Frequency higher then plasma frequency

[Thierry12]

Electromagnetic wave with higher frequency then plasma frequency are barely attenuated. I always hear that the reason is the electrons can't oscillate fast enought... how does that actually work? (how can an electron not be able to oscillate at the same speed?)

 

[Born2bwire]

Because the behavior of a plasma is non-linear. Take a look at the "Convective Derivative." A normal force equation, assuming constant mass, is just $$m\frac{\partial \mathbf{v}}{\partial t}$$ but when we talk about plasma, there is an extra term that arises resulting in the force being $$m\left[\frac{\partial \mathbf{u}}{\partial t} + \left(\mathbf{u}\cdot \nabla \right)\mathbf{u} \right]$$. This non-linearity gives rise to the ponderamotive force and other effects that will affect the propagation. In addition, electrons have mass and thus will time-lag the electric feld due to their inertia. If the frequency is low enough, the lag is negligible but at very very high frequencies they cannot keep up with the oscillations. This is a common assumption with the ions but it can apply to the electrons too.

 

[granpa]

I just read this on a wikipedia page:
At the plasma frequency and above, dielectrics behave as ideal metals, with electron gas behavior.

I thought 'plasma frequency' referred to conductors not dielectrics.

 

[Born2bwire]

Plasma frequency applies to many things. It is not necessarily tied to one type of material and is derived based upon plasma physics, an ionized gas. So anytime that you can achieve a plasma in a material, the plasma frequency can arise to describe various phenomenon. A lot of materials, when subjected to a very high frequency electromagnetic wave, will behave like a plasma. Conductors will do this, I believe silver's plasma frequency is in the terahertz, but it does not surprise me that it can occur in dielectrics too.