Really Definitely Finished Now

46 pages containing 10,044 words and 44 images. 5 days of 24/7 work (yes, even while I was asleep). I could have written double the amount!

And Now For The Report

I now have to write a report on this project. It's got to be ~40 pages long, so it resembles an uber-lab report. I have until next Friday to get it done... Which is why I'm still awake, typing it...

Finished!

I did my final presentation today - I'm finished my project! I still have to write my report for next Friday, but I have no more classes this term. I think my talk went pretty well overall. I wish I had been asked more questions - I had so much to say!

I did run the 980nm laser through the taper yesterday. I observed the exact same effects as with the 1460nm laser, but the fibre was not jumping (which is good). An example video is shown below with a modulated field and oscillating clump of beads. The camera can sense the 980nm light so we can see visually when the light field switches on and off.

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.

I have GOT To Stop Dreaming About This Project

I've dreamt of this project solidly for a week. It's a bit mad. Or, well, I'm a bit mad. I have an idea though - two 'modifications' to the experiment which I think will work. I dreamt about them last night and it worked beautifully in dreamland (a simple translation to reality isn't too much to ask, right?). I saw all the beads being propelled towards and along the fibre. I'm so optimistic about this that I have included the following slide in my presentation:

There will be a second movie of trapped beads in there by Wednesday!

Second Last Chance

Today was my second last chance to get this experiment working - I am going to try once again next Tuesday using a few ideas I've been thinking about as well as Sile's idea of enhancing the electric field by applying a voltage along the fibre.
Here's a video of the spheres next to the fibre not being trapped but undergoing Brownian motion. Damn beads. Even at ~90% power they wouldn't budge!

Argh

I spent the day using my final set-up to try and trap the polystyrene beads. No such luck. I don't think my laser is powerful enough. And of course, there are all those problems I explained in the post below. I did come across one very strange thing during my first attempt at trapping. I was scrolling along the taper looking for beads and I came across two very large spheres stuck onto the side of the taper. I thought they were bubbles at first so I lowered to objective onto them to burst or move them, but they stayed put. I tried moving them with a micropipette and they didn't move. Johnathan thought they were two microspheres but we couldn't see why they would end up in my water.
While I was making a new sample and fibred slide to see if these spheres were an optical effect, I broke the taper in half - and I never got a photo of them! So then I tried trapping with half tapers. I also had no luck with this. I tried to trap the beads with a looped section of the half taper (inspired by Yuqiang 'lassooing' a microsphere with his taper) but I couldn't focus on the loop as the objective kept pushing it around in the water.