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.

Laser Cleaning of Wall Paintings

Summary of an article from yesterday:

The Laser Cleaning of Wall Paintings

Maria Carolina Gaetani, Ulderico Santamaria; J. Cult. Heritage 1 (2000) S199-S207

 The authors attempt to determine the proper laser parameters for cleaning of frescoes via direct experimentation. The laser cleaning of paintings is an order of magnitude more complicated than the cleaning of stone because of the presence of pigments. In order to maintain the painting, obviously one desires to preserve the pigments. However, the difference between constituent pigments in the paint layers along with their thicknesses and the superimposition of consolidants/protective films requires significant and case-specific laser parameters. 

 The authors attempted cleaning with an Nd:YAG laser at both 532 and 1064 nm. The pulse width was 5-6 ns. Their delivery system could use either a focusing or diffracting optic, leading to irradiation areas of .018 or .38 cm^2. The authors also counted on direct particulate absorption, which depends on the stratigraphic (strata) composition and chromatic characteristics of the materials. The authors tested their results by monitoring pH and specific conductivity, as well as measuring the surface temperature after ablation and carrying out colorimetric tests.

 The first painting the authors attempted to clean was a wall painting representing The Visitation. The laser treatment was applied to a portion of the sky which was in a relatively good state of preservation, but which was blackened due to the stratification of various layers superimposed over the original image during previous restorations. Organic solvents were attempted, but did not allow a selective removal of layers. The authors found that the optimal cleaning condition was 532nm, 10 pulses/s (#pulses unstated) fluence of 1.5 J/cm^2 neither focused nor diffused.  The laser removal did, however, lead to yellowing in one of strata due to photo-oxidation of an oily layer. To completely remove that layer required fluences far superior to the damage threshold of the original paint layer, so a solvent was used. The yellowing was, however, less for 532 nm than for 1064.  The authors attempted the use of water as a cleaning moderator, but found that the bubbles formed damaged the paint layer.

 The second painting was a representation of the Nativity. Mary's cloak had been repainted several times during previous restoration, and consequently had a fragmentary surface. The authors found that using a diffuser, 532nm light at a fluence of .56 J/cm^2 and a 10 pulse/s rep rate yielded the best cleaning without harming the underlying original paint layer.

 The last painting was a representation of The Circumcision. The non-original substances were mainly removed with a pH alkaline saline solution, but small residues remained firmly attached to the surface in tiny fragments. The authors found that the optimal operating conditions were 1064nm light with a fluence range of .8-1.4 J/cm^2. Fluences above 1.4 J/cm^2 caused damage to the original substrate. 532nm light produced no appreciable results. 

 The authors claim that laser cleaning is a valid tool for painting restoration, but requires piece-specific calibration.

 

Translation stage: check

 Good news on the lab front: we now have a motorized translation stage. This is slightly less imperative now that we have a functioning shutter fast enough (hopefully) to let only a single pulse pass, but it should still make life a little easier. The next hurdle is getting beam time and finding out just how good the beam looks and how fast our shutter actually is. If it's not fast enough, we might have to just take Rod's suggestion and use it to dice carrots.

 I attempted cleaning some of the Japanese coins Bianca purchased using two very scientific methods:

1. Toothbrush and soapy water 

2. Q-tip and soapy water

 Neither of which worked particularly well. I currently have two coins soaking in soapy water in cups on my desk. Bianca also suggested methanol, which I went to get, but the lab workers were very hesitant to let me have any (saying "it's toxic" about 10 times. Yes, I know, I promise I won't drink it or let it touch my hands/skin) so I didn't get very much to the point that by the time I got back to the office there was very little I could do with it.

Speaking of the coins:

The resolution is a little cruddy because I took the shots with my laptop's camera, but in any case these are the first photos on this blog of anything directly (physically) related to my experiment. The characters on the coin (read NSEW) are:

寛永通寳

かんえいつうほう (Kanei Tsuuhou; the correct pronunciation of the first word would be something like kan'ei where the n is a syllable by itself)

Kanei was an Era in Japanese history from 1624-1643. Japanese history is broken into "eras" corresponding to the different emperors, for instance, this current year is Heisei 22 since the emperor ascended the throne in January 1989. For more info click here.

