Photonic Crystals Show Fast Response Speeds

A technique to restructure liquid crystals could lead to the development of fast-responding liquid crystals suitable for next-generation displays and advanced photonic applications such as mirrorless lasers, biosensors, and fast/slow light generation. The technique was developed by an international team of researchers from Pennsylvania State University (Penn State), the Air Force Research Laboratory, and the National Sun Yat-sen University in Taiwan.

The team worked with blue-phase liquid crystals, which typically self-assemble into a cubic photonic-crystal structure. The researchers believed that if the crystals were reconfigured, they could develop properties not present in their cubic form. After nearly two years of experimentation, the researchers found that by applying an intermittent electrical field and allowing the system to relax between applications and dissipate accumulated heat, they could coax the crystals into stable and field-free orthorhombic and tetragonal structures. The team’s approach makes use of the Repetitively Applied Field Technique to gradually transform the initial cubic lattice into various intermediate metastable states, until a stable noncubic crystal is achieved.

A close-up image of a mm-size blue-phase liquid crystal during its formation stage. Courtesy of Khoo Lab, Penn State.

“The most important thing about this research is the fundamental understanding of what happens when you apply a field, which has led to the development of Repetitively Applied Field Technique,” professor Iam Choon Khoo said. “We believe that this method is almost a universal template that can be used for reconfiguring many similar types of liquid crystals and soft matter.”

The resulting liquid crystals show a photonic bandgap that can be tailored to anywhere within the visible spectrum and possess the fast response speed necessary for a variety of next-generation displays and advanced photonic applications. The addition of a polymer to the crystals could stabilize them within a wide operating temperature range — from freezing to nearly boiling point, compared to typical liquid crystals, which are stable within only a 5° range. The polymer scaffold could also speed up the switching response.

The team is applying the lessons it learned in this study to create new crystal structures and orientations using the electric field from a laser source. The reconfigured liquid photonic crystals show promising properties for information display, electro-optics, nonlinear optics, microlasers, and biosensing applications.

The research was published in Nature Materials (www.doi.org/10.1038/s41563-019-0512-3).