Bound-state third-order optical nonlinearities of embedded germanium nanocrystals

TL;DR

This paper theoretically analyzes the third-order nonlinear optical properties of embedded germanium nanocrystals, revealing size and wavelength dependencies, and compares them with silicon nanocrystals to identify potential for optical switching applications.

Contribution

It provides a detailed theoretical characterization of the nonlinear susceptibilities of germanium nanocrystals, including size and wavelength effects, using an atomistic pseudopotential approach.

Findings

Ge nanocrystals have smaller nonlinear susceptibilities than Si nanocrystals of the same size.

Two-photon absorption threshold extends beyond half the band-gap, enabling wider wavelength tunability.

Refractive index increases as nanocrystal size decreases, similar to Si nanocrystals.

Abstract

Embedded germanium nanocrystals (NCs) in a silica host matrix are theoretically analyzed to identify their third-order bound-state nonlinearities. A rigorous atomistic pseudopotential approach is used for determining the electronic structure and the nonlinear optical susceptibilities. This study characterizing the two-photon absorption, nonlinear refractive index, and optical switching parameters reveals the full wavelength dependence from static up to the ultraviolet spectrum and the size dependence up to a diameter of 3.5 nm. Similar to Si NCs, the intensity-dependent refractive index increases with decreasing NC diameter. On the other hand, Ge NCs possess about an order of magnitude smaller nonlinear susceptibility compared to Si NCs of the same size. It is observed that the two-photon absorption threshold extends beyond the half band-gap value. This enables nonlinear refractive index tunability over a much wider wavelength range free from two-photon absorption.