You have probably heard of this before but im just curious what laser they are using to do this. They do t appear to be ionizing air. It seems the one laser is just for color on the particle but what is the other? I would like to try this at home and make a dot for fun but i dont quite understand whats going on here. Any ideas?
The full letter is here:
They have 4 lasers: an RGB beam (3 lasers) that provides colour and an invisible 405nm beam that traps the particle. The RGB beam is pretty self-explanatory: you shine a beam of the colour you want the line to be.
The trapping beam is more complicated, by using lenses with particular imperfections you can focus the beam so that there is a dark volume near the focal point. Particles in this volume will be kept there by the photophoretic force. There is more information in this paper:
Playing with invisible lasers can be pretty dangerous