Asymmetric Trap: Correction Attempts and Existing Questions

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Asymmetric Trap Strength: An Unsolved Mystery

If you have looked at both our Brownian motion data as well as our Stokes drag force data you will have noticed that the x and y trapping force and trap force constant are far from indentical. Consistant with previous groups, for the Brownian Motion calibrations, the x trap force constant is much larger than the y trap force constant. Conversely for the Stokes drag force calibrations, the y trapping force is actually stronger than the x trapping force. These asymmetries in trap strength have led us to beleive that at the focal point of the laser is elliptically shaped, with the long axis of the ellipse oriented along the y axis. This would allow for greater y direction oscillations in brownian motion, and allow for a larger trapping area for the Stokes measurements. The source of the elliptical beam spot was investigated. As a reference to future groups, the corrections there attempted were:

    - Rotate Fiber Collimator within its mount
    - Rotate the 1/2 waveplate 90 and 180 degrees
    - Adjust the position of our beam expanding lens so that the beam passes through the center of each lens.
    - Keep the microscope objective mount parallel, rotate the objective about its vertical axis.
    - Rotate the microscope objective off of it's vertical axis.

After each attempted correction, a test force calibration was performed. Either of adjustment had no effect, or it made the system not capable to perform trapping. Some remaining possibilities are:

    - Some inherent issues with the tracking software or CCD/camera lens system
    - Aberrations inherent to the mirrors and lenses.
    - Properties like polarization of the microscope cover slip could deform the beam in some manor.

We recommend for the next group to verify the actual shape of the beam using a diagnostic tool such as a beam profiler. This information would at least confirm the asymmetric trap findings and could shed light onto the causes of the asymmetry.