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.