NCN Project

"Investigation of light propagation properties of photonic crystal fibers infiltrated with ferroelectric and antiferroelectric liquid crystals"

NCN Project No. 2011/03/B/ST7/02547 (OPUS)

Photonic liquid crystal fibers (PLCFs) have been of great interest for many research groups on the world. PLCFs are a combination of photonic crystal fibers (PCFs) with liquid crystals (LCs), whose properties and susceptibility in external physical fields, temperature and pressure, give the possibility to modify propagation properties of the medium.  

Fig.1 A cross-section of the Photonic Liquid Crystal Fiber 

The vast majority of research works in this topic were devoted to infiltration of PCFs with nematic LCs, which resulted in some practical applications including tunable fiber optic filters, polarization controllers or long period fiber gratings. However, only some initial studies included PCFs infiltrated with chiral nematic or cholesteric LCs were conducted. Chiral smectic C LCs (SmC*), also called ferroelectric liquid crystals (FLCs) combine the electric polarization and anisotropy of ferroelectric crystals with the hydrodynamic properties of liquids. 

Fig.2 Schematic alignment of SmC* liquid crystal molecules

Due to this aforementioned physical properties, FLCs are considered to be the future of modern photonic technology.

In the following research, we have used isotropic and birefringent PCFs and micro-capillaries (MCs) made of silica glass (UMCS, Lublin, Poland and NKT Photonics, Denmark).  In order to infiltrate the PCF microstructure, we have used SmC* LCs synthesized by the group of Prof. D±browski from MUT. Also, we have used a commercially available SmC* LC FD4004N (Dainippon Ink & Chemicals, Japan).

During preliminary research, we have analyzed an alignment of FLCs inside single silica glass microcapillaries. We have noticed that it is not trivial to align FLC molecules in the desired way. According to obtained results, we have decided to use an aligning material in order to obtain a uniform alignment of the FLC molecules. At first we have investigated a 3, 6 and 13 um MCs with thermoaligning layer on the inner sides and infiltrated with MT2_B15 FLC. We used two thermoaligning materials: SE-130 and SE-1210 (Nissan Chemicals). Due to improper balance between anchoring energy normalized to the MCs diameter and elastic energy of the FLC helix, we were unable to obtain uniform planar alignment in 3 and 13 um MC, Only for 6um MC the result was quite satisfactory. 

In the next stage, we have investigated an MCs covered with a commercially available photosensitive azo-dye material SD1 (Dainippon Ink & Chemicals, Japan). The sulfonic azo-dye used in our research offers variable anchoring energy depending on the irradiation energy and thus provides good control on the FLC alignment in MCs. The good and stable FLC alignment has been observed only when anchoring energy normalized to the MC diameter is less than the elastic energy of the FLC helix. We have conducted a research with a 6um MC with a prepared SD1 layer with different times of linearly polarized UV light exposure. The samples were infiltrated with FD4004N FLC. The have analyzed the quality of the alignment under the microscope with crossed polarizers. The results are shown in Fig.3.

Fig. 3 A 6um MCs with  uniformly aligned FLC with different irradiation doses [J/cm2]   (f), (g) and (h) show the MC with irradiation dose of 2.84 J/cm2 with MC easy axis parallel polarizer (f), tilted at 45 degrees (g) and parallel to the analyzer (h).  

The other advantage of the SD1 layer is that it is possible to realign FLC molecules by use of additional exposure on UV light with linear polarization with different azimuth. We have used an amplitude mask and additional UV irradiation procedure in order to generate periodic changes of the FLC molecules inside MC. The results are presented in Fig. 4.

Fig.4. Periodic alignment in MC infiltrated with FLC molecules after double step irradiation process.

In the next step, we have carried out experiments with PCFs with photoalignment layer and infiltrated with FLC molecules. The PCF samples were prepared in the same way as MCs, and next selectively infiltrated with FLC FD4004N. In result, the selective light propagation based on photonic bandgap effect was obtained (Fig. 5). As a light source, we have used broadband white light source (Supercontinuum SuperK Compact, NKT Photonics).

Fig.5. Light propagation based on photonic bandgap effect in the isotropic and birefringent PLCF.

Furthermore was observed the change of state of polarization under the influence of external electric field. We have used a square shaped signal with a frequency of 1kHz and amplified 200 times. Depending on the electric field intensity (ranging from 0V/um to 9V/um), we were able to observe changes of the state of polarization of light guided in the PFLCF. The results are shown in Fig. 6.

Fig.6. State of polarization changes shown on the Poincare sphere. 

It was shown that photonic ferroelectric liquid crystal fibers are capable of being applied in designing and creating new fiber optic devices for telecommunication and sensing technology. It should be pointed out, that it is necessary to continue this research line in order to improve the quality and repeatability of achieved results. Moreover, this project may have a great impact on other disciplines, like i.e. optofluidics.

Acknowledgement 

The research team would like to thank Prof. Roman D±browski (MUT, Poland) for providing an FLC mixtures, Dr. Pawe³ Mergo (MCSU, Poland) for PCFs and MCs. Also, we want to thank Prof. Vladimir G. Chigrinov (HKUST, Hong Kong) for providing FLC and photoaligning materials. 

The Head of the project would like also to thank Dr. Abhishek K. Srivastava for collaboration and many interesting discussions.

This project was supported by the National Science Center under Grant No. 2011/03/B/ST7/02547