Integrated optics with optofluidic PDMS channels and liquid crystals

Industry is looking for inexpensive, flexible and easy-to-build optical reconfigurable and switching devices to be used in several areas such as optical switching, sensing and lab on chip applications. In this stream, polydimethylsiloxane (PDMS) microfluidic channels offer interesting features such as the spontaneous homeotropically alignment of liquid crystals (LC) molecules to its surface. Leveraging on this effect, which is due to the interface hydrophobic interaction between PDMS and LC, it is thus possible to easily make tunable optical waveguides. A sample of such kind of devices was designed, made and characterized. The theoretical study of the waveguide inner structure was performed by Monte Carlo simulations of the molecular ordering inside the microchannels and the calculations were based on a Lebwhol-Lasher (LL) lattice spin model to emulate the LC cell by imposing homeotropic boundary conditions at each wall of the channel. The simulation results confirmed how the ordering inside the system is affected by the aligning surfaces whereas this ordering is constant along the channel. PDMS microchannels were built using standard casting and molding techniques and the LC, in its isotropic phase, was infiltrated at 80°C under vacuum by capillarity. As expected no alignment was needed to obtain molecule orientation on the LC:PDMS surface, being this result experimentally confirmed during the characterization of the device, in which it was shown that light transmission is allowed through crossed polarizers only along the nano-structured region on the edges. The characterization, carried out at the infrared wavelength of 1550 nm confirmed switching potentialities of such structure which presented an extinction ratio of the output power level of over 20 dB. Moreover the device is polarization insensitive, since the transmitted light power variation with polarization angle is limited to less than 0.3 dB. Such solution, relying on the efficient electro-optic and non-linear optical effects in the LC allow the design and fabrication of switchable and tunable devices for integrated optics. As an example we defined a basic directional coupler structure consisting of PDMS channels filled with nematic liquid crystal (NLC) with a square cross section of 2 micron x 2 micron. Calculations show that a directional coupler with a coupling length of 45 micron and a waveguide gap as short as 300 nm can be obtained. One of the benefits of this approach with respect to classical integrated devices is the major power budget reduction in terms of both energy dissipation and driving power.