Portable displays that brought about the informational revolution have been enabled by a deceptively simple device, an electro-optical phase retarder comprised of a slab of a nonpolar nematic liquid crystal and a pair of transparent electrodes. The electro-optical response is through the realignment of the average molecular orientation, called the director, which is also the optical axis of the nematic. The director realignment is relatively slow, milliseconds, which is sufficient for applications such as displays, but is too slow for emerging developments in artificial intelligence, optical switching, augmented and virtual reality. For more than 100 years, nonpolar nematics were the only known liquid crystals suitable for displays and other electro-optical devices. In a dramatic turn of events, in 2017-2020, new polar nematics, called ferroelectric nematics, have been discovered, in which all molecular dipoles point in the same direction and thus produce a spontaneous electric polarization. The goal of the project was to establish how the ferroelectric nematic can be aligned by surface interaction, how they respond to the externally applied electric field, and whether and how they can be used in electro-optical devices.

Intellectual merit: The ferroelectric nematic liquid crystals are new to the field of electro-optical devices, as the study of their coupling to the electric field is in its infancy.  The project uncovered that the structure of the ferroelectric nematics and thus their interaction with the electric field are dramatically different from the properties of the well-studied solid ferroelectrics. In particular, since the ferroelectric nematics are fluids, the domain walls in them are not rectilinear but adopt a curved shape that minimizes the so-called bound electric charge. The research demonstrated that these domain walls are of a shape of conic sections such as parabolas and hyperbolas. The avoidance of the bound charges produces spontaneous chiral ground states of ferroelectric nematic films. The finding is striking since the constituent molecules do not contain any chemical chiral groups. The spontaneous polar ordering was found also in thin monomolecular films of ferroelectric nematics, which demonstrated that the polar ordering is caused by electrostatic attraction of molecules with a chain of positive and negative charges along their long axes.

The research team developed electro-optical phase retarders based on the newly discovered ferroelectric nematics as working elements with dramatically improved response times (one microsecond) and strong birefringence change (about 20%), which is triggered by moderate electric fields. This technology is based on the electric field modification of the degree of molecular ordering in the nematic phase of ferroelectric material sandwiched between two glass plates with transparent electrodes. Theoretical and experimental studies of the electro-optical devices filled with oblique helicoidal liquid crystal demonstrated an electrically controlled total reflection of light of a preselected wavelength that can be tuned in a broad range from ultraviolet to infrared, swiping the entire visible range. One of the unsolved issues of electro-optics of ferroelectric nematics was the value of their dielectric permittivity. Prior reports by numerous research teams claimed extraordinarily high values on the order of 10,000. The research team, working in collaboration with Dr. Jakli at Kent State, uncovered that the correct value of the permittivity is much smaller, on the order of 100.

Broader impacts: The project resulted in deeper understanding of the fundamental properties of the newly discovered ferroelectric nematics, such as their alignment at different substrates, shapes of their domain walls and the relationship between different molecular arrangements and the optical response under an externally applied electric field. The project's transformative value is in the potential for new design concepts for electrically tunable and switchable optical devices with fast response, such as optical switches and electrically tunable selective reflectors of light. These advances can potentially bring enormous societal benefits in future technologies, such as artificial intelligence, optical switching, augmented reality, and deep learning. The results created incentives to explore new physics and chemistry of ferroelectric nematics and chiral nematics with oblique helicoidal structure, their interactions with the external fields. The results will be important in various fields of optics, electro-optics, and materials science, since the development of devices based on new advanced materials is of crucial importance for emerging technologies.

The project engaged and educated a new, diverse generation of multi-lingual scientists, including women, with fundamental and technological cross-disciplinary backgrounds in optics, electro-optics, engineering, and materials science. Participating male and female graduate students successfully defended their PhD theses and continued research careers in academia and industry. A PhD student Olena Iadlovska supported has been awarded the Glenn H. Brown Prize of the International Liquid Crystal Society for the best PhD thesis on liquid crystals and the Facebook Reality Labs (FRL)-ILCS Liquid Crystal Research Excellence Gold Award for her studies of electro-optics of liquid crystals.

Last Modified: 10/19/2024
Modified by: Oleg D Lavrentovich