Full 3D+1 modelling of the tilted-pulse-front setups for single-cycle terahertz generation

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especially when pushing for high optical-to-terahertz conversion efficiency. A 3D+1 (x,y,z,t) numerical model is necessary in order to fully investigate the terahertz generation problem in the tilted-pulse-front scheme. We compare in detail the differences between 1D+1, 2D+1 and 3D+1 models. The simulations show that the size of the optical beam in the pulse-front-tilt plane sensitively affects the spatio-temporal properties of the terahertz electric field. The terahertz electric field is found to have a strong spatial dependence such that a few-cycle pulse is only generated near the apex of the prism. The part of the beam farther from the apex contains a large fraction of the energy but has a waveform that deviates from a few-cycle. This strong spatial dependence must be accounted for when using the terahertz pulses for strong-field physics and carrier-envelope-phase sensitive experiments such as terahertz acceleration, coherent control of antiferromagnetic spin waves and terahertz high-harmonic generation.Comment: a typo of the minus sign and the ratio of f1/f2 is correcte

High-energy mid-infrared sub-cycle pulse synthesis from a parametric amplifier

High-energy phase-stable sub-cycle mid-infrared pulses can provide unique opportunities to explore phase-sensitive strong-field light-matter interactions in atoms, molecules and solids. At the mid-infrared wavelength, the Keldysh parameter could be much smaller than unity even at relatively modest laser intensities, enabling the study of the strong-field sub-cycle electron dynamics in solids without damage. Here we report a high-energy sub-cycle pulse synthesiser based on a mid-infrared optical parametric amplifier and its application to high-harmonic generation in solids. The signal and idler combined spectrum spans from 2.5 to 9.0 μm. We coherently synthesise the passively carrier-envelope phase-stable signal and idler pulses to generate 33 μJ, 0.88-cycle, multi-gigawatt pulses centred at ~4.2 μm, which is further energy scalable. The mid-infrared sub-cycle pulse is used for driving high-harmonic generation in thin silicon samples, producing harmonics up to ~19th order with a continuous spectral coverage due to the isolated emission by the sub-cycle driver

A compact THz-driven electron gun

Novel all-optical terahertz (THz)-based accelerators promise to enable new science by providing ultrafast and bright electron bunches at a small footprint. While practical prototypes of THz-based devices have been demonstrated and have shown exceptional capabilities to accelerate and manipulate electron beams on sub-ps timescales, the development of practical THz-driven photoguns has lagged behind due to challenges associated with physical miniaturization and the high THz pulse energy required. This thesis significantly advances the development of such THz-driven guns by setting a dual focus: First, systematic parameter scans are performed on tilted pulse-front setups providing crucial insights into the non-collinear interactionlengths, parameter sensitivities and physics inherent to high-energy THz generation. In addition, spatio-temporal manipulation of the pump pulse is explored both experimentally and by simulations and found as viable tool to scale tilted pulse-front based THz sources to such energies required for the next generation of compact particle accelerators. Application of the findings resulted in robust extraction of THz pulses with energies up to 400 μJ while operating well below the optical damage threshold. Secondly, these setups are used in in three different experiments aimed at developing a practical compact THz-driven gun. The extraction of multi-keV electrons from a triggerable THz photogun is demonstrated for the first time, and the parameter space and resulting performance are explored. Subsequently, multi-layered structures are developed that extend the interaction between electrons and THz pulses. A novel single-sided pumped THz accelerator is tested and a segmented dual-sided pumped THz photogun is developed. Finally, the instrumentation for commissioning and scaling THz driven gun technology to energies beyond 100 keV, comparable to the performance of modern compact DC electron guns, is presented and discussed. This work represents a critical step in the development of practical all optical THzdriven electron guns and paves the way towards more compact accelerators with fs bunch durations, low emittance and orders of magnitude higher brightness to power future ultrafast electron diffraction experiments and compact X-ray sources

Enhanced soft-X-ray high-harmonic generation driven by mid-IR pulses mixed with their third harmonic

