TJIIRRS: Number 10 of an Ongoing Series;

Commercial laser dyes can cost over $100 per gram, and are out of the reach of most DIY laser builders. Most chemical companies refuse to sell to individuals these days, in any case. Although Fluorescein is often available on eBay, it is not necessarily very pure; it is difficult to excite with a nitrogen laser because it has little absorption at 337 nm; and it fills only one part of the spectrum.

This page explores dyes for DIYers, with an eye toward affordability, availability, and good performance. Most of these have been tested with nitrogen laser pumping, and some have been tested with flashlamp pumping.

A good laser dye has high fluorescence efficiency, and absorbs pump light well. In addition to these general characteristics, a dye that is good for DIY use dissolves in solvents that are readily available and relatively nontoxic. It does not undergo intersystem crossing to any great extent, and thus does not exhibit much triplet-triplet excited-state absorption. (Frankly, this is not a problem with most of the common dyes, e.g., Rhodamine 6G, Rhodamine B, or Fluorescein; I have encountered it with only one or two exotic dyes that are not of interest to DIYers in any case, as they are expensive and toxic.)

This table suggests one or more dye sources for various parts of the spectrum. In each case, I have listed the ones that seem to provide the best performance in my testing, but as of Autumn, 2007 the column for flashlamp-pumped dyes is very iffy; I need to do more testing to verify performance. In addition, this table is only just begun; I hope to fill in more of it over time.

KEY:

• “BH” = Blue Highlighter (various brands may work)
  
  
• “VH” = Violet or Purple Highlighter (various brands may work)
  
  
• “PH” = Pink Highlighter (various brands should work)
  
  
• “OH” = Orange Highlighter (I have seen Foray^&tm; markers in this color)
  
  
• “SAH” = Sanford’s Sharpie^® ‘Accent’ Highlighter, yellow-green (other brands also work well)
  
  
• “FYH” = Foray yellow-green Highlighter, yellow-green (other brands also work well)
  
  
• “DTC” = Dharma Trading Company “Optic Whitener”
  
  
• “POPOP” = 1,4-bis-(5-phenyloxazole-2-yl)-benzene (A common organic scintillator; also a common laser dye. Occasionally available on eBay. Dissolves best in nonpolar solvents, though there is a chance that POPOP may dissolve well enough in 99% isopropanol for use in flashlamp-pumped lasers; I haven’t had a chance to test that yet. Moreover, although POPOP has been used by a number of researchers, I have not yet managed to get it to lase as of 10 November, 2007. Working on it.)
  
  
• “PPO” = 2,5-diphenyloxazole (Another common organic scintillator; also a common laser dye. Occasionally available on eBay. Dissolves best in nonpolar solvents. I have lased PPO.)

Note: Until the questionmarks disappear, you should take this table with a grain or two of salt. (The questionmarks mean that I haven’t finished testing yet.)

Color	N2 Laser	Flashlamp
Near-IR		BH??
Far Red
Red
Red-Orange	PH	PH
Orange	PH; OH	PH; OH
Yellow-Orange	PH; OH; SAH; FYH?	PH; OH; SAH; FYH?
Yellow	OH?; FYH?; SAH	OH?; FYH?; SAH
Yellow-Green	SAH; FYH?	SAH; FYH?
Green	SAH; FYH?	SAH; FYH?
Blue-Green
Blue	DTC?	DTC?
Indigo (“Deep Blue”)	DTC	DTC
Violet	DTC?; POPOP?	DTC?; POPOP?
Near-UV	POPOP?; PPO	POPOP?; PPO

Note: The Sharpie “Accent” highlighter and Dharma Trading Company’s &ldquoOptic Whitener” are the best performers I have seen so far, under nitrogen laser pumping. DTC will dissolve in ordinary drugstore rubbing alcohol (70% isopropyl), and a lifetime supply costs only about $4 plus shipping.

Notice that the green ink is thoroughly incompatible with isopropanol.

Some colors won’t lase, probably because they contain other dyes, which absorb the output; or because they have too much “junk” in them (if the solution is cloudy, it won’t lase or won’t lase well); or because they do not fluoresce efficiently enough. With regard to cloudiness: I noticed that when my yellow-green “Accent” solution got cold it became cloudy, and I had to warm it up to get decent lasing. If a solution contains solid particles even at reasonable temperatures, you can centrifuge it or filter it (though a filter that is good enough for laser work tends to be expensive), or just let it stand for a long time and then very carefully decant the clear liquid off the top, leaving the sludge behind.

Another possibility is that the dye may have insufficient extinction at 337.1 nm, in which case the pump light just goes through it. If the beam goes even as far as 1 millimeter in, there is very little chance that you’ll obtain lasing from the dye. Here are three dyes that fluoresce nicely, but simply won’t lase with transverse nitrogen-laser pumping. (I have never seen a setup for longitudinal nitrogen-laser pumping, but that might be one way to lase dyes of this sort.)

