I got to see a Ti:Sapphire beam!!

  I've spend the last two days almost completely in the laboratory, which is absolutely fine by me. Bianca and I found that the two pinholes which had been used to align our system were of slightly different heights, and also that the He-Ne we were using to align everything was a little unstable in its holder. The latter problem is not so easily fixed, but we realigned everything, and have 2 pinholes at the end of our system now which are definitely at the right height. Today, Lei and I got (for the very first time) the beam we're going to be performing experiments with, and aligned it with the system. We also got a photodiode and oscilloscope and found that the chopper is quick enough to pick off one pulse. Rod's going to help us measure our pulse duration tomorrow morning. We still need to insert the lenses and the CCD camera, and also to measure beam width, but it feels like progress is being made.

  In my spare time, I've been reading a paper by W. Lamb about a QM treatment of the photoelectric effect without the use of photons. Essentially he treated the electric field of the incident light classically and the metal in a quantum fashion, simplified to one dimension, applied perturbation theory, solved for the photoelectron density matrix elements, and re-derived Einstein's equation (E=hv-W). I understand his approach, although he skipped quite a few steps in the derivation, so his math was difficult to follow.  The paper can be found as the 8th footnote on the wiki page about the photoelectric effect.

  I played badminton last night. I played pretty poorly, which might be related to lack of sleep during the previous night (I have a hard time sleeping when it's hot and it has been hot and humid the last few days. Thankfully, last night and today are better). I need to get in better form before going back to school. My backhand is failing something awful. Everybody here plays with plastic birdies, which are much less responsive than feathered birdies. By less responsive, I mean that more force is needed for a similar change in momentum. The upshot is that shots I think should be going just over the net are hitting it halfway between the tape and bottom.

 I intended to renew my Navigo last night and forgot. I need to do it tonight or else tomorrow morning is going to be hellacious.

Scans

 This weekend was pretty low key. I found a gym near the Cite to play badminton and I did that Friday night. It's the first use I got out of my racquet in a couple weeks and it felt fantastic to be able to blow off some steam. The guys at the gym are around my level or lower, but the guy who seems to be in charge told me of another place to go where the people are much better. He also told me about a tournament on Saturday, which I went watched for a little.  The two gyms operate on different days, so I may be able to play at both. It's going to cost 20e to play for 2 months, which is a lot less that I was expecting.

After watching a little bit of the tournament, I went to Place de Bastille and Pere Lachaise. Shame on the people who run Pere Lachaise, because Fourier isn't on their list of famous people buried there. His grave was easy to find, luckily. Chopin's, however, was not so easy to find.

Last night around 0300 the fire alarm in our building went off. It was turned off around 0315. And promptly went off again. And was turned off 10 minutes later. And promptly went off again. This continued for a while. The upshot was that around 0340 we all came back inside at which point the alarm went off AGAIN and we ignored it, but it's hard to fall asleep when the alarm is right outside one's door. It was shut off for good, thankfully, around 0400. I'm feeling a little sleep-deprived as a result.

Below are high-res scans of the coins I'll be ablating some time in the next month.

Top row: Three Kanei Tsuuhou Japanese coins. The first one (leftmost) was kept as-is from the seller, while the middle and rightmost were scrubbed and then soaked in soapy water for 2 days. Nonetheless, the rightmost coin still looks quite dirty. I'm going to name all the coins here for future shorthand/reference. The one on the top left I'll call KETH kanji, because the characters are easily and clearly visible compared to the middle two.  The middle one I'll call KETH kurai, since kurai is the Japanese word for dark. The last one I'll call KETH kitanai because it's quite dirty. I'm interested to see what happens when we ablate around the edges of the characters, especially the somewhat-complicated "kan" on the coin's north. Also, I believe that the coins may be from different mints. I was reading online that the length of the leg on the "kan" character is indicative of different mints. Look carefully at that character. In the leftmost coin, the leg stops before the edge of the coin's central square, whereas with the other two, the leg is flush with or just exceeds the square. Moreover, the characters, especially the "hou" on the coin's East seem more well defined for the leftmost coin. I wonder if the different mints would have different concentrations.

The lower two coins are two American silver dimes. Because the years are vastly different, I'll just use those as tags. Thus the left dime will be ASD54 and the right ASD46. The coins are nominally 90% silver and 10% copper. ASD54 seems to be in particularly bad shape, and ASD46 has a greenish corrosion covering most of FDR's face.

