Trapping particles in a laser beam (photophoretic trapping)

To make a long story short this topic to me is very confusing but I would really like to achieve the effect. Basically in photophoretic trapping from what I understand it is the lowest density section of the laser beam (when focused) that is able to trap a particle. Ive seen videos of particles being trapped from crayola markers, graphite, tungsten, and cellulose.

I already read a few OSA articles on the subject and yet from watching videos, what to do is still unclear. In diagrams you need several types on lenses and mirrors etc like this:

But couldn't I take say a 405nm laser, a plano convex lens where the fl = 25.4mm, and then maybe 25mW to 3W. with these couldn't I just put a particle in the highly focused beam and trap the particle or do i need my setup to be more complex. Please respond with maybe how this works, and how this can be setup that I can test it. From what i have looked up this answer will be of great help to me and this community.

10/10/18 UPDATE: from what I now understand the more power you have the better chance you have of keeping particles within the system, but you may keep more particles then wanted but that's ok for now. Please look at the following diagram:

Here they are using a 532nm laser but I would like to use a 405nm laser. The wattage they use varies from 30mW to 200mW to which mine will be a happy medium. Now I see he focused the beam in the beginning (f=50mm) so that the beam can expand to 16mm (starting beam width was a few mm) the f2 is 200mm. What is the point of getting a bigger beam? Finally the beam is focused down with a f=50mm lens (mine would be 25mm). They trap ink toner particles and small iron particles which I too will try and replicate. I also imagine all the lenses are plano convex. Please let me know if my setup is correct (I might actually try the 532nm to do this exactly) but the answers to the questions I asked would better help me understand what I'm doing!

9/11/18 UPDATE: please refer to the following diagram.

This will be the experiment I will try (looks like the same image as above except you can now see the plano-convex lenses and the focal lengths). If anything changes it will be the focal length of the last lens, because if i can get it to be less then 25mm that would be better. One thing I'm worried about is the laser losing power over distance its traveling and all the expanding and culminating processes it will be running through. The laser I will be using is here 532nm Laser 100mW if this seems like a bad choice for a laser please do inform me, or if my setup seems insufficient because on the beam diameter being so small and expanding it to what I want would be a little lengthy (using my lens process. experiment errors or successes will be posted when complete.

10/13/18 UPDATE: from my post on laserpointerforums I see a person had tried this but ended up burning the particle. He never listed the mW or W of the laser but now I'm wondering how much is to much. My 5mW laser can burn black objects so how many milliwatts do I need for particle-trapping without burning the particle right away? Should I go for 5mW or should I use my helium neon laser which is 0.95mW?

I bought 2 lasers, the first one is 532nm 20mW And the second is red 100mW My hope is that by testing these with my optical bench I can produce results good enough to answer this question!

• "What is the point of getting a bigger beam? " -> you get more aperture and therefore your laser is focused to a smaller spot – Manu de Hanoi Oct 11 '18 at 3:14
• @ManudeHanoi thank you very much i did not know that! Now looking back on it that makes sense – Reese Houseknecht Oct 11 '18 at 3:15

Using simple optics i was able to use the 532nm 25mW laser (which when measured was actually 20mW) and trap a particle, heres how. There are 2 lenses in the photo above, a focal lens where fl=50mm and another focal lens where fl=25mm. The one that is directly after the laser beam is the 50mm focal lens which focuses the beam but its true purpose is then expand the beam diameter to about 1cm. The beam shines through a hole made of black tape so that the main beam passes through and the small fuzzy particles get blocked. The final lens is the fl = 25mm lens which focuses the beam from such a larger beam diameter that you can achieve photophoretic trapping. Here are some pictures (i use a black sharpie marker to show the trapped particles Here is a youtube video of the trapping photophoretic trapping

Also there is a tutorial here that i made tutorial