The science and technology of liquid crystals has advanced rapidly during the past half-century. As a result, liquid-crystalline materials are increasingly ubiquitous in various optical and photonic devices. Since the 1970’s—a historical mark for the significant development and later flourishing industry of flat-panel liquid crystal displays, a great number of molecular structures of liquid crystal compounds have been designed, novel functioning materials synthesized, new mesophases and phenomena discovered, and state-of-the-art applications have been manifested or even seen in various manufactures, not to mention more numerous suggested potentials awaiting their maturity for marketable products. As a multidisciplinary open-access journal with quick time-to-publication and an emphasis on advances in optical material development, Optical Materials Express is an appropriate venue for a feature issue dedicated to the recent progress in the optics of liquid crystals, especially because it has been more than a decade since we first published a focus issue on “Liquid Crystal Materials for Photonic Applications” in 2011.

This special theme issue aimed to inspire and encourage further research into liquid-crystalline materials and systems with new and improved photonic properties and applications. It highlights two invited full-length articles and 10 contributed papers on optical behaviors as well as thermo-optical and electro-optical properties of primarily thermotropic calamitic liquid crystals, although an earlier effort was exerted to solicit original papers on a wider spectrum of thermotropic and lyotropic liquid crystals, composites, and colloidal systems.

In her invited paper entitled “Physical properties of hydrogen bonded nematic liquid crystals and electro-optical properties in terahertz waves [1],” Yamaguchi explores the physical properties, including the ordinary and extraordinary refractive indices, orthogonal components of dielectric constants, and three elastic constants of splay (K₁₁), twist (K₂₂), and bend (K₃₃), of hydrogen-bonded liquid crystal mixtures in the nematic phase at room temperature. For terahertz applications, the electro-optical characteristics of the homogeneous and twisted-nematic cells are numerically investigated by means of measured physical properties. In comparison with the counterparts of two conventional nematic single compounds, the well-known cyanobiphenyl 5CB and phenylcyclohexane 5PCH, this study underlines the potential of dichroism-free hydrogen-bonded liquid crystal for terahertz devices.

Focusing on minimization of light leakage in oblique viewing directions for a bright image displayed in a dark background—namely the notorious and yet unpreventable halo mura—in homogeneously aligned liquid crystal modes [2], Kim and colleagues in their invited paper report on a novel optical design to greatly promote the contrast ratio in both the normal and oblique viewing directions by introducing a thin film of vertically aligned dichroic dye either on top of the second polarizer or dispersing the dye into a positive C-plate for viewing-angle compensation. The simulated and experimental results suggest that this approach is applicable to all liquid crystal displays (LCDs), including high-dynamic-range LCDs based on the local dimming technology.

In the study of reflection of anisotropic optical slabs [3], a research group led by Yang of the Materials Science Program and Advanced Materials and Liquid Crystal Institute at Kent State University examines a stratified anisotropic optical film where the optic axis of each layer is twisted a bit compared with the one beneath it. The authors analyze the helical component of the dielectric tensor by using Fourier transform and calculate the reflection spectrum with the Berreman 4 × 4 matrix method. The results indicate that both the right-handed and left-handed helices can coexist in the stepwise twisted structure, producing a simultaneous reflection of oppositely handed circularly polarized light. This feature enables optical films with superior reflection properties, such as high reflectance, broad bandwidth reflection, and short-wavelength reflection. Although there is a lack of relevant experiments for further verification of these theoretical findings against real-world data, a significant impact of such stratified optical media can be expected on flat-panel display and photonic devices.

Three papers deal with lasing and coherent light control [4–6]. In the article of the Editor’s pick [4], Lin et al. present chirality control of defect-mode laser beams from a dye-doped isotropic lasing medium sandwiched by two polymeric cholesteric liquid crystal layers as chiral reflectors, thereby empowering a chiral distributed Bragg cavity. The Taiwanese group successfully demonstrates the controllable polarization states of the output laser, both experimentally and theoretically. The potential applications for the findings can extend to bidirectional asymmetric laser components, presenting immense potential for the development of diverse micro-laser devices. The second paper, illustrating directional switching and dynamic wavelength tuning in cholesteric liquid crystal laser, is contributed by a joint team led by Feng in the author list [5]. Such laser devices based on the reconfigurable chiral photonic (liquid) crystals can be applied in the field of photonics as highly versatile light sources. The third study focuses on the random lasing and stimulated emission of the whispering gallery mode in radially oriented, dye (0.3 wt% of Pyrromethene 597)-doped nematic liquid crystal (E7) droplets dispersed in an aqueous solution of 1-wt% sodium dodecyl sulfate [6]. Uchida and coworkers in this study investigate lasing behaviors that are strongly dependent on the irradiation position of the pumping light and droplet size. The coexistence of the two types of laser permits a dye laser in a nematic droplet to emit two lights in distinct wavelength ranges controllable by changing the irradiation position and temperature.

