August 7th, 2009

Alas, today was the last day of Woody and the REU program. We sat in on the REU project talks. In addition to those of Mara, Max, and Tom, there were numerous others, as there were ten REU's in total. The other projects delt with a wide range of topics including the detection of cosmic rays, testing of detection hardware for CERN's Large Hadron Collider, and the age of a specific class of stars. It was a fun day of interesting presentations.

August 6th,2009

Our lunchtime talk had to be moved back from Wednesday, hence the delayed writup. Today, there was no official lunchtime talk, but rather, a series of practice presentations by Mara, Max, And Tom, who desired a chance to warm up for their REU talks tomorrow. They all did quite admirably. The one major qualm that I had was that Max neglected to add any of his equations, an integral part of the field of optics.

August 4th, 2009

The REU talk this week was a tour of the LTC. We scrambled to set up numerous assorted demonstrations that we believed the REU's would enjoy. These included A fiber optic cable, various examples of diffraction, The Mirage toy + laser pointer demo, Taking a circular polarizer and looking at one's face through a mirror, and a spectacular laser light show set up by Mara and Tom.

August 3rd, 2009

I finally finished my array! After three weeks of attempting collinearization, I finally succeeded. The CVB generated in my array became an HG mode upon passate through a linear polarizer. More than this, when the polarizer was rotated, the HG rotated with it, the hallmark of a true CVB. Previously, whenever the polarizer was passed at 45 degrees, a fringe pattern emerged (a thought on the fringe formation is that without proper allignment, the "CVB" produced is simply a glorified which way experiment). There were some irregularities at times, but the two dot structure was maintained throughout rotation.

July 31st, 2009

Earlier today, it was thought that my CVB generation setup was not theoretically apt. Luckily for me, however, I found an article that proved otherwise! After fiddling with the setup and searching for articles, it was time for me to call it a day.

July 30th, 2009

Unfortunately, Laser Sam had to leave today, but before he did, he showed off some pretty interesting demonstrations before he did. The first was a Hewlett-Packard laser from the seventies. It turns out that it was the oldest laser rangefinder. It measured distances in terms of wavelengths of light, as opposed to meters. It seemed quite accurate at short distances, but I doubt that It would have functioned at long ranges (that being said, it was only intended for short ranges in the first place). After that, he demonstrated the use of a focused HeNe to burn electrical tape (it was very fun).

July 29th, 2009

Laser Sam arrived today, and gave a talk on the different laser varieties over lunch. Laser diodes, the most common variety of laser today, operate using an excited semiconductor chip as a lasing material. Laser Diodes come in many different sizes and powers (the average size of the ones in laser pointers is roughly that of a grain of sand, although Laser Sam is the proud owner of one a few centimeters across with an output of 35 W!), however, there are currently no commercially viable green laser diodes. The gas laser (the HeNe especially) is considered by many to be the standard laser, and the closest laser to the ideal case of a simple oscillator cavity. The HeNe is on average the most well collimated of any commercial laser. Dye lasers, despite a wider range of wavelengths than most solid state or gas lasers and prevalence in the early days of the field, are used much less often than the other varieties available. Considering the toxic organic dyes used to power lasers of this type, and the necessity of recirculation of the dyes, it is little wonder why they are unpopular in many applications. Solid State lasers make use of substrate materials in the solid state of matter. The first laser ever invented was, in fact, a solid state ruby laser. Finally, laser sam described a Fabry-Perot interferometer. A Fabry-Perot interferometer is essentially a hollow laser cavity (or any two precisely aligned reflecting mirrors) that one fires a laser into. What is measured is the interference caused by the reflecting beam with itself over the length of the cavity. The day was finished with a demonstration of a homebuilt version of said device by Laser Sam.

July 27th, 2009

Today, I attempted to colinearize the two beams. Although I did not achieve this, I came closer than ever before (through a process of walking the beam using the CCD camera to precisely align the two beams). This task will not be easy, as The beams must be perfectly colinear, coaxial, and with the proper phase relative to one another.

July 23rd, 2009

Alas, my first experimental data is not coming out as planned, and my setup may end up needing revision. Despite this, I observed some very interesting vortex-like patterns in some of the images. these will be presented below.

