Polarization properties of near-infrared light confined in nematic liquid crystal channel waveguides embedded in SiO2/Si grooves

Linear and non-linear optical properties[1] of liquid crystals (LC) make them extremely appealing for use in a wide range of different optoelectronic applications other than displays. Their high optical anisotropy (in some cases higher than 0.2) implies large phase shifts in very short optical paths. Furthermore, their strong electro-optic effect allows rapid reorientation of their optical axis with indeed very low voltages in the range of only a few volt and hence compatible with current silicon technology. In particular, this last property makes them very attractive for low power consumption photonic applications. In this communication, light polarization dependence of channel waveguides made of SiO2/Si grooves filled with the commercial nematic liquid crystal E7 is shown. Grooves were obtained by initially wet etching phosphorous-doped (1 0 0) silicon substrates and then by thermally growing SiO2 up to a thickness of about 2 μm[2]. The cross section of the groove resulted with this method was of trapezoidal shape due to the preferential etching plane of silicon at 54.7° with respect to the wafer plane. The upper width of the groove as defined from the lithographic mask was 10 μm. The processed silicon substrate was subsequently assembled into a typical LC cell with a sodalime glass plate on the top. The inner surface of the glass had been previously spin-coated with Nylon 6, which was then rubbed for the alignment of the LC molecules approximately along the grooves. Propagation of infrared laser light (1550 nm) launched at one end of the waveguide by butt coupling was observed for a channel length of 2 cm. The signal at the output of the waveguide could be also coupled to a second optical fiber. The guided light showed a good optical confinement both in theory and the experiment. The simulated output profile by means of a typical beam propagation method was found to be very similar to the one grabbed at the output of the waveguide. Furthermore, the experimentally observed polarisation dependence of the light confinement into the waveguide was correctly predicted from our model. In particular, near infrared input light was controlled by using a fiber pigtailed sequence of 1/4wave-1/2wave-1/4wave plates. A measurement of light intensity versus the angle of input light polarization will be also reported. Such measurement shows that the orthogonal polarization state, for which the optical electric field “sees” only the lower LC refractive index (ordinary), was suppressed by more than 20 dB. This was justified by the observed pre-tilt angle of the molecules with respect to the propagation direction along the channel.