Despite the coin being labeled as such, such coins were minted into the 19th century. All the searches I've done for 寳 seem to indicate that the character is slightly outdated and is more typically written as 宝　(treasure) with the same pronunciation. The one on the right means "street" or "way" but as a verb it can also mean something to the effect of diffuse/circulate. So the coin basically says "Kanei Era circulating treasure." Appropriate enough for money. The ones we have were minted in the 1760s. The value on the coin is 4 mon, which wasn't very much.

 I've been looking for a good source for the metallurgical content of the coin and I haven't been able to find much in English, and my Japanese language skill is not up to par with reading metallurgical articles in Japanese without the aid of a Kanji dictionary, which I left at home because it weighs about 5 lbs. The best I have been able to find thus far is this abstract.

Google scholar, UMich and Rochester's databases have turned up nothing on this journal. I may send a very polite email to the corresponding society.

And ask very politely if they have old journals available online somewhere.

Fun fact: the Japanese write "copper" as either 銅 (meaning "same as gold") or 赤金 (meaning "red gold"). The latter is slightly archaic, and stems from a very early period in Chinese history when gold, silver, and copper were all considered gold. "red" was used when the materials needed to be distinguished; silver was "white" and gold was "yellow". The first character was more recent, although by no means modern, and was based on the notion that gold and copper shine in the same manner.

I also read an article on the laser cleaning of wall paintings, but there are some very nice charts in there which are probably going to require a post in and of themselves.

3rd article

A summary of the laser paper I read yesterday

"Lasers Cleaning of Patrimonial Plasters" (LaserS not a typo)

E. Tanguy, N. Huet, A Vinçotte

Due to mineralogical and physical characteristics, plaster becomes dirty quickly. Moreover, traditional plaster cleaning techniques often remove fineness of detail within the artwork.

 The authors fabricated a plaster sample, mixing the powder with only water (no additional additives). As a natural dust contamination process would be extremely lengthy, the authors deposited powdered carbon graphite as a contaminant.

 The authors thermally treated samples of their plaster to determine what modifications of phase could be induced by the ablation pulses. The samples were viewed with SEM after treatment, and were then ground and analyzed via X-ray diffraction. The results are below.

 The authors first attempted using the first harmonic of an Nd:YAG laser (1064), which resulted in "an intense yellowing" of the plaster, while use of the 3rd harmonic (UV) recovered a color "close" to the original plaster.  The fluences used were 2.88 J/cm^2 for the 1064 nm light and .72 J/cm^2 at 355. The discrepancy, not alluded to by the authors, may be enough to cause the yellowing of the plaster. The authors note that cleaning by the fundamental harmonic also morphologically modifies the surface, although the exact modification is not stated. The authors estimate the ablation thresholds of the "dirt" as .16 J/cm^2 and .23 J/cm^2 at UV and IR, respectively. The authors then cleaned a plaster pieta piece using UV laser light at .41 J/cm^2 and achieved "satisfactory results," preserving the details on the piece.

Getting Closer...

Today, we (Lei and I) succeeded in getting the shutter functioning with some help from Rod. After consulting the engineering guys, and then (and more importantly) consulting Rob, we figured out that the trigger signal we were trying to use had a frequency far too high for the shutter. We got a signal generator from Rod, and we now have the shutter configured so that it can open and shut as quickly as possible on command. The next logical step is to figure out how fast the shutter is, but we don't have a beam yet.
We're now missing a translation stage driver, two lenses, a CCD camera (which Lei knows how to find) and a beam. Rod's crew are tinkering with a pulse shortening mechanism, so we don't have a cleaning beam yet. However, when their mechanism is finished, we're potentially looking at pulses as short as 5-6 fs, which would be pretty cool. Rod also gave me a tour of their setup, which was cool, but a lot of it went over my head because I know nothing about plasma physics. Also, the Japanese (mon) coins that Bianca purchased came in today, so I'll be stopping by Carrefour tonight and picking up Q-tips to do some very very light mechanical cleaning as a prelude to ablation.

I also read three papers today, which I will summarize now.

