Periodically loaded transmission line with effective negative refractive index and negative group velocity

Media with negative refractive index (NRI) are expected to exhibit properties that are unusual compared to materials with a positive index of refraction. However, until recently, these properties were not experimentally observed, since no NRI material occurs naturally. Periodic structures with NRI have been constructed. Our group created artificial NRI materials by loading a cellular network of transmission lines with series capacitors and shunt inductors (Eleftheriades, G.V. et al., IEEE Trans. Microwave Theory and Techniques, vol.50, no.12, p.2702-12, 2002; Microwave and Wireless Component Lett., 2003). We have extended that work to design a medium that exhibits negative group velocity (NGV) in addition to NRI. To achieve the NGV, a resonant circuit is embedded within each loaded transmission line (LTL) unit cell. The resonance produces a region of anomalous dispersion in which the group delay, and thus the group velocity, is negative. The NGV means that the peak of the output pulse emerges from the LTL prior to the peak of the input pulse, though much reduced in magnitude. Note that the front of the output pulse does not precede the front of the input pulse; that is, the output pulse front suffers the usual positive delay. The proposed transmission line is fabricated using coplanar waveguide technology. Scattering matrix measurements verify the theoretical predictions.