October 2, 2023

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Growing simple crystals for nonlinear 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 thing at exorbitant prices. But beyond that, some crystals have seemingly magical powers – eg The ability to convert light from one color to another.

None of this is magic, of course. Rather, as optics enthusiasts [Les Wright] He explains, Nonlinear optics is all about physics. Big physics, too, like the kind that made the National Ignition Facility the first fusion research group to reach the “break even” point, at least in terms of light energy. To do this, they need to convert megajoules of infrared laser radiation all the way through the visible spectrum into ultraviolet light, relying on massive crystals of defatted potassium dihydrogen phosphate (KDP) to do so. Depending on how they are cut, crystals of these types have nonlinear optical properties such as second harmonic generation, which combines two input photons into a single photon with twice the energy of the original. This results in the input wavelength being halved, doubling the frequency.

While the process used in NIF produces crystals of gigantic proportions, [Les] More modest needs and therefore a simpler process. Their KDP is a ready-to-use chemical, and there’s nothing fancy about it being added to boiling water to make a saturated solution. A little solution is poured into a watch glass to make seed crystals, and the whole thing is allowed to cool slowly. A nice seed crystal is attached to a piece of monofilament fishing line and suspended in a saturated solution, and with enough time good sized crystals form. When placed in the path of a 1,064-nm IR laser beam and carefully rotated relative to the beam, the crystal readily produces a bright green laser output.

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This is great stuff, and we look forward to seeing where [Les] goes with this. Polishing the crystals to make them optically cleaner might be a good next step, as is growing larger crystals.