Some more Research Related Stuff

I will talk about a few things in this post:  (1) Initial thoughts on my research project, (2) other things that I have been doing at work, and (3) some other reading that I have been doing that is optics related.

So the main idea of my project is as follows: say you have a mirror and you shine light at it. There are two things that happen. First, only a percentage of this light is reflected. Second, the light is delayed. The amount of reflection and delay differ for different frequencies (i.e. red light and blue light are reflected differently). It is easy to measure reflectivity but it is hard to measure delay. There is a (somewhat complicated) relationship between these two quantities. The issue is that to calculate delay from reflectivity, you must know the reflectivity for all different frequencies. My current understanding is that my goal is to approximate the delay using only some (but not all) knowledge of the reflectivity. (Ex. you might get a pretty accurate answer by just using the reflectivity data for visible light and not other frequencies like ultraviolet). 

Overall, I am pleased with this project. It combines theory and experiment in a nice way for me. The approximations that I will need to make require me to both know a lot of math and understand the trends in the actual reflectivity data. The math that I have learned in the past year that is related to approximating integrals and integral transforms may be useful for my projection. Additionally, the approximations that I make will require me to understand the trends of the actual data. I will have to know what data can easily be measured and what areas cannot be neglected even if it would be mathematically convenient to do so. 

Anyways, at my first real day of work. Franck introduced us to a program called IMD that simulates multilayer mirrors. For now, we are just looking at a periodic mirror of Mo and a-Si. We are varying the number of times we repeat layers of those two substances, the ratio of those two substances, the thickness of the period and the roughness (i.e. there is some error involved in making the mirrors so we introduce some error into the sizes of the layers). Then we are looking at the reflectivity as a function of wavelength. By bragg's law, we know that the maximum should be around double the period of the multilayer mirror. 

Another thing that I noticed yesterday is that when you look at these plots, there is a maximum and the center, but then there are local maxima on the higher wavelength side of the global max. After playing around with this, I noticed that when you increase the number of layers, then the number of ripples increases. When you change the incidence angle, the ripples are on different sides of the central maximum.  This isn't too important, but it was fun to play around with a bit. In order to see the ripples, you have to put the reflectivity on a log scale.

I also asked more about automating the process of using the program to simulate the multilayer mirrors. It would be nice to generate a reflectivity graph, then feed that result into a loop, and then adjust the layers with some method and then see what the new reflectivity would be. Apparently one of the people here made a program that simulates the reflectivity in matlab. Depending on the nature of my project, it might be worthwhile for me to either learn matlab or to learn how to make the program in mathematica. 

After we played around with things, our professor came in and talked to us for a bit. I hope that he continues to check up on us and see how we are doing. He seems very knowledgable about the program that we were using. He also explained complex reflectivity to me. The idea is that if you have a light pulse that reflects off a surface, then you get the same frequency with a different amplitude and phase. It turns out that if you represent the incoming wave as a complex exponential, then the math works out nicely and when you divide the outgoing complex exponential by the incoming exponential, you get a complex number whose magnitude is the ratio of the amplitudes and the argument is the phase difference. This is an interesting representation. 

I have been having so many experiences that I haven't had the time to talk about the physics related stuff that I have been doing. For the past few days, I have been taking some time to read a long paper on general attosecond physics. The main thing that it talked about was the specifics behind high harmonic generation and the simulated raman scattering schemes for generating short pulses. I focused more on the high harmonic generated because it seems like my lab is going to focus on that light in that range. 

The other interesting things that I have been reading about involve measuring these attosecond pulses. Since they are so short, you need to generate new methods to measure them. One idea is streaking. So what I read about was that you have a light pulse and you send that through a photocathode. This knocks off electrons. Then you have a rapidly varying electric field and so the electrons excited by the start of the pulse are moved a different amount by the electric field than the electrons excited by the electric field later in the pulse. This converts the image of the pulse into electrons. The intensity of the pulse is measured through the number of electrons. I don't think that this can be done with attosecond pulses because a professor said that this is more useful on the order of picoseconds. I am not sure if there are streaking cameras that can measure attosecond pulses. However, apparently these apparatuses are very expensive. 

