The optical response of ring-shaped gold nanoparticles prepared by colloidal lithography is investigated. Compared to solid gold particles of similar size, nanorings exhibit a redshifted localized surface plasmon that can be tuned over an extended wavelength range by varying the ratio of the ring thickness to its radius. The measured wavelength variation is well reproduced by numerical calculations and interpreted as originating from coupling of dipolar modes at the inner and outer surfaces of the nanorings. The electric field associated with these plasmons exhibits uniform enhancement and polarization in the ring cavity, suggesting applications in near-infrared surface-enhanced spectroscopy and sensing.
The optical properties of gold nanodisk arrays prepared by colloidal lithography are studied experimentally.
The arrays exhibit short range translational order and weak interparticle interactions. Tunable localized surface
plasmon resonances are achieved by varying the diameter of the disks at constant disk height. The macroscopic
optical properties are well-described by modeling the gold disks as oblate spheroids in the electrostatic limit.
The optical sensing capabilities of the disks are investigated by varying the surrounding refractive index. It
is found, in agreement with theory, that more oblate disk shapes have higher sensitivity. This suggests that
nanodisks prepared by colloidal lithography are of interest as substrates for optimizing optical biosensing
methods at the nanometer scale.
Elastic scattering measurements show that isolated nanometric holes in optically thin Au films exhibit a localized surface plasmon resonance
in the red to near-infrared region. The hole plasmon red shifts with increasing hole diameter or increasing refractive index of the surrounding
medium, analogous to a dipolar particle plasmon. A pronounced blue shift is observed when the distance between holes is decreased, indicating
an enhanced coupling between holes mediated by surface plasmon polaritons of the intervening flat film surface.
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