A laser beam hits a transparent crystal, and the crystal answers with a whole rainbow — from infrared to invisible ultraviolet. This isn't a trick with mirrors; it's the work of a nonlinear crystal, which can add light frequencies the way a musician plays not just the fundamental note on a string but also its overtones.
In the experiment, they used a beta-barium borate crystal and a single beam from a titanium-sapphire laser with a wavelength of 800 nanometers. The crystal turned it into five additional waves — with frequencies two, three, four, five, and six times higher. The highest, the sixth harmonic, falls into the vacuum ultraviolet range: such light is absorbed even by air, so it's usually produced in vacuum chambers.
The secret is that the waves don't appear independently: the second harmonic combines with the third to produce the fifth, and then the sixth. Such chains are called cascade processes. They work even where the crystal is barely transparent — that's why such high frequencies could be detected.
For practice, this means a simple and cheap way to get ultraviolet light for spectroscopy — a method that determines what atoms a substance is made of by the color of its glow. Instead of complex vacuum setups — one crystal and one laser.
Beta-barium borate is the very crystal often used in laser shows to convert infrared radiation into green light.
In Arthur C. Clarke's novel "The Light of Other Days," light technology lets you see through walls — and here, a crystal "sees" ultraviolet, invisible to the eye.