Effect of non-specificity in shape, size, and dielectric properties on electromagnetic extinction and optical field enhancement from spherical nanolayered metal-dielectric particles

摘要

Metal-dielectric composite nanospheres can amplify the scattering, emission, and absorption signature of molecules in their vicinity. Their ability to redistribute electromagnetic fields and produce pockets of greatly amplified fields is the dominant cause in achieving enhancement effects, for example, for surface-enhanced Raman spectroscopy. Extensive use of the field amplification has been made in devising ultrasensitive tag (label)-based spectroscopic techniques. For example, we have recently proposed nano-layered alternating metal-dielectric particles (nano-LAMP)-a symmetric implementation of which is a nanoparticle consisting of alternating metal and dielectric shells. Exceptional spatial and spectral control onamplification can be achieved by designing the size and location of metal and dielectric layers in this geometry. Theoretical understanding exists and an engineering optimization approach can be adapted to design a palette of probes exploiting this control and tunability. However, current fabrication techniques are limited in their ability to achieve the required specificity in the spherical configurations. Hence, we investigate here the effects of variability, introduced by fabrication approaches into the structure of nano-LAMPs, on their spectroscopic signature. In particular, theoretical results are presented for the effects on enhancement due to variability in size, shape, and dielectric environment in the cases of gold-silica, silver-silica, and copper-silica nano-LAMPs. The results obtained show that the shape and dielectric properties of the metal shell play a crucial role in experimentally realizing the specificity of the magnitude of the enhancement and determine the key parameters to control and test in experimental validations.