# Light Propagation in Confined Nematic Liquid Crystals and Device Applications
## 2 Materials and LC Waveguide Structures
### Nematic LC Confined in SiO\({}_{2}\)/Si V-grooves  
A channel waveguide can be obtained by infiltrating a nematic LC in a SiO\({}_{2}\)/Si groove as shown in Figure 2a. The ITO layer and the n-Si substrate act as electrodes to apply an external electric field. A silicon V-groove is made by exploiting anisotropic etching of silicon. Typical groove width ranges from 5 to 15 \(\upmu\)m. A native thermal oxide SiO\({}_{2}\), obtained in an oven at about 1100 \({}^{\circ}\)C acts as a low index cladding, considering that a nematic LC has a refractive index higher than 1.45. The V-groove is fabricated by means of a micromachining process whose details are described in ref [44].  
Figure 2b shows a scanning electron microscope picture of the V-groove in which smoothness of SiO\({}_{2}\)/Si surfaces is able to minimize defects in the embedded LC molecules, consequently attenuating light scattering in the final LC waveguide.  
The etching of Si creates a V-groove with an angle \(\alpha=54.7^{\circ}\) using (100) Si wafer as schematically shown in Figure 3a. A glass cover of 0.5 mm is then placed on top of the V-groove [4]. A D263 borosilicate glass substrate is used with a refractive index \(n_{\text{D263}}=1.516\) (at the at wavelength \(\lambda=1.55\)\(\upmu\)m) intermediate between the values of the extraordinary and the ordinary refractive index of the LC used, such as E7 (\(n_{e}=1.69\), \(n_{o}=1.5\) at wavelength \(\lambda=1.55\)\(\upmu\)m), which allows the creation of an optical switch [4]. A thin film of Nylon 6 is spanned and rubbed on top of the ITO film, to align the LC along the groove. Nematic LC E7 in the isotropic phase is infiltrated in the covered V-groove by capillarity in vacuum at 80 \({}^{\circ}\)C and then cooled down to the room temperature. Figure 3b shows the orientation of nematic LC molecules, whose tilt angle \(\theta\) is controlled by the applied voltage between the ITO and n-Si.  
Figure 3: Schematic images of an LC waveguide in SiO\({}_{2}\)/Si groove: (**a**) cross-section indicating the etching angle of the Si groove; (**b**) longitudinal section showing the orientation of the nematic LC molecules under applied electric field between ITO and n-Si.  
Figure 2: Optical waveguide made of LC infiltrated in a SiO\({}_{2}\)/Si V-groove: (**a**) Three-dimensional exploded schematic, showing the orientation of the nematic LC molecules along the V-groove; (**b**) Electron microscope scanning of a SiO\({}_{2}\)/Si V-groove.  
Light is coupled to LC waveguides in Si/SiO\({}_{2}\) grooves, referred to as LCW (Liquid Crystal Waveguide), by fibre butt-coupling, as shown in Figure 4. Since LC-NOA61 interface was used, total coupling losses, including input and output, of 4.5 dB or better were measured, as a result of minimization of LC molecular random orientation representing a source of light scattering at the input and output faces of the LCW, which determined higher coupling losses in previous samples without LC-NOA61 interface [45]. Propagation losses resulted to be about 6 dB/cm which can be further lowered by using photoalignment techniques, able to improve the optical quality of the LCW [46; 47].