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.

Cité and Day 3

I realized only now that I didn't describe the Cité, despite the fact that my last post promised that I would. We're situated in the German house. Tom and I, as well as Sean and Lee have rooms on the 4th floor. They're a little small, especially in comparison to the ones the girls got, and seem like they will be boiling hot in July, but the wifi here is 10x better than at the hotel. Also, there are no towels available here to buy/use as was described, so I had to go out and buy one today.

I spent the morning reading part of Leis et al's "Basic Investigations for Laser Microanalysis I. Optical Emission Spectroscopy of Laser-Produced Sample Plumes." I consider myself quite comfortable with laser operation and principles of lasers, but I know relatively little of the materials aspect of it.On the way to lunch, I ran into a French Ecole student holding a badminton racquet. It seems that the school has a club which plays quite frequently. I asked Corinne Chen to look into it for me; she's pretty friendly and seems to know the ropes very well.

I spent the first hour or so of my afternoon reading and taking notes on "Simple Technique for Measurements of Pulsed Gaussian Beam Spot Sizes" by J.M. Liu. The process is remarkably simple. It turns out that the surface of a silicon crystal will become amorphous (in the chemistry sense) at a threshold fluence of .20 J/cm^2. The first amorphous area is simply circular, and evolves into a ring pattern with increasing fluence until a fluence of .26 J/cm^2 is reached, at which point the material reverts to a single crystalline state. In other words, there exists a non-crystalline state where the crystal was hit with between .26 and .2 J/cm^2. Letting the outer and inner radii of the amorphous pattern be defined as ra and rc respectively, and assuming a TEM00 spatial Gaussian beam profile:

Where Ea is the threshold amorphous-region-causing fluence (.20) and Ec is the crystalline fluence (.26)
Taking the logarithm of both equations yields:Which can be plotted on a semi-log plot as follows (taken from Liu):
Where the open circles are the outer radii and the filled-in circles are the inner radii. Ec and Ea may be determined from linear fits of the data. Note that the equations for the radii squared may be combined, killing the Eo terms, and the spatial beam waist, rho, may be solved without calibration for Eo. Note that one can express Ea and Ec in terms of Eo in arbitary constants, i.e.
Ea=A Eo
Ec= C Eo
And when one subtracts the two equations, one finds that the dependence of Ec and Ea is
ln(Ec)-ln(Ea)=ln(Ec/Ea)=ln(C/A)
Completely eradicating the Eo dependence.

Thus, one can determine the beam width without calibrating the peak energy.

I spend the remainder of the time at work reading Tam et al's "Laser-cleaning techniques for removal of surface particulates." Michael read this last year and took notes on it, but I wanted to read it to be thorough. I'll summarize it here after I finish it. We left work a little early to go buy cell phones and a towel.

Days 1 and 2 of Work and Cité Universitaire

On Monday, John Nees accompanied everybody to Ecole Poly so that we could meet with our advisers. Johanna and I both have Bianca as our adviser, so we met with her. I also have the privilege of meeting and shaking the hand of Dr. Gérard Mourou. I was actually a bit surprised that he was around, as I gather he's very widely desired by more important members of the scientific community.

Since our badges weren't ready due to some bureaucratic/paperwork black hole, Johanna and I went to a student picnic with Bianca and Julien (one of the graduate students working with Bianca).

In the afternoon, Bianca showed me around one of the labs. It seems like I'll be spending some time spending another researcher there do experiments related to laser damage, which will tie in pretty well to my project related to laser cleaning. We're using a ~1% tap from Rodrigo's Ti:Sapphire ~20fs pulse laser; I think Bianca said we'd be working with 10 micro-joules at a 1000 kHz rep rate. One of the biggest problems would be separating the pulse train to make sure we isolate a single pulse, but Rodrigo managed to snag an electric shutter for the project, which simplifies life a bit.

I was a bit late to work on Tuesday because some idiot (me) had his ATM card eaten by a BNP Paribas ATM. Sean, John, and I went to the bank and got it straightened out pretty easily. When we got the Ecole, we found that ID cards had arrived, so Bianca, Johanna, and myself went and dealt with that before lunch. In the afternoon I started reading a paper on laser-material interaction, and was shown a terahertz lab run by another researcher who's name I forget. I have to do laser safety training before I can work in the lab, and apparently the guy who does it, Ivain, is on holiday, so it will be next week until I can do labwork. I'll spend the rest of the week reading papers most likely.

On either Thursday or Friday I intent to make a post here detailing what I've learned from the week's readings.