STIRAP Research Journal
Below is a journal of the research I did during the 2012 summer where I helped Yuan Sun, a graduate student, on his experiment STIRAP (Stimulated Adiabatic Rapid Passage). More information about Yuan, and Dr. Metcalfs research group can be found here
Monday 12 August 2012
When I walked into the lab today Yuan had the SEO Ti:Saph cavity open because he was cleaning it. He was also looking at the cooling system involved in regulating the Ti:Saph crystal's temperature. Although we were not sure how to fix that cooling system problem when he took the cavity cover off it was too dusty to just put back. This allowed me to get a close up view of the cavity and the optics inside it. It was very interesting and the instruments seemed top of the line. The basic principle of the cavity is that of a ring laser and one can adjust the frequency of the output laser using the etalons inside. It was really awesome to look into this expensive piece of equipment and see the inner workings.
To run the experiment it is necessary to allow for the lasers to warm up so once we turned them on we allowed at least two hours before trying to lock and use them. Time must also be allowed to let the vacuum system cool down both temperature and pressure wise. From experience we know that the vacuum system is cooled and steady at 2.10 Torr. Other difficulties arise because all three experiments in this lab share equipment.
After waiting for everthing to warm up and and cool down we finally were able to run. Our 1083nm laser that is used for the optical molasses passes through an AOM which we use a radio frequency generator to adjust the output beam frequency. I learned that for our specific experiment we hit resonance when that frequency generator is at 60MHz. From this we can adjust the frequency above resonance to obtain optical heating (forcing the atoms out) and below resonance we get optical cooling (pushing the atoms in) and if our alignment is correct the effect, heating or cooling, is uniform in both directions. Today we were experimenting with the n=30 transition with optical heating and were able to observe ionization peaks once all the lasers were locked and stayed locked. Yuan and I then compared these peaks to data taken last week when the 1083nm laser was blocked. As expected with heating the peaks were shorter and wider when compared to the peaks with the 1083nm laser blocked. The area under the peaks should all be the same as with their associated non-1083nm peaks, which we will anaylze at a later date. Eventually the SEO Ti:Saph would not remain locked for longer then a couple seconds and we were not able to continue experimenting. Once everything was turned off we opened this cavity again and cleaned it.
Wednesday 8 & Thursday 9 August 2012
I spent the day looking deeper into the problem of coupling a 780nm diode laser into a fiber-port. I spent awhile talking Gaussian Optics with Prof. Metcalf to learn that if you have a beam incident on a lens and that lens is placed at the beams waist the new waist occurs at the focal length of that lens. Because of this the Gaussian Beams behavior after can be approximated as a ray with a small adjustment defined by the matrices describing that beam. In terms on my set up this correction was only on terms of 1mm and Prof. Metcalf pointed out I do not know that this lens' focal length is EXACTLY 200mm so it is fair enough to approximate the beams propagation as you would a ray. This meant that I was able to keep my telescope set up as it was, and that the magnification of the beam would be the ratios of the lens' focal length. The fiber-port's optimal beam diameter had a range from 1.2mm-2mm. Since the beam diameter just past the cavity was ~3mm all I needed was to telescope this beam to half its diameter. The telescope consisted of a 200mm lens, placed closest to the cavity, and a 100mm lens placed after the 200mm lens at a distance equal to their combined focal lengths (300mm). I measured the beam diameter after the cavity, with no optics placed in front, for a total of 40cm in the direction of propagation. Past 10cm from the cavity the beam retained a diameter of 3mm, which when calculated gives a Rayleigh length of ~9m. This meant I all I needed to do was place the first lens f=200mm anywhere inside that range.
I really enjoy setting up all the optics and creating with my hands. Unfortunately my set up consisted of only two lens' and 2 mirrors so it was not too complex and did not take long. Prof. Metcalf reminded me that one always needs two mirrors to "walk the beam" since the beams propagation must be perpendicular to the surface of the object its going into. Meaning it can not come in at an angle and that the head of the Gaussian Beam should be the point of highest intensity. Which results in 4 degree of movement that is why two mirrors are used. I placed a CCD camera at the fiber output and was able to see the saturation of the light inside the fiber on a small monitor. As I walked the beam in steps, vertically, then horizontally, then vertically and so on it was very easy to tell the best adjustment because I would see higher saturation on the monitor. After awhile it became clear of the max saturation I was able to get just from walking the beam. I then took a power meter and determined the the power (in Volts) of the beam right after the cavity (~9.64 V) and of the output beam after the fiber (~3.4mV). That is an efficiency of less then 1%. It became clear there was more needed to be done, and Brain suggested that I play with the cavity since the beam profile is elliptical in the horizontal axis and then rotated up at about 30˚, almost like this / expect at a smaller angle and elliptical not straight. This means there is a larger abberation due to astigmatism. He also told me that in previous calculations Alex determined there was a local max of the beam profile different from the global max. I may be shining the local max into the coupler head on instead of the global max. I quickly realized how tough this problem was because to get a good output efficiency one has to make note of every single thing, including the specs on the fiber-port as well as the shape and propagation of the beam incident on it. It seems though that this laser to fiber connection is not the best as all of the grad students as well as Dr. Noe have played with this and none have reached an output efficiency of at least 50%. I will defiantly play around with the cavity in the next coming days. Despite the difficulty and minimal results from this problem I have learned so much about fiber couplers and what it takes to do it right.
