Photonic Crystal Fibers (PCFs) have appeared as a new class of optical waveguides, which have attracted large scientific and commercial interest during the last years. PCFs are microstructured waveguides, usually in silica, with a large number of air holes located in the cladding region of the fiber. The size and location of these air holes opens up for a large degree of design freedom within optical waveguide design. Further, the existence of air holes in the PCF gives access close to the fiber core and by introducing new materials into the air holes, a high interaction between light and hole material can be obtained, while maintaining the microstructure of the waveguide. In this paper, we describe what we call Liquid Crystal Photonic Bandgap Fibers, which are PCFs infiltrated with Liquid Crystals (LCs) in order to obtain increased fiber functionality. We describe a thermo-optic fiber switch with an extinction ratio of 60dB and tunable PBGs using thermo-optic tuning of the LC. These devices operate by the PBG effect and are therefore highly sensitive to the refractive index distributions in the holes.
©2003 Optical Society of America
Standard optical waveguides guide light by total internal reflection, which requires that the refractive index of the waveguide core is larger than that of the surrounding (often homogeneous) cladding. Such waveguides have been extremely successful as transmission fibres  in world-spanning telecommunication systems or in the emerging field of integrated optics . With the appearance of photonic crystal fibres  (PCFs) and, more generally, three-dimensional periodic structures for control of electromagnetic waves , a more diverse field of optical waveguides has appeared, relying on the properties of structural repeatable index contrasts often in air-solid combinations . The waveguiding properties of photonic crystal structures may (depending on structure) either be explained as modified total internal reflection , or as Photonic BandGap (PBG) guiding, in which a periodic cladding structure exhibit strong reflection through a two-dimensional destructive interference process. PCFs have shown numerous unique properties, e.g., as non-linear fibres with highly extended dispersion control , as air-guiding fibres  or Bragg fibres  allowing for transmission of light at new wavelengths, or as fibres with very high numerical apertures, which are specifically attractive in very-high-power cladding-pumped fibre lasers .
Photonic crystal structures have also opened for a highly extended access to the guided light through the hollow voids in or around the core region of the waveguide.
Important examples include the demonstration of large waveguide dispersion in a PCF with high-index liquid  filled holes, or as devices combining Bragg grating technology and Microstructured Optical Fibres (MOFs) with polymer-filled air holes . The work by Kerbage and Eggleton  is particularly interesting, because it shows the possibility of realising active waveguide control, by dynamic positioning of a microfluid inside a MOF. Further, Yablonovitch  states that a combination of liquid and photonic crystals may hold new potential also for 3-dimensional waveguide structures.
The perspectives of design and active control of optical waveguides are further widened, if we bring the idea of self-organised material components into action. A very interesting candidate is the use of Liquid Crystals (LCs), which according to theoretical work by Busch and John  may be used in three-dimensional photonic crystals to obtain tuneable light localization, or to allow mirrorless lasing through the formation of a self-assembled PBG material . It is, therefore, fundamentally interesting to study the potential application of LCs as means of introducing enhanced periodicity and controllability, and in a further sense make the two-dimensional PCF a three-dimensional structure. Even more, it is relevant to consider the fundamental differences between optical waveguiding resulting from total internal reflection in contrast to the PBG effect, because enhanced active waveguide control must be expected, if e.g., switching between these two waveguiding principles is brought in action.
Here we report the first liquid-crystal-filled photonic crystal fibres, which guide light by the photonic bandgap effect as shown on Fig. 1. We demonstrate the first use of an actively controlled photonic bandgap fibre device, and describe how liquid-crystal-filled PCFs can serve as a highly sensitive, tuneable signal-processing element operating by the photonic bandgap effect. We demonstrate a tuneable filter function and low-voltage controlled broadband optical switching with an extinction ratio of as much as 60 dB, and an insertion loss of only 1dB.
Mesophases of liquid crystals
Liquid crystals are organic materials consisting of geometrically anisotropic molecules, leading to long-range orientational ,  order and a number of mesophases, which are thermodynamic phases with physical properties intermediate between those of pure liquids and pure solids. Phase transitions occur either when temperature is varied (thermotropics) or when the concentration of solute molecules is varied (lyotropics). Thermotropics are mainly used for electro-optic applications, such as flat-panel laptop displays. Lyotropics occur abundantly in nature, such as in DNA and cell membranes. We are presently concerned with thermotropic, rodlike (calamitic) LCs and examples of mesophases within this class are shown in Fig. 2. Nematics have only long-range orientational order, while smectics A and C in addition have long-range positional order in one dimension, resulting in a structure of thin (2–5 nm) layers; several hundred smectic layers thus average up to determine the local optical properties. In case of chiral molecules, all phases become noncentrosymmetric, leading to a helical superstructure in chiral nematic (N*=cholesteric) and chiral smectic C (SmC*), but not in chiral smectic A (SmA*). The helical structures are self-organized PBG materials, provided the helical pitch p is on the same order of magnitude as the wavelength of the light.
Liquid crystal photonic bandgap fibres
In this work, we have filled LCs into the circular voids of a triangular structured PCF, with inter-hole distance of 7µm and hole size of 3.5µm. We examined the alignment of the LC by polarizing microscopy observations of a single hole. We have used two different types of LCs: MDA-00-1445 from Merck and TM216 from BDH. The first is a short-pitch cholesteric having a N* to isotropic phase transition at 94°C, a reflection wavelength of 470nm and extra-ordinary and ordinary refractive indices of ne,589nm=1.6844 and no,589nm=1.5070, respectively. The latter has both a chiral Smectic A and a cholesteric phase, with the phase transition temperatures SmA* 26.2°C N* 42.3°C Isotropic. TM216 is a short-pitch LC with a highly temperature sensitive pitch around the SmA* to N* phase transition .
