Giant enhanced optical nonlinearity of colloidal nanocrystals with a graded-index host

TL;DR

This paper theoretically investigates how a graded-index host enhances the linear and third-order nonlinear optical properties of metallic colloidal crystals, revealing sharp resonance peaks and broadening effects due to layer interactions and host gradient.

Contribution

It introduces a self-consistent method to analyze local electric fields in graded-index colloidal systems, highlighting novel resonance behaviors not seen in homogeneous or random colloids.

Findings

Sharp resonance peaks in optical properties due to layer interactions

Broadening of peaks with increased packing density

Enhanced nonlinearity from graded-index host effects

Abstract

The effective linear and third-order nonlinear optical properties of metallic colloidal crystal immersed in a graded-index host fluid are investigated theoretically. The local electric fields are extracted self-consistently based on the layer-to-layer interactions, which are readily given by the Lekner summation method. The resultant optical absorption and nonlinearity enhancement show a series of sharp peaks, which merge in a broadened resonant band. The sharp peaks become a continuous band for increasing packing density and number of layers. We believe that the sharp peaks arise from the in-plane dipolar interactions and the surface plasmon resonance, whereas the continuous band is due to the presence of the gradient in the host refractive index. These results have not been observed in homogeneous and randomly-dispersed colloids, and thus would be of great interest in optical nanomaterial engineering.