Transparent electrodes for advanced liquid-crystal tunable devices

Joint research project

Project leaders: Dimitrios Zografopoulos, Vera Marinova
Agreement: BULGARIA - BAS - Bulgarian Academy of Sciences
Call: CNR/BAS triennio 2019-2021 2019-2021
Department: Physical sciences and technologies of matter
Thematic area: Physical sciences and technologies of matter
Status of the project: New

Research proposal

Liquid crystal (LC) technology is ubiquitous in modern society thanks to the worldwide diffused market of LC displays (LCD). Moving beyond the globally recognizable LCD technology, LC-tunable devices are constantly infiltrating more and more technological sectors, from photonics to terahertz (THz) science, providing low-cost and low-power consumption versatile components. Thanks to their inherent anisotropy and the capacity to dynamically control their electromagnetic response by applying an external voltage, LCs are the key medium in numerous reconfigurable systems for electromagnetic wave modulation, switching, filtering, or steering.

In order to fully exploit their potential, LC-tunable devices rely on the integration of electrodes for the application of the necessary voltage to switch the LC molecules. The traditional material of choice for their fabrication is indium-tin-oxide (ITO) thanks to its sufficient conductivity and high transparency in the visible (VIS) and infrared (IR) spectrum. However, the massive deployment of ITO as the standard electrode for electro-optic -not only limited to LC-based- devices is hindered by the scarcity and, hence, material cost of indium, as well as its very low transmittance at wavelengths higher than the IR, e.g. in the rapidly advancing THz sector.

The project aims to address the issue by demonstrating a series of advanced LC-tunable components based on low-cost, transparent electrodes in both the VIS/IR and THz spectra. In particular, the project synergistically merges the extensive expertise on the growth of aluminum zinc-oxide (AZO) [1] and graphene [2,3] mastered by BAS with that of CNR-IMM on the theoretical design [4], fabrication, and characterization of LC-tunable components from the IR [5] to THz spectrum [6,7].

AZO electrodes are shown to have comparable performance to ITO, while involving easier fabrication and greatly reduced material cost [1]. They will be fabricated by atomic layer deposition and integrated in optimized photonic light valves, namely LC-tunable devices for spatial light modulation based on photorefractive or photoconductive (PR/PC) materials [8]. The design and performance of the electrodes and the complete devices will be first analyzed using a finite-element method multiphysics simulation toolbox, specifically developed for the target applications, which involve the physics of electromagnetic wave propagation, LC driving and switching and the PR/PC effect.

Graphene electrodes will be grown via low-pressure chemical vapor deposition (LPCVD for single layer graphene) and atmospheric pressure chemical vapor deposition (APCVD for multilayer graphene) on various substrates, both standard (glass) and optimized for THz applications (cyclo-olefin polymers, high-resistivity silicon). Advanced configurations, such as electrodes on flexible substrates [9] or lithographically patterned electrodes will also be demonstrated. Samples will be characterized via VIS/FTIR/THz spectroscopy and their performance will be assessed in planar LC cells. Given the scarcity of transparent conductive materials at THz, graphene is considered as perhaps the most promising material for THz electrodes [10]. Its performance will be benchmarked against THz electrodes based on conductive polymers, in particular, PEDOT:PSS.

Finally, metamaterial-based LC-THz components, such as tunable absorbers or phase modulators, will be designed and experimentally demonstrated. Such devices will be based on extremely subwavelength resonant cavities, thus reducing by orders of magnitude the response time compared to standard free- space LC-THz configurations [6,7]. Patterned graphene will be used for the electrical interconnection of metallic elements in order to apply the LC driving voltage.

The project is based on a comprehensive and inter-disciplinary approach that involves rigorous theoretical investigations, material science and growth techniques, LC physics and technology, and THz science and spectroscopy, perfectly coupling the corresponding know-how and expertise of the two partners. Moreover, its objectives are aligned with international projects to which both partners participate, i.e. the EU COST Actions CA16220 on "Microwave Photonics" (Zografopoulos), MP1402 on "Atomic Layer Deposition" (Marinova), CA15207 on "Nano-Carbon Composite Materials" (Marinova), and a bilateral project between the Taiwan National Chiao Tung University and BAS on "Multifunctional Optical and Magnetoelectric Materials and Applications" (PI: Marinova). Hence, there is clear potential for multi-lateral networking and the project will provide a solid basis for long-term cooperation and possible joint applications for future scientific projects, for instance, in the frame of the Horizon 2020 program.

In parallel, the knowledge and scientific results gained through this project will lead to a mutual transfer of knowledge and know-how between CNR-IMM and BAS and the mobility will contribute to the training of the young researchers involved in LC science and advanced material growth techniques. The scientific outcome of the project will be published in high-impact international peer-reviewed journals in the fields of photonics, material science, and THz technology, thus attracting the interest of the scientific community and maximizing the project's overall impact.

[1] Y. C. Su et al., Opt. Quant. Electron. 50, 205, 2018.
[2] S. Petrov et al., Opt. Data Process. Storage 3, 114-118, 2017.
[3] V. Marinova et al.,J. Phys.: Conf. Ser. 794, 012009, 2017.
[4] D. C. Zografopoulos et al., Phys. Rev. E90, 042503, 2014.
[5] D. C. Zografopoulos et al., Lab Chip 19, 3598-3610, 2012.
[6] G. Isic et al., Phys. Rev. Applied 3, 064007, 2015.
[7] D. C. Zografopoulos and R. Beccherelli, Sci. Rep. 5, 13137, 2015.
[8] V. Marinova et al., Opt. Quant. Electron., 48, 270, 2016.
[9] A. Ferraro et al., Appl. Phys. Lett 110, 141107, 2017.
[10] L. Wang et al., Light: Sci. Appl. 4, e253, 2015.

Research goals

The main scientific goal of the project is the design, fabrication, and characterization of LC-tunable devices for photonic and terahertz applications, based on low-loss transparent electrodes.

In particular, the project's objectives are:

[O1] The development of a finite-element multiphysics platform for the theoretical modeling of photonic LC devices including the photorefractive and/or photoconductive effect.

[O2] The fabrication and characterization of micro-patternable AZO and graphene sheets with low resistivity (90%) for LC devices in the VIS/IR and THz spectrum. Benchmarking of the fabricated electrodes against standard solutions, i.e. ITO at VIS/IR and PEDOT:PSS at THz.

[O3] The design and experimental demonstration of free-space and/or metamaterial-based LC-tunable terahertz phase-shifters based on graphene electrodes.

[O4] Transfer of know-how and expertise between CNR-IMM and BAS, and training of young researchers in LC science and advanced material growth techniques.

[O5] Dissemination of the results in the form of scientific publications and talks in relevant scientific networks (e.g. COST Actions CA16620, MP1402, CA15207).

[O6] Establishing a fruitful collaboration between CNR-IMM and BAS, as a prerequisite for future joint projects, combining their complementary expertise in advanced growth techniques and LC and THz technology.

Last update: 15/07/2025