Characterization of Laser Cleaning of Limestone

M.I. Cooper et al, Optics & Laser Technology Vol 27 No 1 1995

Limestone, which was a commonly used building material in Europe especially for monuments is composed primarily of calcium carbonate and is chemically susceptible to sulphur dioxide and nitrous oxides, which are common pollutants from the combustion of fossil fuels. Chemical reaction between the stone and pollutants lead

to the formation of a gypsum crust. From there, a buildup of soot particles with small amounts of metal, rubber, and asphalt results in the formation of a black crust, the removal of which is desirable.

The authors made use of the different absorption spectra of the limestone and crust; at 1060 nm (Nd:YAG) clean limestone absorbs about 30% of the radiation while the black crust absorbs 90%. The cleaning is thus a self-limiting vaporization process. The authors used a Nd:YAG laser with a 300mJ/pulse limit, pulse duration 6ns, and a rep rate of 10 Hz focused to "a desirable" (i.e. unstated, and probably not measured) fluence. The authors also extolled the virtues of applying a thin layer of water, the explosive evaporation of which removes particulates at lower fluences. When the crust is removed, boiling of w ater due to heating of the limestone is minimal.

The experimental layout used by the authors is shown below.

The calorimeter was used to monitor the output energy of the Nd:YAG laser. The HeNe laser was used to monitor the amount of removed material by the intensity of HeNe scattering recorded. The acoustic monitoring was an interesting idea. Ap parently when material is ablated and rapidly released from a material, the ejection creates an audible shock pulse. The authors found a linear relation between material ablated and shock pulse amplitude, with the slope being greater by an order of magnitude for "wet" cleaning. However, the shock amplitude is dependent on the angle from the sample's normal at which the microphone is held, so care has to be taken to maintain angle during the experiment. In terms of laser ablation, the experiment was successful, and preserved detail on the limestone quite well, with the cleaning being more effective upon the addition of water.

Laser Cleaning in Art Restoration

Gobernado-Mitre et al, Applied Surface Science (1996) 474-8

The author and his compatriots used a 7ns pulse Nd:YAG laser at 1064 at a rep rate of 20 Hz in an attempt to clean limestone from the Santa Cruz Palace. They characterized their samples using X-ray diffractometry, IR spectroscopy, and micro-Raman spectroscopy. The authors also used optical microscopy and SEM to morphologically analyze their samples.
The authors found that the samples they were using were mainly composed of dolomite CaMg(CO3)2, small amounts of gypsum CaSO4 x 2H2O and quartz, SiO2. The gypsum was probably due to weathering of the building to airborne pollutants, leading to chemical degradation via exposure to SO2 in the air or rain.

The authors used various numbers of incident pulses and studied ablation depth. The results are below.

The author observes that a natural "over-painting" exists on the cleaned stone for fluences of 200 mJ or less, but which is ablated by 2 400 mJ pulses.

The last paper I read was on plasters, but I'm falling asleep at the keyboard now, so any summary I write is going to be unintelligible. Will update tomorrow.

Table of Results

The above table lists the articles I've read in the past week along with the results obtained within. The three at the end labeled "no applicable results" were theory papers or background info and contained no data.

End week 2

I meant to post this last night but our wireless died around 10. It just now went back up.

I've been feeling tired these past few days, so I've fallen behind with the blogging. Sean and Johanna were (and still are) sick this week, and Michelle was sick before them, so I'm trying to avoid whatever's going around with OJ, sleep, and food.

On Thursday, I finished reading Chichkov et al's "Femtosecond, picosecond, and nanosecond laser ablation of solids" article. His results are fairly predictable to me now that I've read quite a few papers; fs lasers reduce thermal effects and make for cleaner ablation than ns pulses, although ps pulses also fair pretty well. Chichkov actually went into some of the thermodynamics behind the 2 step model, which I followed as best I could, but he skipped quite a few steps and my grasp on Thermodynamics is par at best. I also spent some of Thursday and then Friday reading a paper on the formation of corrosion products of Copper in the presence of sulfur. Since I'll be ablating primarily wartime nickels, primarily copper, and since sulfur was strongly present during the war years due to its use in an industrial setting, that seemed like a good choice for starters. Bianca asked me to make a table of the results of the papers I've been reading, so that should be up here today, hopefully.

