# Light Propagation in Confined Nematic Liquid Crystals and Device Applications
## 2 Materials and LC Waveguide Structures
### Nematic LC in PDMS Waveguides  
Optical waveguides can be made by infiltrating a nematic LC in polydimethylsiloxane (PDMS) channels by capillarity. Such waveguides are referred as LC:PDMS waveguides. Use of PDMS is motivated since it is a material often used for microfluidic applications with high optical quality [48]. PDMS is optical transparent, with low surface energy in the range of 20-23 mJ/m\({}^{2}\) at 20 \({}^{\circ}\)C [49], low dielectric constant in the range of 2.32-2.40 [50] and Young's modulus, which is between 0.57 MPa and 3.7 MPa [51], and thermally and optically enabled polymerization. PDMS is also reliable for soft lithographic fabrication of many microfluidic and micro-optical devices [52]. Flexibility is another attractive feature of PDMS which is effectively used in robotics, automation, consumer electronics and in novel applications, such as health care and biomedical technologies. PDMS is used in optical interconnections for datacom applications to replace metallic connections whose performance in terms of bit rate are very low and suffer problems related to high heat dissipation [53]. PDMS channels to be filled with a nematic LC are fabricated by the cast and moulding technique involving several technological phases as shown in Figure 5 and described in ref. [31]. The PDMS channels are filled with a nematic LC by capillarity which results in homeotropically alignment as shown in Figure 6a. Homeotropic alignment of LC is due to the hydrophobic characteristic of the PDMS inner surface. A fabricate sample including LC:PDMS waveguides with widths of 8, 10 and 15 \(\upmu\)m is reported in Figure 6b. The optical characterization to check the LC alignment in the LC:PDMS waveguides is performed using a polarized microscope. Polarized light is transmitted through the channels, revealing the homeotropic arrangement of the LC molecules. Such orientation of the LC is proven by the black part in the central region for the waveguides indicating that polarization of input light is not changed when transmitted through the LC and is stopped by the crossed analyser, as it can be observed in Figure 6c. Light leaks through the edges of the waveguides because of the optical retardation induced by the LC molecules homeotropically aligned on the vertical sides of the PDMS channels. Figure 4: Picture of a single-mode optical fiber which is butt-coupled to a chip, including a set of LC waveguides on SiO\({}_{2}\)/Si grooves. Waveguide width is 8–12 \(\upmu\)m.  
LC molecular orientation inside the PDMS channel induces a refractive index distribution which determines polarization-independent light propagation [31]. Green laser light at a wavelength of 532 nm and red light using an He-Ne laser source at a wavelength of 632.8 nm were efficiently fibre butt-coupled to the LC:PDMS waveguide as shown in Figure 7a. Propagation in the C-band was also studied using a tunable laser source between 1510 nm and 1590 nm. Light transmission versus polarization orientation was measured using a set-up including a polarization controller, consisting of a half-wave plate between two quarter-wave plates, an optical power meter and a polarizer at the output of the waveguide. It has been observed that light polarization does not vary as it propagates along the LC:PDMS channel for any polarization at the input. A polarization differential loss of only 0.3 dB, as shown in Figure 7b, was measured with propagation losses of about 8 dB/cm. It is possible to control LC:PDMS waveguides by means of coplanar electrodes. Preliminary measurements show that LC molecules can be switched by applying a square voltage of 1 kHz with an amplitude of about 2 V. In this case, only change of modal propagation was affected without a substantial light transmission of light, since the core refractive index remains above the PDMS refractive index. By choosing a PDMS with a refractive index intermediate between the ordinary and the extraordinary refractive of the LC, light modulation or switching is possible.