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Phys Rev Lett. 2008 Oct 17;101(16):167401. Epub 2008 Oct 15.

Causality-based criteria for a negative refractive index must be used with care.

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Blackett Laboratory, Imperial College, Prince Consort Road, London, United Kingdom.


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 omega. It also allows us to relate the global properties of the loss (i.e., its frequency response) to its local behavior at omega. 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.

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