Tuesday 7 August 2012
Yesterday, Monday the 6th, Yuan ran while I was away. He focused on the 26s transition. When he ran the experiment he saw very similar data to the data we took last week with regards to the optical molasses and its effects on the ionization peaks. Our result was a slight dip when the optical molasses beam was blocked, on level with the electrical noise. Overall less than 5% of the total amplitude of the ionization peaks. Today we are looking at the 30s transition and observing the effects the molasses has on it. Each time we run we need to realign the optical molasses beam such that it is retro-reflected, thus causing a standing wave in the interaction chamber. At first we obtained optical molasses at 1 MHz below atomic frequency but Yuan realized quickly that yesterday he ran at 1 MHz above atomic frequency so for consistency we had to realign until we saw another max push on the atomic beam. At first we repeated the steps taken yesterday at ran for 33P2 to n=26s state. After some grief from one of the Ti:Saph lasers we obtained ionization peaks. When we observed these peaks the noise to amplitude ratio was outstanding. We then blocked the molasses beam and compared the peaks to the data taken yesterday. The difference in amplitude between yesterdays peak and today's molasses blocked peak was ~5%.
Yuan said that he noticed slightly better results if the optical molasses pushed the atoms to the right, if your looking down the line of propagation. The only way I can think that this helps is because it physical separates itself from the He* that did not get excited to a Rydberg Atom. This of course is nowhere near the actual separation that occurs when a bichromatic force is applied. After realigning the optical molasses to do so we tried experimenting with the n=30s transition. Unfortunately the UV Ti:Saph laser would not stay locked. We believe it is due to the cooling system that maintains the temperature of the Ti:Saph crystal. If the crystal has a temperature that is fluctuating then the frequency produced will change. We ended here with plans to continue working with the n=30s next week as well as fix the cooling system.
Thursday 2 August 2012
This morning I continued reading a past graduate students thesis, Xiaoxu Lu, which is titled Excitation of Helium to Rydberg States Using STIRAP. At the start of her laser systems chapter she discusses several techniques used to lock a laser. I found these techniques very interesting as well as useful so I spent the time looking each one up and trying to understand how it functions. The different methods are the Pound-Drever-Hall technique, Hãnsch and Couillaud Technique, and Saturated Absorption Spectroscopy. All the methods consist of detecting resonance in a Fabry-Perot, and if the laser is off resonance then the reflected light from the Fabry-Perot converted to an electronic signal which gives a measure of how far the laser is off resonance. I am thinking about setting up a Saturated Absorption Spectroscopy just to do it and see if I can understand all the optics and electronics that make it work.
Afterwards I went back to coupling the laser into a fiber. I got some great suggestions from Brian, who taught me how to telescope the collimated beam which can also magnify it, and from Dr. Noe who told me to look into the spec's of the Fiber-Port because each one has a lens that focus' the beam into the fiber of extremely small diameter. He said that the lens has an optimal beam diameter that when passed into the lens will be most efficient in focusing it into the fiber. With these suggestions and a lot of reviewing Gaussian Optics I have come up with a set up to do such. When I measured the collimation of the diode beam right after the cavity for 60cm in the direction of propagation the beam diameter stayed ~3.5mm. This is very nice because it means I do not have to use optics to collimate it. Collimation occurs in the cavity based off the angle the diode beam hits a diffraction grating. I am not entirely sure how this works but I plan on looking into it. When I looked up the specs of the Fiber-Port it turns of the optimal coupling input beam diameter is 1.2mm-2mm. This is nice because all I need is a telescope that magnifies the beam by a half, shrinking its diameter to roughly 1.5mm-1.75mm in theory. From here I am going to sit down and calculate the necessary components of the Gaussian Beam to understand its waist and Rayliegh length after each optical device. Once I do this I can go back into the lab, align everything and see how it works. The whole process of just hammering through it and understanding each detail has really given me a lot of knowledge into this. This will be my plan for next week and hopefully by the end I have sufficient output beam power to be detected on a photometer.
