Wednesday, June 9, 2010

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.

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