potassium dihydrogen phosphate

2 Articles

Automated Rig Grows Big, Beautiful Crystals Fast

December 12, 2024 by 8 Comments

We haven’t seen [Les Wright] in a while, and with the release of his new video, we know why — he’s been busy growing crystals.

Now, that might seem confusing to anyone who has done the classic “Crystal Garden” trick with table salt and laundry bluing, or tried to get a bit of rock candy out of a supersaturated sugar solution. Sure, growing crystals takes time, but it’s not exactly hard work. But [Les] isn’t in the market for any old crystals. Rather, he needs super-sized, optically clear crystals of potassium dihydrogen phosphate, or KDP, which are useful as frequency doublers for lasers. [Les] has detailed his need for KDP crystals before and even grown some nice ones, but he wanted to step up his game and grow some real whoppers.

And boy, did he ever. Fair warning; the video below is long and has a lot of detail on crystal-growing theory, but it’s well worth it for anyone taking the plunge. [Les] ended up building an automated crystal lab, housing it in an old server enclosure for temperature and dust control. The crystals are grown on a custom-built armature that slowly rotates in a supersaturated solution of KDP which is carefully transitioned through a specific temperature profile under Arduino control. As a bonus, he programmed the rig to take photographs of the growing crystals at intervals; the resulting time-lapse sequences are as gorgeous as the crystals, one of which grew to 40 grams in only a week.

We’re keen to see how [Les] puts these crystals to work, and to learn exactly what a “Pockels Cell” is and why you’d want one. In the meantime, if you’re interested in how the crystals that make the whole world work are made, check out our deep dive into silicon.

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Growing Simple Crystals For Non-Linear Optics Experiments

July 6, 2023 by 18 Comments

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.

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