Visnyk of the Lviv University. Series Physics 60 (2023) с. 90-100 DOI: https://doi.org/10.30970/vph.60.2023.90 Photoelastic properties of ammonium fluoroberyllate crystals B. Mytsyk, B. Horon, N. Demyanyshyn, V. Stadnyk, P. Shchepanskyi, Ya. Kost

(не менше 1800 знаків) All independent piezo-optic \pi_im coefficients of ammonium fluoroberylate (NH₄)₂BeF₄ crystals were determined by the interferometric method using experimental setups based on a single-pass Mach--Zehnder interferometer. Note that under the action of uniaxial pressure on the sample placed in one of the shoulders of the interferometer, the expression for determining the coefficients \pi_im has the form \pi_im = - \frac{\lambda}{n_i³\sigma_im^o} + 2S_km\frac{n_i - 1}{n_i³} (n_i is the index of refraction, S_km} is the coefficient of elastic compliance, \sigma_im^o\sigma_imd_k is the control stress, \sigma_im is the half-wave stress and d_k is the thickness of the sample in the direction of light propagation). The geometry of the experiment corresponds to the indices i, k, and m, which indicate the directions of polarization and expansion of light and the direction of action of uniaxial pressure, respectively. Based on \pi_im and elastic stiffness coefficients C_mk, all independent elasto-optical coefficients p_ik were calculated using the product p_ik = \pi_imC_mk. It was found that some coefficients \pi_im have an atypically high value (\sim 10-13~Br). It was established that all the largest values of piezo-optic and elastic-optic effects correspond to the main coefficients \pi₁₁, \pi₂₃, \pi₃₃ and p₁₁, p₂₂, and p₂₃. This makes it possible to attribute ammonium fluoroberyllate to the best acousto-optical materials for the ultraviolet region of the spectrum, since the lower limit of the FBA transmission spectrum is in the deep UV region $\sim$~250~nm. The relative errors of determining the main piezo-optic coefficients are mainly within 13-18 %. For coefficients \pi₄₄, \pi₅₅, \pi₆₆, the absolute and relative errors are slightly larger than the corresponding errors of the main coefficients \pi_im. This is due to the fact that the expressions for calculating such coefficients include complex sums of \pi_im and S_km coefficients, the errors of which get added up.

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