I am excited to get back into the lab with Yuan because this week he took some data and has been figuring out why the optical molasses has not improved our STIRAP efficiency. Some progress and understanding is going on which is always good. I have learned that nothing will work the first time you try it and working away at a specific problem will yield the desired answer after much effort. The whole experimenting procedure requires much time and effort to be thought about and implemented and I am very fortunate to be apart of it and learn the ins and outs.
Wednesday 1 August 2012
Unfortunately I was not able to come into the lab on Tuesday when Yuan experimented. When I asked him what he did he said that he realigned some of the optics in the path of the UV laser and upon testing the change he saw some clear optical heating. When we run the experiment we use liquid nitrogen (LN2) to cool the atomic beam down. It is necessary to wait a day after running because water molecules in the air can condense on the source chamber because of its low temperature, even after the liquid nitrogen is gone. Therefore we wait at least a full day after experimenting for this garbage gas and "dirt" to evaporate from the system. Unfortunately we could not run for the rest of the week so we have plans of setting up the bichromatic force next week.
The rest of the day was spent listening to the LTC students giving a practice talk for their final presentation on Friday. From hearing these talks last week, I got to see the changes the students made over the week. Ariana started with her talk on Moiré Patterns which I found just as interesting as the first time I heard it. I understood aliasing a little better which is when a pattern, or signal is more pixalated then the computer resolve, so it only samples every couple of pixals. This led me to understand the Nyquist Frequency which is the sampling frequency divided by two such that anything less then this number is free from aliasing. In terms of an electronic signal, any frequencies lower then this wont undergo aliasing when sampled. For example the human ear hears at about 20KHz so all recording devices record at +41KHz.
John's talk was titled: A Multi-Pinhole Interferometer for Characterizing Optical Vorticies. An optical vortex is a light wave that has a azimuthally varying phase eiø with orbital angular momentum of lħ per photon. He used this multi-pinhole interferometer to project such vorticies and captured them using a CCD camera. Melia & Marissa's presentation was equally as interesting which was on the subject of Bessel Beams. Bessel Beams are non-diffracting beams which have a thin-infinitely long core. They come from mathematical formula and cannot be produced experimentally but as they presented one can get close. A very interesting aspect of Bessel Beams which I learned from their presentation was that barcode scanners create Bessel Beams because of its non-diffracting property you can scan and item for several meters away. I wish they went more into that because it seems amazing that these very common instruments, barcode scanners, create a Bessel Beam when Melia & Marissa were using far more complicated setups.
After the talks, Prof. Metcalf sat down with the graduate students, Dr. Cohen, Thomas and myself and talked about his trip to Paris and ICAP (International Conference on Atomic Physics). He outlined almost every talk he went to each day for us, three a day! They sounded very interesting and solidified my interested in Atomic, Molecular, and Optical Phyiscs because the topic has so much application and overlap into other topics. I feel so grateful for this opportunity here this summer because I have learned so much, and it has helped me in figuring out where my interests lie.
Monday 30 July 2012
At this point with STIRAP, Prof. Metcalf feels that just observing the ionization peak is not enough data to determine the optical molasses effects STIRAP efficiency. He suggested to Yuan and I that we choose to use another 1083nm laser and deflect the metastable ground-state He using a bichromatic force after interaction with the STIRAP beams. This gives us two peaks, one showing us the amount of metastable ground-state He we have and the other amount is how much Rydberg atoms we produced. From here we can get an efficiency ratio, knowing the original amount of He that entered the system. This will require us to realign the bichromatic beam because it has not been used in awhile. Other problems arise from the fact that we now need to use two fiber amplifiers, which are already being used by John so basic but problematic setup and space issues arise. Yuan gave me a quick run through of the 1083nm laser setup and showed me how we are going to split that beam into two, one used for molasses, and the other used for the bichromatic force. Since both beams need to travel from one room to another we use fibers and fiber amplifiers to maintain output power, or to increase it.
I did not get a chance to go into the lab and work on my laser coupling set up but I read into several articles and procedures done by other universities and technology companies on this topic. The problem is that a Diode laser gives an elliptical beam which gives astigmatism aberrations. I am going to play around with different lens combinations to try and obtain a collimated beam which will be coupled into the fiber.
Thursday 26 July 2012
Today Yuan and I ran the experiment. After going through the normal procedure, we outlined our goals of seeing if the optical molasses has any effect on the ionization peaks, and to change our intermediate state to 3P1. When we got the laser's locked we used the CCD camera to observe the optical push on the atomic beam through the method of blocking and unblocking the beam. We saw clear, observable push on the atomic beam and equal on both sides. When we turned the STIRAP beams on and observed the ionization peaks, we saw no difference in strength of the peaks when the optical molasses was present, and not. We had to come to the conclusion that this will not improve the efficiency.