There are two papers discussing liquid crystal lenses [7,8]. In the comprehensive work entitled “Theoretical model of an electrically tunable liquid-crystal-based contact lens [7],” Reshetnyak et al. establish a theoretical model for a liquid crystal contact lens for the correction of presbyopia. A comparison to the experimental data validates the theory. They believe that the model can be adopted for engineering optimization of the existing prototype. Also concerned with tunable liquid crystal lenses, the other paper is contributed by Mohajerani and coworkers [8]. Here the tunability is unconventionally achieved by the thermal effect originating from surface plasmon resonance. According to the authors, the demonstrated thermo-plasmonic liquid crystal lens is structurally simpler and more economical in comparison with conventionally electronic lenses, rendering greater stability than other all-optical counterparts.

Laudyn et al. describe in their paper some relevant aspects of thermal solitons and their interaction with nematicons, viz. reorientation solitons [9]. The interplay between reorientational solitons and thermal solitons results in a reduced and (or) an enhanced nonlinear response in nematic liquid crystal. With concentration on certain conditions giving rise to a more significantly combined nonlinear response, their findings open up new possibilities for diverse applications, including advanced photonic devices, all-optical signal processing, and innovative techniques for optical communication systems.

In an effort to study spectral features of J-aggregates of an anionic cyanine dye (TDBC, 1,1’-disulfobutyl-3,3’-diethyl-5,5’,6,6’-tetrachlorobenzimidazolylcarbocyanine sodium salt) dispersed in the nematic liquid crystal 5CB [10], Lisetski and his Ukrainian colleagues present interesting optical-fluorescent and electro-optical properties of the mixture, boasting enhanced photostability and longer lifetime than in water. Compared with that of the neat counterpart, the Fréedericksz transition appears to be somewhat higher and sharper in the liquid crystal concoction impregnated with TDBC J-aggregates, together with a promoted optical contrast. The obtained results are promising for development of novel luminescent liquid crystal materials, suggesting ideas for further detailed studies.

The only paper involving optical materials for photovoltaics and energy applications is written by D’Alessandro et al., who clarify that the optical and electrical characterization of a liquid crystal photovoltaic spatial-light modulator (as a complex liquid crystal device integrated with an organic solar cell as a power source to be addressed directly by light) can be carried out with a non-invasive optical method [11]. The researchers are able to extract critical parameters such as the pretilt at the solar-cell–liquid-crystal interface, along with the voltage induced by the cell under different light intensities. Additionally, reported is the use of this method on large areas to probe local responses and detect potential defects or inhomogeneities of the solar cell, making this method interesting to monitor the aging of such systems.

The last paper reveals several two-dimensional patterns of topological defects in vertically aligned nematic liquid crystal cells with pads, crossed-strips, and porous electrodes [12]. In this fascinating work based on the use of a nematic liquid crystal possessing negative dielectric anisotropy and observations under a polarized optical microscope, the Tsung group discovers that the pads and crossed strips produce alternative radial and hyperbolic defects, whereas the porous electrodes generate topological defect dipoles. The dipoles enable a new technology platform for defect array design. They believe that switchable dipole alignment holds great potential for applications in precision optics and quantum devices.

We intend to provide a timely topical and highly intriguing overview of recent advances in liquid crystal science and technology in this focus issue on “Liquid-Crystalline Materials for Optical and Photonic Applications” of Optical Materials Express. To give the reader a taste of what’s inside, we have included brief summaries for 12 papers compiled in the collection. We are grateful to all of the invited and contributed authors who have effortlessly prepared their full-length papers to make this special issue possible. Our many thanks are due to all the peer reviewers for their invaluable time and useful comments, which definitely helped enhance the scientific content of each published paper. Finally, we wish to thank Stavroula Foteinopoulou, present Feature Editor of Optical Materials Express, and staff members in the Optica Publishing Group for their coordination and dedication to have this issue published.