By some coercion, bribery, devilry, or blackmail, Corinne manage to get me an opthamologist appointment for Friday, which I happily took. We got lost on the way there, one of the procedures made me very dizzy and light-headed, and I got lost finding my way to the RER station afterward (promptly followed by a second trek of the day up the Palaiseau hill) but I finally have lab clearance.

On a personal note, we went to Versailles on Saturday with students from Paris tech. The gardens are quite fantastic.
Hey, these statues look somewhat dirty/corroded. Maybe they'd let me irradiate them. I mean, really, what's the difference between a 60 year old coin and a 300+ year old statue?

Laser Safety Step 1: Don't Stick Your Head in the Beam Path

I went to see Yvres-Bernard this morning and picked up the laser safety video which I watched after lunch time. I went over the English version of the laser safety test that he gave me and asked Bianca to check answers with me. I then went to see him again and actually took the test with a bunch of other people, including Lee. Since my French is horrendous (translation: nonexistent) I just took the English language version. I finished in about 10 minutes, but Yvres waited for everybody to finish and discussed the answers (kindly in English and French) with everybody, so the entire test process took a little more than an hour. Afterwards, he gave me laser safety goggles on the condition that I return them at the end of July.

Just before taking the test, I talked to Valerie at LOA, who confirmed for me that if I managed to get an outside opthamologist to do the testing, my expenses would be covered. Using the HTH's doctor database, I found 8 opth. near Paris that I could go to, and with Julien's help (I'm really going to owe him by the end of the summer) I got in touch with 5 of them. I found out that 2 don't have the equipment, and of the 3 that can, one offered an appointment for 6/28, one cost about 200e, and one required that I come in for a consultation (after a month's wait) and then wait 2 months before the opth. appointment... We're going to try the other 3 tomorrow, but neither Julien nor myself are holding out much hope. Yvres-Bernard suggested asking Corrinne to get in touch with the people who usually do it and try to get my appointment moved up. I'll ask her, but I'm not sure how successful she's going to be.

On the research side of things, I finished reading another paper today.

Applied Surface Science 233, 2004

The authors begin by stating the reason for the suggestion of an incubation model, namely, materials irradiated with multiple pulses in succession were damaged at lower fluences. According to the authors, though, "the fundamental physical mechanisms of material removal during laser ablation are not clearly understood as yet."

The authors then mention 3 possible types of ablation processes: vaporization, normal boiling, and explosive boiling (also known as phase explosion, wherein materials are superheated-->vaporization). For short pulse lasers, the first two are apparently insignificant.

The authors next mentioned the morphological phenomenon of surface rippling, although they didn't say much about it. The next paper I'm reading, by Fauchet among others, deals with this explicitly.
The authors then described two ablation "phases", gentle and strong. During gentle ablation, which occurs at fluences just above the threshold the ablation rate is low, and ripples appear. Strong ablation is at fluences well above threshold, and results in rougher surface effects. The author also mentions that they are using the two step model for ablation. The model is as follows (fom Qiu and Tien, 1992, which I've half read and will finish reading soon...hopefully):
1. Incident photons interact with free electrons in the metal, exciting them to higher energy levels. The excited electrons are extremely far from thermal equilibrium and may be thought of as a free electron gas.2. The electrons diffuse through and heat the metal lattice (comprised of phonons) via electron-phonon collision. Due to the difference in mass, it takes 10s of collisions to transfer significant energy from electrons to phonons. The approximate collision time at room temperature (ambient) is 20fs, so the electron phonon relaxation time is on the order of picoseconds. If the laser pulse is much longer than this time then the electrons and phonons will reach local thermal equilibrium and a 2 step model is unnecessary. If however the pulse is shorter, the electron-phonon interaction is significant.
The authors measured the inclubation coefficients and ablation thresholds for four materials (below) using a Ti:Sapphire laser, 775 nm central wavelength, 150fs pulses, for differing numbers of shots. As predicted, as the number of incident shots was increased, the diameter of the ablated spot also increased.

The incubation model is as followed. Assuming a Gaussian beam, the diameter of the ablated spot may be described as:Where wo is the 1/e^2 beam waist, phio is the peak fluence, and phith is the ablation threshold fluence. For N shots,
One can determine the ablation threshold by plotting the first equation (for D^2) and finding the value of Phith for which D^2 is 0, since phi0 may be calculated via measuring the total pulse energy.

