Extinction spectra and electric field enhancement of silver chiral nano-flower shaped nanoparticle; comparison of discrete dipole approximation results with experimental results

摘要

In this work, by using the discrete dipole approximation (DDA), we have proposed a model for the growth of chiral nano-flower shaped sculptured nanoparticle in different sizes and different symmetries (e.g., 3-, 4- and 5-fold symmetries) and calculated the extinction spectra for these nanostructures identifying their plasmon peaks. Our results show that the extinction spectrum obtained experimentally for a chiral nano-flower sculptured thin film can be the result of the overlap of extinction spectra from an assembly of chiral nanoflowers forming the deposited thin film. Furthermore, we have investigated carefully the distribution of electric nearfields and have deduced the physical nature and direction of oscillations related to different plasmon peaks in the extinction spectra. Results show the existence of dipolar and quadrupolar oscillations in all structures while for the 4-fold symmetry chiral nano-flower, we have also observed octopolar oscillations. The calculated results are compared with the experimental results reported by us in [H. Savaloni, F. Haydari-Nasab, M. Malmir, Appl. Surf. Sci. 257, 9044 (2011)]. In addition, using the homogenization theory we have shown that the optical response of metallic structures is affected by a limited thickness of thin film and further increase of the film thickness has no effect on the optical spectra.