Anomalous plasmon resonance from confined diffusive charges: high quality and tunability from mid to far infrared wavebands.

For heavily-doped semiconductor nanocrystals, a new class of plasmonic materials, the abundant dopants provide sufficient free carriers, but also act as scattering impurities, which have been limiting the merit of localized surface plasmon resonance (LSPR). To resolve this intrinsic contradiction, we propose a core-shell (heavily-lightly doped) semiconductor nanosystem to generate plasmon resonance from the highly mobile diffusive charges rather than the strongly scattered charges. We develop a complete theory to describe the spatial distribution of the diffusive charges as well as their resonance under the light excitation and predict a LSPR tunable from mid to far infrared band with a 15-fold field enhancement near the resonance. Meanwhile, the physical nature of such LSPR, which is different from the bonding and antibonding plasmon mode in typical core-shell structures, was revealed by studying the polarization charge distribution. Hopefully, these results can help realize high quality plasmonic structures for applications in infrared waveband.