Next we went to change the 3P2 intermediate state to 3P1 and observe the difference in amplitude of the ionization peaks compared to the polarization of the UV light. Since our new intermediate state has a different angular momentum, it has a different energy associated with that transition. Once we scanned to the associated frequency and saw ionization peaks Yuan and I started taking data as we changed the polarization of the UV light in steps of 5˚. After we took several data points the UV laser would not remain locked and we decided to end for the day.
I made some progess in coupling the light into a fiber today as well. Using the Thomas' suggestion I took out the power meter, and placed a CCD camera after the output of the beam because it is much more sensitive than the power meter. I also had to realign the spherical lens such that the beam passes through the center of it. Once I set that all up I observed some light being detected on the camera, which is connected to a small monitor. I then aligned the camera as closely as I could so that it faced directly towards the output of the fiber. I then messed with the alignment of the beam using the two mirrors and maximized the size and brightness of the light I saw on the monitor. It became clear after a certain point I could not get any brighter or bigger of an output beam with this set up. The problem comes from the fact that the spherical lens is focusing the beam, and the coupler works better with collimated, parallel, light. I am going to ask the grad students opinion and look up telescope designs that collimate a beam well.
Wednesday 25 July 2012
Today the LTC had its weekly group meeting. We were joined by Dave Battin, who is a laser hobbyist and holographer and has been a mentor to the LTC since 2011. As usual each LTC member gave a short presentation on their recent work and findings. Ariana started first and her topic fascinated me. She recently got involved in researching moiré patterns, which started from a work of art Dr. Noe showed her. Moiré patterns are interference patterns when, at its simplest example, two grids are overlaid at an angle, or when the spacing between lines is different. When you observe the grids, light passing through interferes and that is what gives you a distinct pattern. She then worked the wagon wheel effect, and aliasing in to her presentation. I found both of these topics very interesting and these are the examples of phenomena that arise in optics. The wagon wheel effect is an optical illusion in which a wheel appear to rotate differently then its true rotation, as observed by the spokes on the wheel. This effect can also be observed by shining a strobe light on a continuous motion. Aliasing is an effect when different signals become indistinguishable when sampled. For example say we take one sine wave at a certain frequency. That sampled points on the wave can be reconstructed by another wave at another frequency, but is not the actual fit to the first wave. This occurs often when reconstructing video and audio signals. I really enjoyed her presentation because it was thorough and captured many interesting phenomena that arises.
I went into the lab after with goals of putting togather a new set up to couple the laser into the fiber. I went in, took down the set up I had previously and went ahead with the graduate students suggestions. I tried to set up the simplest lay out with a 1083nm diode laser and a spherical lens in front of two mirrors in an L configuration leading up to the fiber input. I used a lens of focal length = 20cm, placed the first mirror 5.5cm after that at a 45˚ angle to the incoming beam. Only 4.5cm after that I placed the second mirror, again at a 45˚ to the incoming light, which reflected the beam another 11cm into the fiber input. On the output side I placed a photodetector right after the output fiber. Once aligned into the fiber input, the power meter detected nothing. I started playing around with the alignment, thinking maybe the tail needed to be aligned better but nothing seemed to work. I had the same difficulty the previous undergrads had. I clearly saw the laser light on the face of the lens that is in the fiber input. After much realignment I figured it give it up for the day and ask the grad students for tips. Thomas helped me out by letting me know that the power meter needs to be calibrated to the frequency of the laser light. He also suggested using a SSD camera viewing the output because it has a much higher sensitivity then the power meter. I am going to try this tomorrow and see if I can succeed.
Monday 23 July 2012
Today Yuan and I ran the experiment fully with goals of observing the optical molasses' effect on the ionization peaks. If the peaks fall when we block our beam, and rise when the beam is present on the atoms then we have succeeded. To start we turned on the atomic source, and once cooled by N2 we went to observe the optical push on the atomic beam through methods we have previously used. By viewing the atomic beam through a CCD camera and then blocking and unblocking the molasses beam we saw there was clearly a push, equal in both left and right directions, on the atomic beam. This did not occur at first, so we had to realign our beam. This proved to be relatively easy. Once everthing was aligned correctly, it was clear the optical molasses was working correct when we viewed its push. After that we set out to lock the UV laser ~389nm. The UV laser is a Ti:Sapphire laser that goes through a frequency doubler generating the ~389nm ("blue") wavelength we use. We choose to scan the other Ti:Sapphire ("red") over a range of ~790nm to ~830nm.
