The title of my paper refers to "very high power densities", and this implies here, of course, optical power, but first I should quantify what I mean by "very high". For the purpose of this paper, it shall mean power densities at and beyond which optical glasses are no longer transparent. This occurs basically due to the creation of free electrons by multi-photon absorption, which then cause stress and subsequent physical damage, such as cracks, in the glass. There is a threshold for this phenomenon, typically of the order of 1010 W/cm2 but strongly material dependent. For comparison: this is 7 orders of magnitude higher than what we can ever achieve by focussing light from the sun, our most powerful natural radiation source. It is, needless to say, readily achieved with current technology solid state pulsed lasers.