# Trapping a particle in a laser beam (optical tweezers)

So upon watching videos and reading articles i understand few things. For one when the laser beam is focused to a tight enough point when placing a material in there (people use crayola markers) it will trap and burn that particle for a little while. My question is how can i do this without having to touch a marker to it? Can I achieve a particle effect through something else or atleast something that wont burn off the particle right away. Also is this effect as easy as talking a 405nm laser, focusing it with some sharp lenses and then having that particle get trapped. I have seen a few diagrams but still it is unclear all the equipment they use and then in a video it is just a laser focused to a point. Such as these:

UPDATE: Upon reading more i see the mW required is only around 5mW which isnt bad at all. I also see people use graphite but im not sure how ling that particle lasts? Forever until the beam is broken or only for a certain amount of time. To use graphite could i simple take a block of it and wave it in front of the laser at its lowest density point to capture the particle? My other confusion is the lens required what is so special? Is there maybe an equation that includes a class of particles, wavelength, power, and focal length of lens to see where i stand with this. I know this isnt what stack overflow is for but i just want to set this up and see it and understand it rather then just read so maybe a good diagram that i or any member coming here could understand in order to setup this experiment.

I was able to order a 25mW laser in the 532nm wavelength and trap a particle using the following optics: a 50mm focal lens which after it was focused I allowed it to expand to a 1cm diameter beam width. Then I used another focusing lens of fl=25mm and that focused beam is where the trapping begins. By touching a black marker to it (black because any light colors will simply be reflected and not absorbed) and when the black pieces of sharpie marker would burn off they would be trapped into the laser beam at its lowest density point. As for how long it lasts without burning, I don't think it is burning that is the problem, its wind and air. The slightest breath can cause the particle to move away. Sometimes you can have a particle stay for minuets and some just a second.

• Out of curiosity, what beam quality does your laser source have? – OpticalResonator Nov 3 '18 at 17:42