Lithium niobate, LiNbO3 (LN) attracts considerable attention both from the point of view of fundamental sciences and of applications because of its extremely large second order nonlinear susceptibility. In several z-scan works light-induced change of absorption and refraction in LN – important phenomena from the point of view of nonlinear optical applications – have been studied.
If LN is exposed to intense ultrashort light pulses in the visible frequency range, two-photon absorption may lead to free carrier generation, in turn resulting in the formation of small polarons. Small polarons are charge carriers trapped by the self-induced lattice distortion, which extends over their closest surroundings. Electrons self-trapped at the regular NbNb5+ ions of the LN lattice form the most simple polaron species, free small polarons, while electrons captured at NbLi5+, a defect characteristic for the non-stoichiometric LN, are bound small polarons. Nearest neighbour pairs NbNb5+ - NbLi5+ can accommodate two electrons forming a bound bipolaron, which can be dissociated optically as well as thermally. The polarons affect strongly the linear and nonlinear optical properties of LN, thus understanding their microscopic structure and the related physical properties is essential.
Studies of nonlinear interactions of small polarons with short, intense laser pulses in LN have been already performed by different research groups. However, the formation time of small polarons and the delay with respect to the two-photon absorption was unknown so far. Creatively, by varying the pulse length, H. Badorreck and co-authors determined these with good accuracy using the z-scan method complemented by their own developments.
The well-designed experimental setup consists of a prism pulse stretcher/compressor, a spatial frequency filter, and the common z-scan configuration. The concept of the work is the examination of nonlinear absorption by means of open-aperture z-scan measurements using pulses of duration varying from 70 fs to 1 ps, keeping the pulse energy at a constant value. For the evaluation the standard z-scan model is complemented by a model of the temporal evolution of the free carriers and of the small polarons, and by their effect on the sample transmission. The entire set of z-scan curves relative to different pulse durations is successfully fitted with a single parameter set. From the fitting the two-photon absorption coefficient, the small polaron absorption cross-section and characteristic times for electron-phonon relaxation, for interband relaxation, and for the formation of small polarons, are determined.
The authors performed the fitting with the standard model as well, in this way taking into consideration only two-photon absorption. A good fit was found for the shortest pulse duration, about 70 fs, which is closest to the time delay between the optical excitation of free carriers by two-photon absorption and the appearance of small polarons. For longer pulses, on the contrary, the fitting to the standard model leads to worse fits and a dependence of the best-fitting value of the two-photon absorption on the pulse length. This verifies the reliability of the model elaborated by the authors of this Optics Express article.
These interesting and important results have great importance in many other fields of research, such as nonlinear frequency conversion. They are especially interesting in the field of high-energy THz generation. In oxide crystals and semiconductors at high pump energies multiphoton absorption may lead to free carrier generation causing strong absorption in the THz frequency range. The results presented in this article may be important for research aiming to find the optimal THz generator material and optimal pumping conditions.
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