Microoptofluidics using PDMS and liquid crystals: fabrication technology and devices

Recently polydimethylsiloxane (PDMS) gained a lot of interest to fabricate microfluidic flexible channels to make inexpensive, flexible and reconfigurable devices for many applications ranging from datacom to sensing and biomedical lab on a chip applications. Optical waveguides made of PDMS channels filled with nematic liquid crystals (LC), referred as LC:PDMS waveguides, were demonstrated showing polarization independent transmission of light at both visible and near infrared wavelengths. LC molecules are homeotropically aligned to the PDMS surface, without using any alignment layer as usually required in LC standard electro-optic devices. This is due to the interface hydrophobic interaction between the PDMS inner surface and the nematic LC molecules. Such optical waveguides can be made through a standard casting and molding technique, combined with filling procedure by capillarity to infiltrate the LC in its isotropic phase at 80 °C under vacuum in the PDMS channels. Such solution allows the design and fabrication of switchable and tunable devices by exploiting the efficient electro-optic and nonlinear optical effects in LC. One advantage of such approach with respect of classical integrated devices is a strong reduction of the power budget in terms of both energy dissipation and driving power. We show fabrication techniques and characterization of LC:PDMS waveguides, including fabrication of ITO electrodes deposited on PDMS surfaces in order to obtain low consuming power integrated optic devices. We report characteristics of directional couplers based on LC:PDMS waveguides with an extinction ratio of the output power level of over 20 dB, as a basis for low power optical switches. Directional couplers with a length of 45 μm and a waveguide gap as short as 300 nm are also feasible in view of compact and low power flexible optical switches operating at the fiber optic communication wavelength of 1550 nm.