The other stuff that I have been reading about is related to the methods of FROG and SPIDER.  (Again we see the crazy acronyms that have become standard in this area of physics).  FROG stands for frequency resolved optical gating. I don't really understand what this is doing, but here are some general ideas. This method splits the beam up into two parts. Then one can use mirrors so that the second pulse is delayed in a controlled fashion. Then the pulses are able to interact in a nonlinear medium. I asked the professor/researcher that has been showing me around about this. 

So some things that he explained to me were as follows:   There is a model of a medium where we consider the atoms like little oscillators. When you subject these oscillators to an external EM field, they oscillate with the same frequency as the field however there is a phase delay. These oscillators emit photons at the same frequency. Since this is happening over many little oscillators, this accumulated phase delay over a distance can just be thought of as the light pulse traveling slower in the medium.  Then he proceeded to talk about in a nonlinear medium that at the start, the potential for an electron is parabolic as we might expect from a first order Taylor expansion but when we get into higher order terms, the potential becomes wider than one might expect. That is to say, if the external EM field has a high amplitude, then there is a nonlinear effect and there is an additional phase delay because the potential widens. So the idea here is that if the amplitude changes as a function of position, then the wave separates itself.   Loosely speaking, you could imagine that if you have a known pulse and an unknown one, then when you put it through this nonlinear medium, you could deduce something about the unknown pulse based on how much it slows down.

The FROG says that given two split pulses where we vary the time delay, then we can find the autocorrelation of the original pulse. However, there is something additional that we can get that is more than an autocorrelation. The things that I do not understand are: what are we physically measuring when we do the frog experiment. I asked John a bit about this and he gave me the explanation above that vaguely, you can imagine that in a nonlinear medium, you get some terms that are the product of the electric fields instead of just the sum. Another thing that I don't understand is what information you can extract from the autocorrelation of an electric field.

There is also something called SPIDER. I need to read more about this, but the idea is that it is another method that is used to measure the pulses.   One of the problems of these methods that some people have problems with is that you are using your own signal to probe the signal. All of these things are fuzzy in my head now but it seems that it isn't that important that I understand them for my project.

Wanderings near Port d'Orleans and First Day at Work

So I have a lot of experiences to recount from last night and then I will discuss my first day at work.

One of my friends and I decided to go exploring. There is a road nearby so we decided to walk a few blocks up and down that road in order to see what was going on. First we went south. The area wasn't particularly nice but we did go by a public park.They had a little skate park so we asked on of the kids if we could borrow his skateboard. My friend did a few turns and then almost hit a little kid who did not have the sense to move off the ramp. After that, we continued on and went into a mall. It was not very busy but we looked around in a plant store and we got some food at McDonald's. Apparently there was a different selection of plants at this store than the US. I took a picture of a carnivorous plant. It cost 2 euros for a double cheeseburger.

We went back and we continued past our hotel and went north.  This was a much nicer area and it was a lot of fun to look at all the shops. We went to a grocery store that seemed to be geared towards healthy eating. There was a lot of cheese, fruit and vegetables. We continued on and saw a luggage store that was having a clearance sale. I looked around a bit because I wanted to get a case for my laptop, but the cheapest laptop sleeve was around 24 euros, which was very expensive. We walked around the area and took a few side streets. Among other things, we saw a barber shop, a nut shop, a lot of clothing stores, and a few movie theaters. One of the more interesting experiences is that there was a church so we decided to go inside. We watched about 10 minutes of a service. I could understand a few words while the preacher was singing but for the most part, I could not follow him. My friend remarked that in the US, you would not hear clinking in the money collection baskets because people use small bills when they donate. But there are 1 and 2 euro coins so the money baskets did make noise.

So we came back and went to dinner as a group. We spent a long time looking for a place to eat and I had to get back to talk to my girlfriend so I broke off and had dinner on my own.  I didn't feel bad about this and I was able to get back fine on my own. I expected to have a boring dinner but it actually turned out to be a really cool experience.   So I just went to some random Chinese place near our hotel. I sat outside at looked at people. After I was mostly done with food, a guy and a girl two chairs away from me asked me how I was doing. I told him that I was an american student and he offered me a beer and asked me to tell him about myself. I rarely drink alcohol and I attempted to politely refuse but he insisted. I decided that this was an opportunity that I didn't want to give up. How cool would it be to sit at a restaurant in France and converse with French people. Anyways, it was a lot of fun to talk to them. One cool aspect about the conversation is that I was able to speak mostly in French where they We discussed a variety of subjects. I told them about what have done so far on my trip, my feelings about France, what I think of Obama and what I like about the United States. They told me about their education, what they thought about the US and what they thought about American politics. Also the only place that the man had been in the US was in the Chicago area and the girl was a Spanish student who was finishing her 5th year of French literature studies. She was at cite universitaire, the place that I will be living for the rest of the summer.

