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Generation of continuous-wave ultraviolet radiation by sum-frequency mixing in an external ring cavity.

We have generated several milliwatts of single-frequency cw UV radiation near 265 nm by mixing the outputs of a rhodamine 6G ring dye laser and a 488-nm argon-ion laser in a crystal of ammonium dihydrogen phosphate. The dye-laser intensity inside the crystal was enhanced by a frequency-locked passive ring cavity, yielding at least a tenfold improvement in UV power over earlier mixing schemes. An envisioned extension to wavelengths near 243 nm is of particular interest for high-resolution two-photon spectroscopy of atomic hydrogen.