Journal
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Today Peter helped me get started on my web site. I added the about me section and this journal section. I also read a little bit about html and Linux in a couple of reference books I found lying around the Laser Teaching Center.
I continued today doing some background reading about optical tweezers both on the Internet and several photocopied papers I have.
I used a web service called Science Direct to look up a few different papers involving Optical Tweezers. I also reviewed some basic optics that I had forgotten from my physics course in school. This will be important in studying optical tweezers because an optical tweezers setup uses numerous lenses and mirrors.
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Today I searched for a specific project with optical tweezers to work on. Dr Metcalf gave an interesting talk about the history of physics that is going to lead up to a discussion of quantum mechanics tomorrow.
Peter and I worked on the optical tweezers setup after Dr. Metcalf finished his talk on quantum mechanics. We studied the various connections between the power supply and the laser control box. Then we began lowering all posts in the tweezers setup to reduce vibrations in the setup. That will have to be completed next week. It will require a periscope to raise the beam up to the level of the microscope.
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Peter and I discussed with Dr. Noe how to make the laser beam on the optical tweezers setup more circular. Having the beam more circular would allow for more of the beam to go through the microscope objective and help us in trapping.
Noah Corwin, a student at the Laser Teaching Center last year, gave a presentation today on computer generated holograms. Working with Alex, who has experience using the CCD camera, Peter and I tried to get a picture of the laser from the optical tweezers setup. Our goal was to take pictures of the beam initially and then after it went through two lenses designed to make the beam more circular. This would allow us to see if a significant amount of trapping power is lost because the beam is not truly circular. We also determined the focal lengths of the two cylindrical lenses we have.
I worked again with the CCD camera today. Yesterday the readings from the camera would often plateau leaving us with inaccurate readings for the intensity at the center of the beam. By adjusting the exposure time down I obtained more accurate measurements.
Today I began reading QED: The Strange Theory of Light and Matter by Richard Feynman. Thanks to Evan for letting me borrow it.
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We attended an interesting lecture in the Life Sciences building about the workings of the brain and neurons. After lunch Peter and I finished aligning the optical tweezers setup. Everything was thrown off because we disassembled it to take pictures of the laser with the CCD camera. We also attempted to trap some yeast cells without too much success.
Today we successfully trapped numerous yeast cells. The trapping power seems very low however and we can only drag them along very slowly.
Today Sam Goldwasser visited the laser teaching center. He presented some of the research that he is doing and he showed us two web sites that he created, "Sam's Laser FAQ" and "Sam's Appliance FAQ". He also demonstrated some interesting modes with the He Ne laser.
Sam continued his visit today but left before lunch. Peter and I continued working with the tweezer setup but it remains very difficult to trap particles.
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Today I attended a seminar on the trapping of francium atoms given by a scientist from Italy. This is not the same as the optical tweezers system which traps only particles that are much larger.
Peter and I are having some difficulties with the optical tweezers setup. We seem to be losing much intensity due to the fact that we reflect the laser beam off 4 separate mirrors and through 4 lenses. We are going to try to measure the beam strength to see if it is still adequate to trap particles because there seems to be barely any trapping power at all.
Today we listened to the REU undergraduate students practice their presentations. They have to give presentations tomorrow along with all the other REU Physics students. Good luck. Peter and I also found some first surface mirrors that are already mounted at 45 degree angles so this will simplify the periscope design that we use to raise the vertical height of the beam. We had lowered the entire laser beam path to reduce vibration but the microscope objective was too high so we need to raise the beam.
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Although I have not added to my journal since last week Peter and I continue to work on the tweezers. We found a paper that covers how to build an optical tweezers setup with a diode laser. This will be valuable to us because it is the first article devoted only to a device with a diode laser. Many of the problems we have been experiencing have been based on the properties of the diode laser. For one it diverges a large amount. To help stop that there is a converging lens build into the box the laser is mounted in. This lens should collimate the beam, that is make it parallel. However it is difficult to position the lens to do this.
I joined the students in the Simons program on a trip to Brookhaven National Laboratory today. We saw the STAR detector and the Tandem Van Der Graaf generators. At the STAR detector, which is part of the RHIC device, we met with Dr. Gene Van Buren who told us a little about the detector and life as a physicist.
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Today I talked to Dr. Noe about working on a project involving using hollow beam lasers with the optical tweezers. First however I need to get the optical tweezers working with the current laser setup. Using a hollow beam should allow for significantly more trapping power even if the beam is less intense. Making a hollow beam can be accomplished several ways. The way I am most likely going to be using involves shining the laser through a coherent fiberoptical bundles. The question I will be looking at is do the advantages of a hollow beam outweigh the costs of less laser power, which is the result of shining it through the fiberoptical bundle and losing intensity.
Today we finally figured out exactly how to collimate the laser beam. Tina Shih came and gave a presentation on her research at CREOL. After lunch she came to the lab and talked to everyone about their projects. Tina helped Peter and I figure out what was going on with the collimating lens. We also epoxied the diode to the aluminum tube it was sitting in.
I collimated the laser beam this morning. After lunch I began aligning the tweezers. Hopefully they will be functional again early next week. I wont be in the lab tomorrow because I have my road test. Wish me luck.
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Much has happened since last time I wrote in my journal. The tweezers were working for two days. Then I bumped the laser and knocked the entire system out of alignment. Somehow I destroyed the collimation of the beam when I bumped the laser. The laser does not seem to want to collimate again. It has been a very frustrating couple of days.
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The optical tweezers are once again functional. They are operating fairly easily and powerfully, it seems. My main problem now is going to be figuring out how to measure the force that the tweezers are exerting on the particles I trap. I found a paper that describes the process in detail but the method it uses seems odd and inexact. The paper, titled Optical Tweezers: Measuring Piconewton Forces, recommends using the formula F=kx with k being a constant, the "trap stiffness". Of course there is no way to know the trap stiffness of my setup so I am not sure how accurate this method would be.
Dr. Noe gave me a copy of Sandra Nudelman's Intel paper. I was able to find one of the papers she refered to, Biological Applications of Optical Forces by Karel Svoboda and Steven Block. It contained a detailed description of 4 ways to measure the trapping force. One of those ways involves pulling the medium, water, around the particle and seeing how fast you can do that before it breaks loose. The paper gives equations that can be used to then calculate the force applied by the tweezers.
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I have learned some things about using the formulas to calculate the drag and thus the force on my particle. I have switched to using latex spheres which are much more uniform than the yeast cells I was using before. The samples I make are only useful for a few minutes before the spheres begin to settle down to the bottom of the slide chamber. By focusing the laser beam on the top and bottom of the slide I can determine which spheres are in the middle of my sample, those are the ones I take readings of. I am measuring the force at a wide range of laser output settings. Hopefully later today I will begin putting into place the fiber optic cable that I will use to create a hollow beam.