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| Book/Dissertation / PhD Thesis | PUBDB-2026-02357 |
; ;
2026
Verlag Deutsches Elektronen-Synchrotron DESY
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2026-02357
Report No.: DESY-THESIS-2026-013
Abstract: Compact electron accelerators powered by laser-generated terahertz (THz) radiation have the potential to enable novel laboratory-scale X-ray sources. Accelerating electrons to highly relativistic energies in dielectric-loaded waveguides requires narrowband THz pulses at frequencies between 0.1 and 0.5 THz with pulse energies of several millijoules. Achieving such energies remains challenging for sources based on optical frequency downconversion in periodically poled lithium niobate (PPLN), owing to the limited optical-to-THz conversion efficiency of approximately one percent and the restricted aperture size of available PPLN crystals. In this thesis, narrowband THz generation driven by THz-rate trains of ultrashort laser pulses is investigated with the aim of improving the conversion efficiency through tailored temporal shaping of the optical pump. In an experimental study employing custom large-aperture PPLN crystals and pulse trains with energies of up to 80 mJ, nonlinear absorption of the infrared pump light is identified as a major factor limiting the conversion efficiency. The highest THz pulse energy of 82 μJ is therefore obtained with the longest available pulse train, demonstrating that distributing the pump energy over many pulses effectively suppresses parasitic nonlinear effects. A record THz generation performance of 6.9 × 10−3 %/(mJ/cm2) without efficiency saturation is achieved in a 5 cm long crystal pumped by a train of 128 pulses. This represents more than a threefold improvement over previously demonstrated approaches and corresponds to an internal conversion efficiency of 1.4 % at a sustainable pump fluence of 200 mJ/cm2. Because millijoule-level THz pulse energies would require Joule-class femtosecond laser systems, which are not easily accessible, an alternative approach for generating suitable laser drivers with ultrashort pulse durations is demonstrated. It is based on electro-optic phase modulation of nanosecond pulses from a narrowband Nd:YAG laser in PPLN. The modulation is induced by a strong THz field at 0.35 THz generated by a lower-energy pulse-train laser, thereby extending electro-optic modulation beyond the frequency range of conventional radio-frequency traveling-wave modulators. After conversion of the phase modulation into an amplitude modulation, the modulated pulses are again used for THz generation. The record THz generation performance of the 5 cm long modulation crystal enables modulation of laser pulses with a peak fluence approximately ten times higher than that of the pulse-train laser. This approach therefore effectively multiplies the available pulse train energy. The methods developed in thiswork represent a significant step towards highly energetic laser-driven narrowband THz sources. Combined with large-aperture PPLN crystals, they pave the way for cost-effective THz sources with sufficient pulse energy to power compact THz-based linear accelerators.
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