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

Strong Near-Field Coupling of Plasmonic Resonators Embedded in Si Nanowires

The strength of localized surface plasmon resonance (LSPR) near-field interactions scales in a well-known, nearly universal manner. Here, we show that embedding resonators in an anisotropic dielectric with a large permittivity can substantially increase coupling strength. We experimentally demonstrate this effect with Si nanowires containing two phosphorus-doped segments. The near-field decay length scaling factor is extracted from <i>in situ</i> infrared spectral response measurements using the “plasmon ruler” equation and found to be ca. 4−5 times larger than for the same resonators in isotropic vacuum or Si. Discrete dipole approximation calculations support the observed coupling behavior for nanowires and show how it is affected by the resonator geometry, carrier density, and embedding material (Si, Ge, GaAs, etc.). Our findings demonstrate that equivalent near-field interactions are achievable with a smaller total volume and/or at increased resonator spacing, offering new opportunities to engineer plasmon-based chemical sensors, catalysts, and waveguides

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