(30 May, 2004)

Note: I culled the tuning photos from three series, all of which I took this evening. The tuning range of this dye (4-Methyl-Umbelliferone, which in this case is dissolved in 95% ethanol, and made basic by the addition of a drop of strong ammonia-water) is so huge that I can’t reach all of it without manually rotating the mirror mount that carries the diffraction grating — its adjustment range is just not sufficient. (4-MU can lase all the way from the near UV to a very yellowish green.)

First, here’s a quick look at the dye laser:

The setup is rather informal (ahem), but at least I have the grating attached to a mirror mount. The round-trip transit time for light inside this laser is probably about 100-120 psec, so there is time for perhaps half-a-dozen transits during the pump pulse (which comes from a room-pressure nitrogen laser, and is less than a nanosecond long). This is clearly sufficient to tune the dye laser, as the photos below demonstrate.

I can assure you that these photos fail to do justice to the colors, which are rich and speckly “in person”.

I should note that this is not actually the violet end of the range — the dye appears to tune down to perhaps as low as 420 nm in this setup. I should also note that the ASE (Amplified Spontaneous Emission) from the dye, which appears as a violet band or cloudy triangle in some of these pictures, looks more like a partial rainbow "in person" when the tuning is at the violet end of the range, though of course it lacks the longer wavelengths. I’m not sure why only the violet shows up in the photo here. (You’ll note that the ASE largely disappears near the peak of the tuning curve.)

One other rather peculiar thing I’ve noticed with this dye: when I first mix the solution (at least when I’m using 91% isopropanol as the solvent; don’ recall whether it’s the same with 95% ethanol, though I would presume that it is), it has a reasonably wide tuning range. After the solution sits for perhaps half an hour, however, I find that the tuning range is considerably broader. I can only guess that either the dye-ammonia complex takes a while to form, or perhaps some sort of dye-ammonia-alcohol complex forms in the solution (again, rather slowly), or something truly arcane is going on.

Another nitrogen-pumped dye laser page

A flashlamp-pumped dye laser

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