Ultrashort extreme-ultraviolet pulses are a key tool in timeresolved 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 3 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.Surface-plasmon polaritons (SPPs) are described as the electromagnetic wave propagating along a metal-dielectric interface that results from coupling between incident photons and surface plasmons 10,11 . In nanostructured tapered waveguides, SPPs can be controlled to adiabatically follow the geometric shape of the waveguide and asymptotically stop at the tip of the taper where the local crosssectional dimension becomes infinitesimally small 12,13 . As a result, SPPs can be focused beyond the diffraction limit in three dimensions on a sub-wavelength spot, with drastically enhanced optical intensity [12][13][14] . The effect of SPP adiabatic nanofocusing has been confirmed, studied and also tested for nanometre-scale microscopy in a series of earlier experiments [15][16][17][18][19] .In our investigation, this intriguing phenomenon of SPP adiabatic nanofocusing is used to generate ultrashort extreme ultraviolet (EUV) pulses directly from near-infrared (NIR) pulses. As depicted in Fig. 1, a three-dimensional waveguide was devised to concentrate the incident NIR pulses into a sub-wavelength spot, allowing highharmonic generation of EUV light pulses to take place with high spatiotemporal coherence. The waveguide is a metallic nanostructure made of silver and has a hollow hole that takes the shape of a tapered cone with its elliptical cross-section decreasing from the inlet aperture (minor-axis diameter, 2 mm) to the exit aperture (minor-axis diameter, 100 nm). The incident NIR pulses are focused on the inlet aperture at a repetition rate of 75 MHz with a moderate intensity of 1 × 10 11 W cm 22 . While each NIR pulse propagates through the tapered hole towards the exit aperture, the electric field intensity inside the hole undergoes a substantial boost that is sustained by the SPPs driven by the incident NIR pulse. Consequently, high-harmonic EUV pulses are generate...
Recent experimental data of high-order harmonic generation (HHG), obtained by use of the plasmonic field enhancement of nanostructure bowties and funnel-waveguides, are presented. Emphasis is laid on reproduction of previous experimental results and also elucidation of the fundamental limitations associated with the nanostructure thermal damage, small laser-gas interaction volume, and atomic line emission in the plasmon-driven HHG process. In addition, the dominance of coherent harmonics is quantitatively verified by implementing a two-beam interference experiment using a pair of funnel-waveguides. This study proves that funnel-waveguides are a superior plasmonic device capable of providing not only high thermal immunity but also sufficient atom emitters to produce practically usable extreme-ultraviolet (EUV) radiation in a reproducible
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