The Effect of Epsilon-Near-Zero (ENZ) Modes on the Casimir Interaction between Ultrathin Films

TL;DR

This paper explores how epsilon-near-zero (ENZ) modes in ultrathin films influence the Casimir force, revealing persistent repulsive contributions that depend on film thickness and could be used to control nanomechanical motion.

Contribution

It is the first to analyze the force distribution over real frequencies for ultrathin films, highlighting the role of ENZ modes in modifying Casimir interactions.

Findings

ENZ modes cause significant repulsive Casimir contributions.

Force contributions are persistent around the ENZ frequency regardless of separation.

Thickness of ultrathin films strongly affects the Casimir force density.

Abstract

Vacuum fluctuation-induced interactions between macroscopic metallic objects result in an attractive force between them, a phenomenon known as the Casimir effect. This force is the result of both plasmonic and photonic modes. For very thin films, field penetration through the films will modify the allowed modes. Here, we investigate the Casimir interaction between two ultrathin films from the perspective of the force distribution over real frequencies for the first time and find pronounced repulsive contributions to the force due to the highly confined and nearly dispersion-free epsilon-near-zero (ENZ) modes that only exist in ultrathin films. These contributions are found to persistently occur around the ENZ frequency of the film and are irrespective of the inter-film separation. We further associate the ENZ modes with a striking thickness dependence in the averaged force density for conductive thin films, a metric signifying a thin-film's acceleration due to Casimir effect. Our results shed light on the role of the unique vacuum fluctuation modes existing in ultrathin ENZ materials, which may offer significant potential for engineering the motion of objects in nanomechanical systems.