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8Eclipse z-scan for sensitivity increasing of cubic nonlinearity measurements in THz frequency range
S.A. Kozlov1*, A.O. Ismagilov1, A.O. Nabilkova1, D.V. Gushchin1, M.S. Guselnikov1, M.V. Melnik1, A.N. Tsypkin1
1-ITMO University, Saint Petersburg, Russia * [email protected]
The development of radiation sources has enabled the observation of nonlinear optical effects when working with different matter. The study of nonlinear optical properties of matter is of interest due to the potential applications. The emergence of powerful radiation sources in the terahertz radiation range has made it possible to study nonlinear properties in this range as well.
This work is novelty usage the eclipse z-scan method to measure the optical nonlinearity in the THz range. The eclipse z-scan method employed in this study represents a modification of the traditional z-scan method, which is used to analyse the third-order optical nonlinearity of a variety of materials, including liquids and crystals [1]. This method enhances the sensitivity of the system by an order of magnitude by blocking the central region of the beam and observing only its outer edges using a disk aperture. Despite the advantages of this method, our group is one of the first to use it to measure the nonlinear refractive index in the terahertz (THz) range. The terahertz radiation source, TERA-AX (Avesta Pr.), was used. The generation of terahertz radiation in this system is based on the optical rectification of femtosecond pulses in a lithium niobate crystal. The output pulse energy of the THz radiation was 400 nJ with a pulse duration of 1 ps and a centre wavelength of 0.6 mm (0.5 THz). As previously stated, this radiation source exhibits a wide, asymmetric spectrum, and its characteristics have been examined and illustrated in Fig. 1(a) [2].
The spatial dimension of the radiation at the generator output was 17.5 mm. In order to enhance the intensity of the terahertz beam, a short-focus parabolic mirror with a focal length of 12.7 mm and a large numerical aperture was used. The peak intensity of the radiation in the terahertz beam caustic was equal to 0.5*108 W/cm2 [3]. Subsequently, the radiation is directed towards a disk that attenuates 80% of the radiation and focuses it on a detection system in the form of a bolometer (Gentec-EO). In this study, a negative uniaxial congruent lithium niobate (cLN) crystal cut along the plane (001) with a thickness of L = 0.52 mm was used as a measurement sample. The refractive index (n0) of this crystal has been determined to be 6.5 within the frequency range of 0.25-1.25 THz. The THz eclipse z-scan curve of this sample is illustrated in Fig. 1(b) [4].
Frequency ('
Fig. 1. a) Spectrum of THz radiation source, b) THz eclipse z-scan curve of cLN.
The demonstrated increased sensitivity of the eclipse z-scan allows us to conclude that it is possible to evaluate the properties of materials exhibiting lower nonlinear refractive indices, thereby expanding the applicability of the method to characterize a variety of nonlinear optical materials. This work was supported by the Ministry of Science and Education of the Russian Federation (Passport No. 20190903).
[1] T. Xia, et al, Eclipsing Z-scan measurement of V104 wave-front distortion, Optics letters, 19.5, pp. 317-319, (1994).
[2] I. Artser, et al, Radiation shift from triple to quadruple frequency caused by the interaction of terahertz pulses with a nonlinear Kerr medium, Scientific Reports, 12.1, p. 9019, (2022).
[3] A. Tcypkin, et al, Giant third-order nonlinear response of liquids at terahertz frequencies, Phys. Rev. Appl., 15.5, p. 054009, (2021).
[4] A. Nabilkova, et al, Sensitivity enhancement of cubic nonlinearity measurement in THz frequency range, IEEE Transactions on Terahertz
Science and Technology, ACCEPTED, (2024).
