Electrically tunable ring resonators incorporating nematic liquid crystals as cladding layers

Abstract

We have demonstrated electrical tuning in ring resonators fabricated from silicon-on-insulator wafers by incorporating nematic liquid crystals ͑NLCs͒ as the waveguide top and side cladding material. Photolithographically defined electrodes aligned around the ring resonator were used to control the orientation of the NLCs to modulate the cladding refractive index and, hence, the resonant wavelengths of the ring resonator. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1630370͔Microring resonators, fabricated with conventional semiconductor processing methods in silicon, offer significant advantages over the existing telecommunication filter technology and may be the foundation of future dense-wavelengthdivision-multiplexing ͑DWDM͒ filters. 1-5The high refractive index ͑RI͒ contrast available in silicon-oninsulator ͑SOI͒ ring resonators enables low loss and high-Q filters fabricated with radii down to a few microns.6,7 Such resonators can be designed as notch filters for adding or dropping individual channels in the telecommunication bands and can be densely integrated in photonic networks. For reconfigurable DWDM systems, and to compensate for temperature changes, it is desirable to tune the precise channel frequency dropped by such resonator add/drop multiplexers.Two primary methods exist to control the optical path length of a ring resonator and thus tune its resonant frequency. To statically tune a ring resonator one can either adjust the physical dimensions ͑in particular its circumference͒ or the refractive indices of the constituent materials of the resonator. Dynamically tunable resonators provide another level of functionality over statically tuned resonators and are most practically obtained by controlling the refractive indices of the constituent materials. Dynamic tuning is commonly achieved by thermally changing the RI, traditionally by introducing a heater close to the resonator. 8 However, power dissipation may provide a serious problem in such tunable ring resonator designs, especially when many resonators have to be integrated in a DWDM multiplexing system. In this letter we demonstrate the dynamic tuning of a ring resonator by changing the RI of its cladding via the orientation of the nematic liquid crystals ͑NLC͒.The resonator system under study, as shown in Fig. 1, was fabricated from a SOI wafer with silicon thickness of 205 nm and oxide thickness of 1 m, ring radius of 5 m, and ring and waveguide widths of 500 nm. The resonator was coupled to one waveguide, which served as both the input and output port and was separated from the resonator by a 100 nm gap. Modulation electrodes were then photolithographically defined and deposited using standard lift-off processing. The left and right electrodes were approximately 4.0 m wide and were spaced about 400 and 300 nm from the resonator, respectively ͑Fig. 1͒. The modulation electrodes were designed to preferentially orient the directors of the NLC molecules parallel ͑azimuthally oriented͒ to the resonator. To minimize their electrostatic ene...

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