If you want to see a demonstration of this, read my page about our Molectron laser, and note the photos a little more than halfway down the page. There are four of them in a row, and a fifth one a few lines further on; they show Fluorescein, first not quite lasing, and then lasing nicely when I add a small amount of a coumarin dye to it. Even though I didn’t have the camera aimed straight into the beam, taking the fifth photo in the set damaged the sensor. You will not be able to get a new sensor for your eye if you do this to it...

Top to bottom: Foray Violet (in iso); Sharpie “Accent” Purple (in iso); Foray Green (in water). All taken with the pump laser at full intensity.

Here, so you can see the other side of the coin, is a dye that lases very nicely (4-Methyl-Umbelliferone, in alcohol and ammonia). In order to take these next two photos I had to attenuate the pump laser beam to a tiny fraction of its usual intensity. Even moderate beam strength was enough to lase the dye, which would have prevented me from taking the picture on the right at all; and the brightness of the fluorescence would have made it impossible to see what was going on in the photo on the left.

For more photos, including some that show the tuning ranges of a couple of these, please see Report 10A and Report 10B. Also, see the detergent photo that is further down on this page.

Fortunately I was able to get a double set of these, which gave me the chance to compare water against 91% isopropanol. Here are the results:

Marker Color	In Water	In 91% Isopropanol
Hot Pink	Cloudy	Somewhat clotty
Orange	Cloudy	Somewhat clotty
Yellow-Green	Excellent	Rather Cloudy
Green	Probably not sufficiently fluorescent	Almost entirely insoluble
Blue	No visible fluorescence	No visible fluorescence
Violet	Very Good	Excellent

The violet was the big surprise here. I didn’t actually expect fluorescence, but it looks very nice in a preliminary test. Unfortunately, as you can see from the first pair of photos, above, it will never lase under transverse nitrogen pumping on its own, because it doesn’t absorb the pump light strongly enough. My hope is that this dye and the corresponding “Accent“ dye (shown in the second pair of photos) may lase in a flashlamp-pumped dye laser.

Here is what the tests looked like in room light plus blacklight:

(Each pair shows water on the left and isopropyl alcohol on the right. I have omitted the blue dye because it didn’t fluoresce.)

(Note, added on 23 October, 2007: It is not even necessary to bother with detergents if you don’t care to; see below for information about concentrated optical brighteners.)

During a conversation about detergents in mid-2007, in which we were comparing results, Jarrod Kinsey mentioned dry detergent powder, which could, at least in principle, provide a more concentrated dye solution than commercial liquid detergents; but he noted that it doesn’t dissolve well, even in water. This suggested to me that I should try extracting the brightener from some dry powder detergent with isopropanol. (Up to that point, we had both been using ordinary liquid detergent.)

The no-dye, no-perfume dry powder version of Arm and Hammer® gave me a very concentrated solution that lased extremely well, even in a cuvette with misaligned walls. (If the walls are aligned well, the dye can use them as mirrors, so it is easier to reach threshold in a correctly aligned cuvette.) I have tested 91% isopropanol from the drugstore and also 99+% iso from an electronics supply store, both of which worked beautifully when I used a small homebrew TEA nitrogen laser as a pump. I have not tested 70% isopropyl Rubbing Alcohol, but there is a good chance that it would also work.

Note: I have, so far, done this with three brands of dry detergent. Only Arm and Hammer gave a dye solution that was concentrated enough to lase with a nitrogen-laser as the pump source. Tide® should give you a solution that you can lase with flashlamp pumping; but unless you evaporate some of the solvent to concentrate the dye, it probably won’t work with nitrogen-laser pumping.

The following protocol produces enough dye solution to fill a small cuvette once or twice. It should scale up nicely if you have a flashlamp-pumped dye laser, but I have not tested that.

Protocol:

Take a small jar. (I used one about an inch in diameter and 2 inches tall.) Fill the jar about half full of dry Arm and Hammer powder. Add enough isopropanol so that after all of the detergent is wet, there is still a layer of iso on top of the powder, perhaps 1/3 of the powder depth. (You can try various amounts to find out what concentration your nitrogen laser needs.) Cover the jar and shake it for 5-10 seconds, then let it settle for about half an hour. Pour the liquid off the powder, and centrifuge it to remove any powder that remains suspended. (In order to lase well, the solution needs to be very clear. Filtering can work as an alternative way of removing suspended solids from the dye, but I have not tested it myself; Jarrod Kinsey, however, reports that he has had success with it.) Pour the supernatant into a cuvette, and try pumping it. If it is clear, if your nitrogen laser is working well, and if the concentration of dye is high enough, it should lase nicely. (See photo, below.)

Here is a photo of a jar of detergent and isopropanol that has settled for several hours. At this stage it is probably still too cloudy to lase well, but should be easy to clean up. The iso in this jar was originally several millimeters deeper than you see here, btw; before I took this photo I removed some, centrifuged it, and used it to take the photo below that shows the solution lasing.

Judging from this photo, you might well be able to settle the stuff for several hours, decant it carefully into a cuvette, allow it to settle overnight, and just lase it.

