Efficiencies of nonlinear optical-to-terahertz (THz) conversion below one percent remain a limiting factor for applications of multicycle THz radiation like THz-driven acceleration and inspired the use of multi-line pump spectra. To overcome the difficulty of phase stabilization of multiple narrowband sources required by the multi-line approach, we exploit its temporal analog, i.e., regular pulse trains with THz repetition rate, in which the THz waves generated by rectifying the individual pulses add coherently. The optical setup producing the pulse trains consists of motorized interferometers and enables precise control over the pulse train parameters like pulse spacing and amplitude. It is operated with a laser providing 400 fs pulses and energies of up to 110 mJ, which is the highest yet attempted for a pulse-train-type experiment. Opposed to earlier work, pulse division is done after amplification making the system more flexible in terms of tuning the pulse number. We present initial results of an experimental campaign of multicycle THz generation in custom periodically poled crystals with large apertures up to 10x20 mm2. The available pump energy allows filling these apertures at high fluences, promising increased THz yields. We investigate the dependence of the conversion efficiency on the single pulse duration and aim to find the optimum pulse number for different crystal lengths to determine the efficiency limitations in a regime avoiding laser-induced damage. Since crystal length and pulse number define the bandwidth of the THz pulses, this work demonstrates a path to an optimized THz source tunable to different requirements of applications.
We demonstrate efficient generation of terahertz radiation (THz) by nonlinear down-conversion driven by trains of pulses of tunable duration, spacing, amplitude and number. The optical setup, based on cascaded interferometers, provides fine control over the pulse train parameters allowing precise control over the conversion efficiency of the process as well as over the THz frequency and pulse duration. Our approach enables thorough optimization of the conversion process, which theory predicts can reach multiple percent. We perform experiments and simulations of THz generation in periodically-poled lithium niobate and KTP to validate theory and determine conversion-efficiency limitations in a highly-optimized scenario.
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