Investigations into liquid crystal based optofluidic polarization devices

• Main Author: Ranjini Radhakrishnan
• Other Authors: Murukeshan Vadakke Matham
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/56097
• Institution: Nanyang Technological University

Optofluidics, refers to the special type of optical devices fabricated by integrating optics and microfluidics with the aim of manipulating fluid-light interactions. This has developed into one of the latest research areas in the recent past which allows the integration of multiple tasks in a single chip leading to lab-on-a-chip devices. One of the major challenges in developing optofluidic platforms or components is the correct choice of fluid. Though a variety of fluids are used depending upon the applications, the possibilities of incorporating an anisotropic fluid were not well explored. It is to be mentioned that such fluids generally offer additional degrees of freedom and tunability of intended parameters. Based on the analysis of various tunable optofluidic systems reported in literature, it is found that the devices with tunability of optical polarization are a poorly explored area. Further, the inclusion of an anisotropic fluid, liquid crystal, offers a novel method of refractive index modulation. In this context, the main objectives of the thesis focus into the (i) research and development of tunable optofluidic polarization devices, with a novel method of attaining tunability by integrating liquid crystalline materials, (ii) illustration of dynamical properties of liquid crystalline materials and possible effects due to the interaction with light. Tunability in the state of polarization, based on the flow induced refractive index modulation in the fluid medium, is comparatively a new method which can offer various possibilities in lab-on-a chip devices fabrication. Also liquid crystals, an efficient choice of fluid for the same are not well explored based on an optofluidic platform. Even though liquid crystals exist in the fluid state at room temperature, an applied shear force or flow can induce an orientational as well as translational motion of the molecules. Thus, due to the molecular realignment there can be refractive index modulation (birefringence) and hence tune the state of polarization of the light beam transmitted through the medium. In order to validate these concepts, both lyotropic and thermotropic liquid crystals are analysed. For lyotropic liquid crystals, hydrated crystals formed are monitored and found to be birefringent which can be controlled with the applied flow. Also the possible fluid-light interactions that can contribute to the transmitted light intensity are investigated. Thermotropic liquid crystals, in the aligned and non-aligned state are analysed based on the same flow driven perturbations. Scattering losses, apart from polarization is found to be significant for non-aligned liquid crystals. Therefore, to make the liquid crystal medium homogeneous and thereby reducing the scattering effects, a preferential alignment is applied to the liquid crystal molecules. The molecular reorientation occurring across the aligned liquid crystal medium is analysed using conoscopic imaging. The corresponding change in the state of polarization is monitored using optical transmission. Jones matrix of the liquid crystal medium is also formulated based on the alignment of molecules and thereby validating the change in the state of polarization of the transmitted beam. The proposed concepts and optical configurations for tuning SOP by flow induced molecular reconstruction in liquid crystal can be improved for specific applications in future. Future research will hence be targeted mainly to enhance the device performance and also to explore different device configurations such as variable attenuator, tunable phase retarder and tunable filter. Spectroscopy, imaging and biosensors are the possible application areas in which the flow induced polarization device can be integrated into lab-on-a-chip devices. It is envisaged that the outcome of this thesis can enable future research and development of relevant tunable devices for their potential lab-on-a-chip devices applications.