Improving the brightness of high-harmonic generation (HHG) sources is one of the major goals for next-generation ultrafast, imaging and metrology applications in the extreme-ultraviolet spectrum. Previous research efforts have demonstrated a plethora of techniques to increase the conversion efficiency of HHG. However, few studies so far have addressed how to simultaneously minimize the divergence and improve focusability, which all contribute to an increased brightness of the source. Here, we investigate how to improve both photon yield and divergence, which is directly linked to focusability, when adding the second harmonic to the fundamental driving field. We study the effects of the relative polarization in two-color HHG and compare the results to a one-color configuration. In a perpendicular two-color field, the relative phase between the two colors can be used to suppress or enhance recombination of either the long or the short trajectories. This allows to exert control over the divergence of the harmonics. In a parallel two-color field, the ionization rate is modified through the two-color phase, which selects trajectories during the ionization step. This enhances the total yield. We elaborate on the underlying mechanisms for parallel, perpendicular, and intermediate polarization angles, and confirm our experimental observations with simulations.

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We introduce extreme-ultraviolet high-harmonic generation from nanostructured silica. Generation inside micron and nanoscale sub-wavelength structures with periodic as well as aperiodic features leads to emission of structured extreme-ultraviolet pulses, useful for spectroscopy and imaging.

The generation of high-order harmonics in bulk solids subjected to intense ultrashort laser pulses has opened up new avenues for research in extreme nonlinear optics and light-matter interaction on sub-cycle timescales. Despite significant advancement over the past decade, a complete understanding of the involved phenomena is still lacking. High-harmonic generation in solids is currently understood as arising from nonlinear intraband currents, interband recollision and ionization-related phenomena. As all of these mechanisms involve or rely upon laser-driven excitation we combine measurements of the angular dependence of nonlinear absorption and high-order harmonic generation in bulk crystals to demonstrate the relation between high-harmonic emission and nonlinear, laser-induced ionization in solids. An unambiguous correlation between the emission of harmonics and laser-induced ionization is found experimentally, that is supported by numerical solutions of the semiconductor Bloch equations and calculations of orientation-dependent ionization rates using maximally localized Wannier-functions.

Solid-state high harmonic generation (SSHHG) is a relatively recent, [1][2][3][4] non-destructive technique that offers new insight into the dynamics of strong-field light-matter interaction. [1][2][3][5][6][7][8] At the same time, SSHHG holds promise for being a viable route to engineering innovative, flexible, compact sources with emission in the extremeultraviolet (XUV) spectral range. 9, 10 The technique has already been shown to yield XUV light, 3, 11-13 albeit with low conversion efficiencies, as compared to the more traditional gas-based high harmonic generation (HHG) sources. 3,11 In this work we demonstrate that a non-collinear, multicolor SSHHG arrangement leads to spectra in the XUV with a high degree of tunability, and a considerable enhancement of the output flux. The observed behaviour can be understood in terms of perturbative optical wave mixing over more than one order of magnitude of the drive intensity. In addition, a model based on the recently-introduced injection current 8 allows accurate predictions over the entire experimental range.