If you want to use a centrifuge and you don’t have one, there are ways to make simple ones; see the January, 1998 “Amateur Scientist” column in Scientific American magazine for an example. (All of these columns are available on a CD-ROM now, and can be purchased from several sites on the Web, including The Surplus Shed and American Science and Surplus.)

Here is the extract being pumped by a small TEA nitrogen laser. You are looking from behind the cuvette and off to the side; the bright blue stripe at the right edge of the cuvette is the dye being pumped. (The bright stripe on the left side of the cuvette is a reflection.) The blue spots on the paper to the left of the cuvette are the output or, to be more precise, half of the output: the other half went off to the right, out of the picture. The camera cannot do justice to the color...

There is an easier method, which is probably not quite as good but can certainly be made to work: acquire some “no-dye, no perfume” laundry detergent, the ordinary thick liquid sort. It’s probably best to use one that has as few ingredients as possible, but contains optical brightener[s]. (The best “organic” ones don’t, so be sure to check the label.) I have tried this with several detergents, and they all worked; but Jarrod Kinsey reports particularly good results with the 2x concentrated version of Arm and Hammer.

For pumping with a nitrogen laser, you should try the detergent straight out of the bottle. For flashlamp pumping, however, you’ll almost certainly have to dilute it at least a little; try very cautiously adding some distilled water. Your objective here is to add only enough water to allow the solution to flow through your system. (Once you get it lasing you can dilute it more if that seems to be appropriate, but if you dilute it too much at the outset you won’t be able to bring it to threshold.)

Because the solution gets warm when you pump it, and the refractive index changes with temperature, you’ll probably get only one or two pulses with N₂ pumping, and then lasing will quit until you let the solution cool down. Detergent is so viscous that this can take several minutes. With flashlamp pumping you’ll want to flow the dye solution, and you’ll probably have to wait a short while between pulses, so that the flow of dye solution can cool down the dye cell.

Yet another method, this one somewhat questionable: I have tried diluting liquid detergent with alcohol, in an effort to reduce the viscosity so I can pulse it more often. This often just causes the detergent to crystallize out, which makes the stuff cloudy. If you are handy, however, you can probably use a combination of 99% isopropyl alcohol and chilling in a refrigerator or freezer to concentrate the dye by separating out the detergent from it. The objective is to add the alcohol, stir thoroughly, chill the solution and allow the detergent to crystallize out as thoroughly as possible, and then filter out the detergent crystals while holding the solution at low temperature. If this works, you may get a solution that flows well, fluoresces brightly, and is not cloudy at room temperature. Good luck — you’re going to need it.

Note: I have, more recently, found a 3X concentrated version of All® “Free Clear”, which seems to react differently — it gets slightly cloudy when I add isopropyl alcohol, but slowly clears again. (This takes days, so be patient.) Even when significantly diluted (about 1 part isopropanol to 2 or 3 parts concentrate), it lases quite nicely.

Note, added on 25 July, 2007: Jarrod Kinsey finds that when he pumps liquid detergents with his TEA nitrogen laser, he does not see the “fatigue” effect that I note above; the stuff just lases again and again. I am not yet sure what the difference is, though I was originally pumping with a reduced-pressure nitrogen laser rather than a TEA laser, and that may have had some influence.

In a discussion on the LASERS mailinglist a week or two back, Jacob Thomas pointed out that Rit makes an “Optical Whitener and Brightener” product, which he has since lased. The Rit product appears to consist of brightener, table salt, and some sort of powder that does not dissolve in alcohol. You can extract the Rit brightener with isopropyl alcohol, the same way you can extract powdered laundry detergent; the difference is that it is much more concentrated.

As soon as I saw Jake’s posting I went looking on the Web for sources. It turns out that Rit products are fairly easy to find. In that search, however, I also came across The Dharma Trading Company, which carries a liquid “Optic Whitener” in 8-ounce bottles, so I bought one. There seems to be some detergent in it, and I found that when I added a small amount to some 99.8% pure isopropanol I got a very turbid and almost completely opaque result. If you are stuck with extremely pure iso, it should be possible to filter or centrifuge the suspension to remove the solids; but this product is intended for use in water, so I just added some distilled water, and it cleared very nicely. (Note, added on 28 October, 2007: Jarrod Kinsey found that he had no trouble with ordinary 91% isopropanol from the drugstore, and I have verified that it also dissolves just fine in 70% isopropyl rubbing alcohol.) The solution lases beautifully when I pump it with a small TEA nitrogen laser. A very small drop of the concentrate is enough for a cuvette of dye solution if you are pumping with a nitrogen laser.

I have not yet pumped this dye with a flashlamp, so I don’t yet know what kind of concentration is appropriate for that. I am, however, entirely confident that it will lase.

On to a followup about alignment

On to a second followup, about tuning

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Contact Information:

My email address is a@b.com, where a is my first name (jon, only 3 letters, no “h”), and b is joss.

My phone number is +1 240 604 4495.

Last modified: Mon Nov 12 02:34:14 EST 2007