title: Observation of wave packet distortion during a negative-group-velocity transmission.
abstract: In Physics, causality is a fundamental postulation arising from the second law of thermodynamics. It states that, the cause of an event precedes its effect. In the context of Electromagnetics, the relativistic causality limits the upper bound of the velocity of information, which is carried by electromagnetic wave packets, to the speed of light in free space (c). In anomalously dispersive media (ADM), it has been shown that, wave packets appear to propagate with a superluminal or even negative group velocity. However, Sommerfeld and Brillouin pointed out that the "front" of such wave packets, known as the initial point of the Sommerfeld precursor, always travels at c. In this work, we investigate the negative-group-velocity transmission of half-sine wave packets. We experimentally observe the wave front and the distortion of modulated wave packets propagating with a negative group velocity in a passive artificial ADM in microwave regime. Different from previous literature on the propagation of superluminal Gaussian packets, strongly distorted sinusoidal packets with non-superluminal wave fronts were observed. This result agrees with Brillouin's assertion, i.e., the severe distortion of seemingly superluminal wave packets makes the definition of group velocity physically meaningless in the anomalously dispersive region.
authors: Laboratory of Applied Research on Electromagnetics (ARE), Zhejiang University, Hangzhou 310027, China.
authors: Dexin
authors: D
authors: Ye
authors: 1] Laboratory of Applied Research on Electromagnetics (ARE), Zhejiang University, Hangzhou 310027, China [2] Institute of Electromagnetic Fields (IEF), ETH Zurich, 8092 Zurich, Switzerland.
authors: Yannick
authors: Y
authors: Salamin
authors: Laboratory of Applied Research on Electromagnetics (ARE), Zhejiang University, Hangzhou 310027, China.
authors: Jiangtao
authors: J
authors: Huangfu
authors: Zhejiang University City College, Hangzhou 310015, China.
authors: Shan
authors: S
authors: Qiao
authors: 1] Biomedical Engineering, University of Connecticut, Storrs, CT 06269, USA [2] Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, USA.
authors: Guoan
authors: G
authors: Zheng
authors: Laboratory of Applied Research on Electromagnetics (ARE), Zhejiang University, Hangzhou 310027, China.
authors: Lixin
authors: L
authors: Ran
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