Investigation of Scalable Concepts for Intense Terahertz Pulse Generation

Abstract

The study and control of materials with extremely strong THz fields, or the acceleration and manipulation of electrons and protons are emerging applications which require THz sources with unprecedented parameters. Besides the pulse energy, an excellent focusability is also essential to achieve the highest possible field strengths. Optical rectification of ultrashort laser pulses with tilted pulse front in lithium niobate (LN) has become a standard technique for efficient THz generation. Conventionally, a prism-shaped LN crystal is used with a large wedge angle equal to the pulse-front tilt (63°). Such a source geometry results in a nonuniform pump propagation length across the beam, which can lead to a spatially varying interaction length for THz generation. This negatively affects the THz beam quality and, consequently, the focusability, thereby limiting the achievable field strength. Lateral beam (and eventually waveform) nonuniformity is especially problematic in high-energy THz sources, where a large-diameter pump beam is needed. Different approaches have been proposed to mitigate limitations of tilted-pulse-front pumped THz sources. Recently, a modified hybrid approach to provide uniform interaction length across large pump and THz beams have been proposed. The setup uses a plane-parallel LN slab as the nonlinear medium, which is equipped with an echelon structure on its input surface. Inside the LN slab, a segmented tilted pulse front is formed with an average tilt angle as required by phase matching. Intense nearly single-cycle terahertz (THz) pulses can be used in materials science and for the acceleration of electrons and protons. The waveguide and resonator structures proposed for electron acceleration can be more efficiently driven by multicycle THz pulses. Multicycle THz pulses also could be used as drivers of coherent X-ray generation and electron beam diagnostics. In THz spectroscopy experiments, multicycle narrowband THz pulses with high spectral brightness can selectively address different lattice, electronic, and spin degrees of freedom.