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.