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I S S U E   1:
Some Comments on Laser-Pumped Dye Lasers

[Work related to Broadside #6]

(2013.1010 &ff)

I recently tried extracting and lasing a Sharpie® ACCENT® highlighter “with Smear Guard® technology”. I had not previously encountered this particular variant, and I am guessing that it is relatively new. The dyes [yellow-green ACCENT markers clearly contain two] are probably the same as those in the original version. As a first attempt, I tried extracting with isopropanol; the initial result was a solution with blue fluorescence. The yellow-green dye dissolved only rather slowly, and the result became turbid. I suspect that it will clear after a while, but I haven’t checked yet. In the meanwhile I tried distilled water on another marker, and it quickly produced a viable extract:

I also tried a higher concentration of “DTC” (in 70% isopropyl rubbing alcohol) than I have previously used, and was pleased to find that it lases extremely well.

There are several intertwined issues here, the first of which is absorption depth. Unless your pump laser is ridiculously powerful (in the MW or multiple MW range), all or nearly all of the pump beam needs to be absorbed in the minimum possible depth of dye solution. In most cases, even 1 millimeter is too deep. If you are trying to lase a dye that does not absorb well at the pump wavelength you’re using, the obvious way to get the beam absorbed in less depth is to increase the concentration of dye in the solution.

This brings up the second issue, which is concentration quenching. If you increase the dye concentration too much, you eventually find that the fluorescence, in addition to shifting to longer wavelengths (which is expectable from the physics), dims out. In many cases this appears to involve aggregations of dye molecules, and the addition of a surfactant often holds it off to some extent, particularly in aqueous solutions. (I will note that DTC already contains a detergent of some sort.) Even with assistance from a surfactant, however, some dyes are very difficult to lase.

It is sometimes possible to use an auxiliary dye that absorbs well at the pump wavelength and emits at a wavelength that your target dye absorbs. I tend to think of this as “cascade pumping”, though that could imply more than one step. Inasmuch as you lose a certain amount of energy at each step, it is not easy to do more than one or at most two.

Only a few combinations are viable; if the auxiliary dye emits at a wavelength that the target dye doesn’t absorb, it is unlikely to help much. (See previous graf about the fact that a cascade of three or more dyes is not likely to be easy to arrange...) It is also possible for two dyes to be chemically incompatible. I have even run into a situation in which the auxiliary dye I wanted to try was not sufficiently soluble in any solvent that was suitable for the target dye. This is rather annoying. (The auxiliary dye almost certainly has to be good enough to lase on its own, and has to be at sufficient concentration to do so. It is probably easiest to start by making a working solution of the auxiliary, and adding the target dye to it. You may find that at some combinations of concentration, both dyes will lase simultaneously. (I have occasionally seen a very nice turquoise beam from a cuvette that contained a blue dye and a green dye. In principle, this could be caused by the target dye absorbing the shorter wavelengths that are emitted by the auxiliary dye, leaving the longer ones; it depends on what the auxiliary dye is capable of, and on where the absorption of the target dye peaks, and it seems mildly unlikely to me, at least with the dyes I have on hand.)

One viable combination for nitrogen pumping is Coumarin 1 [7-Diethylamino-4-Methyl-Coumarin] assisting Fluorescein. This combination works in high strength isopropanol, and probably also in 95% ethanol.

Note: The common Rhodamine dyes also have very little absorption at 337 nm, and are difficult to lase with nitrogen pumping. Unfortunately, they don’t absorb well in the blue, so it is not as easy to find a good downshifter to combine with them. Also: the fluorescence efficiency of Rhodamine B correlates directly with the viscosity of the solvent. It is mediocre in methanol, decent in water, and very nice in glycerol or in PMMA plastic. I haven’t seen a mention of this with regard to any other Rhodamines, so I presume that the effect is too small to matter with them.

Another related issue is the Stokes shift, which is (IIRC) the difference between the absorption peak and the emission peak. A large Stokes shift means that the emitted photons have a lot less energy than the pump photons, so it chews into the efficiency of the dye. Most dyes have moderate Stokes shift, which means that the nitrogen laser is a viable pump for most laser dyes from the near UV [for example PPO (2,5-Diphenyloxazole), PPF (2,5-Diphenylfuran), and αNPO], to the blue or possibly blue-green. Oddly, though, several of the Quaterphenyls appear to have large enough Stokes shift that they don’t absorb all that well at 337 nm. The Stokes shift is most of the reason why it is difficult to use a nitrogen laser to pump dyes that emit green, yellow, orange, or red: those dyes have their major absorption bands in the visible or very long UV. It is sometimes possible to pump a dye into a higher electronic state, but there are some drawbacks. More energy is lost, so the efficiency is lower, and there is more chance of photodegradation because the higher-energy pump photons are more likely to break bonds in the dye molecule. (The latter is probably more of an issue with pump lasers at shorter wavelengths than 337 nm, but it’s something you can bear in mind, especially if you have the opportunity to pump with an excimer laser.)

More as it transpires.

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Last modified: Fri Oct 11 12:01:24 EDT 2013