Attenuation of waveguide modes in narrow metal capillaries

TL;DR

This paper investigates waveguide modes in narrow metal capillaries, revealing that their attenuation lengths are significantly longer than classical predictions and that they can efficiently transfer energy to the fundamental mode, with implications for laser pulse guiding.

Contribution

It introduces a universal structure of waveguide modes in metal capillaries and demonstrates their enhanced attenuation lengths and energy transfer efficiency, regardless of capillary material.

Findings

Attenuation length is two orders of magnitude longer than classical estimates.

Up to 98% of initial energy can transfer to the fundamental mode.

Mode structure depends only on the cross-sectional shape, not material.

Abstract

The channeling of laser pulses in waveguides filled with a rare plasma is one of promising techniques of laser wakefield acceleration. A solid-state capillary can precisely guide tightly focused pulses. Regardless of the material of the capillary, its walls behave like a plasma under the influence of a high-intensity laser pulse. Therefore, the waveguide modes in the capillaries have a universal structure, which depends only on the shape of the cross-section. Due to the large ratio of the capillary radius to the laser wavelength, the modes in circular capillaries differ from the classical TE and TM modes. The attenuation length for such modes is two orders of magnitude longer than that obtained from the classical formula, and the incident pulse of the proper radius can transfer up to 98% of its initial energy to the fundamental mode. However, finding eigenmodes in capillaries of arbitrary cross-section is a complex mathematical problem that remains to be solved.