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5The 30th International Conference on Advanced Laser Technologies N-I-20
ALT'23
Electrical-field tunable diffraction optical elements based on lithium niobate single crystals
A. Akhmatkhanov1, A. Esin1, V. Shur1, V. Pavelyev2
1-Institute of Natural Sciences and Mathematics, Ural Federal University 620002, Ekaterinburg, Russia 2- Samara National Research University, 443086, Samara, Russia
E-mail: [email protected]
Periodically poled lithium niobate (LN) single crystals are widely used for nonlinear optical applications for quasi-phase-matched laser light frequency conversion. The quasi-phasematching process in this case is realized due to opposite values of second order susceptibility tensors in neighboring domains of periodical domain structure. On the other hand, these domains possess opposite signs of electrooptic coefficient, which allows creation of electric field controllable diffraction optical elements (DOEs). The utilization of this effect led to demonstration of the following devices: optical beam deflectors [1-2], diffusers [3], Fresnel zone plates [4] and Shack-Hartmann sensors [5]. Creation of effective DOEs requires precise control of LN domain structure.
In this work we present the results of creation and characterization of various LN-based DOEs. The DOE creation process was based on deep study of the features of domain structure kinetics in LN. The dependence of domain wall velocity on field and orientation was analyzed. We have revealed that the walls oriented along Y crystallographic directions were more than four orders of magnitude slower than the X-oriented ones. This effect hampers creation of some types of DOEs as they require creation of domain structures with wide range of domain wall orientations. The methods of overcoming of this problem are discussed. We have designed the structure of 2D hexagonal zone plate, which represents the best approximation of circular zone plate by the set of hexagonal ferroelectric domains with Y-oriented walls. The creation of 2D hexagonal zone plate with minimal ferroelectric domain size down to 7 ^m and period of 20 ^m has been demonstrated. The obtained half-wave voltage of DOE was 2.9 kV. We have shown that additional heat treatment at temperature about 150oC allows to compensate built-in internal field and to decrease the zero-phase-shift voltage down to 0.4 kV.
The following DOEs were created and characterized: (1) 2D diffraction grating, (2) hexagonal zone plate, (3) bidomain element for TEM00-TEM01 mode transformation [6], (4) DOE for generation of transformation of TEM00 mode into angular-momentum modes. The 2D diffraction grating allowed decreasing the zero-order diffraction maximum intensity by 80%. The hexagonal zone plate has focused up to 25% of the laser light into a spot of 0.13-mm-diameter. The measured response time of the bidomain element for TEM00-TEM01 mode transformation was below 0.3 ^s (limited by the photodiode bandwidth).
The possible applications of bidomain element for channel multiplexing in communication systems are discussed.
The research was made possible by Russian Science Foundation (Project №21-72-10160).
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