Ricerc@Sapienza

We present optoelectronic tunable and switchable devices for integrated optics based on easy-to-build optical waveguides. The devices take advantage of the spontaneous homeotropically alignment of liquid crystals (LC) molecules to the surface of a polydimethylsiloxane (PDMS) microfluidic channel (LC:PDMS). This behaviour was investigated by means of Monte Carlo simulations, which allowed to evaluate the molecular organization and ordering inside the cell. A set of rectangular PDMS waveguides, filled with LC were designed and the calculations were performed using a Lebwhol-Lasher (LL) lattice spin model to emulate the LC cell, imposing homeotropic boundary conditions on each wall of the channel. Simulations confirmed that the aligning surfaces affects the ordering, being this ordering constant along the channel. Samples of the structure were made using cast and moulding techniques to build the PDMS rectangular microchannel waveguides, which were infiltrated with LC in its isotropic phase at 80 °C under vacuum by capillarity. As forecasted, no alignment was needed to obtain LC molecule orientation on the PDMS inner surfaces. Characterization, performed at a wavelength of 1550 nm, confirmed a substantial insensibility to polarization despite liquid crystal anisotropy, being the variation of the transmitted light power with polarization angle less than 0.3 dB. Based on this simple structure, we demonstrated LC:PDMS optical directional couplers with a coupling length of 45 µm and 300 nm waveguide gap. Switching capabilities of the device were confirmed, with an extinction ratio of the output power level higher than 20 dB [4]. To obtain the electro-optical control of the directional couplers we fabricated flexible electrodes by sputtering ITO on the PDMS, with different deposition times of 30 s, 60 s, and 90 s, both at 100 °C and 150 °C. Preliminary conductive ITO layers with a thickness of about 27.5 nm were obtained.