The obverse of the above described coins. The waves on the back of the Japanese coins look like a good place to irradiate without worrying about scattering from coin features. ASD54 was minted in San Francisco, which is pretty cool, especially since minting stopped there the following year for generally circulated coins.

Above are two world war era American nickels. The concentration is nominally 56/35/9 Copper/Silver/Manganese. As these two coins are from the same year, I'll have to be more creative than with the dimes. I'll call them AN44L for the left one (for light-grey) and AN44D for the right one (for dark-grey). I'm actually very interested in these two coins because they exhibit similar coverage by differently-colored corrosion products. I want to try irradiating the coins in the same area, Jefferson's collar looks promising, with fluences well below the ablation threshold for copper (1.06 J/cm^2) and see if the "gunk" of either pedigree gives way.

Obverse of the above nickels. 

Two other world war two nickels. The one on the left is in particularly bad shape and seems to have scratch marks, perhaps from earlier cleaning attempts. The left side exhibits reddish corrosion, while the right exhibits black. Again, this could be an interesting basis of comparison for two types. I'll call the left coin AN43. The coin on the right is in fairly good shape, and seems to have corrosion of the same color as AN44L. If cleaning on AN44L is moderately successful, I'd like to have a go at trying to clean this coin completely. I'll refer to is as AN44CC for that reason.

Obverse of above nickels

Front of a Roman coin minted in Thessalonica somewhere between 348 and 351 AD, during the reign of Emperor Constans or shortly following his assassination. The coin is 17mm and 1.51 grams, but other than that, I've been able to find astonishingly little about the metalurgical composition of this coin other than "bronze".  Most of the coins Serafinites et al cleaned were minted earlier, so while they might be good for an approximation, they cannot be considered gospel. An image of what the un-corroded would look like is here. 

Obverse of the Roman coin. I really don't have a plan for this guy yet. The thin green corrosion layer is similar to what Serafinites dealt with, but he was more successful at shorter wavelengths and we're using ~800nm. Perhaps the fact that we're using fs instead of ns pulses will yield different results (less thermal effects).

End week 4

 For the first day this week, I went in to the office in Palaiseau. I feel like I didn't accomplish much this week, although the Laser 50 conference was enjoyable.

I read two papers today, and scanned the samples I'll be ablating (hopefully...) soon. I'll post 

the pics in a separate post as blogger seems to throw a fit when I try to put too many images into a post.

Measurement of Gaussian Laser Beam Radius Using the Knife-Edge Technique: Improvement on Data Analysis

 de Araújo, Silva, de Lime, Pereira, de Oliveira; Applied Optics Vol 48, No 2, 10 January 2009

 The authors begin by assuming the incident beam has a Gaussian spatial profile, as follows:

Where w is the beam radius at the point where the intensity declines as 1/e, and xo and yo are the coordinates of the center of the transverse profile of the beam. If one were to drag an edge (usually a knife via a precision micrometer) in a direction, say x, such that the knife slices out lines of constant x, the power which reaches passes by the knife edge is given by:

Where x' is simply a variable of integration. The denominator is for normalization and the top involves an error function in x. The general solution to the above equation is given by:

The difficulty with this equation is that the error function is impractical for use in fitting experimental data. Alternative methods include using the derivative of P with respect to x, but such differentiation results in amplification of fluctuations and consequently increases error.  None of this is new. The authors reference a previous paper by Khosrofian and Garetz, who suggest using a purely analytic which approximately represents P(x) to fit the data. The function they suggest is:

Where the a's are simple coefficients. To extend the results to negative s, K&G claim that f(-s)=1-f(s), that is to say, the function is odd with respect to P(x)=.5 as seen in the graph below.

The addition of the authors of this paper is that claim that p(s) may only have odd order terms, the original authors having included both odd and even order terms. Frankly, I'm not sure I buy this yet, and I want to give it some more thought over the weekend. In any case, the authors find:

a1=-1.5954086

a3=-7.3638857e-2

a5=6.4121343e-4

Which they then use to determine w of a HeNe beam experimentally to an accuracy of +/- .06 um.

Measuring a Narrow Bessel Beam Spot by Scanning a CCD Pixel

Tiwari, Ram, Jayabalan, Mishra, Measurement Scence and Technology 21 (2010)

Although the authors specify their measuring technique as useful for Bessel beams, the same precision makes the process useful for Gaussian beam profiles as well. I will summarize this procedure very generally, since it seems a little computation heavy/involves some software munching as compared to what we're doing.