Today we found that the UV laser would not stay locked for longer then a minute. After some grief we realized that the coolant used to control the temperature of the Ti:Sapphire crystal is low. This caused the temperature of the crystal to fluctuate on magnitude of one kelvin, which causes problems in obtaining a locked frequency. More coolant had to be added but that can only be done when everything is turned off so we continued the experiment and planned to fill it after. For today we continued by trying to get a five minute window in which the lasers stayed locked, and observe the difference in the ionization peaks when the molasses was on or off. When we did get that window, and blocked-unblocked the molasses beam we saw little difference in the peaks. The change was smaller then electrical noise detected on the wavemeter. We concluded this as just the first preliminary results since we cleaned out the atomic source and the oil in the pumps. On Wednesday we plan on investigating the effects the molasses has on STIRAP efficiency deeper, with overall goals of taking more and better data.
Friday 20 July 2012
It feels really good to be working here full-time since I've been done with summer classes. It is also overwhelming how much there is to learn, but to be overcome by that would get me nowhere. This week I outlined a lot of articles I want to read and my goals for the next couple weeks. Which include starting to read some Quantum Mechanics books as well as couple a laser into a fiber.
On Tuesday Yuan and I were in the lab making the final checks on our atomic source. This consisted of checking if the discharge was working over a long period of time, it was, as well as observing the beam in the detection chamber. By eye we could see a bright green dot as a result of the phosphorus screen but on the CCD camera we saw a very dim spot. After some observation we realized that the brightness was turned down on the TV we were using and that was all. After this we wanted to see if our molasses beam was still aligned and working properly. Once we turned on the laser and wanted for it to lock, we blocked the beam and viewed the atomic beam on the CCD camera and saw no change. It seems that the molasses is off and needs some fixing. We saw that the reflected beam came back in-line with the transmitted so it may be that the frequency after the AOM is not correct or the laser is not locked. This will be our main goal on Monday.
I took the other undergrads set up to couple a laser into a fiber and completely took it apart. Even though they were not able to get it, I wanted to build it from the ground up. I started late yesterday and ended by setting up two cylindrical lens' in hope of creating a telescope. When I brought this up to the grad students today they suggested to forget the telescope set up, couple the laser into the fiber despite the output efficiency and once I do that, then I can work on getting the efficiency up.
Monday 16 July 2012
Today when I walked in Prof. Metcalf was giving a talk to the LTC students so I sat in on it. They were talking about each specific experiment and concluded with Prof. Metcalf saying how he never understood Bell Inequalities until he saw this speech given by Dr. Joseph Eberly when he recieved the Frederic Ives Medal in 2010, the highest award given by the Optical Society of America. In this talk titled; When Malus tangles with Euclid, who wins? he explains the details of the Bell Inequalities and then uses it as an example in the case of a photon polarization experiment. The example shows how the Bell Inequalities violate quantum mechanics. The violation comes from the fact that the inequality is defined from the exisitence of specific states, and according to quantum mechanics the polarization state does not actually exist if we do not observe it. In this case we choose not to observe one of three analyzer loops which means that the photons are in both polarization states. I did not fully understand the talk but it was the second time I saw it and compared to the first time I understood a bunch more.
Prof. Metcalf then had the weekly talk with the other undergrads and myself, which is when we get the chance to ask him questions we've encountered over the week, whether it be defining a term or explaining the physics behind an experiment. Two weeks ago he was teaching us about optical molasses and he asked us to figure out why the velocity of the atoms never reaches zero. I kept thinking about it and was puzzled when clearly the procedure was effective enough such that it can cool atoms to near 0 Kelvin. Alex had a near correct hypothesis that the kick the atom feels from the force may "step" it near zero velocity (since temperature is based off the average kinetic energy we associate zero velocity with zero temperature), but then another kick occurs and the atom now has negative velocity. The kick the atom feels is from the force but as it nears zero velocity it is subjected to collisions from subatomic particles that behave in a manner described by Brownian motion. Brownian motion is the random motion of a particle suspended in a fluid. It is a result from the bombardment on the particle by fast moving atoms or molecules of the fluid. Having little knowledge of Browniam motion before he told me about it I looked it up online and it has wide applications in many fields. For example it is used to describe fixational eye movements (maintaining the visual gaze on a single location), the random movement of molecules in liquids and gases, as well as gambling and probability estimations. I found it very interesting to see how many applications it had, as well as satisfied now knowing the answer to the optical molasses question.
Here is a series of pictures showing the vacuum chamber involved in obtaining Rydberg Atoms through STIRAP
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Thursday 12 July 2012
Since Tuesday we have let the turbo pumps run so that we know they are working properly and also it "cleans" out the vacuum of any garbage gas. Today our goal was to turn on the atomic beam and let the discharge warm up and run for at least 10 minutes. Yuan said that through his experience he knows that if you let this discharge warm up and see it run for an extended period of time, the next time we use it, it will stay constantly ionizing He for a period of time.