Anyways, I had to cut our conversation short because I had a call to make. I had drank about 3/4ths of the beer that they gave me. I didn't realize that it wasn't polite to finish the beer. The guy wanted me to drink all of the rest of the beer in one sip. I didn't do this and took my time. He didn't seem too happy about this besides this, I had quite a good time. I would consider going to a cafe and asking some people to talk. It seems that the socially acceptable way to do this would be to buy them a beer. So who knows. I do feel a bit odd because I basically haven't have anything to drink in the past few years but I didn't really feel any different after drinking.


As for work, we didn't do a whole lot today. My professor and the three of us working with him had a plan to meet at 2 pm. However some of the other students had earlier meetings so we all went as a group to the area where our research is taking place. We spent a lot of time walking around. There is a large area that contains a lot of research. The area is very pretty and I took a good number of pictures. We had a very tasty lunch although it was somewhat expensive because we were visitors. I had apple sauce, strawberries, bread, cheese (the moldy green kind), pasta, salad, chicken, and vegetables. I was very full by the end. My lab mentor seemed very nice. We filled out some forms and he gave us a bit of an overview of the work that is done in his research group. We asked him a number of questions and talked. I expressed the fact that I was interested in doing a more theoretical project and he said that there would be plenty for me to do. I am looking forward to really getting to work tomorrow on my research.

First Day in Paris

So I have had such a great time so far. The international flight was relatively uneventful. I brought ear plugs and when I had them in, I couldn't hear anything. I tried to go to sleep immediately and it seemed like I was half asleep for the first 5 or 6 hours of the flight. In the last hour or so, I read a long review article that was related to my research.

We got off the plane and went to buy train tickets. My credit card got rejected but luckily I had bought some euros at the airport before my international flight. This threw me off for a bit, but for the most part, I felt confident because I know enough French to communicate with people here. For example, when we were on the train, a lady asked me to put my bag up on a rack near the ceiling and I was able to understand her. Another instance was when I was walking on the street, a lady asked me where "rue d'Histoire" was. Even though I didn't know the answer, I was still able to understand her question. In general, I have found that it is very easy for me to ask questions but it is harder to understand their responses. Also many French people have just started speaking English when I have asked them something.

Anyways, a few hours after we got here, some French students from ENSTA (ecole nationale superiere de techniques avancees). They showed us around Paris and they were really fun to hang out with. They had a number of crazy games that they played with us. One of them was where we were on the train and a bunch of us sat in a line and then the last person dived on top of us and we used our hands to push them to the back of the line and then the front person did the same. I appreciated how they could do crazy fun things like this. We also played a game where if somebody yelled "Pigeon" we had to stop what we were doing and crouch down and cover our necks. If somebody yelled "water", then we had to run to the nearest place where we could stand on something that was not the sidewalk. We also played a game where we ran in front of tourists who were taking pictures and posed with them.

While playing all of these games, we went around and saw a lot of tourist attractions. I don't know the name of some of these places, but we went by a few amazing churches and we got to stroll around a French neighborhood. There were lots of street vendors and it was fun to look up at the walls and get the feeling of a French area. We continued along and passed by an art museum. There were a lot of people around and a lot of street performers. Among other things, I stood side by side with a mime and tried to copy his pose. Whenever a little kid came near him, he would bow and the kids would run away. We also went by the Louvre, the Champs-Elysees, the Arc de Triomphe, and the Eiffel Tower. We didn't spend to much time at any particular tourist attraction. The goal was just to show us around a bit and I think that we will be coming back later. When we were at the Eiffel Tower, we played a few games. One of them was where you stand in a tight circle then one person moves their left foot so that it touches the left foot of the person to their left. Then the person to their left does the same. This is repeated until somebody either cannot get their foot to do that or somebody falls. This was a lot of fun. We also played mafia. Apparently the french have a similar game called werewolves.

