Simulation of Optofluidic LC:PDMS Directional Couplers for Photonic Switching

Integrated optofluidics is a new technology field which combines photonics with microfluidic techniques to fabricate devices for applications in many fields such as datacom, biosensing and lab-on-chip [1]. Such devices are inexpensive to fabricate, and can be flexible because polydimethylsiloxane (PDMS) can be used as substrate material. It is possible to create switchable and reconfigurable devices by combining PDMS channels filled with nematic liquid crystals (NLC) used as core material, referred as LC:PDMS optical waveguides. The process to fabricate well defined PDMS channels for simple waveguides or directional couplers in which a liquid crystal can be infiltrated by capillarity, is very simple [2]. The mold to create a micro-channel is obtained through soft photolithography with SU-8 2005, which is a negative photoresist, on a 2.5 cm x 2.5 cm silicon crystalline wafer. The wafer is preliminarily cleaned in a solution of 2% of HF to remove the native oxide and then SU-8, is spinned on the treated surface so that a thickness of 5 μm is obtained. PDMS (Sylgard 184 by Dow Corning) is used for the fabrication of waveguide cladding. In our simulations we have studied the directional coupler structure sketched in Fig. 1 with a square cross section of 2 μm x 2 μm as a basic structure to make an optical switch [3]. The orientation of the NLC molecules in the PDMS channels has been studied through the Oseen-Frank free energy minimization in a model built by using Comsol Multiphysics® in order to derive the related distribution of the refractive index of the NLC. The results have been imported in a BeamPROP’s device simulator to observe light propagation at the wavelength of 1550 nm. The NLC infiltrated in the waveguides has an extraordinary refractive index of 1.689 and a ordinary refractive index of 1.5, the PDMS used for the waveguides has a refractive index of 1.3997. We simulated three directional couplers with three different gaps between the waveguides of 1 μm, 0.75 μm and 0.5 μm which provide a complete exchange of optical power between the two waveguides at the lengths of 500 μm, 300 μm and 125 μm respectively (Fig 2). Furthermore we calculated the extinction ratio (ER), as a figure of merit, for every directional coupler and plotted in Fig. 3. The ER is defined as the ratio between the power levels at the output of the directional coupler and we found that an ER over 20 dB can be obtained.