Where I can find photos of nuclear explosions just after detonation (before 5-10 ms, the shorter the better)?
Never underestimate the power of googling.
I googled "high speed nuclear explosion photos" and got a large number of them. Here is one:
p.s. I should have remembered one frame cameras, since I had been using "gun cameras" from WWII to get images of cosmic rays in spark chambers by triggering, back in 1967.
The picture below shows an explosion after about 1 ms. (source) It's called the "rope trick" image, because the explosion is taking place at the top of a tower and the spikes at the bottom are ropes being vaporised by the radiation travelling out from the explosion.
I found it by searching Wikimedia Commons for "nuclear detonation" and then scrolling down until I saw it, because I've seen the image before and knew what it looked like.
There is also the following photo (source), which is of a different explosion, taken after 16 ms:
I haven't seen many other pictures like this, so I suspect these are among the very few in existence.