Negative n from simultaneously negative e and m: Negative Index Media made so far consist of arrays of wires and split-ring resonators. These systems have overlapping, narrow resonances in effective m and e as shown. In a narrow frequency region above the two resonances, both m and e can have negative real parts and small imaginary parts (low-loss "transparency range" with negative phase index).

However, a paper by Pendry et. al. in Physical Review Letters (PRL, V.76, 25, 4773 June 1996) claimed that the real physics behind the e resonance is the lowering of the (quantum) plasmon mode in metal wires to extremely low frequencies because the self magnetic field of electrons in a thin (less than 10 microns) wire makes them as heavy as nitrogen atoms.

Walser, Valanju and Valanju's comment in PRL, V.87, No. 11, 119701-1 10 Sept. 2001 proved this "extermely low frequency plasmon" concept to be completely wrong since it neglected the dominant collisions and violated gauge invariance. It was shown that there are no "low frequency plasmons (classical or quantum)" and no "electrons as heavy as nitrogen atoms" in these wires, and none are needed to explain the observed electromagnetic responses.

Pendry et. al. reply to our PRL comment was not accepted or published by Physical Review Letters (PRL).

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