Institute of Solid State Physics

1. LC pressure infiltration in PCF Schematic of the molecular configuration of one full pitch p of a chiral nematic LC showing the director n and the helix axis H. a b c Institute of Solid State Physics BULGARIAN ACADEMY OF SCIENCES Electrically tunable chiral nematic liquid crystal photonic crystal fibersM. PETROV, B. KATRANCHEV, H. NARADIKIAN, T. ANGELOV, K. PANAJOTOV and A. ZHELTIKOV Introduction Photonic Crystal fibers filled with liquid crystal –LC-PCF have appeared as a new class of optical waveguides, which have attracted large scientific interest during the last years.PCFs are fibers with an internal periodic structure made of capillaries, filled with air, laid to form a hexagonal lattice. For the effective using of LC-PCF, as an hopeful alternative of both conventional and PCFs, two basic operations must be done: the infiltration the PCF with liquid crystal and in turn a stabilization and detection of the alignment of the confined LC. The choice of the liquid crystal substances with suitable material constants, like refractive indices, dielectric permittivity, electroconductivity, chirality etc, in order to realize an effective tunable waveguide system is also important task and our goal will follow such a direction. We account all these properties of LC-PCF and search the optimal conditions for electric driving of the waveguiding photonic fiber properties in the case of chiral LC. A combination of LC and photonic crystals may hold new potential also for 3-dimentional waveguide structures. Optical micrograph of the end facet of the PCF. The PCFs outer diameter is  13.3µm, and the inner one is 3µm. The inner core diameter is 49.2µm. Results The polarized micrographs of the infiltrated LC-PCF capillaries. amorphous Linearly polarized light that is incident parallel to the helix axis of N can be considered as two circularly polarized waves with opposite handedness. A PBG occurs for the circular polarized mode that matches the rotation sense of the helix when the wavelength is of the order of the pitch. For the other circular polarized mode with opposite sense handedness, the light is transmitted. Measurement of the transmission (or reflection) spectrum defines the PBG, where the decrease (or increase) in light intensity indicates the range of wavelengths forbidden to propagate. The bandwidth of the reflection band  is proportional to both the helix pitch P and the birefringence n, so that n.P. In the case of planar (homogeneous orientation) we observe cholesteric texture consisting of cells surrounded by oily streaks (bundles). By applying a.c. or d.c. electric field we observe the cells colour changes between green and red, indicating the helix axis direction (perpendicular to the FCP capillary axis out of the electric field) rotation by electric field (from perpendicular to tilted with respect to axis) becausea is big and positive. In the case of homeotropically initial orientation of the mixture (no is perpendicular to the fiber capillary axis) we observed striped texture typical for the cholesteric substance, which indicates the H direction coaxial with the fiber tube axis. Applying the electric field we observed the texture colour change between green and red and the stripe period increasing indicating the H rotation from parallel to tilted with respect to the fibre axis. Using the measured pitch p values towards the electric field magnitude and frequency and fixing the temperature at 65C we estimated the bandwidth of the reflection band  (for n=0,2) to be 400 nm. Thus we can control the electric dependence of the bandwidth of the reflection band . We note that changing the temperature we can vary the bandgaps width also. amorphous 1 We keep the electric field low than the electric correlation length =(K2/a)1/2/E since at a certain critical field Ec, p and we obtain a N*N structure transition. Using the threshold frequency trend in the planar case, we found the minimum ( 2.3V for 2000 Hz) of the electric threshold voltage ensuring suitable driving allowing for in-fiber functionalities with a minimum of insertion loss. 10 nm S 3 10 nm amorphous crystalline Ta2O5 Conclusions We indicated that the small, high-performance optical elements may be designed that requires a minimum of the external (e.g. electric) influence to obtain tunability and allow for in-fiber functionalities with a minimum of insertion loss. The high optical nonlinearity of the liquid crystals (LCs) makes them an candidate for all-optical signal processing devices, such as tunable Bragg grating, and by using the self-organizing photonic bandgap (PBG) effect of the LC, the possibility of designing more advanced signal processing elements may open up. We indicated that the chiral LCs are suitable medium for photonic crystal fiber electric control both of reflection and transmission spectra, including the bandwidth of the reflection and transmission bands.Combing this with the knowledge of LCs in biological systems, such DWA and cell membranes, could open up for a new class of PBG fiber based biosensors with sub-microliter sample volume. Acknowledgements:This study was supported by Grant No.F-1307 from the Ministry of Education and Science of Bulgaria.