When you focus a laser beam in water (because that’s something normal people do…) you concentrate the energy of the laser light into a small volume of water. This can cause the water to rupture, which is what we call vaporising. This is similar to the bubbles that form on the bottom surface of your saucepan when you heat it. When the laser fires, we dump all this energy into the water, which creates a large bubble. The bubble then collapses (because the surrounding pressure is too large for it to be sustained) and the collapse generates many smaller bubbles. This is what you see in the video below.
What it looks like
How we do it
This concept of laser induced bubbles is nothing new (see, for example, the works of Lauterborn in the early 1970s). For our particular setup, we needed three lenses. The first is a diverging lens, which spreads the beam out. This is beneficial for two reasons. Firstly, it increases the converging angle of the final beam, meaning the energy density (energy / volume) is higher. Secondly, it reduces the energy density (now energy / area) on the outside of the water tank, reducing the risk of melting or damaging the tank walls. The next lens collimates, or straightens, the laser beam, which then passes through the final lens, which is the focusing lens.