Terahertz-driven, all-optical electron gun

Ultrashort electron beams with narrow energy spread, high charge, and low jitter are essential for resolving phase transitions in metals, semiconductors, and molecular crystals. These semirelativistic beams, produced by phototriggered electron guns, are also injected into accelerators for x-ray light sources. The achievable resolution of these time-resolved electron diffraction or x-ray experiments has been hindered by surface field and timing jitter limitations in conventional RF guns, which thus far are <200 MV/m and >96 fs, respectively. A gun driven by optically-generated single-cycle THz pulses provides a practical solution to enable not only GV/m surface fields but also absolute timing stability, since the pulses are generated by the same laser as the phototrigger. Here, we demonstrate an all-optical THz gun yielding peak electron energies approaching 1 keV, accelerated by 300 MV/m THz fields in a novel micron-scale waveguide structure. We also achieve quasimonoenergetic, sub-keV bunches with 32 fC of charge, which can already be used for time-resolved low-energy electron diffraction. Such ultracompact, easy to implement guns driven by intrinsically synchronized THz pulses that are pumped by an amplified arm of the already present photoinjector laser provide a new tool with potential to transform accelerator based science.Comment: 24 pages, 9 figure

Overcoming Bifurcation Instability in High-Repetition-Rate Ho:YLF Regenerative Amplifiers

We demonstrate a Ho:YLF regenerative amplifier (RA) overcoming bifurcation instability and consequently achieving high extraction energies of 6.9 mJ at a repetition rate of 1 kHz with pulse-to-pulse fluctuations of 1.1%. Measurements of the output pulse energy, corroborated by numerical simulations, identify an operation point that allows high-energy pulse extraction at a minimum noise level. Complete suppression of the onset of bifurcation was achieved by gain saturation after each pumping cycle in the Ho:YLF crystal via lowering the repetition rate and cooling the crystal. Even for moderate cooling, a significant temperature dependence of the Ho:YLF RA performance was observed

Spectral Phase Control of Interfering Chirped Pulses for High-Energy Narrowband Terahertz Generation

Highly-efficient optical generation of narrowband terahertz (THz) radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation (DFG) driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband DFG using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses.Comment: 25 pages, 5 figures, updated to the state before review at Nature Communications (updated the affiliations, title, some content, methods, etc.

Dual-crystal Yb:CALGO high power laser and regenerative amplifier

This paper reports on a high-power dual-crystal Yb:CALGO laser head with greatly reduced sensitivity to thermal lensing in the gain medium. In continuous-wave operation 23 W of power were extracted from 2% doped crystals, and tunablity between 1018 nm and 1060 nm was demonstrated. This is the highest output power reported from a bulk Yb:CALGO laser to date, as well as the demonstration of the broadest tuning range. 4 mJ pulses at 1040 nm were achieved in cavity-dumped operation with quasi-CW pumping at 1 kHz repetition rate with nearly diffraction-limited beam quality. When seeded at 1030 nm with stretched femtosecond pulses, 3 mJ were achieved

Towards an Ytterbium based optical waveform synthesizer

Molecular and atomic structures and dynamics have been unraveled with the development of ultrafast, high-energy optical lasers, delivering pulses from the infra-red to the X-rays. Soft X-Rays attosecond pulses can be generated via high-harmonic generation from an optical high-energy, single-cycle laser. Coherent pulse synthesis of few-cycle, high-energy pulses is a promising technique to generate isolated attosecond pulses for its scalability in spectral bandwidth and energy. Here we consider pulse synthesizer based on OPCPAs. Four major parts compose a waveform synthesizer: first a pump line scalable to high energies, second a broadband carrier-envelope phase (CEP) stable front-end, third a sequenceof parametric amplification stages to amplify the front-end pulses to high energies, and fourth synchronization and stabilization of the pulses. The state of the art waveform synthesizers rely on Ti:sapphire pump lasers, which are advantageous for the mature technology and the ultrashort pulses, but are intrinsically limited in achievable average power. This limitation in the waveform synthesizer pump line can be overcome by using alternative laser materials, like ytterbium doped hosts. In this thesis, the developments toward an ytterbium based waveform synthesizer are presented.The pump line of the synthesizer realized in this work consists of a seed oscillator with chirped fiber Bragg grating pulse stretcher and two main amplifiers. The pulse energy of the regenerative amplifier reaches 6.5 mJ at 1 kHz repetition rate. Its output is split in two: one part is compressed to 615 fs transform-limited pulses to drive the front-end. The second part seeds a multi-pass amplifier based on composite thin-disk technology, whichboosts the energy up to 72 mJ. With the compressed pulses of the regenerative amplifier, the front-end based on white-light generation is demonstrated with a passive CEP stability of 90 mrad over 11 h. The best adapted parameters for white-light supercontinuum generation with sub-picosecond long pulses were found after an experimental study. A narrow-band fraction of the super-continuum is parametrically amplified. The complete electric field of the amplified signal was retrieved from a FROG measurement. The smooth and well-behaved phase is a proof that the broadband pulse generated by white-light continuum remains a single, compressible pulse. The corresponding CEP stable idler generates a CEP stable supercontinuum, which is split in the channels of the waveform synthesizer. These broadband pulses are then amplified to the μJ level with parametricamplifiers. The pulse synthesis and the dispersion management is discussed