Negative Group Velocity and Spin-Flip in Microwave Adaptors

TL;DR

This paper demonstrates a microwave interferometer setup that exhibits negative group delay and velocity due to polarization shifts and spin-flip phenomena in adapted waveguides, revealing novel wave propagation effects.

Contribution

It introduces a passive dielectric medium with negative group delay created by polarization shifts in microwave adaptors, and explains the role of spin-flip in achieving negative group velocity.

Findings

Negative group delay observed in microwave waveguides

Polarization change causes spin-flip and negative group velocity

Experimental validation in frequency and time domains

Abstract

A Fabry-Perot like interferometer with two microwaveguide adaptors as reflectors creates a passive dielectric medium with a negative group delay time due to polarization shift. A rotational strain of the polarization vector by one of the adaptors is coupled with a drastic negative group velocity. The adapted rectangular and circular waveguides have the same dispersion. The input rectangular waveguide mode is linearly polarized, whereas the basic mode of the adapted circular waveguide is circularly polarized. A 667 wavelengths long circular waveguide connects the input with the output adaptor. Experiments are performed in the frequency and in the time domain. We describe, how the helical polarization change and the spin-flip of the two different circular wave modes produce the observed negative group velocity.