Giant Core-Shell Plasmonic Enhancement of Spintronic Terahertz Emission

Plasmonic metasurfaces are widely proposed as means to enhance nonlinear light-matter interactions in optical devices, mediated by an extreme form of field localisation. In the domain of optical-to-terahertz conversion, metastructures demonstrated the ability to enhance the local optical conversion or even to elicit conversion in cases where the native material does not support effective conversion mechanisms [1]. One of the most modern approaches towards ultrafast terahertz sources has been recently enabled by the development of spintronic emitters, in which an ultrafast thermal transient combined with an inverse-spin-Hall effect is responsible for the formation of intense terahertz current sources. A recent seminal work explored this aspect by directly depositing gold nanorods on the spintronic structure [2], showing field enhancement despite the structure being seemly an optically-opaque multiple-layer assembly. An interesting question is whether sparser distributions of plasmonic structures exhibiting stronger local enhancement can be used to enhance the spintronic conversion. In this work, we demonstrate that the distribution of core-shell plasmonic nano-resonator electromagnetically coupled with the surface via a thin silica layer interface introduces a substantial enhancement of the local ultrafast heating process, boosting the spintronic terahertz emission [3].