負屈折媒質は本来，ポインティングベクトルと波数ベクトルが逆方向となる媒質として定義されている.しかし，群速度と波数ベクトルが逆の媒質と定義している文献も多い.エネルギー速度と群速度は異なった概念であり，損失媒質中では値も一致しないし，負群速度媒質では向きが逆にさえなる.ここでは混乱した負屈折媒質の定義を見直す．%The negative refractive medium is characterized as an electromagnetic medium in which the Poynting vector (or the energy flow) and the wavevector point opposite directions. In many articles, however, it is considered as a medium in which the group velocity and the phase velocity have opposite signs. The group velocity and the energy velocity are conceptually independent quantities and could point opposite ways. We reconsider the confused definitions of negative refractivity.
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