RESUMEN
Open-aperture Z-scan measurements have been carried out to investigate the three-photon (3â PA) and four-photon absorption (4â PA) coefficients at 800â nm and 1030â nm wavelengths, respectively in congruent and stoichiometric lithium niobate (cLN, sLN) with different concentrations of Mg doping. The laser pulse duration at the two wavelengths were 40 and 190 fs. The peak intensity inside the crystals varied between approximately 110 and 550 GW/cm2. The 3â PA and 4â PA coefficients were evaluated using a theoretical model and the results suggest that their minima are at or around the Mg doping level corresponding to the threshold for suppressing photo-refraction for both cLN and sLN. This result can be attributed to the contribution of crystal defects to the 3â PA and 4â PA processes. Furthermore, the 4â PA at 1030â nm exhibited greater nonlinear absorption than the 3â PA at 800â nm under the same intensity level. Possible reasons for this unexpected behavior are discussed. Overall, comparing the 3â PA and 4â PA values of these crystals will enable for selection of the optimum composition of LN crystal for efficient THz generation and for other nonlinear optical processes requiring high pump intensities.
RESUMEN
Optical rectification of femtosecond laser pulses has emerged as the dominant technique for generating single- and few-cycle terahertz (THz) pulses. The advent of the tilted pulse front pumping (TPFP) velocity matching technique, proposed and implemented two decades ago, has ushered in significant advancements of these THz sources, which are pivotal in the realm of THz pump-probe and material control experiments, which need THz pulses with microjoule energies and several hundred kV/cm electric field strengths. Furthermore, these THz sources are poised to play a crucial role in the realization of THz-driven particle accelerators, necessitating millijoule-level pulses with tens of MV/cm electric field strengths. TPFP has enabled the efficient velocity matching in lithium niobate crystals renowned for their extraordinary high nonlinear coefficient. Moreover, its adaptation to semiconductor THz sources has resulted in a two-hundred-times enhancement in conversion efficiency. In this comprehensive review, we present the seminal achievements of the past two decades. We expound on the conventional TPFP setup, delineate its scaling limits, and elucidate the novel generation TPFP configurations proposed to surmount these constraints, accompanied by their preliminary outcomes. Additionally, we provide an in-depth analysis of the THz absorption, refractive index, and nonlinear coefficient spectra of lithium niobate and widely used semiconductors employed as THz generators, which dictate their suitability as THz sources. We underscore the far-reaching advantages of tilted pulse front pumping, not only for LN and semiconductor-based THz sources but also for selected organic crystal-based sources and Yb-laser-pumped GaP sources, previously regarded as velocity-matched in the literature.
RESUMEN
A novel, to the best of our knowledge, compact, imaging-free, tilted-pulse-front (TPF) pumped terahertz (THz) source based on a LiNbO3 slab with a small wedge angle (< 8°) and with an echelon microstructure on its input surface has been demonstrated. Single-cycle pulses of more than 40-µJ energy and 0.28-THz central frequency have been generated by 100-mJ, 400-fs pump pulses with 4.1 × 10-4 efficiency and excellent focusability. The peak electric field value focused by a single parabolic mirror was 540â kV/cm. Using 200-fs-long pump pulses, the efficiency increased to 1.0 × 10-3, which is in qualitative agreement with the measured increased diffraction efficiency in the velocity matched diffraction order. A further â¼8x increase in efficiency is expected by pumping a cryogenically cooled wedged echelon with appropriate step sizes, better microstructured surface quality, and antireflection coating on both the input and the output sides. THz generation efficiency maxima were found at â¼2.7-mm crystal thickness for both pump pulse durations. The focused THz beam was diffraction limited within 5% accuracy. Compared to conventional THz sources, this setup is very compact, easy to align, can be pumped by larger beam sizes maintaining the high THz generation efficiency, and produces THz pulses with superior focusability.