Plasmonic generation of ultrashort extreme-ultraviolet light pulses

Ultrashort extreme-ultraviolet pulses are a key tool in time-resolved spectroscopy for the investigation of electronic motion in atoms 1 , 2 , molecules 3 and solids 4 . High-harmonic generation is a well-established process for producing ultrashort extreme-ultraviolet pulses by direct frequency upconversion of femtosecond near-infrared pulses 5 , 6 , 7 . However, elaborate pump–probe experiments performed on the attosecond timescale 8 , 9 require continuous efforts to improve the spatiotemporal coherence and also the repetition rate of the generated pulses. Here, we demonstrate a three-dimensional metallic waveguide for the plasmonic generation of ultrashort extreme-ultraviolet pulses by means of field enhancement using surface-plasmon polaritons. The intensity enhancement factor reaches a peak of ∼350, allowing generation up to the 43rd harmonic in xenon gas, with a modest incident intensity of ∼1 × 10 11 W cm –2 . The pulse repetition rate is maintained as high as 75 MHz without external cavities. The plasmonic waveguide is fabricated on a cantilever microstructure and is therefore suitable for near-field spectroscopy with nanometre-scale lateral selectivity. Researchers use plasmonic nanofocusing of near-infrared pulses in metallic tapered gap waveguides to generate ultrashort extreme-ultraviolet pulses. They calculate that the electromagnetic field intensity is around 350 times higher than that of a reference untapered waveguide, allowing harmonics up to the 43rd to be realized at a modest incident intensity of ∼10 11 W cm −2 .