High-speed, single-shot velocity-map imaging (VMI) is combined with carrier-envelope phase (CEP) tagging by a single-shot stereographic above-threshold ionization (ATI) phase-meter. The experimental setup provides a versatile tool for angle-resolved studies of the attosecond control of electrons in atoms, molecules, and nanostructures. Single-shot VMI at kHz repetition rate is realized with a highly sensitive megapixel complementary metal-oxide semiconductor camera omitting the need for additional image intensifiers. The developed camera software allows for efficient background suppression and the storage of up to 1024 events for each image in real time. The approach is demonstrated by measuring the CEP-dependence of the electron emission from ATI of Xe in strong (≈1013W/cm2) near single-cycle (4 fs) laser fields. Efficient background signal suppression with the system is illustrated for the electron emission from SiO2 nanospheres.

Ultrashort extreme-ultraviolet pulses are a key tool in timeresolved
spectroscopy for the investigation of electronic motion in atoms, molecules and solids. High-harmonic generation is a well-established process for producing ultrashort extreme-ultraviolet pulses by direct frequency upconversion of femtosecond near-infrared pulses. However, elaborate pump–probe experiments performed on the attosecond timescale 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 ∼1x10^11 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.

Collective electron dynamics in plasmonic nanosystems can unfold on timescales in the attosecond regime and the direct measurement of plasmonic near-field oscillations is highly desirable. We report on numerical studies on the application of attosecond nanoplasmonic streaking spectroscopy to the measurement of collective electron dynamics in isolated Au nanospheres. The plasmonic field oscillations are induced by a few-cycle near-infrared (NIR) driving field and are mapped by the kinetic energy of photoemitted electrons using a synchronized, time-delayed attosecond extreme ultraviolet (XUV) pulse. By a detailed analysis of the amplitudes and phase shifts, we identify the different regimes of nanoplasmonic streaking and study the dependence on particle size, XUV in streaking spectrograms photoelectron energy, and emission position. The simulations indicate that the near fields around the nanoparticles can be spatiotemporally reconstructed and may give detailed insight into the build-up and decay of collective electron motion.

We have theoretically studied and optimized the field enhancement and temporal response of single and coupled bimetal Ag/Au core–shell nanoparticles (NPs) with a diameter of 160 nm and compared the results to pure Ag and Au NPs. Very high-field enhancements with an amplitude reaching 100 (with respect to the laser field centered at 800 nm) are found at the center of a 2-nm gap between Ag/Au core–shell dimers. We have explored the excitation of the bimetal core–shell particles by Fourier
transform-limited few-cycle optical pulses and identified conditions for an ultrafast plasmonic decay on the order of the excitation pulse duration. The high-field enhancement and ultrafast decay makes bimetal core–shell particles interesting candidates for applications such as the generation
of ultrashort extreme ultraviolet radiation pulses via nanoplasmonic field enhancement. Moreover, in first experimental studies, we synthesized small bimetal Ag/Au core–shell NPs and compared their optical response with
pure Au and Ag NPs and numerical results.

The production of extreme ultraviolet (XUV) radiation via nanoplasmonic field-enhanced high-harmonic generation (HHG) in gold nanostructures at MHz repetition rates is investigated theoretically in this paper. Analytical and numerical calculations are employed and compared in order to determine the plasmonic fields in gold ellipsoidal nanoparticles. The comparison indicates that numerical calculations can accurately predict the field enhancement and plasmonic decay, but may encounter difficulties when attempting to predict the oscillatory behavior of the plasmonic field. Numerical calculations for coupled symmetric and asymmetric ellipsoids for different carrier-envelope phases (CEPs) of the driving laser field are combined with time-dependent Schrödinger equation simulations to predict the resulting HHG spectra. The studies reveal that the plasmonic field oscillations, which are controlled by the CEP of the driving laser field, play a more important role than the nanostructure configuration in finding the optimal conditions for the generation of isolated attosecond XUV pulses via nanoplasmonic field enhancement.

Collective electron motion in condensed matter typically unfolds on a sub-femtosecond timescale. The well-defined electric field evolution of intense, phase-stable few-cycle laser pulses provides an ideal tool for controlling this motion. The resulting manipulation of local electric fields at nanometre spatial and attosecond temporal scales offers unique spatio-temporal control of ultrafast nonlinear processes at the nanoscale, with important implications for the advancement of nanoelectronics. Here we
demonstrate the attosecond control of the collective electron motion and directional emission from isolated dielectric (SiO2) nanoparticles with phase-stabilized few-cycle laser fields. A novel acceleration mechanism leading to the ejection of highly energetic electrons is identified by the comparison of the results to quasi-classical model calculations. The observed lightwave
control in nanosized dielectrics has important implications for other material groups, including semiconductors and metals.

We designed, fabricated and characterized a nano-periodical highly-efficient blazed grating for extreme-ultraviolet (XUV) radiation. The grating was optimized by the rigorous coupled-wave analysis method (RCWA) and milled into the top layer of a highly-reflective mirror for IR light. The XUV diffraction efficiency was determined to be around 20% in the range from 35.5 to 79.2 nm. The effects of the nanograting on the reflectivity of the IR light and non-linear effects introduced by the nanograting have been measured and are discussed.