July 22nd, 2009 setup is finally complete! To finish it, I placed two linear polarizers in a block of styrofoam in front of each beam. to rotate the polarization in the arm without the dove prism, I placed one polarizer at 45 degrees off the state of the laser, and another at 90 off. A similar setup was used to attenuate the other arm, but with the final polarizer being at the same angle as the initial state. It seems a little haphazard, and I am considering revising it in favor of a more accurate setup. However, until a fatal flaw is found, I will keep it. I am hoping that when I take my first data tomorrow, it is positive (ie: a CVB is apparent).

July 21st, 2009

My experimental setup is finally beginning to come to fruition! At this stage, I have the beams aligned and the dove prism at the correct angle to rotate the beam by 90 degrees. To align the beams, Dr. Noe and I used a procedure known as walking the beam. It is necessary to have a standard procedure to align the beams, as without one, it is possible to never properly align them, or worse, to cause them to diverge. I will now describe walking the beam in the context of a rectangular Mach-Zehnder Interferometer, the array with which I am currently working. In said array, the farther mirror from the final beamsplitter is denoted the position mirror; the closer one is denoted the angle mirror. to walk the beam, one places a screen far away from the final beamsplitter, and then adjusts the angle mirror to overlap the dots. After this, one moves the screen very close to the final beamsplitter, and adjusts the position mirror in the same fashion. One simply repeats the process multiple times to create an aligned beam. The dove prism was rather finicky, because of the fact that the faces were not exactly equiangular with respect to one another. Despite this, however, eventually the correct alignment was stumbled upon.

July 20th, 2009

Today, we attended a thoroughly informative lecture by Marty LiGare about two photon interference. It very effectively demonstrated the differences between a quantum and a classical field. It was a very enjoyable talk. Upon the conclusion of the talk, I had a phenomenal chat with Dr. Jack Marburger. We discussed the nature of light, and asymmetry in physics (he gave an example from high energy physics, I gave one from aeronautics).

July 16th, 2009

Today I tried to utilize Tom's interferometer setup with the open cavity laser (OCL). Unfortunately, the OCL is orthogonal to the input direction of the interferometer. When I tried to reflect the light into the setup, an interference pattern emerged in one arm even before the recombination of the beams! It is clear to me that the setup will need to be moved in front of the OCL to work effectively.

July 15th, 2009

Dr. Metcalf gave yet another talk today, however, unlike yesterday's talk this one was on polarization. He discussed the Pointing vector's’ significance, the property of birefringence, the different states of polarization, the true nature of natural light, and the Jones Calculus. It was a very informative and fun discussion, and I learned a lot (especially about natural light and the Jones Calculus).

July 14th, 2009

Today, we found that the light off of a cholesteric liquid crystal array is circularly polarized. Amazing! Later, we aided Dr. Metcalf in his REU talk. I also found out that by orienting a linear polarizer 45 degrees off the direction of a laser's polarization, and then place another 90 degrees off of the laser's initial polarization, it is effectively the same as a 1/2 waveplate!

July 13th, 2009

Today we learned about liquid crystals. Dr. Metcalf is giving a talk tomorrow on the subject, and we were his test subjects. Basically, a cholesteric liquid crystal is a series of sheets stacked one on top of the other. The cholesterol molecules are all oriented in the same direction within a plane. The liquid crystal array consists of many of these planes, each rotated a certain constant angle off of the previous one's orientation. This forms a loose helix. Each orientation is capable of reflecting a certain linear polarization. The frequency reflected depends on the distance between layers (referred to as the pitch), which is, in turn, dependent on the temperature of the array. Interestingly, as the temperature of the liquid crystal increases, the pitch decreases. This means that the wavelength reflected by the liquid crystals is shorter when the temperature is higher. A popular use of cholesteric liquid crystals is in mood rings, thermometers, and other devices requiring cheap, temperature sensing.