Our procedure today consisted of turning on the He beam which we then waited 20 minutes before we added liquid nitrogen to the system to cool the atoms. Once the liquid nitrogen has cooled the system, which is determined when the behavior of the liquid nitrogen's rate of evaporation has slowed down significantly. What we really wanted was to observe ionization for ten straight minutes, then we can be confident of letting it run for an hour to fully warm-up. The first ten minutes are when the discharge has the highest probability of turning off by itself. After several time of observing this, we could not get it to stay discharging for longer then 2 minutes. We ended by keeping the He source on for the night with hopes that this will clean out any remaining garbage gas in the system.
Tuesday 10 July 2012
Today when I came into the lab, Yuan told me we were going to clean the atomic source, as well as the discharge needle that excites ground-state He to metastable ground-state He, prior to encountering the STIRAP beam in our set up. This gave me a chance to understand this fundamental part of our set up. The basic lay out of this apparatus is a long discharge needle that is connected at the end to a turn screw that basically elongates or contracts the length of the needle. Over the needle goes a glass casing which is open to the inner workings of of the apparatus. This inner workings consists of three main valves. One connects to the vacuum pressure gauge, another one connects the electronics that apply a voltage to the needle which in turn excites the gas to the metastable ground-state. The last valve is the output pump for ground-state He gas. The input pump for the He is not connected to this apparatus but flows towards the discharge needle, and the gas that is not excited enters the glass tube and travels its path towards the output pump.
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Before I came into the lab, Yuan had already detached the apparatus and glass casing. To his surprise he found the glass tube shattered in three places and broken beyond use. Luckily we have another one which we put through thorough cleaning. Then we went to test the electronic input. When we applied a small voltage to the input, and measured the voltage on the needle, when done we saw none. After trying this several times we decided to open up the input value to find that the wire was not connected to the input and was also black, meaning the heat burnt it. At this point we had to switch the insulation cap, which was discolored from the gas deposits as a result of the heating. We did this easily and also cut away at some of insulation on the wire, which will cause us no problem as long as the wire does not touch the container, we fixed it so that did not happen. Once successfully reattached we then tested it by applying a voltage to the input and detected a voltage on the needle, meaning everything was fine. From here we changed the oil on the turbo pumps and then turned on the vacuum system, with our apparatus reattached successfully. After waiting a full hour to flush out any garbage gas we then turned on the turbo pumps and saw that everything was working correctly and that our vacuum system has not been compromised. Tomorrow we will turn on the atomic source and see if our discharge needle is working properly.
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Tuesday 3 July 2012
When I came into the lab today Yuan told me that he had goals of improving our optical molasses set up. We accomplished this by adding a Polarizing Beam Cube or PBC in the path of the 1083nm laser beam, which we use for optical molasses. The PBC consists of two triangular glass prisims that transmits the horizontally polarized portion of the light while the vertically polarized light is reflected. This splits the beams two polarizations and is a nice way to control the polarization of the beam. For the purposes of our experiment we use the vertically polarized light because it is "purer" such that when any reflection occurs, the light becomes polarized according to Brewster's angle, so we know that that light is polarized in the vertical direction with high efficiency In total we added a quarter-wave plate after the AOM and then the PBC after that.
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Once we aligned all this and waited for the liquid nitrogen to cool the system down we then turned the atomic source on. After some alignment we then sought to observe the optical force on the atomic beam by placing a camera which faces the phosphor screen as described in an earlier journal entry. When I blocked and then unblocked the beam we saw no push on the atomic beam. This meant either our alignment was off or the laser was not mode-locked. After some observation both these problems were off so Yuan and I went to fix both. Finally we observed a large push on the atomic beam. Our next goal was to reflect this beam back and create a standing wave in the system. This required even more careful alignment of the transmitted and reflected beam until we saw a push on the atomic beam. We then sought to get the most symmetric force possible such that when the beam was blocked and unblocked we should see an equal push on both sides. Again after much realignment we were able to obtain an atomic beam, that when propagating through the 1083nm laser, felt a push equal on both sides and that is exactly what we saw. Both sides were condensed and the middle began to bulge. With this near-perfect alignment Yuan and I ended for the day. The next time we experiment we will use this improved optical molasses to see if we get more efficient STIRAP results.
Thursday 28 June 2012
Unfortunately due to summer classes I have not been able to focus on research but the semester ends next week. Once those two classes are done with I can be involved full-time with the group. That is why it felt good today to get back into the lab with Yuan and experiment.
