Novel approach to nonlinear susceptibility

TL;DR

This paper introduces a new many-body theoretical formalism that accurately calculates third order nonlinear susceptibilities for complex Hamiltonians, overcoming previous limitations and revealing different results from earlier methods.

Contribution

It develops a novel many-body approach applicable to arbitrary Hamiltonians, enabling precise calculation of nonlinear susceptibilities in nanooptics.

Findings

Proves size-dependent term cancellation for complex Hamiltonians

Provides a new formalism yielding different results from previous methods

Advances the theoretical understanding of nonlinear nanooptics

Abstract

The calculation of the third order susceptibility still is a long standing fundamental problem of particular importance in nonlinear nanooptics: Indeed, cancellation of size-dependent terms coming from uncorrelated excitations is expected, but up to now shown for very simple Hamiltonians only. Using a many-body theory recently developed to handle interacting close-to-bosons, we prove it here for \emph{arbitrary} H. This new formalism actually provides the first clean way to calculate nonlinear susceptibilities, with results different from previous ones.