Numerical analysis and fabrication process flow of a tunable phase shifter based on liquid crystals and polymeric slot waveguide

Phase shifting is a key operation in many optical devices and systems which needs to be implemented in optical integrated circuits with low propagation losses, fast response times and high integration density [1]. A promising process to induce a phase shift takes advantage of the electro-optical effect through liquid crystals (LC). When combined with LC, polymer-based optical integrated circuits can greatly boost performances adding new functionalities in sensing, computation, and communications [2, 3]. In those systems, when used as cladding, LCs show remarkable switching and tuning capabilities with low voltage and low propagation losses. At the same time, the combination of the LC cells with optical waveguides implies an increase in the overall system fabrication complexity. Thus, in addition to the performance evaluation of the devices in terms of phase shifting, it is also necessary to define compatible and accessible materials and fabrication processes. The schematic of the device we developed is represented in Fig. 1, and consists of a SU-8 slot waveguide (n=1.573), a bottom cladding of SiO2 (n=1.4657), and an upper cladding of nematic LC (inset of Figure 1). We have considered two of the most commercially available nematic LCs: 5CB and E7. For 5CB the ordinary and extraordinary indices are respectively no=1.511 and ne=1.678 (Δn=0.167) while for E7 we considered no=1.5 and ne=1.689 (Δn=0.189). The initial alignment of the LC at rest is set along the propagation direction of the waveguide (x axis), and is imposed through a thin layer of either a polymeric photo-aligner or Nylon 6, accordingly prepared. In this work we evaluated the compatibility of SU-8 spacers with Nylon 6, further tests will be performed with a polymeric photoaligner. The phase shifting parameters of the system was numerically analyzed through two techniques: a combination of 2D-FEM and index ellipsoid formula, and a direct 3D FDTD simulation. The transverse profile of the system evaluated by those numerical methods is represented in the inset of Fig. 1, and consists in a slot waveguide, a configuration that allows an improved interaction between the propagating signal and the cladding (especially in the gap). Mainly, TE00 modes were considered in the initial analysis. The results are represented in Table 1. The fabrication process flow is represented in Fig. 2. The proposed device allows a wide phase shift of more than 20 with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of with an applied voltage of about 1.8 V.