smallercap

Smallercap is a term used in nanophotonics to describe cap-like nanostructures in which the cap portion has a smaller radius of curvature than the surrounding body. The geometry concentrates and enhances electromagnetic fields at the cap edge, enabling tailored plasmonic resonances in the visible to near-infrared. The term is descriptive rather than prescriptive, used to classify nanoparticles that combine a larger core with a diminutive cap segment.

Design and fabrication typically involve a metallic core, commonly gold or silver, with a partial shell or cap formed by controlled deposition or selective etching. Realizations use colloidal synthesis with seed-mediated growth, electron-beam lithography with lift-off, or masked deposition on substrates. Key parameters include core diameter, cap height, and cap curvature radius, all of which influence the optical response.

Optical properties are highly tunable: cap size and shape shift the localized surface plasmon resonance and affect field confinement and quality factor. Theoretical models predict strong electromagnetic hot spots at the cap edge, which can enhance spectroscopic signals.

Applications include surface-enhanced Raman spectroscopy (SERS), optical sensing, and integration into plasmonic antennas for nanoscale imaging. Smallercap nanostructures are an active area of study in nanofabrication and plasmonics, with ongoing work addressing reproducibility, orientation effects, and substrate interactions.