Bizarre behaviour of Nd:YVO4/KTP composite crystal

Composite Nd:YVO4/KTP microchip crystal mounted in brass holder
Composite Nd:YVO4/KTP microchip crystal
mounted in brass holder

These days it is very easy to get hold of so-called "microchip" DPSS laser crystals, comprising a tiny tiny bit of Nd:YVO4 attached to a scarcely less tiny bit of KTP, and with suitable coatings already applied, so that all you have to do to make a 532nm DPSS laser is basically to shine an 808nm pump diode at it. On the right is a photo of such a crystal, mounted by means of white spooge in a small brass holder (image is from the rather excellent Odic Force laser parts site, although Odic themselves do not sell these things separately). The crystals are of some random Chinese origin, and appear to be roughly equivalent to the "Casix DPM0101" crystal as described on the also rather excellent Sam's Laser FAQ site. However, they are not the same thing; they are apparently the same size, but are supplied already mounted, as in the image. They are the parts used in yer average bog standard green laser pointer. The tiny green square in the photo is the input face of the vanadate, which is HR coated for 532nm, hence the colour.

So, recently I found one of these things for sale on Amazon - see the original listing here. A fair bit of the information in that listing appears to be bollocks, such as where it claims a possible output power up to 200mW - ITCHY CHIN. But since I knew that, I didn't care, and bought one.

Naturally, when it arrived, the first thing I did with it was shine a pump source at it - what I believe (it's Chinese) to be an 808nm 300mW laser diode, in a mount with a focussing/collimating lens - to see what would happen. And what did happen? Fuck all.

Well - not quite fuck all. After much messing around - incidentally charring a spot on the white spooge when I got the pump beam off target; it can't be as white as all that - I did see a tiny speck of light that was not red. But it wasn't green either. It was yellow.

I made sure that this wasn't simply incandescence from another bit of white spooge getting burnt. It wasn't. By this time my fiddling had progressed from simply holding the mounted crystal in one hand and the pump diode in the other to having the things supported in decently accurate alignment, and the pump beam was definitely impinging on the input face of the crystal and not on anything else. So obviously there was some degree of up-conversion going on, but not of the kind that was expected. And certainly most of the pump radiation was being absorbed; a bit was passing all the way through the crystal and could be seen when I put my hand in its path on the output side, but it was a lot dimmer than without the crystal there at all.

The white spooge does not completely cover the KTP crystal; it is only approximately applied, and about half the length of the crystal, on the vanadate side, is exposed, although you wouldn't guess it just from looking at the thing. But by means of a large magnifying lens it can be seen clearly.

And what could also be seen clearly, with the pump diode on, was the beam inside it. Which was yellow.

It was also dim as fuck, and did not make it out of the output side of the crystal to any visible extent. But inside the crystal it was most definitely yellow.

I did of course try to take a photo of this, but not much to my surprise it was totally useless. The digital camera sensor is much more sensitive to the 808nm pump radiation than is the eye, and all that came out was a big purple splodge obscuring everything.

I suspect - though I have no way to tell; even if I did have a spectroscope the yellow light emission is so small and hard to get at that I still wouldn't be able to measure its wavelength - that what is going on is not SHG, but SFG: that the vanadate is lasing at two wavelengths simultaneously, presumably 1064nm and 1340nm, and these two are being mixed in the KTP to produce the sum frequency, at 593nm. This is, apparently, possible; although googling for references is unnecessarily difficult because the fucking wankers at Google are in bed with the other fucking wankers who want to charge you 40 bucks for every fucking thing you read and so have frigged the search results so you only get the fucking shitey ones that only stupidly rich cunts can read and don't get any actually accessible ones (FUCK YOU GOOGLE YOU SHITEARSED GREEDY FUCKWAD CUNTDRIBBLES), there is enough available to see that some people have managed to do it. (There are also people who have done it with the first Stokes shift line at 1178nm, but I don't think that's what I've got, because that gives you a result at 560nm-odd and what I'm seeing is not nearly green enough to be that.)

What is rather cool about this is that while the various researchers have had to engage in large amounts of frigging about to get such a result, I seem to be getting it for nothing, presumably due to some kind of manufacturing defect in the composite crystal assembly. The sum frequency power may only be fucking minute, but it is still happening to enough of an extent to be seen. Which is quite neat.

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