Everytime I go into the lab I feel more comfortable and my understanding of what each instrument's function is grows. Today we went through our normal procedure of turning on the TiSaph Laser and the blue laser which we then aligned such that each beam is parallel and overlapping. From here we let the vacuum system remove any "dirt" then we pump in the He and cool it using liquid nitrogen. Our main goal of today was to view the difference the Optical Molasses, we set up last time, has on our ionization peaks. These ionization peaks are a result of a Rydberg atom being created so we have a simple detector that scans through different frequencies and observes the strength of each ionization signal, peaks show that at a certain frequency Rydberg Atoms are being produced. We then "locked" the blue laser so that its frequency flucuation is almost none. This is done by setting some reference frequency (the freq. of the atomic transition) and then forcing the laser to lock on to that frequency.
For the purpose of STIRAP we lock the blue laser and then scan the TiSaph laser through a range of frequencies in which the atomic transition frequency is included in that range. Once scanning Yuan and I saw three ionization peaks (one for each time the frequencies were sweept). Therefore at wavelength of 792.2879nm Rydberg Atoms were being produced. Our overall goal is understand how small changes can effect the population transfer, whether the change be a different polarization of the light, or a different intensity for example. Now that we know our procedure was correct we then turned on the 1083nm laser beam for purposes of observing how the optical molasses effected the ionization peaks. We should see a stronger peak since the atomic beam is denser, more electrons should be excited by the two beams. After allowing the 1083nm laser to settled we did a frequency scan of the TiSaph and should have seen another peak when the wavelength was 792.2879nm, but we did not observe this. We checked if the blue laser was still locked. Finally with some adjustment we more peaks but at a wavelength of approx 792.2909nm which is a .0030nm jump from before. Now the frequency of atomic transitions does not change so Yuan indicated that is must be the failure of the wavemeter, which gave us the reading of 792.2909nm. He said the same thing happened to him yesterday and that this only confirms it. Once we saw the peaks, we blocked the 1083nm laser and saw that the peaks dipped a noticeable distance. From here we tried this several times over the course of two hours and saw less and less of a difference from when the 1083nm laser was on and when it was not. We also saw the wavemeter jump another .0030nm when we recorded peaks later after another frequency scan. We ended with plans to conduct the same procedure and see if the optical molasses really is or is not effecting the ionization peaks.
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Friday 22 June 2012
Today was Dr. Metcalf's research group's weekly meeting. We discussed general things and then moved onto each experiment specifically. When we got to STIRAP Yuan talked about how we were successful in imposing a force on the atomic beam through optical molasses. From there we said our next step was to run the procedure of STIRAP and see if the OM gives us a greater population transfer efficiency.
Throughout the week, the other undergrads Alex and Adam have set up the simple experiment of "walking" a laser beam into a AOM (acoustic-optic modulators) which is used because it has two holes, spaced by a couple cm's, in which the laser beam enters and exits. This serves as good practice for aligning a beam because when done correctly it passes through both holes in a manner where the least amount of intensity is lost. Finally I got a chance to set up the experiment and practice "walking" the beam. The procedure consists of setting up two mirrors at a 45 degree angle to the laser beam. With these mirrors we are able to adjust the beams path through two knobs on each mirrors with one controlling movement horizontally and the other vertically. We use two mirrors because the beam may enter the first hole of the AOM but its path may not take it through the other end. When we use two mirrors we control the "head" of the beam and the "tail" so our degrees of freedom allow a beam to pass through the AOM when aligned correctly. When set up properly I then put in a f=20cm lens so that the beam would be focused in the center of the AOM. This required that the center of the AOM be ~20cm from the lens so I had to readjust the two mirrors lengths to obtain this length requirement. After doing that correctly I had Brain, a grad student, come over mess with my vertical and horizontal alignment so I could play with aligning both mirrors vertical, then horizontal orientation, and back and forth so that a photocell, placed behind the AOM read a maximum intensity, with loss found from the AOM. After I did this I took down the whole set up, and set it up again using a f=50cm lens. I played with this set up several times, messing it up and aligning it back again to the point I felt very comfortable "walking" a beam
My goal for next week is to couple a laser beam into a fiber.
Thursday 21 June 2012
Today Yuan and I started by using the Titanium-Sapphire (TiSaph) laser (which is the "red" laser used in STIRAP) as a reference beam to align the 1083nm beam ("blue" laser used in STIRAP) such that both beams are parallel to eachother, and orthogonal to the atomic beam. Then we turned the atomic beam on and allowed some time to pass in order to pump out the "garbage gas" that may be on the inside of the glass tube which holds the atomic beam. After some time we poured liquid nitrogen into the system which makes our atomic beam denser as well as lowers the velocity of the atoms in the vacuum. With the atomic beam cooled and the 1083nm laser locked our next step was to observe if the optical molasses was working proper.
