A ferroelectric single crystal patterned inside glass by means of femtosecond (fs) laser irradiation has been shown to maintain its ferroelectric properties, as reported in the January 2019 issue of MRS Communications. The researchers were also able to control and manipulate the ferroelectric domains of the crystal, crucial for the development of electro-optic devices.

In microelectronics, optics, and optical fiber technology, introducing anisotropic atomic arrangements of a ferroelectric crystal inside an isotropic glass allows for the conversion of glass from a material mostly limited to passive uses—like optical glass fiber—to an active functional material for active applications.

“It was an open question whether crystals which are ferroelectric, [such] as bulk single crystals, would preserve their ferroelectricity when confined within glass. This work provides the first account that such behavior is preserved for laser-induced lithium niobate crystals in glass,” says Keith Veenhuizen of Lebanon Valley College and first author of the article.

The laser-fabricated crystalline channels act as optical waveguides, according to Veenhuizen. Similar to optical fibers, the crystals in glass can confine an information-carrying light signal, demonstrating their potential for use in photonic integrated circuits (analogous to electronic integrated circuits).  

Himanshu Jain and Volkmar Dierolf of Lehigh University and their colleagues from Lebanon Valley College, Corning Inc., and Oak Ridge National Laboratory used femtosecond laser irradiation to grow the lithium niobate (LiNbO₃) crystals 200 μm below the surface of lithium niobosilicate (LNS) glass. The energy deposited by the laser beam was absorbed by the LNS glass material and the local accumulation of heat as well as the rise of the temperature resulted in an environment suitable for the nucleation and growth of the LiNbO₃ crystals. The laser was operated at a pulse duration of 175 fs, wavelength of 1026 nm, and repetition rate of 200 kHz.

One of the challenges the research team faced was to identify a suitable method for initiating ferroelectric domain reversal and to confirm that such a reversal had occurred. Ferroelectric domains are regions with different orientations of the polarization vector that may coexist within a ferroelectric sample. Fabrication of photonic devices relies on the ability to control the structure of the ferroelectric domain of the crystals in glass. Scientists achieve this by performing poling, that is, by forcing the electric dipoles of each domain to orient themselves in a prescribed direction.

“An early attempt involved the use of transparent, planar electrodes for application of high voltage and utilizing Raman spectroscopy for detection. This technique was not well-suited for probing crystals in glass on the micrometer scale,” Veenhuizen says. Performing the work at Oak Ridge National Laboratory afforded the researchers access to scanning probe microscopes capable of applying highly localized fields and detecting changes on this small scale.  

The researchers chose a region of the single crystal with complicated domain structure and used the cantilever tip of a piezoresponse force microscope (PFM) to pattern a new domain structure using a two-step process. First, they applied a spatially localized large positive dc bias with the cantilever tip to switch the spontaneous polarization of the crystal so that it would point toward the sample surface. Then they applied a large negative dc bias over a smaller region within the region poled in the first step so that the polarization would point out of the sample surface.

The team was surprised to find out that domain reversal was unsuccessful at room temperature, but elevating the temperature of the sample to 100^oC allowed domain reversal to occur. “This differs from the ability to perform domain switching in bulk lithium niobate single crystals at room temperature and is good for retention of fabricated domain structures,” Veenhuizen says. PFM scans 24 h after the poling experiments verified the stability of the ferroelectric domains.  

PFM was also used to characterize the piezoelectric behavior and ferroelectric domain structure of the crystals in the glass. The researchers obtained information about the crystallographic orientation by means of electron backscatter diffraction (EBSD). EBSD and PFM data were then correlated to generate theoretical piezoresponse maps, which showed that the piezoresponse correlated with the lattice rotation of the crystal.

Bertrand Poumellec, an expert in laser-induced structural transformations in silica-based glasses and Research Director at the French National Center for Scientific Research (CNRS), who was not involved in the study, says, “The interesting question that is answered here is whether the piezo-electric properties of a crystal would vary, when passing from free to a confined space inside a glass, with the size of a few microns. It is very interesting to see that the properties are preserved; for example, it is possible to imprint the orientation of the spontaneous polarization at the micron-scale, by poling at 100°C, whereas the same process is achievable at room [temperature] in unconfined space. This variation could be the subject of a further study.”

According to Veenhuizen, the exploration of the temperature dependence of the ability to initiate domain switching is one of the next steps of this work. “For one thing, this would dictate the parameter space for the fabrication of optical devices based on controlling the ferroelectric domains,” he says.

Read the abstract in MRS Communications.