Growing Simple Crystals For Non-Linear Optics Experiments

Here’s an exercise for you: type “crystals” into your favorite search engine and see what you get. If you’re anything like us, you’ll get a bunch of pseudoscientific posts about the healing power of crystals, along with offers to buy the same at exorbitant prices. But woo-woo aside, certain crystals do have seemingly magical powers — like the ability to turn light from one color into another.

None of this is magic, of course. Rather, as optics aficionado [Les Wright] explains, non-linear optics is all about physics. Big physics, too, like the kind that made the National Ignition Facility the first fusion research outfit to reach the “break-even” point, at least in terms of optical energy. To do so, they need to convert megajoules of infrared laser beams all the way across the visible spectrum into the ultraviolet, relying on huge crystals of deuterated potassium dihydrogen phosphate (KDP) to do so. Depending on how they’re cut, crystals of these sorts have non-linear optical properties like second-harmonic generation, which combines two input photons into a single output photon with twice the energy of the original. This results in a halving of the wavelength of the input, which doubles the frequency.

While the process used at the NIF produces crystals of enormous proportions, [Les] has more modest needs and thus a simpler process. His KDP is an off-the-shelf chemical, nothing fancy about it, which is added to boiling water to make a saturated solution. A little of the solution is poured out into a watch glass to make seed crystals, and everything is allowed to cool slowly. A nice seed crystal is glued to a piece of monofilament fishing line and suspended in the saturated solution, and with enough time a good-sized crystal forms. Placed into the beam path of a 1,064 nm IR laser and rotated carefully relative to the beam, the crystal easily produces a brilliant green laser output.

This is fascinating stuff, and we’re looking forward to seeing where [Les] goes with this. Polishing the crystals to make them optically cleaner would be a good next step, as would perhaps growing even larger crystals.

18 thoughts on “Growing Simple Crystals For Non-Linear Optics Experiments

  1. Wow, this is cool. I’ve worked with commercial nonlinear optics crystals and never thought you could DIY them. What a cool thing! For what it’s worth, the majority of cheap green laser pointers are exactly this: an IR diode with a NLO doubling it up into green. Often, thermal control of the NLO will help with output efficiency, depending on the crystal.

  2. Um, please be careful with the IR laser. A couple of years ago when the cheap Chinese green laser pointers arrived there were a lot of issues with residual IR in the beam. This was caused by the manufacturers skimping on the IR filters. The IR beam can easily damage your eyes. With these home grown crystals there is no IR filtering whatsoever. Wear IR filtering safety goggles!

  3. Come on, break even? I know the article is about lasers, but you’re the one who brought up NIF. It still took about 4x the energy to create the laser pulse. That laser pulse, measured in optical power, not input power, did better than break even. Now if only we had a 100% efficient, super powerful laser….then we could make a net power output or break even claim. Until then, you should edit the article…

  4. Cool..I grew my own KDP and ADP crystals back in the 80s when I was in university. Polishing them wasnʻt too difficult but the humidity made keeping them that way challenging. I have a 2kg lithium niobate crystal as a paperweight.

  5. Now to feed the output from the crystal back into the crystal again creating a loop. No doubt it won’t get up to gamma ray or even x-ray frequencies, even if it could, the amount of photons that would make that many iterations would be a fairly small percentage of the whole. I guess you’d have to impedance match it, for efficiencies sake, a variable loop length depending on the frequency, some sort of weird flat wedge wave guide perhaps. It’s a shame the temperature has to be so low for the regular hydrogen variant to show a significant reaction to an electric field.

  6. This is really, REALLY cool. I guess I figured frequency doubling required material quality and uniformity that couldn’t be achieved in a home environment. Funny thing about crystals, though – if your technique is good, THEY take care of maintaining uniformity as they grow.

    Great work! Next stop, 355nm?

  7. This is quite amazing!

    Reading it I was immediately reminded of Mr Scott’s Lithium Crystals. It goes to show the importance of reading Science Fiction now, while it’s still fiction.

  8. Just got done harvesting a few honkers of Copper Sulfate and Potassium Ferricyanide :) My rochelle salt xtals continue to shard off poly but still hoping the last seed will come out a strong mono :) I dont use them for optics other than display. Just fun to see what grows and what shapes you can get for me. Cheap hobby too :) I got started at the end of quarantine about 9 months ago and have been testing all kinds of types out. The humidity here just means you really have to stay on top of things as it throws off everything and generally makes life less pleasant in the heat. I was super stoked to see this rando article here on HaD :)

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