Electromagnetic stress at the boundary: photon pressure or tension?

TL;DR

This paper investigates the electromagnetic stress at material boundaries, revealing that the boundary stress can be either tension or pressure depending on microscopic structure, challenging the traditional view of photon pressure.

Contribution

It introduces an ab initio method accounting for microstructure to determine boundary electromagnetic stress, highlighting the role of lattice effects in optical stress.

Findings

Electromagnetic boundary stress is indeterminate with only macroscopic parameters.

Microscopic lattice structure influences whether light exerts tension or pressure.

Helmholtz stress tensor can account for lattice effects in boundary stress calculations.

Abstract

It is well known that incident photons carrying momentum hk exert a positive photon pressure. But if light is impinging from a negative refractive medium in which hk is directed towards the source of radiation, should light insert a photon "tension" instead of a photon pressure? Using an ab initio method that takes the underlying microstructure of a material into account, we find that when an electromagnetic wave propagates from one material into another, the electromagnetic stress at the boundary is in fact indeterminate if only the macroscopic parameters are specified. Light can either pull or push the boundary, depending not only on the macroscopic parameters but also on the microscopic lattice structure of the polarizable units that constitute the medium. Within the context of effective medium approach, the lattice effect is attributed to electrostriction and magnetostriction which can be accounted for by the Helmholtz stress tensor if we employ the macroscopic fields to calculate the boundary optical stress.