= Single-polarization two-dimensional photon density of states
{title2=$g(\omega)/V=\omega/(2\pi c^2d)$}
For an area $A$ with periodic in-plane <boundary conditions>, one polarization contributes $A\,2\pi k\,dk/(2\pi)^2$ <normal modes> of the <electromagnetic field>. For linear <dispersion relation> $\omega=ck$ and effective physical volume $Ad$, this gives the <density of states> in the title. No extra factor two for polarization is allowed. Opposite in-plane directions are already counted in the full circle of <wavevectors>.
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