struggled to remember something something…..metals….resonance frequency….transparency…..

Finally found the thread

-> Plasma Frequency: “Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric fieldof the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it.”-wiki plasmons

      ->Ponderomotive:“However, there are secondary effects on the electrons in the plasma from the magnetic field of the electromagnetic wave. This gives rise to what is called the ponderamotive force. If I recall correctly, the ponderamotive force is what allows the wave to eventually propagate.” - physics forum

“In physics, a ponderomotive force is a nonlinear force that a charged particle experiences in an inhomogeneous oscillating electromagnetic field.” -wiki ponderomotive

                ->Geometric diodes: “We need some sort of non-linear effect to make this work!” -Garret Moddel, my old advisor

hm.

then….

Frisbee.