We observe extreme-ultraviolet (XUV) high harmonics from both epitaxial thin film and bulk crystalline silver when exposed to the intense few-cycle near-infrared light pulses. Silver has a higher damage threshold than metallic titanium nitride, enabling high-harmonics generation up to photon energies of about 17 eV. As well, circularly polarized harmonics are efficiently generated by using circularly polarized incident light. High harmonic emission from metals can be a route to compact frequency combs in the vacuum ultraviolet regime.

Control of high-harmonic beams with nanostructured surfaces" Abstract: Nanostructured surfaces allow to generate high-order harmonics with better efficiency and with controlled properties. In my talk I will present control of XUV high harmonics with a nanostructured dielectric surface and the generation of polarization-controlled deep-UV harmonics from plasmonic nanoantennas.

Intense ultrafast optical parametric amplifiers have opened up new opportunities in high harmonic generation in gases, solids, and liquids. In gases, the cutoff is extended from the extreme ultraviolet to the soft x-ray region, allowing element-specific ultrafast spectroscopy. In solids, polarization analysis reveals the fundamental electron dynamics in the band structure and allows polarization control via the crystalline symmetry. In liquids, non-trivial enhancements are observed, suggesting the underlying rich physics. In high harmonic generation (HHG), the field-driven sub-cycle electron dynamics plays the fundamental role. In the case of gas HHG, the three-step model leads to the cutoff energy scaling of 3.17Up + Ip where Up is the ponderomotive energy and Ip is the ionization potential [1]. This simple relationship has motivated the development of optical parametric amplifies that can produce phase-stable and ultrashort intense pulses in the infrared. We have developed a BiB3O6-based optical parametric amplifier (OPA) system for the generation of intense SWIR pulses (λ~1.6 μm) and extended the spectral range of isolated attosecond pulses to the water window [2]. The hierarchical dynamics of electrons, molecular vibration, and molecular rotation in NO and N2O gases are clearly observed by the transient soft x-ray absorption spectroscopy at the nitrogen K edge (~400 eV) [3]. Further extension of the spectral range of intense OPAs into the medium wavelength infrared (MWIR) region allows the generation of high harmonics in solids and liquids, because the associated low photon energy prevents multiphoton excitation followed by destructive avalanche ionization [4]. The polarization analysis of the high harmonics in GaSe with respect to the crystal orientation allows to understand the sub-cycle electron dynamics [5]. We also confirmed the selection rule for HHG in GaSe with circularly polarized light [6]. In the case of liquids, flat thin water jets are developed, which allow to study destructive laser-matter interaction at higher intensity than with the solid targets with a high repetition rate (1 kHz) light source. We have observed non-trivial nonlinear optical responses and high harmonic generation with 3-μm excitation, where strong absorption by OH vibrations coexists with field-driven electron dynamics.

Conventional optical techniques to study chiral systems have poor sensitivity. We introduce novel chiroptical techniques with enhanced sensitivity using intense femtosecond light pulses.