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Ordinary white light is a jumble of all colors at once, like the noise of a crowd where everyone speaks at their own frequency. But scientists need light where frequencies are aligned with ruler-like precision, like piano keys, where each note sounds distinct and exact. Such "frequency combs" have long been achievable in the infrared range, but in the ultraviolet it's trickier: light is easily lost and hard to amplify.

In the new work, physicists took an infrared comb, passed it through a crystal, and mixed it with a green laser. The result was ultraviolet light with the same precise frequency grid, but at a wavelength of 323 nanometers — a range nearly invisible to the eye, used for studying atoms. The trick is that the mixing happened inside an optical resonator — a pair of mirrors that makes light pass through the crystal many times. This boosted the ultraviolet power a hundredfold compared to a single pass, making it easy to detect with a standard photodiode.

Using this system, the scientists measured the spectrum of rubidium atoms — absorption lines that reveal the atom's internal structure. Such measurements are needed for atomic clocks, quantum memory, and tests of fundamental physics. Importantly, the method isn't tied to one wavelength: by changing the infrared source, you can produce combs anywhere in the ultraviolet range from 300 to 400 nanometers. This opens the door to new experiments requiring precise control over ultraviolet light.

Original abstract

as written by the authors arXiv:2603.06959

A chirped-pulse electro-optic (EO) dual comb system operating near 821 nm is used to generate cavity enhanced dual combs in the ultraviolet (UV) region near 323 nm with an optical bandwidth of up to 90 GHz. The UV combs result from intracavity sum frequency mixing of the non-resonant near-IR combs with a cavity enhanced field at 532 nm in a nonlinear crystal. The cavity is pumped with 1 W at 532 nm and seeded with less than 10 mW in the near-IR region to generate a few uW of UV power within a detection bandwidth of < 20 MHz. The UV power is enhanced by 100-fold relative to single pass methods and is readily detectable using an avalanche photodiode. The system is used to measure the high-resolution UV comb spectra of Rb atoms near 323 nm (9 2P3/2 <- 5 2S1/2). This method is easily extendable across the 300 nm to 400 nm region using telecom and near-IR dual EO comb sources for seeding.