Another interesting aspect of our trip was seeing how tourists are treated. At the Eiffel tower, there were at least 50 people selling the same little Eiffel Towers. I found it very strange that all of these people were selling the same thing but apparently a lot of tourists buy them. The French students that we were with said that there have been vendors like this as long as they could remember. Also there seemed to be a lot of people who were trying to trick tourists and get their money. One example was at one of the churches, there was a little boy holding out bread for the birds. It was very cool to see the birds flock around the bread. So I picked up a piece of bread and held it up. Then an older man grabbed my hand and put some bread into it. It was really awesome to feel all the little birds jump onto my hand. So I appreciated that but at the same time, if my wallet wasn't securely in my pocket, it could have easily been stolen.  There was also an instance of a woman who claimed to be from Romania and she was stuck in the country. There was another woman at the train station who was groveling for money. One other experience that I enjoyed was that there were a lot of performers on the train. I saw a puppet show, a violin, and an accordion player.

This is already a long post so I wont talk too much about what I have been reading. I will get that later today or tomorrow. There were other interesting things that I also hope that I can blog about. For the rest of the day, I am going to try to get some more work done.

Last Full Day In Michigan

So I have heard that some people who are reading my blog are not so interested in the physics so I will try to leave the more technical things to the end of my post.


Anyways, each of us gave presentations about what we think that we are going to do during the program. I enjoyed giving the presentation but it seemed like it was stressful for some other people. I feel like there is a lot of material that I do not understand and I hope that I will have somebody in France that will be able to answer a lot of questions.

Another thing that I am thinking about is how I will manage my time in Paris. I feel like the group mentality is leaning more towards working less and having fun more. I think that it will take a lot of discipline on my part to break myself away from the group and work. I want to be a part of everything that people are doing but one of the things that I want to get out of this summer is a really good independent research experience.   I also want to do a responsible job with studying for the GRE. It would certainly make it easier if I can get a group together that will be willing to study together.  Another person suggested that I get up early and do work in the morning.

Tomorrow, we will be traveling and I will be arriving in Paris Saturday morning  (the flight from Chicago to Paris is about eight hours).


 I spent a reasonable amount of time working on my presentation.  It was interesting to see how people did their presentations. I felt that my presentation was more serious than the other presentations. I suppose that is somewhat reflective of my attitude. I feel like I could spend another 5 to 10 hours reading these papers and I would still not understand everything. I hope that I will make time to do this. I would like to have a better idea of the research that I will be doing. The person who worked with the same professor did a theoretical project. I hope that the professor has some projects that he can suggest for me.

In my presentation, I discussed how one can make multilayer mirrors that manipulate the attosecond light pulses produced by high harmonic generation. I don't exactly understand all of the details, but the idea is that you have a laser that makes pulses on the order of picoseconds into a noble gas (like Ar,Ne) and this produces a much shorter pulse. When this pulse is looked at in the frequency domain, it consists of discrete frequencies that are odd multiples of the frequency sent into the gas. The amplitude of the frequencies start high, drop off quickly, plateau, then eventually die out. When making these pulses, one screens out most of the harmonics. The other interesting thing is that the different harmonics have a phase relationship where there is a nonzero, constant group delay dispersion. (Group delay means that for a pulse, when you break it into frequencies, the different frequencies are out of phase). Anyways, mirrors can be made to compensate for this dispersion and this results in a shorter pulse. This is because if you have a bunch of superposed harmonics in phase, you get a shorter pulse than when they are out of phase. This method can make a pulse of about 90 as and the limit of the shortest pulse is about 74 as.

Anyways, I am looking forward to getting into a routine in Paris. I hope that I will have free time to do enjoyable things in Paris. I also hope to have a close group of people to hang out there.

Another Day in Paris

We are still in Michigan. Today we got an introduction the the Umich Center for Ultrafast Optical Science. They have a laser that is capable of achieving the highest intensities of any laser in the world. The idea of the laser is to take a short pulse, then pass it through a series of optical components that induce a chirp. This has the effect of broadening out the pulse. Then the broader pulse can be amplified and the pulse can be made into a short pulse again. The reason that this technique is necessary is because if you just start with a short pulse and try amplify them, the pulse becomes too strong for the amplifier. So in this method of chirped amplification, the pulse is broadened out and amplified. Since the pulse is broadened, any one place doesn't have a particularly high energy.

