Electrical tuning of optical LC:PDMS waveguides

Polydimethylsiloxane (PDMS), characterized by high optical transparency and low surface energy, electric constant, and Young’s modulus, is widely recognized as an interesting, high-quality organic material for micro- and opto-fluidic functional systems. Other advantage of applying PDMS is its cheap and easy processing owning to the fact that PDMS-based photonic devices are typically fabricated with use of reliable technology based on a standard soft photolithography. In this paper studies on electrical tuning of optical channels formed by the PDMS-based waveguides infiltrated with liquid crystalline material are presented. Inspection of resultant LC:PDMS waveguides under a polarized microscope, as shown in Fig. 1, indicates that NLC molecules are aligned homeotropically with respect to each PDMS wall. This spontaneous molecular arrangement on PDMS surface is obtained mainly due to the low energy surface of the PDMS which forces a minimum contact interface between the PDMS surface and strongly elongated NLC molecules of a rod-like shape. Molecular orientation inside the waveguide channels, as presented e.g. in Fig. 2, has been found by assuming stiff homeotropic boundary conditions on PDMS walls and by performing numerical simulations with use of the finite difference relaxation scheme. The alternative-direction successive over-relaxation (ADSOR) scheme has been applied to find a solution of the Euler-Lagrange equation. ADSOR combined with the multigrid method (MM), has been applied to calculate molecular reorientation under the influence of electric field. Calculation of the spatial electric field distribution in the region within the electrodes Ex and Ey components are simply calculated as the first derivatives of the electric potential with respect to x and y directions shown in Fig. 3b. Fig. 3c shows calculation of the molecular orientation under the influence of the electric applied field applied. In order to obtain the electro-optical control of directional couplers to make optical switches, flexible electrodes have been fabricated by sputtering ITO on the PDMS, with different deposition times from 30 s up to several minutes, at temperatures of 25°C, 100°C and 150°C. For the best depositions (e.g. 30 s at 150°C), preliminary conductive ITO layers with a thickness of about 28 nm have been obtained as shown on Fig. 4. Furthermore thin films of chromium have been also successfully deposited as an alternative solution. Fig. 5 shows the I-V curve for a sample of Chromium sputtered on PDMS for 4 minutes at 25 ℃ with a thickness of about 100 nm. The measured resistance is about 5 kΩ with a resistivity of 0.5 Ω∙cm and a sheet resistance of about 45 kΩ/□. In conclusion, electric tuning of LC:PDMS couplers are feasible based on numerical simulations showing LC molecular reorientation due to electric field applied, as well as on experimental demonstration of the preliminary conductive ITO and Cr layers sputtered on the PDMS surface.