The results from the paper are as follows:
The authors also detailed a method for studying Phith(N) as a function of depth removed per pulse, but since that is somewhat removed from my project, I will not describe it here.

Red tape, red tape

I came in this morning and read about half of two separate articles... I started reading a paper on a 2-level model of laser ablation, but I was having trouble concentrating, so I started reading "The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air" by Mannion et al. I'll summarize both when I finish them.

After lunch, I helped Lei align the He-Ne in the lab. Since the He-Ne is such low power, my untested knowledge of laser saftey is adequate. The system is now almost entirely set up optically except for the lenses and CCD. We need to see an electronics guy about the shutter, since the trigger apparently needs more voltage than will currently reach it. We're also apparently waiting for another translation stage for the sample.

I got back to the office and found out from Bianca that although we do have the bon du commande, the earliest the ophthalmologist with whom the Ecole seems to have a preferential relationship could make an appointment was 7/1, which is frankly ridiculous. I'm going to ask Julien to help me call them tomorrow, but if they can't do anything, I've already found 4 other possible opth.s through HTH's website. The health insurance won't cover it because it's not illness related, and it would be 90e out of pocket with the POSSIBILITY of being reimbursed...because of the relationship between the ecole and the opth. they want me to go to...

So the upshot is that I was really not in a good frame of mind at the end of the day, and that if Julien can't help me convince the nearby people to give me an appointment sooner, I've got the decision between paying quite a bit of money and getting the thing done, or sitting around unable to do research for 3 weeks.


At least the day ended well. Francois invited us to a party at ENSTA, where we were pretty well welcomed by the French students.

Bureaucracy is a French Word. Efficiency isn't.

Today I went to see Yvres-Bernard to get the laser safety exam and watch a laser safety video after which I'm allowed access to the laser lab. However, we discovered two things:
1. The video is currently MIA, or at least somebody has it borrowed from Yvres-Bernard
2. I need to get a retinal scan/eye exam before I can even take the exam.

I'm not concerned about the exam. I've studied for it a bit, and the laser safety stuff we did at CUOS seems to more than cover the material. The retinal scan issue came as a surprise, and apparently there's a problem requisitioning the proper paperwork because apparently I'm technically LOA and Corinne (who's been extremely helpful so far) is ILE... or maybe I have that backwards. In any case, it took all of today to figure out who was actually going to cover the exam, and since we can't make an appointment until we have a "bon du command" which I take to be a declaration of payment, that's going to get done tomorrow. I'd be ok covering it up front with cash and then being reimbursed, but apparently that's even more of a bureaucratic pain. I was looking forward to actually getting some lab time in, but it doesn't look like that's happening within the next few days.
So instead, I spent most of today reading articles.
Article from SPIE proceedings vol. 7027, 2008. (all images taken from above article)

This group of researchers from Athens conducted research on laser-based cleaning of coins, using a Roman coin (minted c. 120 AD) and a Byzantine coin minted some time in the 6th century A.D. The Roman coin was relatively well preserved, but covered in a thin green corrosion product the authors identified as Copper Chlorite, while the Byzantine coin showed evidence of multiple different colors and types of corrosion products.The group used XRF (X-Ray fluorescence) to monitor the amount of each chemical compound in the corrosion products; that is to say XRF was used to monitor the removal of certain products by change in emission spectra.

The group attempted cleaning with several different laser systems, including an Nd:YAG which they used at 266, 532, and 1064 nm with a pulse width of 6ns and a rep rate of 10Hz, an Er:YAG of 2.94 um wavelenth with 190ns pulses at a rep rate of 1 Hz, and a TEA CO2 laser 10.6 um, 80 ns pulses, 1-5 Hz rep rate. The pulsed beams were focused by a lens of f=50 onto the coins. According to the authors, fluences ranged from 2-19.2 J/cm^2.

In terms of general results, the authors found that using a larger number of pulses resulted in a greater display of thermal effects in the material. Lower fluences were preferable, except that below a certain threshold the coins were poorly cleaned, or not cleaned at all. The authors also tested the coins in "wet" and "dry" conditions, although they failed to define what those definitions meant. My understanding is that "wet" means they applied a thin film of liquid to the coins before irradiating them, but the paper was not explicit.