We use a Multi Channel Plate (MCP) & a phosphor screen to observe the atomic beam with position depedence. This occurs when the metastable helium (which has energy of approx 20eV) hits the MCP. The MCP has properties that when enough energy hits (hey metastable helium has got enough energy!) hundreds of electrons will be emitted, specific to that position. These electrons then knock into the phosphor screen, and phosphor is a luminescent, so we will see a glow of green on the screen specific to position that the metastable helium hit the MCP. Thus in terms of an atomic beam, the screen should glow exactly the size of the beam.
To observe the optical molasses effect, we tried blocking the 1083nm laser to observe the optical push on the atomic beam, viewed by the camera, but nothing happened! The 1083nm laser goes through a radio generator so we tried turning that on and off to see if that was the problem but it wasn't. Yuan said the radiative push may be too small compared to the envelope of the beam. Next we used a UV laser to observe its radiative push on the atomic beam. We got the UV laser locked and clearly saw a radiative push on the atomic beam. Our next step was to observe the push with and without the 1083nm laser on. On both observations we saw no force on the atomic beam. This means that the 1083nm laser was not the problem. After talking to another grad student we went back and realigned our 1083nm laser so that it was orthogonal to the atomic beam, which we realized was not the case. Once we aligned the laser beam orthogonal to the atomic beam we clearly saw a force on the atomic beam and knew that we solved our problem.
Our next step is to see if this optical molasses improves the efficiency of our population transfers.
Tuesday 19 June 2012
Today Yuan and I worked on the alignment of the 796nm laser since we have been away from the lab for two weeks. Over the course of several days the temperature and the humidity change causes different materials to expand or contract such that they change the alignment of the optical instruments we use, enough to spend several hours realigning. We mostly spent time aligning the 796nm laser such that as it is transmitted through the vacuum chamber it is reflected back in such a way that a standing wave should be produced. We did this by observing the reflected beam and adjusting two mirrors so that the transmitted and reflected beam were seen as one dot which indicated that physically both beams travel the same path. Once aligned I asked Yuan if the reflected beam would effect the transmitted in any way and his reply was that experimentally they are slightly out of phase, even if a standing wave is produced, so that there is little interefence on the transmitted beam. In the set up there is an AOM (Acoustic-Optical Modulator) and no reflected beam could enter the output of that instrument so we further our conclusion that the reflected beam will do no harm in this case.
We are scheduled to experiment this Thursday to see if when the laser is mode-locked to the atomic beam at resonance, the Doppler Molasses works such that we are efficiently confining the atoms to a dense, localized space.
The other day Prof. Metcalf gave the other undergrad's and myself a lecture on Optical Molasses that I briefly explained before. In the case of a two-dimensional OM, there is a positive and negative force that the atom feels, and its magnitude is related to the velocity of the atom. Prof. Metcalf asked us why does the velocity never reach 0, which obviously violates physical law since when putting temperature in terms of kinetic energy, the temperature would be zero, which is forbidden. After long thought I am still quite unsure of how such atoms never actually hit zero velocity when it is clear the force's are equal and opposite. But thinking about it in terms of a simple harmonnic pendulum one sees that when the mass approaches zero displacement it never abruptly stops at zero. Thus as it passes there is this force pulling it back and forth which I think in terms of the velocity of the atom as it approaches zero velocity. A force is present on it to push the atom past zero velocity, to negative velocity (just relative to the reference frame) and it oscillates there, never actually hitting zero but oscillating back and forth between. I am still unsure about this hypothesis and plan on doing more reading into it.
Monday 18 June 2012
Today was spent in class and working on homework. I am currently enrolled in Linear Algebra and Applied Real Analysis during Stony Brook's first summer session. In Linear Algebra I learned the Guass-Jordan elimination technique to solve a system of equations. We spent most of the class going over the properties of inverse matrices . In Real Analysis we derived the wave equation in one dimension and used it to find the solution to a vibrating spring problem. This led us to the standing wave equation through which we solved an example of a string plucked at a height h in the center and let go at some time t. The result is two functions that oscillate up and down by which the string moves in a motion found from the average of the two functions.
Tomorrow I plan on going into the lab with Yuan to work on the setup of STIRAP in which ultimate goal of achieving 100% population transfer in the three state system. I also want read up more on Doppler Molasses which is process of laser cooling when two perpendicular laser beams, both circularly polarized, intersecting an atomic beam. Counterpropagating beams of circularly polarized light create a standing wave, where the light polarization depends on the spatial location. Atoms moving with a velocity must climb a polarization gradient hill, and in doing so they lose their velocity. At the peak of said hill the atoms are resonant with the molasses beams, absorb a photon and decay into a lower energy magnetic sub-level and lose velocity. Personally I think this is really interesting method and I plan on learning more about its details.