Let's say I produce a thin electromagnetic wave beam that is directed towards a non-conductive large plastic block (let's say 30cm x 30cm x 30cm). Let's say that the EM beam enters into the solid plastic from point A and goes out from the other side of the block at point B. I want to heat (without melting) the plastic particles that is located closest to the line AB. But I want the rest of the plastic cube to stay relatively cold. In fact, I couldn't find solid answers to the following questions:
What would be the best frequency range of the EM wave for this purpose?
What is the name of the phenomenon when the EM beam heats the non-conductive plastic material as it propagates from point A to B?
As the EM beam propagates from point A to B, would the em beam disperse, or would it propagate in a linear way just like a standard laser pointer?
what would be the decay rate of the energy of the EM beam as it travels from A to B in plastic.
Could you help me find answers to these questions and could you provide sources that can help me find answers? Also, it would help a lot if you could give the names of the phenomena that happen in this situation.