Giant negative group time delay by microwave mode adaptors (Vol. 43 No. 6)

Input (black) of a pulse traversing 20 m adaptor cavity and its output (red). The adaptor cavity delay is 2200ns and leads to a negative time shift.

In 1960 Brillouin wrote a footnote in his famous book on ‘Wave Propagation And Group Velocity’ (p.79): “The negative parts of the (theoretical) group velocity have no physical meaning. A negative velocity shows the maximum of the group at the output before it has entered the input of a special medium”. However, since 1985 several physicists measured a negative delay and thus a negative group velocity at a sharp molecular resonance. Recently a giant negative group time delay was observed in a medium of two microwave mode adaptors separated by a 20 m long waveguide when they are not parallel aligned. Instead of +60 ns vacuum time -2.2 µs were measured for the same distance. Such adaptors are used in communication technology to transform rectangular waveguide modes into circular waveguide modes: For instance, in the case of TV reception via satellite. The strange behaviour is based on a 90° shift of the linear polarization of the superimposed right and left circular wave modes in the case of a perpendicular adaptor orientation. Remarkable, the polarization shift of 90° takes place at each reflection and in this way makes the shifted adaptors to reflectors, whenever the distance between the adaptors equals a multiple of half the wavelength.

A. Carôt, H. Aichmann and G. Nimtz, ‘Giant negative group time delay by microwave adaptors’, EPL, 98, 64002 (2012)