July 10th, 2009

What a week! We went through many an informative talk. First on the list was my Simons talk on Tuesday. It was on paleontology in Madagascar. The speaker discussed his findings over the years, including a new saurapod, an interesting new predatorial dinosaur (who, it turns out, was cannibalistic), and many crocodilians, birds, and other species. Next, there were two fascinating talks in the room adjacent to the LTC. The first was by Giovanni Millone, a former student who got hooked on optics here at the LTC and is now working towards his PhD. In his project, he described a second type of optical vortex, known as a cylindrical vector beam (CVB). CVB's are similar to Laguerre-Gaussians in that there is a discontinuity in the center, however, instead of the phase being varied along the wavefront, the polarity is varied. Depending on how the polarity is oriented, the beam is either described as being radially or azimuthally polarized. A third case is that of the hybrid vector beam (HVB). The HVB occurs when one passes a CVB through a one-quarter waveplate. Along the wavefront, there are four points of linear polarization (with opposite points having the same direction of polarization), four points of circular polarization (with two sets of opposite left and right circular polarizations), and varying degrees of elliptical polarization between each pair of circular and linear polarization points. I am thoroughly interested in the topic that Mr. Millone presented, and I hope to do some research into the field. After the lectures for the day were over, he explained how to generate CVB's and HVB's. Before that, however, Matthew Eardley presented a very interesting talk on the use of a micron-scale cantilever to affect a quantum system. The roughly 100 micron long cantilever is to be placed below a magneto-optical trap. A supercold atom gas globule will descend from the MOT and be suspended in front of the cantilever. Apparently, some of the atoms will resonate with the cantilever and be excited out of the cloud. In essence, this means that the mechanical lever affects the quantum atoms! Professor Metcalf stated that if he had not already heard of the research already, he would have deemed it impossible! Also, This week I generated my very first higher mode, Hermite-Gauss beams! I successfully generated a 10 (one row, two columns), a 20 (one row, three columns), a 30 (one row, four columns), and a 01 (two rows, one column). I also successfully used Tom's interferometer array to interfere the aforementioned HG-01 mode. Unfortunately, the mirrors were…misaligned…during my fiddling and, as a result, much of this morning was spent fixing the array. On the upside, however, he did observe the Gouy phase after some more fussing with the interferometer.

July 2nd, 2009

Today I say my first higher-order laser mode. It was a Hermite-Gaussian with a 2x3 profile (in terms of the number of light spots, at least)! It was beautiful! Dr. Noe also made sense of the equation of the complex electric field of a Gaussian beam. He explained it by relating each of the complex exponential terms to a different beam parameter.

July 1st, 2009

We learned about the Raleigh range (the point at which the radius of curvature of a laser beam wavefront is smallest, and also the point where the beam's cross-sectional area is twice that at the beam waist), and elliptical polarization. During lunch, we talked about Rydberg atoms. These atoms are known as the largest single quantum objects. This is because their electrons are in very high energy states (ex: n = 100 instead of n = 1). They are many times bigger than standard atoms and, apparently, exhibit behavior only possible at the very verge of the quantum-classical boundary! We finished the day with a discussion of rainbows and caustics.

June 30th, 2009

We went outside today to conduct many interesting experiments with lenses and mirrors (this quickly devolved into the wanton burning of many holes in sheets of noncombustible paper). We were put to the test by Dr. Noe, who asked why his glasses would not focus light into a point. As he had mentioned that he was nearsighted, I correctly guessed that his corrective lenses must concave. As concave lenses diverge light, they cannot focus the light. After this, we used a mirror and a piece of noncombustible paper with a single hole burned through the center to illustrate the concept of angular diameter. Angular diameter is the measure of the angle between the edge of an object and that of its image. We placed the mirror behind the paper so that the only reflective surface was through the hole. We then measured the diameter of the hole, stood a meter apart, reflected light from the hole onto a piece of white paper, and measured the diameter of the spot. The angular diameter worked out to about 1 cm per meter. Interestingly, the angular diameter decreases slightly as the diameter of the reflector used increases (this is based on the size of the initial light source, the sun in this case).

June 29th, 2009

My first day at the LTC! Today, we toured the lab, met the REU's working with us in the lab, learned about De Moivre's formula, and proved that cos2(x)=(1+cos(2x))/2 using Euler's Formula (which is e^(ix)=cos(x)+i*sin(x)).

Josh Lieber

Laser Teaching Center