We demonstrate a scheme for nonlinear pulse compression at high average powers based on self-phase modulation in a multi-pass cell using fused silica as the nonlinear medium. The scheme is suitable for compression of pulses with peak powers exceeding the threshold for critical self-focusing. At >400  W of input power, the pulses of a Yb:YAG-Innoslab laser system (10 MHz repetition rate, 850 fs pulse duration) are spectrally broadened from 1.6 to >13.5  nm bandwidth while maintaining almost diffraction-limited beam quality. The chirp is removed with a dispersive mirror compressor, and pulse durations of 170 fs at an output power of 375 W are achieved. The compression unit reaches an overall transmission of >90%.

We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 µJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.

Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 × 105 photons per pulse in the 25-to-60-eV range, releasing 1 × 1010 photoelectrons per second from a 10-µm-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.

We present an efficiency scaling study of optical rectification in cryogenically cooled periodically poled lithium niobate for the generation of narrowband terahertz radiation using ultrashort pulses. The results show an efficiency and brilliance increase compared to previous schemes of up to two orders of magnitude by exploring the optimal pump pulse format at around 800 nm, and reveal saturation mechanisms limiting the conversion efficiency. We achieve >10 --3 energy conversion efficiencies, µJ--level energies, and bandwidths <20 GHz at ~0.5 THz, thereby showing unprecedented spectral brightness in the 0.1--1 THz range relevant to terahertz science and technology.OCIS codes : (190.7110 [5], among others. Many of these applications demand the development of narrowband terahertz radiation sources because they can select, switch, or control processes in distinctly defined frequency bands. A well--established source for narrowband and tunable terahertz emission is the gyrotron, a device capable of delivering MW--level terahertz power in continuous--wave operation up to 200 GHz. When pulsed, gyrotrons typically emit pulses as short as nanoseconds at Hz repetition rates, thereby restricting their use to slow or static processes. In the pursuit of accessing future novel ultrafast applications, such as time--resolved microscopy [6] or electron acceleration [7], the promise of generating efficient high--field, narrowband, tunable and ultrafast terahertz radiation lies in laser--based techniques. Generation methods include optical parametric oscillation (OPO), difference frequency generation (DFG) with two distinct narrowband lasers, and optical rectification (OR) of a single broadband pulse (or intra--pulse DFG) in periodically poled (PP) crystals. Previous DFG work has proven successful through different schemes in achieving optical--to--terahertz energy conversion efficiencies in the order of ~10 --5 [8, 9]. OPOs have demonstrated slightly better conversion efficiencies [10], but cast doubt about their frequency and bandwidth tunability, with difficulty to reach bandwidths narrower than a few hundreds of GHz in the lower terahertz frequency regime [11]. Recent work in seeded parametric generation in bulk lithium niobate has shown remarkable progress on wide tuning range terahertz generation with efficiencies up to 10 --4 [12]. Alternatively, OR in PP crystals was also demonstrated to produce frequency and bandwidth tunable terahertz radiation [13--16]. Here, a broadband optical pulse is rectified by a quasi--phase matched (QPM) material with alternating sign of second--order susceptibility (!) [17], which enhances conversion efficiency by achieving longer interaction lengths in collinear geometries. In this process the down--converted central terahertz frequency ( ! ) is primarily determined by the domain period (Λ) and the angle of observation (Θ) -defined as the angle relative to the normal of the lateral surface-, and to a lesser extent by temperature ( ) tuning via temperature dependent index of refraction ( )....