I don't think the opacity is varying per se.
I believe the situation is that a plasma will reflect waves of low frequency, and transmit waves of high frequency.
The frequency at which this transition occurs (reflection vs transmission) depends on the characteristics of the plasma - essentially it's electron density. Thus our ionosphere reflects radio waves, but transmits light. A denser plasma will thus reflect light as well as radio waves, which is apparently what the laser people are taking advantage of.
See for instance:
I think the technical term for this frequency is "plasma frequency", but I'm really not positive.
You might also try talking about this topic on rec.arts.sf.science (that's a usenet newsgroup).