Subject of study. Spatio-frequency modulation of a terahertz field generated in a crystal was investigated. Aim of study. The study aimed to theoretically and experimentally analyze the effect of spatio-frequency modulation of one-and-a-half-cycle and multi-cycle pulses in the terahertz spectral range. Method. The spatial distribution of the terahertz field spectrum was measured using time-domain spectroscopy. The process of propagation of radiation of one-and-a-half-cycle and multi-cycle pulses in air was simulated using the angular spectrum method. Main results. The spatial localization features of broadband terahertz radiation during its generation were demonstrated theoretically and experimentally. A spatio-frequency modulation distinctive to few-cycle pulses was revealed, which involves the propagation of high frequencies closer to the beam axis, whereas low frequencies propagate farther from the axis. This effect was observed exclusively for the pulses consisting of a small number of field oscillations. Propagation of multi-cycle pulses is not accompanied with such a modulation. Practical significance. This effect should be considered in the experiments sensitive to the spatial distribution of the terahertz radiation field. Such applications include terahertz imaging, terahertz holography, and the development of commercial and scientific sources of terahertz radiation.(c) 2023 Optica Publishing Group
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