of evanescent fields, providing insights into near-field heat transfer. Supported by the local density of states theory, the findings emphasize the role of electron density beneath the probe tip in NF signals. These insights into film structure, electron dynamics, and NF thermal radiation, have applications in nanoscale thermal management and infrared sensing technologies.