I thought that this was a cool device because it is able to probe new properties of materials by subjecting them to a high electric field. We also talked about lab safety today and got to see the laser described above. This laser has the acronym HERCULES. I find it amusing that they choose such acronyms.

One issue that I have had with a lot of physics research that I have seen is that it seems like a large amount of attention is given to achieving new extremes: lower temperature, higher pressure, higher intensity, etc. It is hard for me to see that just increasing the strength of our tools really gives us practical knowledge. I asked on of the professors about this issue. He gave a satisfying response. He noted that for this particular area of high intensity lasers, they are reaching the point where the laser could induce pair production in vacuum. That is to say, if you take a space with nothing in it, and subject it to a very large electric field, then an electron-positron pair emerges from seemingly nowhere. I am not sure if this is correct, but it seems that the energy from the electric field goes into creating the electron and positron. This raises the question: if we have a vacuum, do is there actually nothing there? From this, it seems like an electric field does count as something there.

Tomorrow, we are going to be making a hologram. I am looking forward to this. We also are going to be giving presentations about what we think our research is going to be about. The other students seem to be saying that they haven't gotten to reading the papers that their research mentors sent to them. I find it a bit strange that people wouldn't be more responsible about this. However, I am sure that people will get on top of their work.

Besides that, I have been having a good time meeting all of the other students and hanging out with them. Among other things, I hung out with people and tossed a Frisbee around. I do hope to get some more exercise during the trip. Some of the other guys are interested in Frisbee and apparently there is an exercise room at the place where we are staying. We also went out as a group to a Mexican restaurant and had a good time. I definitely feel like I have meet a group of people with whom I would enjoy doing a lot of site seeing. It seems like a bunch of people really want to go see as many countries as possible during their time in Europe. Personally, I would enjoy visiting two or three places and then spending the rest of the weekends in Paris. I think that if we are traveling every weekend, it will get tiring and we won't actually get to see a lot of stuff in the countries that we are visiting. However, we will see how this pans out.

Overall, I am very excited about my trip and the people that I am with. I am looking forward to a great summer.  

Summer in Paris

Since I am going to do be in Paris for nine weeks this summer, I thought that it would be fun to make a blog where I can document my summer. I am not sure what is going to happen with this blog, but I hope to discuss my thoughts about my research project and my experiences traveling.

This summer, I am participating in the Optics in the City of Light REU program through University of Michigan.  (see http://web.me.com/smysmy21/IREU/Home.html). All I can say for now is that I am excited to meet everybody. From what little I can see, I think that I will be in good company. 

My research project is related to: "creating optical components for attosecond pulses in the XUV spectral range". To explain a bit about what this means, you could imagine turning on a laser for one second. that would be a one second pulse. My summer research concerns pulses that are on the order of 10^-18 seconds! 

At first, I thought that this area would be useless, but it turns out that pulses on this time scale theoretically have the ability to look at the dynamics of valence electrons in molecules. (For instance, one source suggested that in a certain biomolecule, the time that it would take an electron to move around the molecule is around 10^-16 seconds.)

Generating such pulses is challenging. There are a number of ways that researchers are trying, but one of the main ones involves shooting a high intensity laser at a gas (ex. Ar, Ne). Apparently, this can produce pulses that are very short. But the drawback is that these pulses have wavelengths such that most mirrors have trouble reflecting them.

Therefore, there is a lot of work going into developing optical components such as mirrors that are able to control these light pulses. The approach that I will be using involves creating layers of different substances and using a computer to optimize the mirror.

When I first attempted to read the papers that my research mentor sent me, I was very confused because there were so many terms that I did not know. Now I am in the process of rereading the papers over again and wikipedia-ing every word that I have not seen. The material is starting to make more sense now and I look forward to actually understanding the details of this research in the subsequent weeks. 

I like to make goals so here are some (optimistic) goals for my summer:

1. be responsible with my research.

2. get to know some of the other people in the program well

3. sleep well

4. see the big tourist attractions in Paris (Louvre, Eiffel Tower, art museums), visit my uncle in Grenoble, visit a friend in Germany, eat good french food  (croissants!) 

5. improve my french  (possibly blog some in french)

Tomorrow, I am traveling to Michigan and meeting the other people in this research program. I'm excited!