Scientists Have Taken a Major Step in the Development of Domain Engineering

10:00, September 29, 2021

The results will be useful in biological and medical research

Ural Federal University scientists were the first in the world to demonstrate and explain the effect of transforming one-dimensional growing (1D) bandgap segnelectric domains in a lithium niobate crystal into chains of two-dimensional growing (2D) isolated domains. An article describing the research has been published in the journal Acta Materialia.

Plates of a 0.5 mm thick lithium niobate monodomain crystal were sequentially irradiated with two pulses of infrared laser. During heating and subsequent cooling, a strong pyroelectric field emerged in the crystal (all segmentelectrics are pyroelectrics). Under the influence of the field upon cooling after the first pulse, narrow band domains with parallel walls, less than a micron wide and up to 200 microns deep, were formed on the surface of the crystal in the irradiated zone. After irradiation by the second pulse, the band domains transformed into regular chains of isolated circular submicron domains.

The amazing effect was explained by obtaining a three-dimensional image of the domains in the crystal volume using an original technique of visualizing the domain walls using optical microscopy, which is based on the generation of the second harmonic, a method that involves "pumping" a pulse of laser radiation.

It turned out that the banded domains on the surface appear to form after the first pulse ridge domains with regular teeth. It was shown that the formation in the volume of domains of teeth with charged domain walls accompanies the elongation of banded domains when cooling after irradiation by the first laser pulse. Upon heating under the second pulse, the splitting of the comb domains into isolated conical domains, that is, the transformation of one-dimensional 1D domains into two-dimensional 2D domains. The revealed mechanism of self-organized formation of comb domains and their subsequent evolution was confirmed by computer simulation.

"It is known that the banded domain structure of ferromagnetics, when exposed to a magnetic field pulse, turns into cylindrical magnetic domains, so the transition of domains from one-dimensional - 1D - state to two-dimensional - 2D occurs. However, in ferroelectrics, which are often regarded as "the electrical analogue of ferromagnets," this effect has not been observed before. At the same time, domain engineering, a technology for creating stable domain structures of a given geometry to improve the nonlinear optical and piezoelectric properties and create devices with record characteristics, is actively developing in the field of segmentelectrics. Naturally, studying the evolution of the domain structure, controlling the shape of domains, and obtaining self-organized structures are of extreme practical interest," says Vladimir Shur, head of the Segnelectrics Laboratory, professor at the Department of Condensed State Physics and Nanoscale Systems, chief researcher at the Department of Optoelectronics and Semiconductor Engineering at UrFU.

The creation of 2D structures can be used for electric field sources of a given geometry, in particular, to improve the method of generation and application of picoliter liquid droplets developed in the laboratory, used in biological and medical research, and in the development of a portable X-ray source.

The described work is the result of 40 years of scientific activity by Vladimir Shur and the world-class laboratory he created to study the domain structure of ferroelectrics. The research was supported by the Russian Science Foundation (Project № 19-12-00210) and with the equipment of the Urals Center for Collective Use "Modern Nanotechnologies" of UrFU.

Reference

Pyroelectrics are materials that exhibit spontaneous electrical polarization, which results in the generation of a field as the temperature changes. Segnetoelectrics are a subclass of pyroelectrics in which the direction of spontaneous polarization can be changed by applying an external electric field. Ferromagnetics are substances that, under certain temperature conditions, are capable of possessing magnetization in the absence of an external magnetic field.

Lithium niobate is well studied due to its exceptional non-linear optical and piezoelectric properties and is as widely used material in optoelectronics as silicon is in electronics. Lithium niobate is used as a model material in studies of the domain structures of segmentelectrics and is also the most popular material for domain engineering.