The authors also mentioned an additional way to characterize the cleaning. The Roman coin in question was a quaternary alloy of Tin, Lead, Silver, and Copper. Apparently there exist two X-rays in Tin's spectrum, denoted La and Ka, where the La X-rays are significantly absorbed if some material exists above Tin, but which radiate strongly if Tin is in strong concentration at the substrate surface. In other words, once the ratio is large ("large" being not well described in the article) the cleaning has made significant process.
Ultimately, the authors found the presence of water helpful, and their best results were obtained by the 532nm Nd:YAG at fluences of about .93-1 J/cm^2

Proceedings of SPIE, vol 7391, 2009

The same heroes from last time, plus a few new faces, study the cleaning of ancient coins, here a coin minted in Cologne around 260 AD and a Roman coin minted in Alexandri in 315 AD. Both coins contain a high concentration of copper; the Cologne coin (designated NMW56 in the paper) was 91% Cu, 3% Ag 3% Sn and 3% Pb roughly, while the Roman coin (R2) was 83% Cu and 17% Ag. The authors stated that composition and structure of the corrosion layers had not
been fully investigated, but that they were mainly comprised of copper corrosion products. The authors used optical microscopy, SEM, and a stylus profilometer to determine cleaning efficiency. The authors used and compared 4 different lasers:

With repetition rates of 10, 1, 490e6, and 5e3 Hz respectively.

For the Nd:YAG laser at 532 nm, the authors reused data from a previous paper (the one about cleaning coins as a function of wavelength). They stated the result of most successful cleaning between .93 and 1 J/cm^2 but stated that the nonhomogeneity of the corrosion products complicated cleaning.

For the Nd:YAG at 1064, in a microscopic photo, the uncleaned area of the coin actually appears more homogeneous that the cleaned area due to melting/reforming of copper and budding exposure of Ag. Most of the corrosion products were successfully removed successfully.

For the GaAlAs Diode laser, the beam had to be tightly focused only to achieve a max fluence of ~.06 J/cm^2. Also, with such a high repetition rate, many more pulses hit each area than with the Nd:YAG laser (appx 10^8 pulses/spot according to the author). Given the author's previous claim that multiple shots lead to thermal effects, I have a hard time seeing this measurement as being comparable with the Nd:YAG results. In terms of results, the authors found that the diode laser successfully removed corrosion products but was not intense enough to remove cover covering the coin's silver outer coating. Personally, I think this sounds quite useful, as it managed to blast off the corrosion products without possibly harming the delicate (um thick) silver coating.

For the Ti:Sapphire laser, the authors found that the pulses removed not only the corrosion products, but also the copper AND delicate silver layer they were trying to protect, and concluded that for this specific operation, it wasn't the tool to use. The pulse rate was such that, at the two scanning speeds the authors attempted, either 1 or 4 pulses hit the same spot on the coin. The authors concluded from the variations in the Nd:YAG trials that perhaps the corrosion product behaved significantly differently for visible and IR radiation, which may have also affected the Ti:S's utility.

It is interesting to note that while this paper purports to compare cleaning as a function of pulsewidth, the repetition rate of the ps laser was such that a great number of pulses hit each spot, and also that the wavelength of the fs laser was such that cleaning may have been impractical regardless of pulse width. The authors even mention in the second to last paragraph that more work needs to be done since the test wasn't a fair comparison of fs, ps, and ns lasers.

As something of an aside, I find the focusing mechanism used in these papers interesting. Every time, the author just seems to mention "a lens of focal length" whatever which causes the beam to focus. But I can imagine that using just one lens would lead to significant beam aberrations especially in larger beams, especially from spherical and chromatic aberrations, particularly for short pulses which have a larger bandwidth. I would be interested in seeing the result of using a well-corrected lens system as opposed to a singlet lens used to focus the beam.

End week 1

The weekend was pretty unproductive for me. Lee, Sean, and myself tried to figure out what sort of sports/activities are available at the Cité on Saturday, and were severely disappointed. You need a membership card to use the pool. The membership card costs 20e and is available at some office which is closed over the weekends, and quite possibly is open 9-5 during the week, which means we'll have to either go in late or leave work massively early to get a pass. You also need a membership card to play tennis or basketball, or pretty much anything. The only thing free is pick-up games on the fields, so we went jogging and then played frisbee for a while on Saturday.

