The GaN hemt would make sense for a second board iteration where you drive the 75V straight through the diode with a very short (variable) pulse like in this EPC reference design:

https://epc-co.com/epc/products/evaluation-boards/epc9172

That way the 1.4 Ohm resistor doesn't eat like 60-80% of the energy (40amps drop around 60V for an instant 240W in the resistor vs the foward voltage of the diode which is like 1.5V or so for the 60W peak).

Just watched a talk on this topic from last December: https://media.ccc.de/v/39c3-of-boot-vectors-and-double-glitc...

I'm more interested in the 3D printed platform made to to hold up the chip and the fact that this seemed to be done in a garage. I thought anything laser related would require much more precision but the article has pretty much everything ordered online.

Previously discussed: https://news.ycombinator.com/item?id=42708682