Spatially Inhomogeneous Structures in a Surface-Doped Lithium Niobate Crystal for Light Beam Transformation.

Spatially inhomogeneous structures (SIS) are important to realize the integrated optical devices transmitting and processing light signals. Nowadays, there are several methods used to create such structures with different characteristics and topologies. However, research on the methods of forming and modifying the characteristics of these structures continues to this day. In this work, we create and study SIS in a lithium niobate crystal surface-doped with copper ions. The results prove that SIS can be created by the point-by-point method using a continuous-wave frequency-doubled YAG:Nd³⁺ laser. The realized structures were formed as diffraсtion and waveguide optical elements with different characteristics and topologies. It is demonstrated that we can change the refractive index up to 10^–3 by the point-by-point illumination of an X‑cut lithium niobate crystal during the structure formation. The properties of the fabricated structures were investigated by diffraction analysis, laser interferometry, and the optical probing method using He–Ne laser radiation. On the formed diffraction structures (DSs), the far-field diffraction patterns (DPs) show that the light power transfers from the incident radiation to first-order maxima. The first-order maxima intensity can exceed the intensity of the zero-order maximum up to several times. The near-field study after the excitation of waveguide structures (WSs) shows that they exhibit the properties of mode filters. The point-by-point method of forming SIS may be useful for creating integrated optical circuits and modifying the characteristics of optoelectronic and photonic devices. [ABSTRACT FROM AUTHOR]