Halil Tarik Olgun
Universität Hamburg, Fakultät für Mathematik, Informatik und Naturwissenschaften (2020)
Abstract: Optically generated, narrowband multi-cycle terahertz (MC-THz) radiation
has the potential to revolutionize electron acceleration, X-ray free-electron
lasers, advanced electron beam diagnostics and related research areas. However, the currently demonstrated THz generation efficiencies are too low to
reach the requirements for many of these applications.
In this project, a MC-THz generation approach via difference frequency
generation (DFG) driven by a laser with a multi-line optical spectrum was
investigated with the aim of increasing the conversion efficiency. For this purpose, a home-built, Yb-based laser source with a multi-line optical spectrum
was developed. This laser source was amplified to tens-of-millijoule using a
regenerative and a four-pass amplifier; it was used to generate MC-THz in
magnesiumoxid-doped periodically poled lithium niobate (MgO:PPLN) and
rubidium-doped periodically poled potassium titanyl phosphate (Rb:PPKTP).
With this laser system, the highest optical-to-THz conversion efficiencies (CE)
of 0.49% with a pulse energy of 30 mJ at 0.29 THz, and 0.89% with a pulse
energy of 45 mJ at 0.53 THz in MgO:PPLN were achieved. These results compare well with 2-dimensional numerical simulations. In addition, Rb:PPKTP,
which has a promising figure-of-merit compared to MgO:PPLN, achieved
a CE of 0.16% with a pulse energy of 3 mJ at 0.5 THz. Next, to scale this
laser system to tens of millijoule MC-THz output, large aperture crystals
for both MgO:PPLN and Rb:PPKTP were investigated using a commercial
laser, producing 200 mJ with a pulse duration of 500 fs at 1030 nm; although
in this case an older method of optical rectification (OR) was used, achieving
less efficiency than the multi-line source. With MgO:PPLN crystals of
aperture size 10x15mm2, a CE of 0.29% at 0.35 THz was achieved with a
pulse energy of 260 mJ. This is the highest known CE value using OR. In addition, wafer-stacks with alternating crystal-axis orientation of aperture size
of 1” for LN and 10x10mm2 for KTP were successfully tested. Two novel
experiments were performed with LN wafers: multi-stage wafer-stacks in
a serial configuration with multi-output THz radiation and back-reflected
seeded MC-THz generation. Both methods improved the efficiency of the
MC-THz generation, compared to a single stack. In particular, for the backreflected seeded MC-THz generation, pulse energies of 280 mJ with a CE of
0.29% was achieved; thus demonstrating the potential of seeded MC-THz
generation. These achievements are an important step for the realization of
next-generation, THz-driven electron accelerators.
Abstract: We generate temporally modulated optical pulses with a beat frequency of 255 GHz, a duration of 360 ps, and a repetition rate of 2 MHz. The temporal envelope, beat frequency, and repetition rate are computer-programmable. A frequency comb serves as a phase and frequency reference for the locking of two laser lines. The system enables beat frequencies that are adjustable in steps of the frequency comb's repetition rate and exhibit Hz-level precision and accuracy. We expect the optical beat pulses to be well suited for versatile multi-cycle terahertz-wave generation with controllable carrier-envelope phase. We demonstrate that the inherent synchronization of the frequency comb's ultra-short pulse train and the synthesized optical beat (or later the multi-cycle terahertz) pulses enables rapid and phase-sensitive sampling of such pulses.
Abstract: High power OPCPAs above 10 W at short-wave IR wavelengths (SWIR: 1.4 - 3 μm) may be limited because of thermal heat dissipation in the nonlinear crystals. In this work we provide up-to-date measurements of the absorption coefficients of the nonlinear crystals used at these wavelengths and simulations of the thermal effects on critical parameters. In particular, power scaling limits will be discussed.