Quantum Dots in an Optical Trap

I worked with an optical trap to manipulate silica spheres and measure microscopic forces from molecular motors in onion cells. Below is a picture of a silica microsphere caught in the optical trap (the sphere is defocused in the microscope when it is caught in the trap).

Silica microsphere caught in an optical trap.

I was inspired by a paper (reference below) to try putting quantum dots (the same as picture in this post) in the optical trap to see if I could induce a two-photon absorption and fluorescence of the trapped dots. The paper used a 5W 1064nm laser to observe the two-photon process. Our optical trap used a 330mW 975nm laser and so the laser power turned out to be far too weak to observe this process in our setup.

I did observe another interesting phenomenon - when the laser was focused into the glass microscope slide instead of the fluid channel and the microscope illumination turned off, fluorescence was observed. This fluorescence clearly came from the glass since it disappeared when the laser was refocused into the water. It also proved to be a useful way to determine the location of the trap - note that the location of the beam spot matches the position of the silica bead above.

Fluorescence from 975nm laser in a glass microscope slide. Laser speckle is also noticeable.

Further Reading

Jauffred, L.; Oddershede, L. B. Two-Photon Quantum Dot Excitation during Optical Trapping. Nano Lett. 2010, 10 (5), 1927–1930. https://doi.org/10.1021/nl100924z.