Universal Zero Conductivity Condition for Optical Absorption

TL;DR

This paper reveals a universal zero conductivity condition at complex frequencies that limits electromagnetic energy transduction in nanoscale optical absorbers, providing a fundamental principle for optimizing various photonic devices.

Contribution

It introduces a universal zero conductivity condition based on causality that sets fundamental limits on optical absorption in nanoscale systems.

Findings

The upper limit of absorption rate is determined by the zero of optical conductivity.

The zero conductivity condition is derived from causality in passive linear response.

An experimental verification method using plasmonic nanoparticles is proposed.

Abstract

Harnessing information and energy from light within a nanoscale mode volume is a fundamental challenge for nanophotonic applications ranging from solar photovoltaics to single photon detectors. Here, we show the existence of a universal condition in materials that sheds light on fundamental limits of electromagnetic to matter energy conversion (transduction). We show that the upper limit of absorption rate (transduction rate) in any nanoscale absorber converting light to matter degrees of freedom is revealed by the zero of optical conductivity at complex frequencies ($\sigma(\omega^\prime + i\omega^{\prime\prime})= 0$). We trace the origin of this universal zero conductivity condition to causality requirements on any passive linear response function and propose an experiment of absorption resonances using plasmonic nanoparticles to experimentally verify this universal zero conductivity condition. Our work is widely applicable to linear systems across the electromagnetic spectrum and allows for systematic optimization of optical absorption in single photon detectors, solar cells, Coherent Perfect Absorbers and SPASERS.