Improving the liquid crystal alignment technique of photonic device

Abstract

Liquid crystals (LC), due to their large electro-optic response, are quite useful for many applications in optical devices. However, the performances of LC based devices rely on active alignment of the LC molecules. For a better performance, microscopic control of the LC molecules within the device is highly required. This can be effectively control through a thin alignment layer. The shape and the interaction energy of surface, influence the molecular orientation of LC. Traditionally, mechanical rubbing method of polyimide (PI) is using in the formation of alignment layer. However, the technique requires direct mechanical contact, which introduces defects, electrostatic charge and dust. To overcome the demerit of rubbing method, the lithographically patterned alignment layer can be potentially used for LC based optical devices. In this study, firstly, we made a comparison on the performances of rubbed and patterned PI alignment layer. It is also very useful to pattern an alignment layer on the same waveguide structure to reduce one fabrication step if the waveguide structure itself can effectively act as the alignment layer. Therefore, a lithographically patterned alignment layer on the top surface of benzocyclobutene (BCB) waveguide was also proposed. In the 2nd part of the study, another comparison with patterned BCB alignment layer was made. In the experiments, LC cell was assembled by sandwiching ITO coated glass substrates with alignment layers and tested under the polarizing microscope. The LC cell is characterized by measuring the rise time of the LC layer. The better performance or lower rise time was found for patterned PI alignment layer and can effectively apply on many optical devices.