Study On The Optical Rroperties Of Metal Nanoparticles With Discrete Dipole Approximation Method

The optical properties of metallic nanoparticles and their application in surface enhanced Raman scattering (SERS) have been a hot topic in nanoscience and nanotechnology. Because nanoparticles are different from bulk materials in the construction, nanoparticles bear some effects, for example, quantum size effect, small size effect, surface effect and so on. These basic properties lead to special optical properties-surface plasmon resonance. resonance. The surface plasma resonance peak positions is influenced by the size, shape, metal composition and surrounding medium, the surface electrical field of nanoparticles are enhanced greatly. The special properties can apply m many areas, for example, sensing the dielectric environments, surface-enhanced spectroscopy of chemical and biological molecules and single molecule detection and so on.The discrete dipole approximation (DDA) is used to investigate the optical properties of metal nanoparticles. Firstly it investigate that the relation of the surrounding medium and the extinction spectra of silver nanoparticles with spherical, ellipsoidal shapes. The calculation results show that the resonance peak position is red-shifted and the resonance peak width is enhanced with the increase of the refraction index of the surrounding medium and the increase of the particle size. In addition, with the increase of the refraction index of the surrounding medium, the plasmon resonance wavelength of the peak appears approximate linear red-shift.For the two dimensional (2D) arrays of metal nanoparticles, it is found that the extinction properly and the surface plasmon resonance wavelength are both influenced by the composition, size of the nanoparticles and the array period and so on. If the period of2D square array is near the resonant wavelength of the single nanoparticle the very narrow plasmon lines appear in the extinction spectra, which results from diffraction of far-field dipole interaction. The position of the very narrow peak is assigned to the conversion wavelength when the grating order changes from evanescent to radiative. The resonant wavelengths of gold and silver nanoparticle square array are different respectively because of the different resonant wavelength of single gold and silver nanoparticle. And the very-narrow peak positions are assigned to different grating orders. So the position of the very-narrow peak of gold nanoparticle appears in the left side of the resonance wavelength of the single nanoparticle. The2D rectangular arrays of silver nanoparticles have different periods in two directions of parallel and perpendicular to the incidence polarization. The resonance peak position and width of2D rectangular arrays could get modulated conveniently by changing the period in the direction of parallel and perpendicular to the incident polarization.