How Very Strange!

I went in to Tyndall today with full expectations of another failure at this optical tweezers malarky. I commandeered Jonathan's lab bench, pulled a micron taper and started again. This time, I used a different glass slide stage, glycerol in my water-bead suspension and a 20X objective. And, I saw some very strange effects. I mean, REALLY strange. And quite funny.

So I skipped lunch and at about 1pm, I switched on the laser, increased the pump current to well above threshold. I watched the beads scatter diagonally from the taper as I increased the current! Deacreasing the current brought them back towards the taper, and with no power they just drifted very slowly, or stayed put and resumed Brownian motion. Jonathan came back and he attached the CCD camera to the microscope so I could take some videos. The video below is of us manually adjusting the pump current and thereby increasing and decreasing the intensity.



Vibrations and movement of the fibre is because I kept knocking into the lab bench while I was dialling up and down the pump current. What is weird is that the evanescent field does not range that far into the liquid. Some other forces could be at work here - some sort of effect from the static charging of the beads or water-glycerol absorption? I'm not really sure.

Jonathan got an intensity modulator to plug into the current driver so that we could oscillate the field strength in a systematic way. We found some even stranger stuff this way. By now, many of the beads were clumping, so we ended up with fantasic looking bead formations... See the video below! (Click here to see it on youtube.com.) The input signal was a square wave and we were in the kilohertz range for the fastest oscillations.



Possible explanations are coherence between the fibre and beads in terms of fluid motion and the static charge effects as mentioned already. We also have the absorbtion of water near my laser wavelength of 1460nm. I'm going to try it once more tomorrow morning using no glycerol and with a 980nm laser.

There is also the big question of why the fibre is vibrating in a uniform way when using the modulator. If it is generating fluid motion to push and pull the beads, then why can I still control them without the modulator where the fibre is not vibrating in accordance with the beads.