When do non-TEM waves occur?

Question

Yesterday, I read about TEM waves on wikipedia. Those are waves that have no electric or magnetic field vectors pointing into propagation direction of the wave.

But on any picture illustrating EM waves, they are showing TEM waves instead of the (normal?) non-TEM waves. Before reading about TEM waves, I thought that there exist only TEM waves (i.e that the characteristic about EM waves is that they are transversal).

When do non-TEM waves get created and what factors do influence the angle of oscillation of the electric and/or magnetic fields of an EM wave? How do these waves behave different in practice?

Thanks for any insight!

Answer 1

If you check out the wikipedia page:

Transverse Mode

You'll see at the start of the second paragraph they say

"Transverse modes occur because of boundary conditions imposed on the wave by the waveguide"

This accounts for the discrepancy you have observed. Most diagrams of electromagnetic waves depict waves in free space, far away from any objects, and in that situation, you will have no electric or magnetic component in the propagation direction. Waveguides are (metal or fiber) tubes that confine an electromagnetic wave and force it in a particular direction. Since you have now introduced materials into the picture, things get more complicated, and you can have a non-zero electric and magnetic fields in the propagation direction.

Most E+M textbooks will have a section on waveguides, and these concepts figure heavily into fiber optics engineering.

Answer 2

http://redshift.vif.com/JournalFiles/Pre2001/V07NO1PDF/V07N1MGU.pdf That PDF has a nice writeup on non-planar EM waves which all have longitudinal components.

Looking at my non-linear optics literature I see that one cause of non-transverse behavior is when the EM field generated alters the dielectric properties of the propagation material. This alteration travels with the wave (thus in the propagation direction).

What a cool question!