Anyway, Mélanie was understanding and gave me the day off yesterday. Not wanting to waste time that I could use to see Things In Paris, I spent the afternoon at Les Invalides, a military-history sort of thingy on the Seine just between the Eiffel Tower and the Tuileries. Lunch was a sandwich jambon crudités on the large lawn in front of the Army Museum.
I walked around the complex until I discovered Napoleon's tomb. There are fifty cities in America where this tomb would be mistaken for the impressive part of a state capitol building.
Thanks to the magic of my international/French student IDs, my ticket for the tomb and museum was free. Military history is perhaps not my passion, but it was fun to see the grandeur that accompanied Napoleon and several other (presumably famous) French military leaders in their sarcophagi. And the Army Museum houses a truly impressive collection of artifacts from the first and second World Wars - uniforms and artillery and leaflets and all sorts of things. I did my very best to appreciate it all properly. By the time I had made it through the equally extensive arms and armor collection, though, I was very tired and decided to end my tour. I think I missed the Napoleon stuff...maybe I shall return and see it sometime. Here is the interior of the tomb:
I had had enough of trains for the day, so I walked back along the Seine to the Notre-Dame RER stop, shortening my public-transportation journey considerably. It was quite a pleasant walk, though I barely made it back to my room before falling asleep for a few hours.
Then Kierstin returned! Hurrah! Her Arras and then UK adventures were rather a mixed bag, from what she tells me, but overall it sounds like she had a good time. I'm glad of that, and glad that she is back in Paris now.
So that brings me up to today. Back to work! I spent a while confirming as systematically as I can that the "full" refractive index of the PMMA-DCM 5% cavities seems to be 1.62, plus or minus a percent. I plotted the frequencies of observed spectral peaks for three different true FP cavities to confirm that they are evenly spaced in frequency, and then superimposed the predicted peak locations based on transmittance calculations and fine-tuned a bit with some Igor functions that I wrote to make life easier. The three cavities agreed with the result given above. This result is mildly unexpected (the bulk index of the material is more like 1.5) but not too startling, since the effective refractive index with dispersion taken into account is 1.64. I think I'm using the index terminology correctly, but I am not entirely sure. So this is a result, which is something that's been in rather scarce supply this summer, and it was fun to do a bit of theory for a change.
The afternoon brought even more theory! Maxime explained to me some of the calculations he's been attempting. He's trying to predict lasing threshold behavior in the tilted FPs using ray optics and thermal lensing effects. If the pump laser heats up the cavity material and causes a portion to expand, the cavity shape will change from a true (ideal) Fabry-Perot to something sort of like a confocal arrangement, with two concave mirrors. This will narrow the beam that bounces around the cavity. As this narrow beam bounces, the tilt in the walls will cause it to 'walk away' from itself instead of retracing its own path exactly, and - presumably - a beam that doesn't overlap itself somewhere can't experience the interference needed to pick out an active, resonant mode and lase. So the net effect should be to increase the threshold energy as the tilt increases, which is a good prediction because we already know the data agrees with it. There are still quite a few wrinkles to be addressed, but I'm enjoying working on the problem with Maxime. Yay physics!
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