I was trying to find $\varepsilon_\infty$ for InSb, and the value I found on Kittel p. 416 was 15.6. But in the literature, even at 6 eV, $\varepsilon (\omega )$ is still in the negative (at -3.835) but that's beyond the optical region and has not reached the positive value of 15.6 shown in Kittel.

Yet, $\varepsilon_\infty$ is supposed to be synonymous with $\varepsilon_\text{opt}$ i.e. that the high-frequency limit is reached at optical frequencies (in visible region).

So really my question is: for finding $\varepsilon_\infty$ it's not enough to go look in optical region right, so the use of $\varepsilon_\text{opt}$ is basically misleading and wrong, right?


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