Title: Dynamically reconfigurable metal-semiconductor Yagi-Uda nanoantenna,
Abstract: We propose a novel type of tunable Yagi-Uda nanoantenna composed of
metal-dielectric (Ag-Ge) core-shell nanoparticles. We show that, due to the
combination of two types of resonances in each nanoparticle, such hybrid
Yagi-Uda nanoantenna can operate in two different regimes. Besides the
conventional nonresonant operation regime at low frequencies, characterized by
highly directive emission in the forward direction, there is another one at
higher frequencies caused by hybrid magneto-electric response of the core-shell
nanoparticles. This regime is based on the excitation of the van Hove
singularity, and emission in this regime is accompanied by high values of
directivity and Purcell factor within the same narrow frequency range. Our
analysis reveals the possibility of flexible dynamical tuning of the hybrid
nanoantenna emission pattern via electron-hole plasma excitation by 100
femtosecond pump pulse with relatively low peak intensities $\sim$200
MW/cm$^2$.