Nonlinear Light Generation in Localized Fields Using Gases and Tailored Solids

n Chap. 18, we demonstrated polarization-sensitive imaging at extreme-ultraviolet (EUV) wavelengths using a gas-phase high-harmonic generation (HHG) source. In a related project, we have investigated new types of gas-phase and solid-state EUV light sources employing field localization in plasmonic nanostructures and structured targets. Whereas our first results indicate that strong field confinement leads to exceedingly inefficient high-harmonic generation in gas-phase targets, for solid-state media efficient high-harmonic generation is possible in localized fields. The latter has great ramifications for new types of high-harmonic generation experiments and technological developments. Therefore, our research efforts aim in two directions: firstly, the development of new types of solid-state sources for high-harmonic generation and, secondly, the application of locally generated solid-state high-harmonic signals for spectroscopy and imaging

Generation and bistability of a waveguide nanoplasma observed by enhanced extreme-ultraviolet fluorescence

We present a study of the highly nonlinear optical excitation of noble gases in tapered hollow waveguides using few-femtosecond laser pulses. The local plasmonic field enhancement induces the generation of a nanometric plasma, resulting in incoherent extreme-ultraviolet fluorescence from optical transitions of neutral and ionized xenon, argon, and neon. Despite sufficient intensity in the waveguide, high-order harmonic generation is not observed. The fluorescent emission exhibits a strong bistability manifest as an intensity hysteresis, giving strong indications for multistep collisional excitations

Continuous-Wave Multiphoton Photoemission from Plasmonic Nanostars

Highly nonlinear optical processes, such as multiphoton photoemission, require high intensities, typically achieved with ultrashort laser pulses and, hence, were first observed with the advent of picosecond laser technology. An alternative approach for reaching the required field intensities is offered by localized optical resonances such as plasmons. Here, we demonstrate localized multiphoton photoemission from plasmonic nanostructures under continuous-wave illumination. We use synthesized plasmonic gold nanostars, which exhibit sharp tips with structural features smaller than 5 nm, leading to near-field-intensity enhancements exceeding 1000. This large enhancement facilitates 3-photon photoemission driven by a simple continuous-wave laser diode. We characterize the intensity and polarization dependencies of the photoemission yield from both individual nanostars and ensembles. Numerical simulations of the plasmonic enhancement, the near-field distributions, and the photoemission intensities are in good agreement with experiment. Our results open a new avenue for the design of nanoscale electron sources

Polarization contrast of nanoscale waveguides in high harmonic imaging

The optical polarization response of a structured material is one of its most significant properties, carrying information about microscopic anisotropies as well as chiral features and spin orientations. Polarization analysis is therefore a key element of imaging and spectroscopy techniques throughout the entire spectrum. In the case of extreme ultraviolet (EUV) radiation, however, both the preparation and detection of well-defined polarization states remain challenging. As a result, polarization-sensitive EUV microscopy based on table-top sources has not yet been realized, despite its great potential, for example, in nanoscale magnetic imaging. Here, we demonstrate polarization contrast in coherent diffractive imaging using high harmonic radiation and investigate the polarization properties of nanoscale transmission waveguides. We quantify the achievable polarization extinction ratio for different waveguide geometries and wavelengths. Our results demonstrate the utility of slab waveguides for efficient EUV polarization control and illustrate the importance of considering polarization contrast in the imaging of nanoscale structures

Electronic and structural fingerprints of charge density wave excitations in extreme ultraviolet transient absorption spectroscopy

Femtosecond core-level transient absorption spectroscopy is utilized to investigate photoinduced dynamics of the charge density wave in 1T-TiSe2 at the Ti M2,3 edge (30-50 eV). Photoexcited carriers and phonons are found to primarily induce spectral red-shifts of core-level excitations, and a carrier relaxation time and phonon heating time of approximately 360 fs and 1.0 ps are extracted, respectively. Pronounced oscillations in delay-dependent absorption spectra are assigned to coherent excitations of the optical $A_{1g}$ phonon (6.0 THz) and the $A_{1g}^*$ charge density wave amplitude mode (3.3 THz). By comparing the measured spectra with time-dependent density functional theory simulations, we determine the directions of the momentary atomic displacements of both coherent modes and estimate their amplitudes. This work presents a first look on charge density wave excitations with table-top core-level transient absorption spectroscopy, enabling simultaneous access to electronic and lattice excitation and relaxation