This work is a systematic study of the enhancement of soft-X-ray high-harmonic generation (HHG) driven by two-color (ω + 3ω) waveforms in argon at different pressures. The observed enhancement due to the two-color driver reaches factors over one order of magnitude when compared to single-color driven HHG. The driver pulses were provided by a kHz, multi-mJ optical parametric chirped pulse amplifier system (OPCPA) at the Massachusetts Institute of Technology. The setup was carefully characterized to provide accurate parameters for simulations. Theexperimental setup and data acquisition were automated to ensure the necessary level of precision and improve measurement speed. Clear modulation of the high harmonic yield with the relative phase of the driving waveform indicates that sub-cycle control over the ionization and electron trajectories on the single-atom level was achieved. The results of the two-color experiments are compared to single-color driven high harmonic spectra at optimized conditions as well as to full 3D simulations. To the best of our knowledge, this is the first report ontwo-color driven HHG using such long fundamental driving wavelengths and thus provides valuable data adding to the very few publications on ω + 3ω mixing in HHG so far

High-Energy Single-Cycle THz Sources for Compact Particle Accelerators and Manipulators

Terahertz-driven (THz) accelerators and manipulators promise to yield short femtosecond electron bunches of high brightness with intrinsic synchronization to the driv-ing laser at a compact and economic footprint. However,development of practical devices requires THz sources thatreliably provide pulse energies in the sub-mJ to mJ regime,which in turn require state-of-the-art pump laser systemsand carefully designed optical transport lines. Here, we in-vestigate both by experiments and simulations on how spa-tio-temporal coupling of pump pulse parameters in tilted-pulse-front based terahertz setups can be used to controlthe position of the “temporal focus”, which is where mini-mum pump pulse duration is reached. This concept opensa pathway to pump tilted-pulse-front setups with arbitrarilystretched pulses which significantly simplifies transportlines for lasers with high peak intensity. This concept is ex-perimentally demonstrated by efficiently pumping a tilted-pulse-front THz source with pulses stretched to 10 ps andextraction of a THz energy of 0.4 mJ while operating well-below damage threshold. Our findings are not just relevantfor THz based particle acceleration and strong-field phys-ics but any application that requires control over the tem-poral focus of beams with a tilted-pulse-front such as othernovel laser-based particle accelerator schemes

Impact of pump beam spot size on semiconductor carrier dynamics in optical-pump-terahertz-probe spectroscopy

Optical-Pump-Terahertz-Probe experiments (OPTP) are widely employed to studythe dynamics of photoexcited carriers in semiconductors. In these experiments, due to the longwavelength nature of the Terahertz (THz) probe radiation, the probe beam can only be focused toa spot size in the mm range. To ensure homogeneous excitation of the probed sample region, asignificantly larger optical pump beam spot size must be used, which is often difficult to implementin the experiment. Frequently used experiment geometries employ beam paths which result in small pump beam spot sizes, leading to spectral distortions of the sample response, translating touncertainties in calculated THz conductivities and fitted Drude conductivity models, for example.We investigate the influence of the pump beam spot size on benchmark OPTP experiments andevaluate model calculations to estimate the induced deviations. We can demonstrate the impactof this effect on the acquired data with different dependencies on the investigated sample andthe employed experimental configuration. This way, we can provide guidelines for optimal configurations for the most commonly employed experiments

Eliten in Deutschland ist ökologische Nachhaltigkeit weitgehend gleichgültig

Kroh M, Schatz R, Wagner GG. Eliten in Deutschland ist ökologische Nachhaltigkeit weitgehend gleichgültig. In: Schatz R, Thomas T, eds. Wachstum 2015. Rapperswil: InnoVatio Verlag; 2015: 133-139

High-energy mid-infrared sub-cycle pulse synthesis

We present the carrier-envelope phase-stable mid-infrared sub-cycle pulses, synthesized from an optical parametric amplifier covering the 2.5-9.0 μm range. The strong-field applications in solids and nano-structures are discussed

Supplementary document for Impact of Pump Beam Spot Size on Semiconductor Carrier Dynamics in Optical-Pump-THz-Probe Spectroscopy - 6489715.pdf

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