Tunable Negative Group Delay in a Birefringent Fabry–Pérot-Like Cavity With High Fractional Advancement Induced by Cross-Interference Effect

A strong and broadband negative group delay (NGD) is demonstrated in a circular waveguide with an asymmetric cross-shaped slotted structure in the millimeter-wave region. For an input of 45° linear polarization (relative to the orientation of the cross-shaped slots), the NGD fractional bandwidth is as high as 3.57%, in which the optimal group advancement obtained is nearly 24 times greater than the delay traveling with the speed of light in vacuum. This further indicates a very high fractional advancement of 12.10%. In addition, the NGD can be controlled by adjusting the polarization azimuthal angle of the incident pulse. The observed tunable NGD effect is attributed to the destructive cross interference between the two polarization eigenstates, which effectively reduces the released field energy at the observed polarization channel. A general theory is provided with excellent agreement in the experimental and simulation results.