Liquid crystal under dual nanocontrol

Liquid crystals made their debut in simple displays. Today, they find applications in a wide range of photonic technologies, and their potential remains far from exhausted. These remarkable materials combine properties that seem inherently incompatible: they flow like liquids while maintaining the high degree of molecular order characteristic of crystals. The key challenge remains understanding how to control their properties most effectively so that they can form the basis of stable, precise, and readily manufacturable devices of the future.

Colourful geometry

A unique type of liquid crystal is the blue phase, in which the molecules locally arrange into so-called double-twist cylinders. These, in turn, spontaneously assemble into a three-dimensional lattice with features measuring hundreds of nanometres. The resulting structure acts as a photonic crystal, selectively reflecting light of a specific wavelength. As the geometry of this three-dimensional lattice changes, so does the wavelength of the reflected light—and with it, its colour.

The key challenge is the precise control of this process. Traditional methods, such as applying an electric field or heating the material, enable changes in the color of the reflected light; however, the effect is short-lived. Moreover, the blue phase itself remains stable only within a narrow temperature range, which makes its practical application more difficult.

“It is precisely the ordered structure of the blue phase that leads to the formation of the so-called photonic bandgap, i.e. a range of light wavelengths that cannot propagate through the material,” says Kamil Orzechowski, PhD, from the Faculty of Physics at the Warsaw University of Technology, the lead author of the article. “Instead, they are selectively reflected, giving the material its characteristic optical properties. This phenomenon is the optical equivalent of the energy bandgap in semiconductors, which controls the flow of electrons through the material.”

“The ability to achieve stable control over such a structure could pave the way for a new generation of photonic devices, in which information would be carried by photons rather than electrons,” says our researcher. “Through the dynamic and reversible control of the properties of the blue phase, such solutions could surpass many current semiconductor technologies in terms of operational flexibility.”

Synergistic effect

Physicists from the Warsaw University of Technology joined forces with chemists from the Military University of Technology, the University of Warsaw, the University of Silesia, and the Polish Academy of Sciences, as well as researchers from Taiwan’s National Sun Yat-sen University. Together, they developed an innovative dual approach based on nanotechnology tools.

The researchers introduced gold nanoparticles coated with specially designed molecules with structures resembling those of liquid crystal molecules into the liquid crystal, ensuring their harmonious integration with the material. Their presence reduced the size of the crystal lattice, causing a “shift” in the color of the reflected light toward the blue end of the spectrum. Importantly, the nanoparticles also increased the thermal stability of the material.

Additionally, the surfaces confining the material were coated with thin, several-dozen-nanometer-thick alignment layers, which imposed a different degree of twisting on the blue phase, modifying the geometry of the entire lattice. As a result, the researchers were able to precisely control the liquid crystal structure and, consequently, tune the color of the reflected light.

The experiments demonstrated that both mechanisms not only operate simultaneously but also mutually enhance their effects, enabling an unprecedented level of control over the optical properties of the material.

Blue-phase liquid crystal sample with tunable reflected light color, photo by Kamil Orzechowski

Potential

Research into the precise programming of liquid crystal properties could significantly expand the scope of their practical applications. The potential for optical innovation remains vast — ranging from telecommunications and fiber-optic technologies, through advanced sensors for detecting chemical substances, temperature changes, or mechanical stress, to sustainable construction using smart windows with adjustable light transmission. Liquid crystals could also find applications in anti-counterfeiting systems, where unique structural colors could provide a difficult-to-replicate alternative to traditional pigments.

More information on the method of controlling the color of light reflected by blue-phase liquid crystals can be found in the article “Synergistic Effects of Nanoparticles and Surface Anchoring on Fine-Tuning the Photonic Bandgap in Blue Phase Liquid Crystals.”

The authors of the publication from the Faculty of Physics at the Warsaw University of Technology are Kamil Orzechowski, PhD, WUT Professor Anna Kozanecka-Szmigiel, PhD, Prof. Tomasz R. Woliński, and graduates Weronika Milewska, MSc, and Aleksandra Neumann, MSc.