STUDIES ON LIQUID CRYSTAL BASED OPTICAL WAVEGUIDES

Abstract

Optical waveguide is a physical mechanism that guides the electromagnetic waves in optical spectrum. Motivation of using liquid crystal (LC) in waveguide is to exploit its property of changing refractive index with the temperature. Light guiding mechanism has lots of practical implication in various fields such as industry, imaging, medical and sensing. The temperature sensing is the one of the keen interest among the researcher. We can utilize the waveguide as a temperature sensor by forming long period grating (LPG) into it. We assume to form LPG in liquid crystal layer only and proposed two different six layer waveguide structures by using two different transparent conducting oxides (TCF). In proposed structure first Ag-ZnO used as TCF and in second one graphene oxide (GO) are used as TCF. First we studied the propagation of light in Ag-ZnO-liquid crystal (LC) waveguide structure. The Ag-ZnO-LC structure has some certain limitation and to overcome these limitation we used GO as a TCF in proposed structure second and got very good and convincing results. Then we studied the response of long period grating (LPG) formed in liquid crystal layer of a proposed waveguide structures as temperature sensor by using couple mode theory. Refractive index of the liquid crystal (LC) is highly sensitive to the environmental temperature. This property of LC could be used for temperature sensing. In both the proposed multilayer waveguide structure the long period grating is formed in liquid crystal layer only. Long period grating is capable of coupling light from fundamental guided mode to cladding modes at a specific wavelength (Resonance wavelength). Thus results in a sharp rejection band in transmission spectrum of the waveguide. The response has been studied for temperature variation ranging from 298 K to 309 K for Ag-ZnO-LC structure, with a very high sensitivity of 41 nm/K. Since beyond 309 K the waveguide does not support any cladding mode, so we proposed second GO-LC waveguide structure. For structure second response has been studied for temperature variation ranging from 306 K to 329 K. The sensitivity of the second proposed sensor is 10.43 nm/K with optimized waveguide and grating parameters. Our numerical simulations based on a coupled mode theory which shows significant changes in resonance wavelength with the temperature in the transmission spectra of the waveguide. ____________________________________________________ We finally conclude that on increasing grating period we can significantly increase the sensitivity of device. We took two different grating periods in GO-LC waveguide to show the significant change in the sensitivity of proposed structure second.