ACS Photon. https://doi.org/c35s (2019)

A miniature broadband deep-ultraviolet light source that emits from 240 to 300 nm has been demonstrated by scientists in Germany and the United States. Liping Shi and colleagues make use of third-harmonic generation (THG) in rectangular nanoholes in a gold film. Localized surface plasmon modes in the holes were excited by a broadband Ti:sapphire oscillator emitting ultrashort pulses centred at 825 nm wavelength. The plasmon resonances not only boost the third-harmonic intensity, but also enable it to be broadened to a spectral width of 60 nm. The structure consisted of 290 nm × 140 nm apertures in a 250-nm-thick Au film on 15-nm silicon nitride, arrayed with 42-nm period over a 50 × 50 μm2 area. Plasmonic resonances at ~760 nm and ~820 nm wavelength were identified as being responsible for the efficient widening of the response. The THG conversion efficiency is of the order of 10−9 and third-harmonic output is elliptically polarized.