Phys. Rev. Lett. 101, 167401 (2008) [4 pages]Causality-Based Criteria for a Negative Refractive Index Must Be Used With CareReceived 7 February 2008; revised 18 August 2008; published 15 October 2008 Using the principle of causality as expressed in the Kramers-Kronig relations, we derive a generalized criterion for a negative refractive index that admits imperfect transparency at an observation frequency ω. It also allows us to relate the global properties of the loss (i.e., its frequency response) to its local behavior at ω. However, causality-based criteria rely on the group velocity, not the Poynting vector. Since the two are not equivalent, we provide some simple examples to compare the two criteria. © 2008 The American Physical Society URL:
http://link.aps.org/doi/10.1103/PhysRevLett.101.167401
DOI:
10.1103/PhysRevLett.101.167401
PACS:
78.20.Ci, 71.45.Gm
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