Optically Abrupt Localized Surface Plasmon Resonances in Si Nanowires by Mitigation of Carrier Density Gradients

Spatial control of carrier density is critical for engineering and exploring the interactions of localized surface plasmon resonances (LSPRs) in nanoscale semiconductors. Here, we couple <i>in situ</i> infrared spectral response measurements and discrete dipole approximation (DDA) calculations to show the impact of axially graded carrier density profiles on the optical properties of mid-infrared LSPRs supported by Si nanowires synthesized by the vapor–liquid–solid technique. The region immediately adjacent to each intentionally encoded resonator (<i>i.e.</i>, doped segment) can exhibit residual carrier densities as high as 10²⁰ cm^–3, which strongly modifies both near- and far-field behavior. Lowering substrate temperature during the spacer segment growth reduces this residual carrier density and results in a spectral response that is indistinguishable from nanowires with ideal, atomically abrupt carrier density profiles. Our experiments have important implications for the control of near-field plasmonic phenomena in semiconductor nanowires, and demonstrate methods for determining and controlling axial dopant profile in these systems

Influence of Dielectric Anisotropy on the Absorption Properties of Localized Surface Plasmon Resonances Embedded in Si Nanowires

We utilize discrete dipole approximation simulations to provide a detailed picture of the scattering behavior of mid-infrared localized surface plasmon resonances (LSPRs) in selectively doped (i.e., <i>i</i>–<i>n</i>^<i>++</i>–<i>i</i>) Si nanowires. Our simulations, and their quantitative comparison to recent experimental results, show that the large refractive indices (<i>n</i> ≈ 3–4) of undoped semiconductors in the infrared and the anisotropic dielectric environment inherent in the nanowire geometry strongly enhance/depress absorption by the longitudinal/transverse LSPR. An examination of “cladding” materials other than Si (e.g., GaAs, Ge, etc.) reveals that this behavior scales with refractive index and that absorption enhancements of at least 35× are possible relative to an isotropic vacuum. We also show how scattering and absorption contribute to the overall extinction and extract a value for the carrier density of Si-based resonators synthesized via the vapor–liquid–solid (VLS) mechanism. Our findings establish a framework for rationally engineering LSPR spectral response in semiconductor nanowires and highlight the promise of the VLS technique for this purpose