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.

Arrival in Paris

We left Michigan around 9:40 last night (EST) and arrived in Paris this morning, local time of about 1130. The flight was a little less than seven and a half hours, and I probably got 3 hours of sleep at most on the plane, so I'm a little wiped out. I had the fortune to sit in a seat where half the leg room was taken up by an air conditioning unit (or some other nondescript black device with vents), so not only did my backpack not fit under the seat completely, but my badminton racquet didn't either, and I spend the entire flight with the racquet between my legs and Svelto's lasers on my lap.

After getting through customs and claiming our bags (which took a surprisingly short amount of time) we bought rail passes. The navigo passes don't start until Monday, so we had to buy 1-way tickets to Paris, which cost 8.50 e.

After getting to the hotel, we met with Francois and Adrian, two Paris Tech students who took us to go see a rugby game. It was pretty helpful to get their input, as they know the area infinitely better than we do.

I'm quite tired from today/yesterday. I don't have plans tomorrow other than going to an Irish church for mass, but until then, I plan to get well acquainted with my bed.

Orientation, Day 2

Today we went on a tour of some of the CUOS labs at UMich. Despite the fact that I am an optics major, it was the first time I've seen a femtosecond laser. It makes this very interesting whining sound after being focused by a lens, and if one looks very very closely, one can see the pulses actually ripping the air molecules. It looks almost like a very very thin spider web.

We then looked at the Hercules laser, currently the most concentrated in the world. I don't remember exactly what order of magnitude it operated at at peak intensity, but I doubt I know the SI prefix for it.

I've been thinking more and more about my presentation on Friday. I've read several articles, most of which seem to be very material specific: we shot xx material with yy fs duration pulses and found an ablation threshold of zz fluence (J/cm^2) . I feel like I have the gist of what's going on, so I'm going to try and put it in writing.

A laser, often Ti: Saphire according to most of my sources, probably Q-switched, emits a fs pulse of light. Outside of the laser cavity, one mode of polarization is selected by a combination Glan (or other) polarizer and half wave plate and attenuated as necessary. The beam is then directed and focused onto the sample; the papers I've read have used copper, marble, aluminum, and even paintings. When the material is struck by the focused femtosecond beam, the outermost layer (to some depth in microns) is either sublimated or more likely ablated, that is, turned into a plasma and ejected. From the ejected plasma, the emission spectra of the ablated material may be determined. Via prior analysis, either chemical or spectroscopic on a small fragment of the artwork/metal, the user knows what materials constitute the artwork and what materials constitute the varnish/dirt layers. Via laser induced breakdown spectroscopy (LIBS) of the ejected plasma particles, a scientist may determine how deep into the varnish he/she has blasted, and also when to stop. An alternate method involves the use of light of a frequency such that the varnish is strongly absorbent, but the artwork is not. This seems more useful, as the process would thus be self limiting, but also more difficult to fabricate and would depend strongly on the molecular components of the artwork and varnish, whereas the former procedure could be universally applied.

My main problem is that I know exactly 0 about plasma physics. I'm going to try to delve into the subject a little bit, and learn more about what happens between the pulse striking the varnish, and a plume being emitted.

As for my project, according to the description, it will be a study of laser cleaning as a function of pulse duration. I haven't run across any papers on the subject, which is exciting.

A Clean Slate

This blog is to be used to chronicle the activities, laboratory or academic, I undertake in Paris as part of the "Optics in the City of Light" iREU program via University of Michigan. My project is on the cleaning of artifacts with femtosecond lasers, as might have been evident from the blog title and URL. Although I'm working for Gerard Mourou, it seems like I'll be primarily working with Bianca Jackson, a postdoc working at Institut de la Lumière Extrême (I'm really going to have to figure out how to get those accents and not just copy paste them).

I flew in to Michigan from Baltimore yesterday afternoon. We spent most of today cutting through red tape; getting ID cards and the like, and also watching a laser safety video. It sounds like tomorrow will be a little more interesting, with a presentation about France as a country to work in on tap.

We have to make presentations (for Friday) detailing what we think the primary question/focus of our research in France is going to be. I've read the articles Bianca sent me, now I just need to mull over what exactly I think I'm going to be doing.