I went to mass again this week at the Irish chapel by the Luxembourg RER stop. On the way back, I successfully gave directions to an Irish guy I met at mass, so I guess I'm finding my way around pretty well at this point. I also did laundry (quite an adventure when there's 2 washers and 1 dryer for several buildings) and skyped with people at home.

On a more work related note, this is a schematic of the layout I'll be playing with for the next few weeks.
Light is emitted from the cleaning laser (which in our case is a pretty small tap off of the laser being used by another group). The light, which is initially linearly polarized, is shot through a half-wave plate and a linear polarizer, the combination of which functions as a variable attenuator. The pulsed beam is then shot through an electronic shutter, which ensures that only one pulse makes it through the rest of the system. The pulse is then reflected off of two dielectric mirrors (reflective at IR wavelengths the pulsed laser is operating at, but mostly transmissive at visible wavelengths) and focused on the sample/subject of cleaning. Simultaneously, a HeNe laser is used to illuminate the sample, and the reflections of said HeNe off of the sample are imaged to a CCD. The reflection profile will be used to monitor changes to the sample.

Day 5.1 (summaries of articles I read yesterday)

"Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward?"

P. Pouli, G. Bounos, S. Georgiou, and C. Fotakis

Springer proceedings in physics

LACONA VI Proceedings, Vienna, Austria, Sept 21-25 2005

This article primarily discussed femtosecond lasers versus nanosecond lasers as cleaning tools for direct ablation of varnish. The author states that it is known that fs lasers are more efficient because:

1. They are able to process even nominally transparent substrates

2. They minimize thermal diffusion throughout the substrate (diffusion in extreme cases can lead to melting and the creation of an even more difficult to clean mess)

3. The pulses are so short that there is no plasma shielding

The author the proceeds to talk about photochemical interactions, which are, as the name might suggest, chemical reactions which are initiated by the presence of light. These are especially important for the case of painted artifacts, due to the photolability (ability or likelihood of a material to change chemically due to interaction with light) of the painted substrates.  

The author next proceeds to define effective cleaning as based on:

1. Spatial resolution (etching efficiency) and ablation threshold

2. Extent of induced photochemical modifications

3. Morphology of ablation spots.

In general, fs lasers can ablate materials at much lower fluences than ns lasers. According to the experimental data presented in this paper, the ablation rate is roughly material independent.

The authors monitored the photoproducts formed in the varnish via LIF (Laser-induced Fluorescence). Apparently the disassociativity and activity of the dopant used are extremely sensitive to alterations in the polymer environment; in other words, the photoproduct activity can lead to a good understanding of photochemistry in the varnish layer. The authors found that product formation via photochemistry was much reduced for fs pulses. The two pulses of different widths behaved similarly under the ablation threshold, but above it, fs length pulses exhibited less thermal effects and less photochemical effects.

In terms of surface morphology, the authors found that fs pulses were also more effective. The fs lasers produced clean, small cuts, while the ns lasers exhibited some melting effects.

In  short, the authors found that fs pulses are better in all respects. Unlike Tam’s article, this process is applied directly to the varnish, probably because the paints would react in a way unpleasing to conservators if the steam cleaning method or some other substrate heating method were attempted. The authors found that the etching depth for both varnishes they used was much less for fs pulses, allowing for more precise cleaning.

“Experimental Study on the Effect of Wavelength and Fluence in the Laser Cleaning of Silvering in Late Roman Coing”

Vlachou-Mogire, Drakaki, Serafetinides, Zergioti, Boukos

14^th International School on Quantum Electronics: Laser Physics and Applications

SPIE Vol. 6604, 66040W (2007)

Due to economic turmoil in the later periods of the Roman Empire, previously silver Roman coins began to be made from an alloy of copper containing led, tin, and silver. The coins were then coated with silver in a film a few micrometers thick. Wear and corrosion can destroy any detail from the silver layer, but more importantly, mechanical cleaning of the coins can easily remove the silver layer, thus the necessity for more precise methods.

The authors irradiated the coins with a Q-switched Nd:YAG laser with 6ns pulses, 10 Hz repition rate, and between .1-7 J/cm^2 possible fluence.  They used both 532nm and the second harmonic 266 nm pulses, comparing the two in terms of cleaning utility.