Polarized microscopy observations of MDA-00-1445 in its cholesteric phase indicated that the helix axis of the cholesteric was aligned in a radial symmetry and perpendicular to the fibre axis. These observations are consistent with the ones found by Kitzerow et al. . Similar observations of TM216 in the SmA* phase indicated a homeotropic alignment of the director in a region extending from the inner walls of the holes and in towards the central region of the holes. This central region had no or little transmission dependence in the polarized microscope. These observations suggest that the LC is aligned radially within the PCF holes equivalent to a smectic layer structure forming concentric cylinders along the axis of the PCF holes, and wrapped around an optically isotropic core region. This alignment is consistent with observations by Lo et al.  and by Mang et al.  and is further substantiated by a theoretical study by Kralj and Zumer .
After the alignment study, all voids of a PCF were filled with LCs, where the relatively high and controllable refractive index of the LC, together with the periodic arrangement of holes in the PCF, gives rise to a fibre cladding with photonic bandgaps . The spectral characteristics of the filled fibre, thereby, depend on the original PCF structure and on the alignment and optical properties of the LC.
To access the potential of controllability, we coated the filled section of the fibre with a thin conducting layer-forming a resistive microheater. By applying a small current through the microheater, the temperature of the LC and, thereby, the photonic bandgaps of the cladding structure could be controlled. The device-principle is illustrated in Fig. 3(a). The inset in Fig. 3(a) shows a polarized micrograph of the SmA* phase of TM216 inside a PCF hole.
While accurate simulation of fibre operation is difficult for the exact LC composition in the holes, Fig. 3(b) qualitatively illustrates the bandgap effects that are in play. The figure illustrates mode indices of allowed states of the fibre as function of wavelength. We have made the approximation of a uniform refractive index material filling the holes. The rods were simulated for a refractive index of 1.59(chosen to be approximately equal to the average value of the ordinary and extra-ordinary refractive indices). Although this is an approximation it serve to explain the basic waveguiding mechanism, but cannot be used for accurate simulation of cladding modes and bandgap edges, since it is expected that the actual alignment of the LC will have a significant influence on these solutions. The fibre has LC-filled holes of diameter, d, equal to 3.5µm, and pitch, Λ, of 7.0 µm. Intuitively, the operation of the LC-filled fibre may be explained as individual rods (the LC-filled holes) forming isolated waveguides at short wavelengths, whereas, for increasing wavelength, the rods become resonant and finally strongly coupled waveguides. Most importantly, forbidden regions with mode index below the low-index background material (n=1.45) appear for narrow spectral ranges. Within these spectral ranges, the fibre is capable of confining and guiding light in the core, despite its lower refractive index compared to the surrounding rods. The spectral position and extend of these ‘light-guide’ ranges are highly dependent on the exact composition of the rod material. Hence, the strong sensitivity of LCs makes them ideal candidates for control of light guidance by very small external influences.
Due to phase transitions, the LC shows large thermo-optical effects, whereby the photonic bandgaps of the cladding may be shifted spectrally. We found bandgap location sensitivities of 3nm/°C and 1nm/°C at infrared and visible wavelengths, respectively. However, when operating the LC around the phase transition temperatures, significantly stronger effects were observed - thereby causing large modifications of the refractive index distribution. This is demonstrated in Fig. 4, where a 20mm length of PCF is filled with a cholesteric LC (MDA-00-1445) and heated from 77°C to 94°C. The dominant bandgap changes from green to yellow, into an off state, and then to blue. The off-state is attributed to a highly scattering behavior of the LC at the phase transition, where the molecules tend to dis-align from their cholesteric phase.
The experimental observation of this effect encouraged us to investigate other LC phase transitions at lower temperatures to explore the potential of low-voltage controlled fibre functionalities. For this purpose, we filled 10mm of the PCF voids with TM216 and examined the characteristics of the photonic bandgaps at the SmA* to N* phase transition. Figure 5(a) shows the transmission spectrum of the filled fibre, when the LC is in its SmA* and N* phase. In the SmA*, case there are 3 bandgaps around 670nm, 1000nm and 1600nm, with the 670nm and 1000nm bandgap having insertion loss as low as 1dB. When slightly heated to above the SmA* phase, which only required a voltage of 5mV, all the transmission bands disappear due to a highly scattering dis-alignment of the LC in the N* phase. Figure 5(b) shows the spectrum of a 974nm pump laser coupled to the fibre, where a 60dB extinction ratio occurs for a temperature interval of only 0.4°C. Further experiments have shown a 60dB extinction ratio, where the required temperature interval only was 0.1°C, but this was difficult to maintain due to thermal fluctuation in the laboratory.
These results indicate that small, high-performance optical devices may be designed that requires a minimum of external influence to obtain tuneability, and allow for in-fibre functionalities with a minimum of insertion loss. Further, the high optical nonlinarity of LCs makes them an ideal candidate for all-optical signal processing devices and by using the self-organizing PBG effect of the LC, the possibility of designing more advanced signal processing devices may open up. Combining this with the knowledge of LCs in biological system, such as DNA and cell membranes, could open up for a new class of PBG fibre based biosensors with sub-microliter sample volume . Further, it has been shown that LCs could act as an optical amplifier of protein-ligand bindings , which further strengthens the possibility of using liquid crystal PBG fibres as biosensors for detection of receptor binding of certain molecules, e.g., proteins.
This work was supported by the NKT Academy in Denmark.
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