Transverse Plasmon Resonance Research Articles

Anisotropic optical properties of arrays of gold nanorods embedded in alumina

A series of thin films comprising gold nanorods embedded in an alumina matrix have been fabricated with lengths ranging from 75 to $330\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Their optical properties, expressed in terms of extinction $\ensuremath{-}\mathrm{ln}(T)$, where $T$ is optical transmittance, have been measured as a function of wavelength, rod length, angle of incidence, and incident polarization state. The results are compared to a Maxwell-Garnett based theory modified to take into account the strongly anisotropic nature of the medium. Transverse and longitudinal plasmon resonances are observed. The interaction between the nanorods leads to the splitting of the longitudinal resonance with the longer-wavelength resonance being forbidden for direct optical observations. The shorter-wavelength resonance related to the symmetric coupling between longitudinal plasma excitations in the nanorods depends on rod length, polarization state, and angle of incidence of the probing light. The impact of electron confinement on the optical properties of the gold rods is also seen and may be incorporated into the Maxwell-Garnett theory by restricting the mean free path of the conduction electrons to produce excellent agreement between observations and the complete theory. Annealing experiments that modify the physical structure of the gold confirm this conclusion.

Starch-Mediated Shape-Selective Synthesis of Au Nanoparticles with Tunable Longitudinal Plasmon Resonance

We report the synthesis ofAu nanoparticles, with tunable longitudinal plasmon band and shape selectivity, mediated by starch in the presence of ultrasonic waves. The synthesis was carried out by reduction of HAuCl4, at various concentrations, using H2O2 as the reducing agent. When the reactions were carried out in the absence of ultrasonic waves, there was no occurrence of the longitudinal resonance band, while the transverse plasmon resonance band shifted toward a higher wavelength. Transmission electron microscopic measurements revealed an increase in particle sizes with increasing higher initial HAuCl4 concentration. On the other hand, in the presence of ultrasonic waves, as the initial concentration of HAuCl4 was increased, while the transverse plasmon resonance band remained the same, the longitudinal plasmon resonance band increasingly shifted toward a higher wavelength. Transmission electron microscopic measurements revealed the change in shape from spherical to triangular to hexagonal particles with increasing initial HAuC14 concentration. We also report that the starch-stabilized nanoparticles could be precipitated from the solution by a starch digesting enzyme which also binds with the particles resulting in its precipitation.

Optical Properties of One-Dimensional Metal Nanostructures

AbstractWe present a theoretical study of the optical properties of one-dimensional (1-D) metal nanostructures including nanorods and nanotubes. Although the optical properties of gold nanotubes are similar to that of gold nanorods, both the longitudinal and transverse plasmon resonances of gold nanotubes show much larger red-shifting and narrower. The E-field calculations indicate that the 1-D gold nanostructures, particularly gold nanotubes have great potential for applications to SERS.

Optical nonlinearities from transverse plasmon resonance in gold nano-rods

The nonlinear refraction of two gold nano-rods with different sizes was investigated by using Z-scan technique with nanosecond pulses at 532 nm. Their optical limiting (OL) effect was also studied at the wavelength. The experimental results show that they possess strong optical nonlinearities when excited near the transverse plasmon resonance, depending critically on the size of nano-rods. The nonlinear refractions arise mainly from the strong excited-state nonlinearity of conduction band electrons. In contrast, the OL effect originates from the transverse plasmon resonance absorption, excited-state absorption and absorption-induced nonlinear scattering.