We demonstrate a compact source of energetic and phase-locked multi-terahertz pulses at a repetition rate of 190 kHz. Difference frequency mixing of the fundamental output of an Yb:KGW amplifier with the idler of an optical parametric amplifier in GaSe and LiGaS2 crystals yields a passively phase-locked train of waveforms tunable between 12 and 42 THz. The shortest multi-terahertz pulses contain 1.8 oscillation cycles within the intensity FWHM. Pulse energies of up to 0.16 µJ and peak electric fields of 13 MV/cm are achieved. Electro-optic sampling reveals a phase stability better than 0.1 π over multiple hours combined with free CEP tunability. The scalable scheme opens the door to strong-field terahertz optics at unprecedented repetition rates. OCIS codes: (140.3070) Infrared and far-infrared lasers; (190.4970) Parametric oscillators and amplifiers; (320.7100) Ultrafast measurements; (320.7100) Ultrafast nonlinear optics; (120.5050) Phase measurementUltrashort pulses in the terahertz (THz) and mid-infrared region of the electromagnetic spectrum have attracted tremendous interest in the past few years as resonant probes of low-energy elementary excitations in condensed matter [1,2]. The combination of CEP-stable pulses with ultrabroadband electro-optic sampling [3][4][5][6][7][8] has allowed for studies of electronic and structural dynamics of molecules and solids, on time scales faster than a single cycle of the carrier wave [1,2]. The recent advent of high-power sources [9][10][11][12][13] has prompted an ongoing revolution of ultrabroadband THz nonlinear optics and resonant THz quantum control of condensed matter [14][15][16][17][18][19][20]. In particular, when the ponderomotive energy exceeds the fundamental bandgap of semiconductors or dielectrics, the carrier wave acts like an AC bias field that can accelerate and recollide quasiparticles [15,16]. It can drive dynamical Bloch oscillations and highharmonic generation [17], or induce tunneling of electrons out of sharp metal tips [18] or through the tunneling junction of a scanning tunneling microscope (STM) [19,20]. In the multi-THz range, non-perturbative dynamics of this nature, often dubbed 'lightwave electronics', have occurred for field amplitudes typically above 10 MV/cm.Optical rectification, i.e. difference frequency generation (DFG) within the broad spectrum of a single femtosecond near-infrared (NIR) pulse, gives rise to passively phaselocked THz pulses [3][4][5][6][7][8]21]. While this concept warrants a particularly stable carrier-envelope phase (CEP), its observed low quantum efficiency has made it a popular choice for the generation of probe pulses [1,2]. Difference frequency mixing between the signal waves of two optical parametric amplifiers driven by the same pump laser, in contrast, has generated CEP-stable few-and single-cycle multi-THz pulses with field amplitudes in excess of 10 MV/cm or even above 100 MV/cm [9][10][11]22]. An innovative in-line scheme of two-color parametric amplification in a single OPA has further improved the long...

We describe a setup for time-resolved photoemission electron microscopy (TR-PEEM) with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215-970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale ("2D nanoscopy"). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump-probe experiments using two independent NOPA beamlines. We extract local variations of the excitedstate dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity (LEER) measurements. Finally, we demonstrate the in-situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.

We report the generation of extremely broadband and inherently phase-locked mid-infrared pulses covering the 5 to 11 µm region. The concept is based on two stages of optical parametric amplification starting from a 270-fs Yb:KGW laser source. A continuum seeded, second harmonic pumped pre-amplifier in β-BaB 2 O 4 (BBO) produces tailored broadband near-infrared pulses that are subsequently mixed with the fundamental pump pulses in LiGaS 2 (LGS) for mid-infrared generation and amplification. The pulse bandwidth and chirp is managed entirely by selected optical filters and bulk material. We find an overall quantum efficiency of 1% and a mid-infrared spectrum smoothly covering 5-11 µm with a pulse energy of 220 nJ at 50 kHz repetition rate. Electro-optic sampling with 12-fs long white-light pulses directly from self-compression in a YAG crystal reveals near-single-cycle mid-infrared pulses (32 fs) with passively stable carrier-envelope phase. Such pulses will be ideal for producing attosecond electron pulses or for advancing molecular fingerprint spectroscopy.

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We discriminate different field dynamics across distances as small as 33 nm within a plasmonic slit resonator using aberration-corrected photoemission electron microscopy and a tunable broadband optical parametric amplifier at 1 MHz repetition rate.