Integrated optical devices based on liquid crystals embedded in polydimethylsiloxane flexible substrates

The contribution of this thesis is to find possible solutions for the creation of interconnections and optical switches to be used in microoptofluidic systems in the frame of the research activities of the Optoelectronic laboratory of the Department of Information Engineering, Electronics and Telecommunications (DIET). The main goal is to explore a new technology for integrated optic based on a low cost technology to produce low driving power devices. Optofluidics is the science which links the field of photonics with microfluidics, for the creation of innovative and state-of-the-art devices. Liquid crystals (LC) can be used for optofluidic applications because they have the possibility to change without external mechanical actions, the average direction of the molecules through the application of electric fields, reorienting the crystal molecules in such a way as to alter their optical properties [1-2]. The research on LC is more than a century old, but only since the ‘80s of the past century these materials were employed in various fields, from flat panel displays used for televisions, tablets, and smartphones, to biomedical and telecommunication applications [3-5]. The results reported in this thesis include simulation, design and preliminary fabrication of optofluidic prototypes based on LC embedded in polydimethylsiloxane (PDMS) channels, defined as LC:PDMS, with co-planar electrodes to control LC molecular orientation and light propagation. Fabrication techniques which were used include microelectronic processes such as lithography, sputtering, evaporation, and electroplating. The simulations were performed through the combined use of COMSOL Multiphysics® and BeamPROP®. I used COMSOL Multiphysics® to determine the positioning of the molecules in a LC:PDMS waveguide. The LC are the core through which light propagates in a PDMS structure. In addition to these simulations, I used COMSOL Multiphysics® to determine the orientation of the LC under the effect of an electric field [6-7] to create low-power optofluidic devices [8], [11]. I used BeamPROP® to explore the optical propagation of various optical devices such as: optical couplers, the zero gap optical coupler, and a multimodal interferometer. All these devices have been simulated through various combinations of geometries which will be extensively explained in the following chapters. The fabrication of prototypes was made in the Microelectronic Technologies laboratory of DIET. The optofluidic prototypes that I designed could be used in interconnection systems on biosensing devices for chemical or biological applications [10-11], wearable [12], or lab on chips [13], which are increasingly being applied in many research fields [14]. Many of these devices need to interface with electronics for processing signals coming from the interaction between the device with molecules, liquids or other biological substances. Moreover it is necessary to create flexible and biocompatible interfaces, whose features are not guaranteed in classic metal tracks. As it will be clear in the first chapter, metal interconnections must be designed with spatial, energy and throughput restrictions. To develop the optofluidic prototypes, I chose to use a combination of two materials for their commercial availability and ease of use: E7 and 5CB LC produced by Merck® as the transmissive medium and PDMS Sylgard 184 produced by Dow Corning® for the cladding [15-16]. The molecules of the LC are anisotropic, whose shape is elongated like that of a cigar. Under appropriate temperature conditions these molecules retain a state of aggregation in which, while retaining some mechanical properties of the fluids, they have the characteristics of crystals such as birefringence or x-ray reflection. These properties are due to two factors that characterize the various phases of LC: the orientational and positional order that vary according to the temperature. E7 was used in its nematic mesophase. The material used for the cladding of my prototypes was PDMS, a thermosetting polymer, flexible, biocompatible, economical, easy to work, and suitable for the creation of optical and optofluidic devices due to its transparency. The thesis is organized in six chapters whose contents are briefly outlined below: • In the first chapter there is a brief description of optofluidics and the transport phenomena of the liquids in the microchannels. The essential parameters for a correct interpretation of the behavior of the materials in the devices will be defined. Some examples of microfluidic devices, Optofluidic Optical Components (OOC) will be mentioned. • In the second chapter, LC’s will be presented, along with their general characteristics and their behavior in the presence of electric fields. An overview of integrated optic devices based on LC will be reported. • In the third chapter the experimental results will be presented concerning the fabrications and the technologies used to obtain electro-optical LC:PDMS waveguides. • The fourth chapter will be dedicated to a brief description of COMSOL Multiphysics® and BeamPROP® simulators, and the implementation of the model of LC channels in PDMS both in 2D and 3D. Also a brief description of Monte Carlo simulations based on Lebwohl-Lasher potential will be mentioned. • In the fifth chapter an LC:PDMS optical directional coupler and the most significant results will be described. • The sixth chapter is dedicated to the multimodal interferometer and its field of application, the theory behind this device and the results obtained from the simulations using the BeamPROP® • In the conclusion, a brief recap of the results obtained in this thesis and future developments will be presented.