The authors found that a fluence in the neighborhood of .93-1 J/cm^2 was most effective for the removal of the corrosion products.  For both silver and copper, the diffusion length is typically less than the corrosion layer depth, but the diffusion length is modified by the presence of the corrosion products, so such an approximation is not always valid, and the user must procced with extreme care (use the lowest effective fluence possible) to remove the corrosion products. The authors also found that at fluences well below the threshold, the material blackened. They postulated that this was due to another chemical reaction in which Cu2OàCuO. In other words, too low a fluence as well as too high a fluence resulted in deterioration of the coin surface

Finally, the authors found that the 532 nm light cleaned far more effectively than the 266 nm light, and postulated that this is due to metals absorbing strongly in the UV as compared to the IR, which would lead to surface melting. 

Work, Day 4

I spent today reading a few more papers which I will summarize here.
"Laser-Cleaning Techniques for Removal of Surface Particulates" by Tam et al

The paper begins by summarizing what it considers the 3 most important attractive forces between particulates and the varnish/object substrate to which they attach. The three forces are the Van der Waals force, the capillary force, and the force of attraction between an induced double layer of charge. The magnitude of each force exceeds the force of gravity.

The Van der Waals is typically predominant for particulates less than a few microns in diameter, and is the force of attraction between an instantaneous dipole in one body and an induced dipole in another. The most important facet of this attractive force is that it scales as d/z^2, where d is the particle diameter and z^2 is the microscopic distance between the particulate's surface and the surface of whatever substance it is attracted to. Although d will decrease for smaller particles, the particle density will greatly increase, and in general smaller particulates are harder to remove. However, this model assumes no compression/deformation of the particulate in the area of contact. Tam says that the VdW force is much greater if there is compression, but fails to mention by how much.

The capillary force is the "suction" force which may be present if a layer of liquid is trapped between particulate and substrate. It scales as d.

The force resultant from attraction between a double layer of charges results from charge transferral upon contact between particulate and substrate, which creates a contact potential of some magnitude at the border region. It scales as d/z.

Note that each of the attractive forces scale as d. Assuming mass ~d^3, employing force balance and F=ma, one needs an acceleration which scales as 1/d^2 to remove the particulate. In other words, smaller particles are more difficult to remove.

Tam then proceeds to describe a few processes for laser cleaning, defining efficiency as the removal of smaller particles with lower fluence and fewer pulses so as to maintain substrate integrity. The first process was dubbed Dry Cleaning, in which the substrate is hit with a laser wavelength at which it strongly absorbs radiation. The substrate then expands thermally and ejects the particulates. The governing equation is:(I haven't figured out how to use Greek letters on blogger yet)
A second related method involves blasting the object with laser light at a wavelength where the particulates strongly absorb. If the fluence is high enough, the particulate will sublimate or ablate.

The process Tam seemed more excited about he dubbed "Steam laser cleaning" wherein the surface of the object under test is coated with a thin film of water or a solution with a high concentration of water and a low concentration of ethanol and then hit with laser light. There are three types of such cleaning, the most efficient of which was called "Strong substrate absorption." The particulate/solution/substrate mix is hit with a laser pulse of a wavelength such that the water and particulates strongly transmit, but the substrate strongly absorbs. The material then superheats the water in the solution, causing explosive evaporation which removes particulates.
It is this ability to be superheated which makes water preferable to a purely ethanol solution, but high surface tension of the water makes the addition of ethanol to allow liquid diffusion between particulates advantageous.

The other methods of steam laser cleaning were film absorption, which is problematic because absorption occurs mostly at the film/air interface and does not result in the ejection of many particles, and partial substrate absorption, which requires a higher laser fluence and may damage the substrate.

Those are the basic principles. An important side note is that the pulse width (temporal) is limited on the lower end by the necessity to superheat the water and on the higher end by worries about damaging the material with high intensity laser light.

I also read Fotakis et al's "Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward?" and Drakaki et al's "Experimental study on the effect of wavelength and fluence in the laser cleaning of silvering in late Roman coins (Mid 3rd / 4th century AD) ". I'll post a summary of those papers tomorrow morning either before I leave for work or when I get there.