 The idea is as follows. First, assume that the number of "counts" per pixel on a CCD is linear with applied intensity. Assume the beam is propagating in the z direction. If one wants to find the beam profile at some z', tag a single pixel on a CCD array. Arrange the pixel at the center of the beam in the y direction by scanning until the maximum number of counts on that pixel is found, applying filters as needed.  The, scan that particular pixel in the y direction to map out an intensity profile of the beam in the z' plane. One may then plot the (normalized) pixel counts as a function of y and compare with the expected profile while varying a parameter. In our case, one would expect a Gaussian profile and would vary the beam width.

 The authors intended to use said method for Bessel beams because the knife method was not available due to the significant amount of beam power contained in the spot rings.

50th anniversary of the Laser, Day 3 and a Train Strike

The 50th anniversary of the laser conference concluded (at least for me; there was a cocktail hour for bigger names which I did not attend) with the talks yesterday. We arrived about 10 minutes into Bloembergen's talk, so unfortunately I missed out on that. Weinreich's talk was entertaining, although I didn't glean much from it. Prof. Tokima's talk was very good, and for the most part I could follow him, although he lost me when he went into QED.

Kroemer's talk on the beginnings of Heterostructure lasers was very good, primarily due to his sense of humor and the speed (followable) at which he talked. I'm taking Optoelectronics next semester, so I hope to learn a bit more about semiconductor physics there.

I was a little disappointed by Dr. Aspect's talk, primarily because it seemed like the 70-80 minute talk he gave at UR condensed into 30 minutes. It was still good, but not as good as it could easily have been if he had been allotted more time.

The last talk I gleaned much out of was the talk on fiber optics given by Prof. Sir Charles Kao's wife and Dr. Desurvire. Apart from a history lesson, I learned that the rate at which fiber is produced per second is approximately 5x the speed of sound, on average.


I didn't go to Palaiseau today because of a train strike. I would have tried to go if Lei had had time to do labwork, but I ran into Lee (Gunderson) this morning who tried to take the RER in to work and found that no trains were running southbound on the RER until 430 PM. I took the opportunity to do some gift shopping instead, since this is probably the best opportunity I'm going to get. Now that I'm back, I think I'll try to read a paper on scattering from a periodic surface, a paper Fauchet references considerably in his paper on periodic surface structures after laser ablation.

50th anniversary of the Laser, Day 2

The series of talks today was quite good. I especially enjoyed the talks given by Drs. Townes and Svelto, although Drs. Zewail and Krausz both gave good talks as well. Dr. Cohen-Tannuodji was speaking about some very interesting things, but he was also speaking quickly and a little above my head, so I'm afraid much of the talk was lost on me.
I can't believe that Dr. Townes is 95. He doesn't look a day over 70, and quite frankly I hope I'm in half as good shape when/if I reach 95. He talked on the history of the maser, the microwave radiation forerunner to the laser. The first day of my Fundamentals of Lasers class at UR also covered this, but hearing it from Dr. Townes was several orders of magnitude more interesting.

Ted Maiman's wife talked about Maiman's contributions, and brought with her Maiman's ruby laser, in the photo below.

The only disappointment was that many of the talks were very "general audience;" with Drs Zewail and Krausz stating and apologizing for as much. I hope the talks tomorrow are a little more detailed. I'm looking forward to Dr. Aspect's talk, although I fear it may be similar to one of the ones he gave at Rochester this past year.




In an unrelated note, GO TEAM USA!

50th anniversary of the Laser, Day 1

The weekend was pretty quiet. Sean and I went to the Grand Palais on Saturday to see their Daoist exhibit. It was pretty interesting, although the blurbs were in French. I could definitely see the Buddhist (pre-Yuan) influence on some of the Daoist pieces, which was pretty cool. After that, we walked the Champs-Élysées for a while. I went to church on Sunday and met Dr. Mourou on the metro. He was heading off to meet Charles Townes for the 50th Anniversary of the Laser conference, so Dr. Mourou and I had a conversation on the metro about my project, mutual acquaintances at Rochester, and racquet sports (he plays squash, I play badminton). Sunday night, Lee, Michelle, Robyn, Sean, and I went to the Luxembourg RER stop and found a place to have dinner. Tom and I tried to go to Pere Lachaise cemetery to visit Fourier's grave, but it was closed by the time we got there.

