Localized surface plasmon based on coupled metallic nanoaggregates has been extensively studied in enhancing light scattering and optical force, which depends on the geometry/symmetry of plasmonic oligomers and the refractive index of surrounding medium. As the interparticle gap distance between nanoparticles becomes smaller than several nanometers, quantum effects can change the plasmon coupling in classical predictions. However, most of the research on plasmonic scattering and optical force has been done based on local calculations even for the gap below ~3 nm, in which the nonlocal screening plays a vital role. Here, we theoretically investigate the nonlocal effect on the evolution of plasmon resonance modes in strongly coupled nanoparticle dimer antennas with the gap down to 1 nm. Then, the refractive index sensing and optical force in this nonlocal system is evaluated and compared with the results in classical calculations. We find that in the nonlocal regime, both refractive index sensibility factor and optical force are actually smaller than their classical counterparts mainly due to the saturation of both plasmon-shifts and near-field enhancement. These results would be beneficial for the understanding of interaction between light and nonlocal plasmonic nanostructures and the development of plasmonic devices such as nanoantennas, nanosensors, and photonic manipulation.
Germanium material based on band gap engineering has aroused great interest for the CMOS-compatible optoelectronic integrated circuits due to its quasi-direct band gap structure. While many technologies have been conquered for germanium light, optimization is the bottleneck due to the excessive threshold current density, low luminescence efficiency and unstable problem in the laser device. The proper understanding of inter-valley scattering mechanisms between direct and indirect valleys in germanium is of paramount importance in view of the optimization of Ge as optical gain medium. The paper focuses on the inter-valley scattering mechanisms in strained Ge in theory based on a time-dependent Hamiltonian describing the electron-phonon interaction. The impacts of temperature and strain on the inter-valley scattering between direct and indirect valleys are discussed quantificationally. For the electrons in direct valley, emitting inter-valley phonon scattering is the dominant mechanism for momentum and energy relaxation of electrons both at the low and room temperature, and they are more likely to be scattered by inter-valley phonons to the L valleys with lower energy. For the electrons in L valleys, inter-valley scattering is important only for electrons with sufficient energy to scatter into the direct valley, which can happen in germanium devices under high electric field. Numerical results also indicate that enhanced indirect-to-direct inter-valley scattering and reduced direct-to-indirect inter-valley scattering are reliable by introducing tensile strain in Ge material at room temperature. The results offer fundamental understanding of phonon engineering for further optimization of the germanium light sources.