The invention features an apparatus providing electromagnetic radiation including: a first non-linear optical crystal for converting input radiation from a laser into intermediate radiation; a second non-linear optical crystal for converting the intermediate radiation into output radiation; and a plurality of optics forming an optical cavity that encloses the first and second non-linear optical crystals, substantially confines and resonates the intermediate radiation, and has a cavity length matching the cavity length of the laser to within less than the coherence length of the input radiation. The apparatus can be used to convert the output from a multimode, diode-pumped, solid-state laser into high power ultraviolet radiation.