Diffraction patterns
observed in surface plasmon resonance imaging
(SPRI) microscopy measurements of single gold nanorods (AuNRs) exhibit
a complex behavior at wavelengths near the longitudinal plasmonic
resonance band. SPRI microscopy measurements at 814 nm from AuNRs
in three samples with resonance extinction maxima at 670, 816, and
980 nm reveal a variety of diffraction patterns with central peaks
that are either positive, negative, or biphasic. A unitless ratio
parameter M
R (−1 ≤ M
R ≤ 1) is created to describe the distribution
of diffraction patterns. A purely negative (M
R = −1) central peak is observed for 30%, 57%, and 98%
of the diffraction patterns in the 670, 816, and 980 nm samples, respectively.
These results along with a theoretical modeling of the diffraction
patterns with an anisotropic complex scattering coefficient suggests
that this behavior only occurs for AuNRs when the laser wavelength
used in SPRI experiments is shorter than the AuNR plasmonic resonance
maxima, that is, in the anomalous dispersion region.
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