Incompatibility of the tunneling limit with laser fields

TL;DR

This paper argues that the Schwinger limit, relevant for longitudinal electric fields causing vacuum polarization, cannot be approached with laser fields, which are transverse and fundamentally different, invalidating tunneling-based predictions in laser-induced processes.

Contribution

It clarifies the fundamental differences between longitudinal and transverse fields and demonstrates the failure of Coulomb-gauge tunneling models for laser-induced pair production.

Findings

Schwinger limit is irrelevant for laser fields.

Tunneling rates do not follow exponential behavior in laser contexts.

Coulomb-gauge treatments fail to predict laser-induced tunneling.

Abstract

The Schwinger limit refers to longitudinal electric fields that are sufficiently strong to "polarize the vacuum" into electron-positron pairs by a tunneling mechanism. Laser fields are transverse electromagnetic fields for which the Schwinger limit has no relevance. Longitudinal and transverse fields are fundamentally different because of the different values of the F^{{\mu}{\nu}}F_{{\mu}{\nu}} Lorentz invariant that characterizes the fields. One aspect of this difference is the zero-frequency limit, that exists for longitudinal fields, but is ill-defined for transverse fields. The goal of approaching the Schwinger limit with sufficiently strong lasers is thus not a possibility. Tunneling transition rates are characterized by an exponential behavior of the form exp(-C/E), where E is the magnitude of the applied electric field and C is a system-dependent constant. Searches for such behavior within a Coulomb-gauge treatment of laser-induced processes are shown to fail.