Thanks DrDu for your help. I'll try to expand.
Nothing practical. Just theory. Monochromatic plane EM wave propagating in vacuum. The wave equation in this case admits a sinusoidal solution, a cosinusoidal solution, and a complex exponential solution.
In EM propagation, the electric field $$\vec{E}$$, the magnetic field $$\vec{B}$$, and other fields, such as the electric displacement $$\vec{D}$$, which interests me particularly, undulate.
When there is no circular or elliptical polarization, the wave equations of $$\vec{E}$$ and $$\vec{B}$$ do not admit the complex exponential solution. Does the wave of the field $$\vec{D}$$ admit this solution?
The answer depends on considering vacuum polarization impossible or possible for any frequency greater than zero. Vacuum disruptive polarization does not occur at any frequency. And what about a non-disruptive polarization, let's say linear harmonic polarization?
I don't dare to deny in advance the possibility of a linear harmonic polarization of the vacuum, without analyzing the subject carefully and in sufficient detail.