Liquid Crystalline Chiral Photonic Crystals for Advanced Photonic Applications

Nov 27, 2019
Focus Image

Prof. Tsuen-Hsien Lin (middle right) and Assistant Prof. Chun-Ta Wang (middle left) and their research team at NSYSU

Professor Tsuen-Hsien Lin from the Department of Photonics at National Sun Yat-Sen University, Taiwan led an international research team, collaborating with professors and scientists at Pennsylvania State University and Wright Patterson Air Force Research Laboratory, USA, has successfully developed the pioneer Repetitively-Applied Field (RAF) technology in the world to stably transform the 3-D cubic blue-phase liquid crystals (BPLC) photonics crystal into non-cubic structure. Their research finding was reported in the world-class distinguished journal, Nature Materials, in October 2019.

Liquid crystal (LC) devices now are everywhere in our daily lives, from the large industrial displays to individual home’s appliances, office’s equipment, automobiles, ubiquitous personal smart phones, and computers. Why LC devices are so widely popular is because the unique physical and optical properties that enable its operations very efficient with low power threshold at low cost. Among emergent liquid-crystalline materials under the development for improved performance, those that also possess photonic-crystal properties present intriguing fundamental challenges as well as tremendous application potentials were not found in or possible with conventional crystals. 

Professor Lin’s research team has been focusing on the fabrication, characterizations and device-feasibility demonstrations of two types of liquid crystalline photonic crystal: cholesteric liquid crystals (CLC) that function as 1-D PC and BPLC that function as 3-D PCs. Both CLC and BPLC are formed by introducing chiral agents into conventional (so-called nematic) liquid crystals used in ubiquitous display screen. Depending on the amount of the chiral agent, the constituent molecules self-assemble into 1-D or 3-D photonic crystals. In a photonic crystal, the dielectric constant is tailor-made to vary periodically in selected dimensions, giving rise to the so-called "photonic band gap” and band-edge dispersion which guides/confines/limits electromagnetic propagation. Photonic crystals in a pristine state, and their variants that contain specifically designed defects structures or photosensitive or electro-optical constituents enable selective reflections, micro integrated photonic circuitries, mirrorless lasers, production of slow/fast lights and other advanced photonic processes/devices.

Prof. Lin's Ph.D. student  in the research laboratory

Several ground-breaking discoveries have been made by Professor Lin’s team for the last two years. Two particularly noteworthy accomplishments concern BPLC, which exhibit 3-D photonic crystals properties in addition to tunable/reconfigurable physical and optical properties of liquid crystals used in ubiquitous display screen. In a paper publicized in Nature Communication (2017), Lin and his co-authors reported the first experimental realization of growing single-crystal BPLC of extraordinary dimensions, and recently in Nature Materials (2019), they reported the first successful demonstration of stable transformation of crystalline structures with a special field-assisted self-assembly technique. The following notes outline have more details of the paper published in Nature Materials and its impact on advanced photonic applications as well as fundamental pursuits and challenges.

 

Nature Materials [10/2019]

URL: https://www.nature.com/articles/s41563-019-0512-3

DOI #:  10.1038/s41563-019-0512-3

Authors:

This work involved collaboration between Prof. Tsuen-Hsien Lin and his Ph. D. student Duan-Yi Guo and other graduate students at the National Sun Yat-Sen University (Taiwan), Prof. Khoo and his Ph. D. student Chun-Wei Chen at Pennsylvania State University, and Chief Scientist Dr. Timothy J. Bunning of Wright Patterson Air Force Research Laboratory. Prof. Lin and Khoo are contact authors; Chun-Wei Chen and Duan-Yi Guo are equal-contribution first authors.

Liquid-crystalline photonic crystals possess and combine all the unique attributes of liquid crystals and photonic crystals. In particular,

Natural self-assembled 3D photonic crystals such as blue-phase liquid crystals (BPLC) exhibit only cubic lattice structures, c.f. Fig. 1. Since band structure varies dramatically with crystal symmetry, the fabrication of new stable photonic crystal lattice structures will enable more flexibility in dispersion and band-structure engineering for advanced applications that require tailor-made band-gap, group velocity dispersion and enhanced effective optical nonlinearity at band edges. In this work, the authors have performed a thorough examination of the dynamical responses and inner workings of the field-induced lattice reconfiguration, and identified key limiting factors for achieving new lattice structures. These detailed studies lead them to develop a Repetitively-Applied Field (RAF) technique to direct the reconfiguration of BPLC from natural cubic form into two new stable crystal symmetries: orthorhombic and tetragonal. By allowing the system to relax and avoid accumulating heat in each of the multiple field applications, and access various intermediate metastable states and evolution pathways, lattice transformation can be gradually built up to an extent not possible with existing techniques that employ single-step continuous applied field/stress.

Using different mixing ratios of the constituents, and a variety of constituents liquid crystals, sample thicknesses, field application directions and initial crystalline orientations, they have also shown that RAF is a generally applicable technique for producing field-free non-cubic BPLC 3-D photonic crystals with tailor-made bandgap (and associated dispersion and band structure) across the entire visible spectrum. These new lattice structures yield large electro-optic responses and can be polymer-stabilized to have a wide operating temperature range and sub-millisecond response speed, promising enlarged applications of BPLC in information display, electro-optics, nonlinear optics, microlasers, and bio-sensing. 

 

Figure 1. [Left to Right]. Photograph of a Blue-Phase Liquid crystals; Cubic lattice structures of BPI and BPII phase; Field-induced lattice transformation as depicted in their respective Kossel diagrams [Light scattering patterns]

 

Future Outlook

Quoting directly from the Nature Materials paper - ….The ability to form new non-cubic BPs in their pristine and polymer-stabilized forms brings about new possibilities and new avenues for theoretical modeling and simulations, and exploration on whether stable non-cubic BPII lattices in the P4222 or other unexpected crystal symmetry can be induced by a martensitic transformation from a stable orthorhombic-BPI or BPX precursor. One could also explore the possibility of using polymer scaffolds of non-cubic BP as templates or seed crystals to grow large non-cubic photonic crystals without need for a post electrical treatment. Stable non-cubic BPLC’s, and the feasibility of fabricating BPLC into massive-sized 3D single crystals also present many possibilities for advanced photonic, electro-optics and nonlinear optics applications, such as 3D emission/absorption control, mirrorless 3D lasers, slow/fast light generation, and nonlinear optical devices with tailored density of states in 3D” ]