On Monday, Johanna and I went to the Louvre. We helped assemble the visiting scientists who came for Louvre tours, which was pretty fun because we got to meet some pretty well known people. We managed to tag along on a tour of the Louvre's Delacroix paintings; it was like being in European History class again. After the tour, we attended Dr. Costas Fotakis' talk on the use of lasers in art conservation. Since the talk was quite general, I admittedly didn't get much out of it, and he showed quite a few more pictorial results than were published in his articles that I've read. I think I might have thrown him off slightly by asking a more technical question after his presentation.

Tomorrow, there are quite a few talks that look very interesting. I'll post summaries of the ones I get the most out of.

End week 3

 I didn't get a chance to do any lab work in the past two days. It's difficult to calibrate your setup and run diagnostics when one has no laser beam. I also found out that we're going to be getting very little laser time. Apparently we'll have time in the morning after Rod's people get here and before lunch, so that's probably 1030-noon.

 I've read a few articles in the past few days which I'll summarize shortly. There was also a picnic outside the main building at ILE today (i.e. directly outside Bianca's/Johanna's/my office as well). The food was quite good (as was the wine) but they set up speakers and played Latin music, the volume of which made it a little hard to concentrate, for most of the afternoon.

Laser Technology for Graffiti Removal

Sasha Chapman, J. Cult. Heritage 1 (2000)

 Two historically significant monuments at Stonehenge and Avebury were subject to graffiti attacks in 1998 and 1996, respectively.

 Eight of the standing stones of West Kennet Avenue were "daubed" with paint. Some of the stones are apparently also a site of scientific interest due to the existence of old lichen colonies. In any case, 2 of the stones had been daubed with white emulsion paint and 6 with black gloss. Conservators used an Nd:YAG laser at 1064nm at a "high" (i.e. unspecified) fluence and monitored the damage threshold acoustically.   The stones consisted of quartz, silica, and ferrous oxides, and the surfaces varied enormously from being "dense and glass-like" to "sugary". The areas of stone which were more porous responded badly to laser treatment, turning either a dark grey or brown as a result of change in the oxides. The authors found that applying a solvent to the stone to remove the majority of the paint and using the laser to remove the remainder was most effective.

 The Heel stone at Stonehenge was attacked with spray-paint, but one of the utilized paint cans was found at the sight, so chemical tests were run with the paint and small quantities of sarsen (sandstone which comprises Stonehenge). In said tests/trials, it was found that acetone successfully removed most of the paint without damaging the stone. On sight, however, removal of paint was more difficult due to heavy lichen growth. The conservators (including M. Cooper, the author of a paper previously summarized in this blog) attempted 532nm as well, and found that it cleaned more effectively than 1064nm at cleaning paint from areas where acetone had been applied. Some residues were left.

 This article was remarkably data-free, but it does illustrate another application of laser cleaning.

Laser Beam Width, Divergence, and Propagation Factor: Status and Experience with the Draft Standard

John M. Fleischer, SPIE Vol. 1414 Laser Beam Diagnostics (1991)

 This article begins by defining several Gaussian beam parameters unambiguously (i.e. beam width is full width, divergence is the full angle and not the half angle). They also define a propagating factor, M^2, as

M^2 = pi * D(o)*theta /4

Where D(o) is the smallest beam width and theta is the full angular divergence.

 The author then describes several practical methods for characterizing Gaussian beam parameters. The first method they describe is the slit-scan method, in which one scans a narrow slit across the beam. After finding the maxima, the user attempts to find the 2 locations where 13.5% of the peak irradiance are incident. The difference between those two is the 1/e^2 beam width. The disadvantage of this technique is that it is impractical for very small beams.

 The next described method is the knife-edge test, wherein one traces a knife edge across a beam and measures transmitted power. The distance between the 10% and 90% points multiplied by 1.56 is the 1/e^2 diameter. This test is less precise than the slit scan and is inaccurate for higher order modes, but is simply to implement.

 The author next describes a diagnostic utilizing a scanning pinhole to map out the transverse beam profile. The diagnostic features large error for higher order modes.

 The final test the author discusses is the "encircled energy" test, wherein one aligns an aperture with the center of the laser beam and varies the area of the aperture. The area at which 86.5% of the power or energy is allowed through has the 1/e^2 diameter. However, the problems with this are:

1. Difficulty and uncertainty in aligning the aperture with the center of the beam

2. Assumption of a symmetric beam, often untrue.

 I also attempted to read "Interaction of Femtosecond Laser Pulses with Tempera Paints" by Gaspard et al, but found myself distracted by the rather loud music outside my office (as well as the US/Slovenia game) and also found the chemistry portion of the article rather difficult and tedious to read. I'll give it another go over the weekend.